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Shrinkage Characteristics of Potatos Dehydrated Under Combined Microwave and Convective Air Conditions
Drying Technology
Abstract
A study of the pysical structure and quality potatoes during microwave and air drying war undertaken. The effect of various experimental variables on volumetric shrinkage including microwove power levels, air temperature and velocity as well as sample geometry was investigated. In both microwave and air drying, the shrinkage of potatoes showed a linear behaviour in relation to different moisture content levels. Experimental data showedthat air velocity and microwave power had a major effect on the degree of shrinkage, also the shrinkage behaviour was independent of sample geometry and air temperature. The use of microwave produced smaller changer in volume (less shrinkage) and henee a better quality finished product.
... Whole Roots Sokhansanj et al., 1999 Onion Slices, Rings, Flakes (Rajkumar and Sreenarayanan 2001;Sutar et al., 2007a;Praveen et al., 2006] Parsley Slices Sobiech, 1980 Potato Cubes, Slices, Shreds, Spheres, Cylinders (Khraisheh et al., 1997;Lebovka et al., 2007;Cui et al., 2004;Wang and Brennan 1995;Srikiatden and Roberts 2006;Sutar et al., 2009) ...
... To overcome the drawbacks of water blanching, steam blanching can be used to inactivate the enzyme activity in roots like potato and carrots (Litvin et al., 1998). The pretreatments like soaking of roots in solutions of NaCl, sucrose, NaHSO3, potassium metabisulphite and sodium metabisulphite are used to supress the enzymatic reactions, to prohibit discolouration resulting from oxidation of polyphenolic compounds during root drying (Ghosh et al., 2004;Khraisheh et al., 1997). The roots are also pre-treated using Pulsed Electric Field (PEF) at various electric field strengths and different pulse durations to enhance the drying rate (Lebovka et al., 2007). ...
... Dipping in 0.45-1% sodium metabisulphate solution for one minute Soaking in 2% NaHSO3 solution for 5 min Blanching in water at 95 °C during 5 min (Khraisheh et al., 1997;Lebovka et al., 2007;Sutar Blanching in water at 95 °C for 4.5 min. Blanching in water at 85°C for 3.5 minutes Electric field strength (E) 400 V/cm, pulse duration 10 -3 s, pulse repetition time 10 -2 s Freezing thawing Blanching in water at 75°C for 15 min Sweet Potato Immersion in 1 % NaCl solution Immersion in KMS and citric acid solutions in the concentrations 0.5:0.5%, ...
... Other new drying methods (barely used at industrial level) such as desiccant drying, infrared drying and supercritical carbon dioxide drying, etc., have also been reported in the recent scientific literature and will be briefly described in this manuscript ( Mujumdar, 2007). First of all, it appears important to highlight that drying behavior of food materials largely depends both on external conditions, such as temperature (in direct relation with the glass transition temperature of food material), pressure, relative humidity, and air velocity, and on internal factors evolving in the course of drying, like food material composition, moisture content, thickness and geometry, initial structure, water diffusivity, as well as its physical (glassy or rubbery) state ( Khraisheh et al., 1997;Mulet et al., 2000;Ratti, 2001;Hatamipour and Mowla, 2002;Mayor and Sereno, 2004;Gornicki and Kaleta, 2007;Yadollahinia and Jahangiri, 2009). In this paper, the effects of some dry drying methods and their combination on the most common physical (color, appearance, particle size and shape), textural, structural (density, porosity, specific volume, etc.), sensorial (aroma, taste, flavor), nutritional (vitamin, phytochemical content, etc.), and functional properties (rehydration capacity, antioxidant activity, powder flowability, etc.) of the final products are described. ...
... A MW drying process consists in three drying periods: (1) a heating-up period in which MW energy is converted into thermal energy within the moist materials and the product temperature increases with time, (2) a rapid drying period during which thermal energy is used for moisture vaporization and transfer and (3) a reduced drying rate period during which the local moisture is reduced to a point that the energy needed for moisture vaporization is lower than the thermal energy induced by MW ( Maskan, 2001;Zhang et al., 2006;Ozyurt et al., 2011). MW drying can be assigned as a "volumetric heating process", MW electromagnetic energy being directly absorbed by water-containing materials and converted into heat by molecular agitation ( Khraisheh et al., 1997;Piyasena et al., 2003). It is worthy to note that MWD has been demonstrated to have moderately low energy consumption ( Tulasidas et al., 1997;Sagar and Kumar, 2010;Motevali et al., 2011). ...
... Certain authors have suggested the use of air temperature below the glass transition temperature of the material to be dried in order to considerably limit the degree of shrinkage ( Karathanos et al., 1993;Kerdpiboon et al., 2007). On the other hand, high values of air velocities (from 1 to 2 m/s for instance) permit to both shorten the drying time and limit the extent of shrinkage ( Ratti, 1994;Khraisheh et al., 1997;Zhang et al., 2014). Furthermore, decreasing the relative humidity of the drying air enables significant decrease in drying time (around 25%) and shrinkage level ( Lang and Sokhansanj, 1993;Kaya and Aydin, 2009). ...
In recent years, fruits and vegetables have received considerable attention, as these materials have been reported to contain a wide array of phytochemicals, which are claimed to exert many health benefits including antioxidant activity. In some cases where bioactive compounds extraction cannot be performed on fresh products, drying appears as a necessary step enabling their later use. Drying is a widely used food preservation process in which water removal minimize many of the moisture-driven deterioration reactions impacting the bioproduct quality. Dried fruits and vegetables and their application in powder form have gained interest in the food industry. Drying and grinding conditions during powder processing greatly influence the quality attributes of biological materials. It implies not only nutritional changes but also physical, textural, sensorial and functional changes. These changes are of great importance and require to be controlled through retroengineering approaches. This paper reviews the effect of the different dry drying and grinding methods on the physicochemical and functional properties of the final products. Overviews of some of the innovative concepts as well as approaches to alleviate the above-mentioned changes are discussed.
... The processing conditions are one of the factors affecting the magnitude of shrinkage. Khraisheh, Cooper, and Magee (1997) have reported that the increase in air velocity produces less shrinkage. ...
... Similar to the study of Kocabiyik and Tezer (2009) (Khraisheh et al., 1997) (Tables 9 and 10). ...
Infrared drying characteristics of kiwifruits under natural and forced drying air convection with different conditions were investigated. An experimental study along with statistical analysis aimed to evaluate quality characteristics of infrared‐dried kiwifruit slices, in terms of drying time, rehydration ratio and shrinkage as a function of infrared power levels, slice thicknesses, slice distance from the infrared lamps, and air velocity. Response surface methodology was used for optimization of drying parameters with employing desirability function. Minimum drying time, shrinkage, and maximum rehydration ratio assumed as criteria for optimizing drying conditions of kiwifruit slices were strongly dependent on the drying conditions. All operating variables had a significant effect on total responses, but slice thickness almost was the most prominent factor. The slices dried at the highest power level, the lowest distance from the Infrared lamp, the least thickness, and air velocity showed a higher rehydration capacity than slices dried at the other conditions. The results of this research can provide a theoretical and practical basis for further design, optimization, and scale‐up of such processes and also the development of models that predict the behavior of infrared food drying.
... The shrinkage increases during drying until the product takes its final skeleton when the availability of moisture becomes limited (less than 1 kg/kg d.b.). These results could be explained by the low porosity of carrots and their high apparent density nature [30,31]. ...
... In a previous work [18], the shrinkage of carrots during infra-red drying was experimentally studied and the shrinkage coefficient (β) was found to be equal to 1.4. In literature, it is reported that the increased density and shrinkage of carrot was dependent on the operating temperature of hot air drying [31]. ...
In this work a two-dimensional convective drying model of heat, mass and momentum transfer is developed taking account of the shrinkage and the quality degradation of carrots. The kinetics of β-carotene degradation in dried agricultural product is considered to follow a first-order reaction. Temperature dependence of the reaction rate constants followed the Arrhenius relationship. The developed model, describes the strong coupling between mass, heat and momentum transfers, shrinkage and quality (β-carotene) changes and takes into account the dehydration of the solid caused by a temperature increase.
... Wang and Brennan (1995) reported that the degree of shrinkage at a low drying temperature (40 C) is greater than at a high temperature (70 C). However, Ratti (1994) and Khraisheh et al. (1997) observed that the volumetric shrinkage of individual particles is affected mainly by air velocity and the temperature had a negligible effect. Other authors found no influence of drying conditions, temperature ( Khraisheh et al., 2004;Hassini et al., 2007) or air velocity ( Hassini et al., 2007) on shrinkage. ...
... At the beginning, the potato cubes maintained their original geometry but they suffered deformations at the end of the dehydration. This is a phenomenon that has been observed by other authors ( Ratti, 1994;Khraisheh et al., 1997Khraisheh et al., , 2004). From the calculation of shrinkage as the ratio V/V 0 , it was observed that, in general, the final volume was close to 17% of the initial volume. ...
Abstract
Shrinkage is one of the most important physical changes that occur during the dehydration of foods. In this work, the effect of the temperature (35, 40, 50, 60 and 70 °C) and air velocity (7, 8, 9 and 10 m/s) on bulk volumetric shrinkage was investigated. Volume changes were evaluated by image analysis. It was found that neither temperature nor air velocity had any significant effects on bulk shrinkage in this system. The bulk shrinkage of the potato cubes was well correlated with the moisture content of the sample during drying (R2 = 97.28). Volume varied linearly with the moisture content changes under the studied conditions. The volume of lost water and the decrease in volume of the samples during dehydration were similar.
Keywords air drying, shrinkage, potato
... The relative area shrinkage (S b ) on potato slices (Fig. 1g) changed in accordance to what was already observed by various authors. [22][23][24][25] They demonstrated that, as drying proceeded and cellular rigidity increased due to moisture loss, potato slices tend to shrink faster, but also to bend upwards and thus to attain an irregular shape. The severity of the phenomenon is reduced wileyonlinelibrary.com/jsfa ...
... In fact, it is fundamental to consider that changes in quality attributes of horticultural products during drying are successfully measurable on the basis of their variations in size and shape. [23][24][25]36,[38][39][40] As expected, models based on relative area shrinkage (S b ) failed in prediction of colour changes (data not shown). Conversely, the dry basis moisture content was successfully predicted through wileyonlinelibrary.com/jsfa the spatial information with RMSEP = 0.31, adj. ...
Background:
The potential of hyperspectral imaging (500÷1010 nm) was evaluated for monitoring of the quality of potato slices (var. Anuschka) of 5-, 7- and 9-mm thicknesses subjected to air drying at 50 °C. The study investigated 3 different features selection methods for the prediction of dry basis moisture content and colour of potato slices using the partial least squares regression (PLS).
Results:
The feature selection strategies tested include interval PLS regression (iPLS), and differences and ratios between raw reflectance values for each possible pair of wavelengths (R[λ1 ]-R[λ2 ] and R[λ1 ]:R[λ2 ], respectively). Moreover, the combination of spectral and spatial domains was tested. Excellent results were obtained using the iPLS algorithm. However, features from both datasets of raw reflectance differences and ratios represent suitable alternatives for development of low-complex prediction models. Finally, the dry basis moisture content was high accurately predicted by combining spectral data (i.e. R[511 nm]-R[994 nm]) and spatial domain (i.e. relative area shrinkage of slice).
Conclusions:
Modelling the data acquired during drying through hyperspectral imaging can provide useful information concerning the chemical and physicochemical changes of product. With all this information, the proposed approach lays the foundations for a more efficient smart dryer that can be designed and its process optimized for drying of potato slices.
... There is a lack of information regarding the mechanism of moisture movement during drying process. Therefore, effective moisture diffusivity, which may include liquid diffusion, vapor diffusion, vaporization, condensation, hydrodynamic flow, and other possible mass transfer mechanisms, was used to represent an overall mass transport property of water in the material (Khraisheh et al. 1997). Effective moisture diffusivity (D eff ) was calculated using Eq. 5. ...
... The calculated values of R 2 and RSME are 0.98-0.99 and ±0.17 to ±0.20, respectively at different microwave power levels. The slight deviation from the linearity seen at lower microwave output powers may be due to more shrinkage in the product, non-uniform distribution of moisture during drying, and variation in effective moisture diffusivity with moisture content (Khraisheh et al. 1997). ...
The effects of microwave drying on quality attribute of okra were evaluated. Microwave power values show significant effect on moisture diffusivity, color, hardness and sensory attributes of dried okra. The total color change () increased whereas , and decreased with an increase in both power and time. The kinetics of and were well expressed by zero-order kinetic models, while and followed first-order kinetic models. Hardness was expressed by a first-order kinetic model. Sensory evaluation showed that okra dried at 800 W was most attractive in terms of color, texture, flavor and overall liking. Microwave drying at 800 W presented the shortest drying time with the least change in hardness, but the most change in color. Thus, high-power microwave drying potentially produces dried fruits and vegetables with minimum changes in hardness.
... Uticaj geometrije sušenih uzoraka na zapreminsko skupljanje nije od značaja [3,6]. Za aproksimaciju eksperimentalnih rezultata predloženi su novi modeli koji opisuju zavisnost relativnog zapreminskog skupljanja materijala od bezdimenzionog sadržaja vlage U: ...
... Sl. 1. Eksperimentalna aparatura 1 -materijal, 2 -lesa, 3 -grejači, 4 -transformatori, 5 -termoparovi, 6 -centrifugalni ventilator, 7 -anemometar, 8 -panelmetar, 9 -sistem za akviziciju, 10 -peć, 11 -vaga, 12 -higrometar Fig. 1. Experimental apparatus 1 -material, 2 -shelf, 3 -electrical heaters, 4 -transformers, 5thermocouples, 6 -centrifugal fan,7 -anemometer, 8 -panelmeter, 9 -data acquisition system, 10 -stove, 11 -balance , 12hygrometer REZULTATI I DISKUSIJA U literaturi je dat veći broj matematičkih modela za aproksimaciju ekperimentalnih rezultata koji opisuju zavisnost relativnog zapreminskog skupljanja materijala od sadržaja vlage [3,4,6,7,9,10]. U ovom radu predloženi su novi modeli koji su dati u tabeli 1. Za svaki model i set podataka, pomoću sedam metoda (Quasi-Newton, Simplex, Simplex and quasi-Newton, Hooke-Jeeves pattern moves, Hooke-Jeeves and quasi-Newton, Rosenbrock pattern search i Rosenbrock and quasi-Newton) određene su vrednosti koeficijenta korelacije. U slučajevima kada su dobijene vrednosti različite, za relevantna se usvaja najveća vrednost. ...
... Considering that date plum fruits have reasonable amounts of nutrients and bioactive compounds, drying could establish to extend the shelf-life and preserve the phytochemicals and antioxidant compounds of date plum fruits. The convective drying or hot air-drying method is an alternative technique to improve the shelf life of food products by exposure to hot air [15,16], preferred in industrial, commercial, and agricultural products due to its lesser operating costs and uniform applications [17,18]. The previous findings highlighted the potential of convective drying as an efficient and safe drying method for fruits and vegetables with the ability to maintain nutritional and antioxidant properties [19]. ...
Date plum (Diospyros lotus L.) fruits are a good source of bioactive compounds and antioxidants. Drying can increase the shelf life of the fruit and its applications in the food development industry. Optimizing the drying conditions can help to produce prime-quality dried date plum fruits and conserve nutrients including phytochemicals. This study used a two-factor graphics-optimal design to optimize convective drying considering the air velocity and drying temperature of date plum fruits. The independent factors considered included drying temperature (43.78–86.21 °C) and air velocities (0.54–1.96 m/s), and the responses included total phenolic content (TPC), total flavonoid content (TFC), ferric reducing antioxidant power (FRAP), and 2,2-diphenyl-1-picrylhydrazy (DPPH) radical scavenging activity of date plum fruit. The optimized drying conditions (68 °C and 1.75 m/s) resulted in desirable TPC, TFC, FRAP, and DPPH values. The findings indicated that long drying time at low temperatures significantly decreased the phenolics and antioxidants. Date plum tea with different decoction times (5, 10, and 15 min) was prepared from fruits dried at optimum conditions. A decoction time of 5 min resulted in the highest catechin, vanillic, epicatechin, syringic acid, and quercetin-3-glucoside content, which were 2.45 ± 0.04, 11.06 ± 0.11, 22.03 ± 0.11, 12.95 ± 0.08, and 9.37 ± 0.10 mg/L; respectively. In vitro gastrointestinal digestion revealed that the tea product can be a source of highly bioaccessible (> 80%) gallic acid, catechin, vanillic acid, and quercetin-3-glucoside. Applying optimized drying conditions to dehydrate date plum fruit can be useful in preparing a highly bioaccessible polyphenol-rich tea.
... The convective drying of fruits is the most applied drying technique to stabilize fruits against physicochemical degradations and increase shelf life. The convective drying method occurs by exposure to heat when the stream of hot air flows over the food, leading to its dehydration [22]. Mathematical modeling is a valuable tool in drying, optimizing, and designing dryers. ...
Autumn olive berries (AOBs) are a good source of natural antioxidants and compounds associated with protective effects on health. The current study investigated the possible impacts of the convective drying process on textural, morphological, phenolic compounds, and antioxidant properties of AOBs. Four drying temperatures (50, 60, 70, and 80 °C) in addition to three air velocities (0.5, 1.0, and 1.5 m/s) were applied. The fresh AOBs contained 77.90% moisture, 4.37 pH, and 16.90% total soluble solids. The phytochemical data of the fresh sample revealed that it contained 1739.95 mg/kg total phenolic (TPC), 4370.69 mg/kg total flavonoid (TFC), 150.69 mg/kg lycopene content (LC), 50.20 mmol/g DPPH and 37.38 mmol/g FRAP. Furthermore, the TPC, TFC, lycopene, DPPH, and FRAP values of dehydrated AOBs were evaluated. It was observed that the convective drying increased the TPC, TFC, and antioxidant properties under all the drying conditions, giving the highest values at 80 °C. The AOBs dried at 80 °C contained the highest level of 96,592.22 mg/kg TPC, 476.97 mg/kg LC, 6.57 mg/kg gallic acid, 9.66 mg/kg catechin, 1.80 mg/kg benzoic acid, 2.13 mg/kg salicylic acid, 37.24 mg/kg ellagic acid, and 8.83 mg/kg quercetin-3-glucoside. Fruits dried at 60 °C contained 1873.09 mg/kg, a higher flavonoid content. The TPC, TFC, LC, and DPPH, increased with dried compared with fresh state. The color values (L*, a*, and b*) of AOBs were decreased by drying, and the highest level of ∆E (10.89) was detected at 50 °C. Furthermore, the dried autumn olive at 80 °C and 1.5 m/s air velocity provided high hardness, chewiness, and gumminess of 6764.23, 1691.83, and 2989.16, respectively. The result of drying air velocities revealed no significant effects on rehydration, textural properties, and total color change. The air velocity of 1.5 m/s presented the highest values of shrinkage, hardness, antioxidant contents, and antioxidant activity for all dried samples than other air velocities. Midilli model was the best-fitted model describing the drying kinetic of AOBs, and the Weibull model described well the rehydration behavior of dried AOBs. The dried AOBs were statistically much more hardness, gumminess, and chewiness than the fresh AOBs. The present study highlighted the potential of dried AOBs as a good source of various nutrients that maintain human health with higher antioxidant properties compared with fresh fruit. These results indicate that the best drying conditions to have dried AOBs with high antioxidant properties were at 80 °C and 1.5 m/s. The dried AOBs obtained at these drying conditions can be used on the industrial scale for multi-purposes.
... Convective air drying is widely used in industries, but it can cause the degradation of valuable compounds, the change of carrot color, and texture because of the use of high temperatures (Marfil, Santos, & Telis, 2008;Figiel, 2010;Chua, Mujumdar, Hawlader, Chou, & Ho, 2001;Khraisheh, Cooper, & Magee, 1997;Liu et al., 2018). Freeze-drying can be used for a better preservation of the product's quality, however this method requires a long processing time and higher costs from energy consumption (Parniakov, Bals, Lebovka, & Vorobiev, 2016;Salazar, Alvarez, & Orrego, 2018). ...
The effect of pulsed electric fields (PEF) on the kinetics of vacuum drying (VD) and rehydration of carrot tissue was studied. The intensity of PEF treatment was E = 0.6 kV/cm and the total PEF treatment duration was tPEF = 0.1 s. The VD was performed at sub-atmospheric pressure p = 3.03 × 10⁴ Pa for different temperatures Td = 25 °C and 90 °C. During VD, the kinetics of moisture removal and temperature evolution inside of untreated and PEF pretreated samples were studied. The color and structure of the samples were also investigated. PEF pretreatment enhanced VD kinetics at different studied temperatures. However, its effect on VD was more evident at lower drying temperatures. With the PEF pretreatment, the drying time was reduced by 55% and 33% at 25 °C and 90 °C respectively. After the drying and rehydration process, the change in color of PEF pretreated samples was less significant compared to untreated ones. After rehydration, the untreated and PEF pretreated samples were nearly restored to their initial form and size.
... However, it causes undesirable thermal degradation of the product quality for which temperature is the main influencing factor (Chen and Mujumdar, 2008). Measurement of color degradation (Chua et al., 2002) and shrinkage (Khraisheh et al., 1997;Mclaughlin and Magee, 1998) during convective drying of potato has been reported by many researchers using destructive approaches. Application of non-invasive technologies for the potato processing mainly focused to sorting and grading based on color, size and shape (Narendra and Hareesh, 2010;Mahendran et al., 2012;Avila and Silva, 1999;El-Masry et al., 2012;Hassankhani and Navid, 2012). ...
Lack of accurate information for the drying kinetics is the main barrier to optimize drying processes effectively. Online monitoring of drying data is not common due to practical issues involved in set-up application. In this study an imaging system was developed for large drying units to investigate the quality changes (color kinetics and shrinkage) combine with weight loss of food product spatially and non-destructively during convective drying. The imaging setup was used in a diagonal-airflow batch dryer and shifted along the dryer length to take data (food images and weight loss) at different positions using potato slices (5 mm thick, 60°C) as drying material. First order model fitted well to value of ΔL* and Δb* while Δa* and ΔE fitted in the zero order model. The experimental and models predicted color kinetics revealed good correlation coefficients (R2 = 0.88-0.99, P ˂ 0.0001). The shrinkage of the samples exhibited two periods; a faster with moisture ration (MR) up to 0.3 and afterwards it became slow employing polynomial cubic regression (R2 = 0.97-0.99, P ˂ 0.0001). The imaging system effectively measured the change in quality parameters spatially along the dryer. The current study would help to develop and implement low cost setup for online measurements used for the assessment of drying processes in large drying units.
... It is therefore important to measure this phenomenon as a function of drying experimental conditions. The introduction of a parameter that accounts for shrinkage while developing drying models seems also to substantially improve their quality (Khraisheh et al., 1997;Lang et al., 1994;Rahman, 2001). ...
Ce travail avait pour objectif l'étude de l'impact des deux procédés : la déshydratation imprégnation par immersion (DII) et le séchage convectif menés séparément ou combinés sur les transferts couplés d'eau et de saccharose et les principaux critères de qualité de fruit (écart de couleur, teneur en phénols totaux (PT), teneur en acide ascorbique (AA), retrait volumique, activité de l'eau) en prenant la poire Conférences comme modèle. L'effet des procédés sur les morceaux de poire a été appréhendé selon deux approches : (i) une étude globale de l'impact des variables de commandes sur la qualité du produit fini et (ii) une étude cinétique des composés d'intérêt nutritionnel et de la qualité (couleur, retrait, activité de l'eau).Un plan d'expériences composite centré à quatre facteurs et cinq niveaux a été établi (concentration en saccharose de la solution osmotique : 25-65%, température de DII : 20-60 °C, durée de la DII : 0,5-6,5 h, température du séchage convectif : 30-70 °C) pour étudier l'effet global des procédés sur le produit fini. En DII, les paramètres « °Brix, « durée » et « la température de la solution » ont un effet significatif (p < 0,01) sur les pertes en eau, le gain en soluté et les attributs de qualité de morceaux de poire. Les pertes en AA sont plus élevées que les pertes en PT et sont essentiellement dues à l'oxydation et à l'entraînement par l'eau. Durant le séchage convectif, les pertes en PT et en AA dépendent plus de la durée du procédé que de la température d'exposition. Elles atteignent respectivement 80% et 34% après 10 h de séchage à 30 °C. Le changement de couleur de morceaux de poire dépend de la durée et de la température de traitement et est plus prononcé en fin de séchage. Des modèles quadratiques prédictifs ont été proposés pour relier les variables de réponse (aw, différence de couleur, perte en PT et durée totale de traitement) aux paramètres opératoires des deux procédés étudiés. Cependant, la perte en AA décroit linéairement avec la durée de DII. Une optimisation multicritères en utilisant la méthodologie des surfaces de réponse (MSR) a été proposée (pertes minimales en PT et en AA, durée totale du traitement et différence de couleur réduites et une aw inférieure à 0,6) pour la combinaison DII/séchage. Les conditions optimales sont : solution osmotique à 28°C et 25°Brix, une durée de DII de 30 min et un séchage convectif à 60 °C. Par ailleurs, les effets de la température (30, 45 et 60 °C) et de l'imprégnation en saccharose par DII (10 et 65 min, 70°Brix, 30 °C) sur les isothermes de désorption et sur la température de transition vitreuse (Tg) de morceaux de poires, de pommes et d'abricots ont été déterminés. L'imprégnation en saccharose du tissu végétal atténue l'influence de la température sur sa capacité de sorption. Elle engendre un effet dépresseur de l'activité de l'eau et une augmentation de l'hygroscopicité de la poire, la pomme et l'abricot à température élevée. Les isothermes de désorption des abricots frais et enrichis en saccharose présentent un croisement des courbes marquant l'inversement de l'effet de la température sur les isothermes. La Tg dépend de la température, de la teneur en saccharose et de la teneur en eau des fruits. Pour une teneur en eau constante, l'augmentation de la température et la durée de DII se traduisent par l'augmentation de la Tg. Les digrammes de phase (Tg=f(X) et Tg=f(aw)) des fruits ont été établis à 30, 45 et 60°C et les teneurs en eau critiques ainsi que les activités d'eau critiques ont été déterminées. Une teneur en eau en fin de séchage inférieure ou égale à 0,02 g/g M.S. est recommandée pour l'obtention de morceaux de fruit stables aux niveaux microbiologique, physicochimique et rhéologique.
... Convective air drying is wildly used because of its low costs, but it must run for a long time and at high temperatures. This can cause degradation of high-value components, changes in color (Marfil et al. 2008;Figiel 2010;Chua et al. 2001), and changes in texture because of considerable product shrinkage (Khraisheh et al. 1997). Vacuum drying (VD) is commonly used because of its low pressure, which reduces the boiling point of water (Parniakov et al. 2016). ...
This study investigates the effect of pulsed electric fields (PEF) on the kinetics of vacuum drying (VD) of carrot and on the preservation of the quality of dried carrot tissue. The impacts of PEF-treatment and VD on β-carotene content and color of carrot samples were studied. PEF treatment was applied with intensity E = 0.6 kV/cm and total treatment time tPEF = 0.1 s to reach a high level of carrot tissue electroporation. The VD was applied at the pressure p = 0.3 bar for different temperatures, Td = 25, 50, 75, and 90 °C. The spectrophotometric method was used to determine the β-carotene content. The color was measured using the CIE L* a* b* method. Obtained results indicated that PEF treatment let to a noticeable decrease of drying time (by 33–55% at Td = 25–90 °C). The activation energy was found to be 18.25 kJ/mol and 13.4 kJ/mol for untreated and PEF-pretreated samples, respectively. The reduction of drying time by PEF pretreatment was beneficial for the retention of β-carotene in dried samples. The application of PEF treatment resulted in smaller changes in color ∆E as compared with untreated samples; this tendency was observed for all studied temperatures.
... If air velocities are very high, drying may make the surface of the sample stiff, limiting shrinkage even at the earliest stages. The surface of the sample does not become stiff at low air velocities until the moisture content has attained very low values (Khraisheh, Cooper, & Magee, 1997). ...
This work aims to estimate quality characteristics of dried pumpkin samples in an infrared dryer under natural and forced drying air convection systems. Drying time, rehydration ratio, and shrinkage as a function of infrared intensity, sample thickness, sample distance from the infrared lamps, and air velocity were optimized using response surface methodology. Maximum rehydration ratio and minimum drying time and shrinkage were assumed as criteria for optimizing drying conditions of the samples. The optimum values of process variables were 1 m/s air velocity, a sample thickness of 2 mm, 2,000 W of infrared intensity, and sample to infrared lamps distance of 550 mm. Based on the significant effect, sample thickness was the most prominent factor. The optimal values of drying times, rehydration ratios, and shrinkages of the dried samples under natural and forced convection were 1.068 hr and 1.017 hr, 10.375 and 9.141 (g rehydrated sample/g dried sample), and 69.38 and 73.17 (%), respectively.
Practical Applications
Pumpkin as one of the tasty, valuable and healthy nutrition vegetables containing a lot of biologically active substances is rich in phenolics, polysaccharides, flavonoids, vitamins, mineral salts, and a good source of carotene and water‐soluble vitamins. Because of the high initial moisture content, it is very sensitive to microbial spoilage. Thus, the drying can be greatly minimized microbial activities and deterioration chemical reaction by the removal of moisture up to a certain level. In addition, choosing the right type of dryer, design, simulation, and optimization of the drying process are largely dependent on the knowledge of the drying characteristics. It is worth mentioning, the developed statistical multivariate model equations accompanied by the optimal process conditions can be very useful for the design and operation of IR dryer when there is no certain multidimensional infrared drying model for high‐value foodstuffs such pumpkin.
... All the dried samples were rehydrated at constant temperature of 30 C for 600 min, to avoid any damage to product tissue. (Giri & Prasad, 2006), apple (Sjöholm & Gekas, 1995), strawberry (Raghavan & Venkatachalapathy, 1999), carrot (Nahimana & Zhang, 2011), papaya (Kurozawa et al., 2012), and potato (Khraisheh et al., 1997;. ...
Green bell pepper was dried under vacuum assisted microwave (VAM) drying conditions. The effect of microwave power (100, 200, 300 W) and vacuum pressure (200, 400, 600 mmHg) were observed on shrinkage and rehydration characteristics of the sample. Shrinkage was modeled as a linear function of volume change of material and its moisture content. A model was proposed where apparent density could be evaluated as a function of product's moisture content. The sample volume in case of VAM drying shrunk by 59 to 78%, while in case of hot air drying, the shrinkage occurred up to 90%. At a given power level of 100 W, equilibrium rehydration values (Req) increased from 5.04 to 5.75 for samples dried at increasing vacuum pressure from 200 to 600 mmHg. Rehydration rate constant decreased correspondingly for samples dried at higher microwave power levels. Higher level of microwave power and vacuum pressure played significant (p < 0.01) role in producing more porous (lesser shrunken) product leading to higher diffusion (and thus better rehydrated product) of water through it. Diffusion coefficient in rehydration was calculated using Fick's second law. The values of “k” and “R²” varied from 1 × 10⁻³ to 6.4 × 10⁻³ and 0.986 to 0.999, respectively.
Practical applications
Shrinkage and rehydration characteristics of dried vegetables are of utmost importance when it comes to culinary preparations, premixes, convenient foods and retention of sensory qualities. The model proposed in the present study will be helpful in evaluating shrinkage as a function of apparent density and moisture content of dried green bell pepper. Several changes such as shape, size, structure, color, bulk density, porosity, and rehydration capacity are induced due to structural alteration in green bell pepper followed by drying. This information could help in safe commercial packaging of dried products. Further, dried green bell pepper must be rehydrated to be used in preparation of foods such as soups, pizzas, stews, casseroles, and many other recipes. Outcome of the current study can also be referred and implemented while accessing the rehydration characteristics of dried and shrunken vegetables.
... Hence better rehydration characteristics may be expected in microwave dried products. Microwave processes offer a lot of advantages such as less start up time, faster heating, energy efficiency (most of the electromagnetic energy is converted to heat), space savings, precise process control and food product with better nutritional quality (Khraisheh et al., 1997;Prabhanjan et al., 1995). The food processing industries are the predominant consumer of microwave energy, where it is engaged for cooking, thawing, tempering, drying, freeze-drying, and sterilization, baking, heating or re-heating (Cui et al., 2004). ...
The present article investigates influence of packaging material, storage condition and duration on quality attributes of osmo-cum-microwave dehydrated Pleurotus sajor-caju mushroom flakes during storage. Oyster mushrooms were dried by microwave drying technique to a moisture content of 6 % (w.b.) succeeding osmosis. The ultimate superior quality dried product obtained through optimization was stored in different packaging material for confined duration of three months at different storage conditions. Various quality attributes that assist in bestowal of overall consumer acceptability were studied during storage period. The present research study revealed that mushroom flakes were highly acceptable upto two months of storage that was packed in high density polyethylene by retaining much of the quality attributes. The outcome of the present investigation perhaps supportive for those involved in the post-harvest processing and value addition of oyster mushrooms.
... The microwave drying mechanism is based on the interaction between water dipolar molecules and the electromagnetic field. Thus, the higher moisture content part of a product can absorb more energy which results in faster drying [6]. The microwave power varies from 170 to 500 watts while the pulsation periods varied from 30 to 120 seconds [4]. ...
Microwave drying provides high heating rate and rapid moisture removal. The objective of this research was to investigate the effect of bed thickness and power levels on drying kinetics and to assess the quality of parboiled paddy under microwave drying. Effect of process variables, namely microwave power (180, 360, 540 and 720 W) and bed thickness (1.5, 3, 4.5 and 6 cm) were studied on drying rate, moisture ratio, moisture diffusivity and activation energy. Drying time significantly decreased with increase in power while increased with increasing bed thickness. Drying rate curves revealed that drying occurred only in falling rate period. The drying data were fitted with several established thin layer drying models. Moisture diffusivity increased with increase in power and decreased with increase in bed thickness. The highest diffusivity of 30.8×10-10 m2/s was found at 720 W and 1.5 cm thickness while the lowest diffusivity of 3.08×10-10 m2/s was observed at 180 W and 6 cm thickness. The highest value of activation energy was found to be 7.718 W/g for 1.5 cm bed thickness whereas the lowest was 2.58 W/g for 6 cm bed thickness.
... The values of ln(MR) were drawn versus the mean drying time (t) of garlic slices at the various experimental condition and the plots for the IR drying curves are illustrated in Fig. 4. The figure revealed that plots have slightly deviated from the linearity for all studied drying conditions. This deviation might be due to various causes e.g. the initial moisture content is not uniformly distributed, a variation of the product temperature during drying, change in moisture diffusivity with moisture content and shrinkage of the product (Adu and Otten, 1996;Khraisheh et al., 1997). ...
Drying of garlic slices in thin-layer have been studied with Infrared (IR) at 0.075, 0.15, 0.225 and 0.3 Wcm⁻² radiation intensity and 0.75 and 1.25 ms⁻¹ air flow velocity. The results showed increasing in drying rate and decreasing at the time of drying with decreasing air flow velocity and increasing IR radiation intensity. The effective moisture diffusivity (Deff) was obtained using Fick’s diffusion equation and its mean values ranged between 5.83x10⁻¹¹ and 7.66x10⁻¹⁰ m²s⁻¹ for all investigated conditions. In addition, a third-order polynomial equation linking the effective moisture diffusivity and moisture content was found. Average activation energy increased with the decrease of IR radiation and increase of air flow velocity. Thirteen different mathematical models were verified with non-linear regression analysis for describing the garlic drying process. Modified Henderson and Pabis model presented the best prediction of the drying of garlic slices.
... During drying experiments, 250 g of bamboo shoot slices were arranged in a single layer on the turntable and placed in the center of the oven. The use of the turntable was necessary to achieve uniform heating of the samples and to reduce the level of microwave power on the magnetron [14]. Three replications of each experiment were performed according to a preset microwave output power and time schedule, and the data given are as an average of these results. ...
The effect of microwave power levels (140, 210, 280 and 350 W) during microwave drying on texture, microstructure and rehydration characteristics of bamboo shoot slices (35 ± 1.75 mm diameter, 3.4 ± 0.2 mm thickness) was looked into. With the increase in power level the hardness of the sample increased due to case hardening and approximately 23% decrease in the protein content was observed when power level was increased from 140 to 350 W. Similarly, water activity decreased from 0.71 to 0.38 during this power level range whereas, rehydration ratio increased up to 280 W after that decreased with the increase in power level. In addition, micro-structural changes studied by scanning electron microscopy showed that with increased power level severe tissue shrinkage, collapse and case hardening took place. Above research findings would be very useful for drying of the highly perishable bamboo shoot slices keeping the quality intact, also enhancing the commercial potential and open new avenues for micro level entrepreneurship and employment generation in the rural sector.
... et al., (2000) investigated the effects of drying methods on the colour of dried potato and reported that, conventional air drying caused extensive browning with a significant drop of the lightness and an increase in hardness and yellowness of dried potato. Khraisheh et al, (2004) also reported the quality and structural changes (in terms of vitamin C destruction, shrinkage and rehydration abilities of potato during microwave and convective drying. They emphasized that the study of drying kinetics is highly essential for adequate understanding of moisture reduction and drying behavior of food products. ...
The effects of varying drying conditions on the physical properties of sweet potato slices were studied. Fresh tubers were peeled, washed and cut into two shapes (rectangular: 50 x 60 mm and cylindrical 60 mm diameter) of two thicknesses (4 and 6 mm) slices. Some slices were blanched in water at 90 °C for 5 mins and some unblanched. Both were either sun dried or oven dried at various temperatures; 50 °C, 70 °C and 90 °C. The bulk density, dimensional changes and moisture loss were investigated. The results shown that, moisture loss and percent shrinkage increased with increased in temperature. Sweet potato of 4 mm thick samples lost more moisture with higher % shrinkage than 6 mm thick samples, although not significantly (P>0.05). Logarithmic equations gave best fit of moisture loss with time at the different temperatures. Generally, in all the samples, thickness had a greater influence on % shrinkage than shape. Blanching minimized %shrinkage although not significantly (P>0.05). Greater shrinkage took place in the sample thickness (up to 63%) than across product diameter or thickness (values up to 26.3%). % shrinkage can be predicted using either the linear or logarithmic equations. The bulk densities of dried sweet potato slices were not influenced by blanching. © 2016, Int. Comm. of Agricultural and Biosystems Engineering. All rights reserved.
... However, the theoretical application of these phenomena to food products becomes difficult due to the complex structure and to the physical and chemical changes that occur during drying. For such purposes, many attempts have been made by several researchers to analyse physical properties during drying for different fruits and vegetables such as apple, banana, carrot, potato and garlic (Baysal et al., 2003;Mayor & Sereno, 2004;Khraished et al., 1997;Krokida & Maroulis, 1997). Koç et al. (2008) studied the effect of different drying methods (conventional drying in fluid bed and tray driers, infrared assisted air drying, osmotic dehydration combined with conventional air drying and freeze drying) on bulk density, substance density, porosity and shrinkage of quince and concluded that all the properties, except substance density, were affected by drying method. ...
... It is therefore important to measure this phenomenon as a function of drying experimental conditions. The introduction of a parameter that accounts for shrinkage while developing drying models seems also to substantially improve their quality (Khraisheh et al., 1997;Lang et al., 1994;Rahman, 2001). ...
The aim of this work was the study of the impact of both drying processes: dehydration-impregnation by soaking (DIS) and convective drying performed separately or in combination on mass transfers of water and sucrose and on the main quality characteristics of the fruit (total difference of colour, total phenols (TP) and ascorbic acid (AA) contents, shrinking, water activity) by taking the pear Conference as a model. The processes effect on the pieces of pear was apprehended according two approaches: (i) a global study of the impact of the variables of commands on the quality of finished product and (ii) a kinetic study of nutritional compounds and the quality (colour, shrinkage, and water activity). Central Composite Design with four factors (sucrose concentration: 25-65 %, temperature: 20-60 °C, immersion time: 0.5-6.5 hr and drying temperature: 30-70 °C) at five levels each was used for pears processing. During DIS, the variables: immersion time, temperature and concentration of sucrose solution had a significant effect (p<0.01) in the loss of water, the solid gain and the quality attributes of pears. The losses in AA are higher than those in TP and are essentially due to the oxidation and the leaching with water. During the convective drying, the losses in TP and in AA depend more on the time of the process rather than the temperature of exposition. They reach respectively 80% and 34% after 10 h of drying at 30 °C. The color change of pear depends on the time and temperature of treatment and is more pronounced at the end of drying. Predictive and quadratic models were suggested to link the responses variables (aw, total colour difference, TP loss and total time treatment) to operational parameters of both studied processes. The losses in AA decrease linearly with the duration of DIS. A multicriteria optimization using response surface methodology (RSM) was proposed (minimal losses in TP and in AA, reduced total processing time and total difference in colour and an aw inferior to 0.6) for the combination DIS/convective drying. The optimal conditions are: osmotic solution at 28°C and 25°Brix, a DIS lasting 30 min and a convective drying at 60 °C. Otherwise, the temperature effects (30, 45 and 60 °C) and the sucrose impregnation by DIS (10 and 65 min, 70°Brix, 30 °C) upon the desorption isotherms and the transition temperature (Tg) of pears, apples and apricots were determined. The sucrose impregnation diminishes the influence of temperature on its sorption ability. It generates a depressing effect on water activity and a rise of pear hygroscopicity, apple and apricot at a high temperature. The desorption isotherms of fresh and sucrose impregnated apricot present a crossing of curves which mark the inverse of the effect of temperature on isotherms. The Tg depends on the temperature, the sucrose content and the fruit water content (X). For constant water content, the rise of temperature and of the duration de DIS result in the increase of Tg. The phase diagrams (Tg=f(X) et Tg=f(aw)) of fruits were established at 30, 45 and 60°C and the critical water content as well as the critical water activities were determined. Water content at the end of drying inferior or equal to 0.02 g/g d.b. is recommended to obtain stable fruit pieces at microbiological, physiochemical and rheological levels.
... Like transport models, mathematical models for deformation in foods can be broadly classified into different categories: (1) boundary tracking method in which it is assumed that shrinkage/expansion is due solely to moisture loss/gain and transport equations are solved in a moving boundary whose position at any given time is related to changes in experimental moisture content (Ramesh, 2003;Bakalis et al., 2009). This approach may be useful in certain situations when information on stress development within the material may not be important; (2) semi-empirical models that use shrinkage/expansion coefficients obtained from experimentally measured volume changes and relate it to the volumetric strain (Khraisheh et al., 1997;Moreira et al., 2000;Ochoa et al., 2002). Here, all different deformation mechanisms are lumped into empirical constants similar to effective diffusivity formulations for transport; (3) deformation models including linear momentum balance of the material and using Hooke's law as the constitutive relationship between stress and strain Chemkhi et al., 2004;Niamnuy et al., 2008;Curcio and Aversa, 2014). ...
A mechanistic framework for quality development such as shrinkage and case hardening in drying would allow new insights making the final quality in a drying process more predictable and controllable. A poromechanics model that includes multiphase (solid matrix/liquid water/vapor) transport and large deformation using hyperelastic constitutive relationship between the stress and strain is developed. Moisture and state (rubbery/glassy) dependent mechanical and transport properties are used. A complex shrinkage pattern that is not simply equal to the amount of water lost is observed at low moisture contents due to glass transition of the material. For high drying rates, the surface dries out faster than the core and forms a case-hardened layer resulting in early deviations in shrinkage. In contrast, for low drying rates, deviations in shrinkage occur at extremely low moisture contents due to a gradual rubbery/glassy transition. Key quality attributes, such as degree of crust formation, are predicted from fundamentals.
... It is therefore important to measure this phenomenon as a function of drying experimental conditions. The introduction of a parameter that accounts for shrinkage while developing drying models seems also to substantially improve their qual- ity ( Khraisheh et al., 1997;Lang et al., 1994;Rahman, 2001). ...
... El secado por convección de aire caliente tiene como ventaja sobre otros tipos de secado que al rehidratar el producto final, las características sensoriales como textura, color y sabor siguen y son muy similares a las del producto original (Setiady et al, 2009). Por otro lado, otros autores han mencionado que la velocidad de flujo del aire es el factor principal que afecta el cambio de volumen (Khraisheh et al, 1997). ...
Resumen El objetivo del presente trabajo fue obtener cinéticas de secado y la propuesta de un modelo de transferencia de masa y calor en medios deformables. Para la parte experimental se utilizó papa como material de estudio. El enfoque propuesto se basa en la mecánica de medios continuos, mediante el acoplamiento de un modelo de los fenómenos de transporte, describiendo simultáneamente la transferencia de calor y masa tanto en la cámara de secado como en el alimento. Se muestran y discuten tanto las cinéticas como la rapidez de secado a dos diferentes temperaturas 50° y 60°C con una velocidad de aire de 1.2 m/s a una humedad relativa del 30%. Se identificaron los periodos de secado en las curvas, con lo cual se observó que al inicio del proceso la superficie del material se encuentra muy húmeda y existe una pronta evaporación debido al calor latente que proporciona el aire caliente. Este vapor de agua se difunde a través de una película límite de aire el cual a su vez es llevado por el aire circundante, y se genera una presión de vapor dentro de la muestra, la cual aumenta y genera un gradiente de presión en el material, con lo cual el vapor de agua se expulsa de la superficie. Considerando los fenómenos descritos, se propone un modelo que considera la deformación del material de forma axial para un análisis 1D. Los resultados mostraron que para una Temperatura de 50 y 60 °C la mayor rapidez de evaporación de agua se presentó en las primeras dos horas del experimento. Se identificaron dos periodos de rapidez decreciente, sin embargo el periodo de rapidez constante fue difícil de detectar debido a la deformación y cambios en la superficie de las muestras. Palabras clave Cinéticas, Deformación, Secado, Papa, Transferencia de masa y calor. Abstract The aim of this study was to obtain drying kinetics and proposed a model of heat and mass transfer in deformable media. For the experimental part potato was used as study material. The proposed approach is based on continuum mechanics, by coupling a model of transport phenomena simultaneously describing the heat and mass transfer in both the drying chamber and food. Some graphics are shown and discussed as kinetic drying speed at two different temperatures 50 ° and 60 ° C with an air velocity of 1.2 m/s at a relative humidity of 30%. The drying periods were identified in the curves, it was observed in the early process that
... The influence of air velocity on particle density can be explained on the basis of effect of variables on the mass transfer. At low air velocities, surface resistance prevails, moisture profiles in the sample are relatively flat and internal stresses are at minimum [12]. If air velocities are very high, drying makes the surface of the samples stiff, limiting the particle density even in the earliest stages. ...
The objective of this paper is an analysis of the influence of drying air temperature and the drying air velocity on the particle density. The particle density of apple, banana, potato and carrot slices during convective drying was experimentally determined. Drying experiments were conducted in a laboratory air-dryer, repeated at different air temperatures and air velocities. The drying air temperatures considered were 40, 50, 60 and 70 °C with drying air velocities of 1, 2 and 3 m/s. Two simple mathematical models for correlating the dimensionless particle density with the dimensionless material moisture content, drying air temperature and drying air velocity are proposed. The models were fitted to experimental data and the correlation coefficients and residual sum of squares were estimated. Nomenclature A, B, C -parameter MRD -mean relative deviation m (kg) -mass n -degrees of freedom R -dimensionless particle density RSS -residual sum of squares r -correlation coefficient V (m 3) -volume SE -standard error of the estimate X -dimensionless moisture content x (kg/kg d.b.) -moisture content Y cal -estimated value Y exp -experimental value Greek symbols ρ (kg/m 3) -density Subscripts 0 -initial a -air ap -apple ba -banana ca -carrot p -particle po -potato s -solid
... Samples were placed at the centre of the turntable in the microwave cavity during treatment (200 W, 10 s). The use of the turntable was necessary to achieve uniform heating of the samples and to reduce the level of microwave power on the magnetron (Khraisheh et al., 1997). After microwave treatment none of the samples had the same moisture content. ...
In this research the effect of drying condition on optical properties of apple slices was investigated. Hunter values (L, a, b) were used to measure the color parameters. Effect of various coating materials such as Carboxymethylcellulose (CMC), Starch and Pectin and drying condition including microwave treatment in presence of coating materials on optical properties with respect to microstructural changes, was studied. Some of microstructural properties and its effects on optical properties were discussed. So scanning electron microscopy (SEM) technique were used to analyze the effects of these processes on microstructure. Color changes values including total color difference (ΔE), and browning index (BI) were measured. Results showed that optical properties of samples were affected by drying condition and coating material. So that coated samples by pectin had a lower L and higher BI, but different results in presence of starch were achieved. Microwave treatment in presence of appropriates coatings could improve optical properties of dried slices.
... Food products have been successfully processed by combining vacuum and microwaves or associating microwaves with conventional convective processes. Some publications were analyzed and the following papers picked out: papers on banana drying: Maskan (2000); Garcia et al. (1988); Drouzas & Schubert (1996); Nijhuis et al. (1998) -on carrot drying: Lin et al. (1998);Prabhanjan et al. (1995) on potato drying: Khraisheh et al. (1997) -on grape drying: Tulasidas et al. (1996) -on apple and mushroom drying: Funebo & Ohlsson (1998) and on American ginseng drying: Ren & Chen (1998). Other authors have specifically recommended splitting the drying process into two stages, the first one being carried out by conventional convective heated air, followed by the application of microwaves during the last stages (Prabhanjan et al.,1995;Feng & Tang, 1998). ...
Whole ripe bananas of the variety "nanicão" (Musa acuminata, subgroup Cavendish), with 3 kg water/kg dry matter, were dried to 0.16-0.23 kg water/kg dry matter, using a domestic microwave oven adapted for a bench scale drying operation. Eleven test runs were carried out with air temperatures varying in the range from 25 to 55 o C and the air-flow rates from 0.8 to 1.8 m 3 /min, whereas the average microwave power density was kept within the interval of 350 ± 50 W/kg bananas. The drying behavior was studied after preparing curves of the reduced product moisture versus time and the drying rate and also by testing the Fick diffusion model. The dried samples were sensory evaluated by determining overall product acceptance, the purchasing intention and the degree of ideality of the color, sweetness and texture. The results showed that the microwave drying processes were well fitted to a simplified diffusion model, with the drying constant K having a value between 0.00861 and 0.01156 min -1 and the drying time varying within the range from 200 to 290 min. The drying rates were observed to decrease for all experiments as a function of decreasing product moisture content, varying from 0.0298 to 0.00248 kg water/kg dry matter. Overall consumer acceptance tests of the product showed good scores, varying between 5.46 and 7.23, for a hedonic scale from 1 to 9. The purchasing intention rates were also good, varying from 2.20 to 3.06 on a 0 to 4 scale. The color, sweetness and texture of the samples were close to the accepted ideal.
... However, the theoretical application of these phenomena to food products becomes difficult due to the complex structure and to the physical and chemical changes that occur during drying. For such purposes, many attempts have been made by several researchers to analyse physical properties during drying for different fruits and vegetables such as apple, banana, carrot, potato and garlic (Baysal et al., 2003;Mayor & Sereno, 2004;Khraished et al., 1997;Krokida & Maroulis, 1997). Koç et al. (2008) studied the effect of different drying methods (conventional drying in fluid bed and tray driers, infrared assisted air drying, osmotic dehydration combined with conventional air drying and freeze drying) on bulk density, substance density, porosity and shrinkage of quince and concluded that all the properties, except substance density, were affected by drying method. ...
The present work aimed at studying the influence of temperature (40, 50 and 60 ºC) and air velocity (0.7, 0.9 and 1.2 m/s) on drying of quince. From the results obtained it was possible to conclude that the quince drying had a similar kinetic behaviour, regardless of the operating conditions. As to the influence of the drying temperature on the process, it was observed that increasing the drying temperature from 40 ºC to 60 ºC leads to a diminishing of 30 % in the drying time. On the other hand the increase in air velocity does not have a proportional effect on the reduction of dimensionless moisture, particularly at higher quince moisture contents. The rate of quince drying is characterised by the absence of the constant drying-rate period followed by falling-rate period, regardless of the conditions. The data for the moisture ratio over time was fitted to different empirical models with the best performances coming from the Modified Page, Henderson & Page and Logarithmic models.
... In addition to improving the drying rate, this outward flux can help to prevent the shrinkage of tissue structure, which prevails in most conventional air drying techniques. Hence better rehydration characteristics may be expected in microwave dried products (Khraisheh et al. 1997;Prabhanjan 1995). Microwave processes offer a lot of advantages such as less start up time, faster heating, energy efficiency (most of the electromagnetic energy is converted to heat), space savings, precise process control and food product with better nutritional quality. ...
For optimization of convective-cum-microwave dehydration process parameters, the experiments were planned in 3-factor Box-Wilson design using response surface methodology (RSM). The independent process variables for convective-cum-microwave drying process were i.e KMS concentration (0.1-0.5 %), drying air temperature (55-75 0 C) and microwave power level (810-1350 W).The moisture content of each samples was reduced to 39% +1% (wb) by convective drying followed by Microwave drying to safe moisture content of 6 % (wb). The dehydration process was optimized for minimum drying time, hardness, shrinkage ratio, specific energy consumption, non enzymatic browning; maximize rehydration ratio, colour (L-Value) and overall acceptability. The optimum conditions obtained by computer generated response surface, canonical analysis and contour plots interpretation were: 0.5 % KMS concentration, 59.41 0 C drying air temperature and 810W power level. The corresponding value obtained for drying time, hardness, shrinkage ratio, specific energy consumption, non enzymatic browning, rehydration ratio, color and overall acceptability was 165. respectively. The analysis of variance (ANOVA) revealed that, among the process variables the temperature had the most significant effect on all the selected responses.
... It is therefore important to measure this phenomenon as a function of drying experimental conditions. The introduction of a parameter that accounts for shrinkage while developing drying models seems also to substantially improve their quality (Khraisheh et al., 1997;Lang et al., 1994;Rahman, 2001). ...
This study was conducted to evaluate quality and structural changes in parallelepipedic pieces of pears during convective drying at different air temperatures (30–70 °C).Submitted to atmospheric O2 conditions, ascorbic acid deterioration demonstrated first-order kinetic behaviour and was found to depend on air temperature and pear moisture content. Loss of ascorbic acid content increased with increasing air temperature. Possible explanation could be the irreversible oxidative reaction occurring during drying. Phenol content degradation fitted a pseudo first-order reaction and was significantly influenced by air temperature.Variations in bulk density, shrinkage and porosity essentially depended on changes in moisture content. Porosity exhibited a nonlinear variation with respect to moisture content. Volume change showed, as expected, a linear variation with moisture content. Drying temperature significantly induced the increase of a* and b* colorimetric parameters due to non-enzymatic browning reaction, which turned the samples more reddish and yellow when the temperature rose.
... It is therefore important to measure this phenomenon as a function of drying experimental conditions. The introduction of a parameter that accounts for shrinkage while developing drying models seems also to substantially improve their quality (Khraisheh et al., 1997; Lang et al., 1994; Rahman, 2001). One of the main purposes in modern food technology is to maximize the retention of nutrients during processing and storage. ...
This study was conducted to evaluate quality and structural changes in parallelepipedic pieces of pears during convective drying at different air temperatures (30–70 °C).
Submitted to atmospheric O2 conditions, ascorbic acid deterioration demonstrated first-order kinetic behaviour and was found to depend on air temperature and pear moisture content. Loss of ascorbic acid content increased with increasing air temperature. Possible explanation could be the irreversible oxidative reaction occurring during drying. Phenol content degradation fitted a pseudo first-order reaction and was significantly influenced by air temperature.
Variations in bulk density, shrinkage and porosity essentially depended on changes in moisture content. Porosity exhibited a nonlinear variation with respect to moisture content. Volume change showed, as expected, a linear variation with moisture content. Drying temperature significantly induced the increase of a* and b* colorimetric parameters due to non-enzymatic browning reaction, which turned the samples more reddish and yellow when the temperature rose.
... Samples were placed at the centre of the turntable in the microwave cavity during treatment (200 W, 10 s). The use of the turntable was necessary to achieve uniform heating of the samples and to reduce the level of microwave power on the magnetron (Khraisheh et al., 1997). After microwave treatment none of the samples had the same moisture content. ...
The effect of coating, hot-air and microwave drying on the texture, microstructure and rehydration characteristics of Golden Deliciousapple slices (22mm diameter, 4mm thickness) was investigated. Blanching was carried out in hot water (80°C, 1 min) to prevent undesirable enzymatic reactions. To induce a porous structure during microwave treatment, coating with a starch (suspension), pectin and carboxymethylcellulose (CMC) solution (20 g/L) with and without CaCl2 (10 g/L) was carried out. A puncture test and a scanning electron microscopy (SEM) technique were used to analyse the effects of these processes on texture and microstructure. Rehydration in distilled water was also studied. Coating, air-drying (70°C, 1.5 m/s) and microwave treatment (300W, 10s) resulted in the production of puffed and porous products. Microstructure of samples coated by starch and pectin with CaCl2 improved. Coating had a beneficial effect on the microstructure of samples after microwave treatment. In addition, microstructural studies showed that coating with pectin had a significant effect on the thickness of the cell wall whereas those treated with starch or CMC did not exhibit such an effect. Adding CaCl2 to the coating solution improved the microstructural properties and prevented central heating phenomena.
... The porosity of food changes during drying, which depends on the composition, and in particular the initial moisture content, as well as the size and drying conditions used. The introduction of a parameter which accounts for shrinkage when developing the drying models seems to substantially improve their quality (Khraisheh et al., 1997;Lang et al., 1994;Rahman, 2001). Zogzas et al. (1994) developed a relatively simple model to correlate density, porosity and shrinkage with moisture content. ...
In the present work the moisture content and apparent density of pears were measured during the drying of pears under different conditions: peeled uncut pears dried by direct sun exposure (a traditional method used in Portugal) and small slices of pear pulp dried in a ventilated drying chamber at constant temperature of 30°C.
Using the experimental results obtained was possible to estimate the parameters of the model proposed by Zogzas et al. (1994), and make previsions of bulk and particle densities as well as porosity variations during drying.
It was possible to conclude from this work that, in spite of being substantially different, the two drying procedures used can be considered equivalent with respect to the structure product developed. In fact, the model parameters were very similar in the two situations tested and the evolution of porosity was also very analogous. Furthermore, it was possible to conclude that the pears developed relatively low porosities during drying, as a consequence of the slow drying rates used in both situations, and the high degree of shrinkage observed.
The study investigated the effect of ultrasound (US) pre-treatment on drying kinetics, color, total phenols content and antioxidant activity of pomegranate peel (PP). The US pre-treatment was carried out at a frequency of 24 kHz at room temperature for two time periods of 5 and 10 min. The pretreated PP were then dried in a convective tray dryer at two different temperatures (70 and 80°C). The US pre-treatment had a significant influence on reducing the drying time (approx. 45% reduction) of PP. Five thin-layer drying models were fit to study the drying kinetics in which Midilli's model gave the best-fit curve. The effective moisture diffusivity (Deff) of PP ranged from 5.44×10−09 to 1.49×10−08 m2/s. The PP activation energy (Ea) decreased with increased exposure time. The color values (ΔΕ*) increased with increasing treatment time and temperature, however, no significant difference among color values was observed. The total phenol content and antioxidant activity were higher in US-treated peel powder than in untreated (UT) PP. The US pre-treatment of 10 min and dried at 70℃ successfully reduced degradation of total phenolic content and antioxidants and is recommended for PP drying pretreated with US
This study aimed to investigate the effect of pulse ratio and microwave power during drying of red beetroot using combined intermittent microwave – hot air at low temperature (40˚C). A significant decrease in drying time was observed by increasing the microwave power from 360 to 900 W and decreasing the pulse ratio from 6 to 2. The highest Deff was obtained at a microwave power of 900 W and a pulse ratio of 6. Dried red beetroots at the microwave power of 900 W and a pulse ratio of 6 had the lowest shrinkage and the highest rehydration. Moreover, the apparent density of the dried samples decreased by increasing the microwave power and pulse ratio. The specific energy consumption was reduced by increasing the microwave power and the pulse ratio. By increasing microwave power and pulse ratio, the content of betacyanins and betaxanthins increased up to 79.47 and 44.20%, respectively. Furthermore, the samples dried at the power of 900 W and a pulse ratio of 6 showed the lowest microstructural collapse. In total, the results of this study showed that industrial dried products with good quality could be produced by considering an appropriate pulse ratio and applied power.
The changes in moisture content and shrinkage ratio of Cordyceps militaris during mid-infrared-assisted convection drying (MIRCD) with different drying temperatures (40, 50, and 60 °C) and velocities of airflow (1 and 2 ms⁻¹) were studied. The relationship between low-field nuclear magnetic resonance (LF-NMR) information and moisture content/shrinkage ratio was modeled using partial least-squares regression (PLSR) and extreme learning machine (ELM). Results indicated that the influence of drying temperature was more pronounced than that of air flow velocity. Both types of models showed good predictive ability with R²>0.90. The ELM models exhibited superior predictive performance than that of the PLSR models.
Chinese eaglewood leaves are dried in a constant environment testing machine, and the curves of drying and rolling deformation are obtained under different temperatures, humidity levels, and restricted space conditions. Results show that drying rate increases with drying temperature but slightly decreases with an increase in humidity or a decrease in restricted space. The edges of the leaves roll inwardly when dried without any limit but roll outwardly under a restricted space. A low restricted height indicates a small leaf roll index (LRI), which is conducive to shredding in the later stage of manufacture of eaglewood cigarettes. When the leaves are dried restrictedly, the effects of drying temperature and air humidity on the LRI are minimal, and the LRI slightly increases with an increase in temperature or a decrease in humidity.
Practical Applications
China Jiangsu Tobacco Industrial Company has added a certain amount of dry eaglewood leaves to tobacco and manufactured cigarettes of eaglewood leaves. The pharmacological effects of eaglewood can reduce the harm of tobacco on the human body. Fresh eaglewood leaves are dried until the moisture content is below 14% and then rehydrated and shredded during the manufacture of cut tobacco. Rolling deformation occurs during drying, and over rolling of leaves produces irregular shredding that affects the quality of cut tobacco. In this study, the drying and rolling deformation characteristics of eaglewood leaves in restricted spaces are explored to reduce the rolling deformation of the leaves during drying.
The drying characteristic and rolling deformation of eaglewood leaves are investigated experimentally, and a model is built based on Fick’s law and stress–strain relations to illustrate the leaf rolling rule. The leaves dehydrate free water and hardly roll during the initial drying period. Rolling deformation is induced by the shrink difference along leaf thickness and occurs when the moisture content reaches a critical level. The rolling index of a leaf that is dried on one side is greater than that of a leaf that is dried on both sides. In addition, the rolling index is influenced by drying temperature and leaf thickness. When a leaf is thick or when the drying temperature is high, the leaf rolls considerably.
This study involves the rolling and unrolling phenomena of Chinese eaglewood leaves during drying and rehydration processes. In the early period of drying, eaglewood leaf rolls slightly and has a low leaf rolling index (LRI), i.e., <0.05, with decreasing leaf moisture content. Once moisture content declines to a critical value (Mc), the drying process enters into a fast rolling stage until LRI eventually reaches a drying equilibrium (LRIe). The leaf will then unroll upon rehydration; however, LRI cannot return to its initial value before drying and it will remain at a rehydrating equilibrium (RLRIe). Mc increases with increasing drying temperature and is hardly influenced by ambient air humidity, leaf size, and forced initial LRI. LRIe increases with increasing drying temperature, leaf size, forced LRI, and decreasing ambient air humidity. Meanwhile, RLRIe decreases as rehydration water temperature increases.
Freezing method, pre-treatment, drying temperature and residual moisture content affect the quality of the dehydrated potato cubes. Blanching of potato cubes at 70°C and soaking in solution containing sugar, NaCl and glycerol followed by slow freezing and drying at 60 and 40°C showed 1:2.2 and 1:1.75 rehydration ratio (RR), 20 and 25 min rehydration time (RT), 34 and 58% shrinkage, respectively. While the blast freezing and drying at 60 and 40°C showed 1:2.1 and 1:1.6 RR, 22 and 24 min RT, 36 and 60% shrinkage, respectively. Above treatment without glycerol and subjected to slow freezing and drying at 60 and 40°C showed 1:1.6 and 1:1.2 RR, 30 and 36 min RT and 46 and 59% shrinkage, respectively. Whereas, blast frozen 60 and 40°C dried samples showed 1:1.4 and 1:1.22 RR, 36 and 35 min RT, 57 and 62% shrinkage, respectively. All the samples blanched in presence of glycerol and subjected to slow freezing and low temperature drying had firm texture. The thermal analysis of the samples revealed an elevation in glass transition temperature (Tg) of the samples subjected to slow freezing, glycerol treatment and dried at 60°C.
Shrinkage-deformation phenomenon of apple slabs during convective drying was studied, aiming to analyse its causes. Apple slabs were dehydrated in an experimental drying tunnel using a factorial design 42. Independent variables were airflow (1, 2, 3 and 4 m/s) and drying air temperature (50, 60, 70 and 80 °C). Relative shrinkage (A/Ao) was evaluated during drying as the ratio of the projected (top view) area of slab at any time during the process (A) to the initial area (Ao) of the same slab. In five punctual measuring zones on the surface of the slab, surface temperatures (ST) and moisture contents (MC) were evaluated. ST and MC were different for each measuring zone, causing irregular dehydration. Fractal analysis was used to evaluate the non-linear (A/Ao) observed pattern and ST distribution. Fractal dimensions for (A/Ao) ratios (FDA) and ST (FDST) were 1.08-1.30 and 1.12-1.54 respectively. Temperatures of 60 ° and 70 ° C, and airflows of 2 and 3 m/s caused the highest irregularity of shrinkage and highest FDA and FDST values. Scanning electron microscopy images for measuring zone were captured and fractal dimension of texture of images were obtained (FDSDBC) it was observed that nonlinear shrinkage was related to irregular microstructure given by FDSDBC.
A microwave-vacuum drying system with the ability of automatic temperature control was developed for strawberry drying in order to obtain a high-quality product, in terms of appearance, color, and texture. Changes in quality attributes of strawberries were determined by measuring the color, texture, shrinkage, and rehydration. Microwave-vacuum drying (47 mm Hg) yielded an elastic product of improved mechanical resistance with just a slight loss of color and appropriate rehydration performance using automatic temperature control around 50 A degrees C and 700-W power. The specific energy consumed during strawberry drying using automatic temperature control varied between 1.30 and 2.30 Wh/g. The efficiency of process in automatic mode was 13.5 times higher than efficiency in manual mode. A drying protocol for strawberries, based on automatic on-off temperature control, was developed. Thus, the drying temperature and dead band could be easily changed during the trials, and there is no need to tune the controller when drying a different kind of fruit.
The acceptance of processed or minimally processed food products by consumers is highly dependent on several quality and nutritional attributes as well as their organoleptic characteristics. Shelf-life is an additional factor that influences consumer behavior. Moreover, consumers now demand high-value food products that are healthier and have high nutritional value. The quality, nutrition level, and perception of processed foods is governed, among other attributes, by the structure of the dried product. Hence, examination of the structure of dried foods is of significant importance. Dehydration methods have a great impact on the structural properties of most food products. The effect of various dehydration methods on shrinkage and porosity of dried foods is discussed in this article following a comprehensive review of the available literature. Novel drying processes and combination of various conventional drying methods are discussed with regard to their expected impact on food structure. Finally, aspects of food formulation during diverse drying processes and recommendations for future research are presented.
Shrinkagedeformation phenomenon of apple slabs during convective drying was studied, aiming to analyse its causes. Apple slabs were dehydrated in an experimental drying tunnel using a factorial design 42. Independent variables were airflow (1, 2, 3 and 4 m/s) and drying air temperature (50, 60, 10 and 80 °C). Relative shrinkage (A/Ao) was evaluated during drying as the ratio of the projected (top view) area of slab at any time during the process (A) to the initial area (Ao) of the same slab. In five punctual measuring zones on the surface of the slab, surface temperatures (ST) and moisture contents (MC) were evaluated. ST and MC were different for each measuring zone, causing irregular dehydration. Fractal analysis was used to evaluate the nonlinear (A/Ao) observed pattern and ST distribution. Fractal dimensions for (A/Ao) ratios (FDA) and ST (FDST) were 1.081.30 and 1.121.54 respectively. Temperatures of 60° and 10°C, and airflows of 2 and 3 m/s caused the highest irregularity of shrinkage and highest FDA and FDST values. Scanning electron microscopy images for measuring zone were captured and fractal dimension of texture of images were obtained (FDSDBC) it was observed that nonlinear shrinkage was related to irregular microstructure given by FDSDBC.
The effect of air drying temperature on shrinkage, rehydration ratio and microstructure of dried onion slices (3±0.2 mm thickness and 30 mm diameter) was studied. The different drying conditions led to distinctive structural changes in the samples, affecting the shrinkage and rehydration ratio. Experiments showed that the intensity of structural changes depends on temperature and drying times. A higher temperature and time caused greater damage to the microstructure of onion slices, resulting in the formation of a highly porous structure. The results also indicated that increasing time and temperature increased the shrinkage and rehydration ratio of samples. In mathematical modeling of shrinkage, the model proposed in this study, which had R2 values higher than 0.978 and SSE values pretty low, predicted accurately the experimental data.
Experimental shrinkage data as a function of water content for particles of potatoes, apples and carrots of different geometry and under various drying conditions are presented. Volumetric shrinkage of individual particles is affected mainly by air velocity; other drying variables have negligible effect. Changes in surface area to volume ratio with water content are practically independent of drying conditions but dependent on sample geometry and the type of foodstuff. A simple model in which volumetric ratio is represented by one or two line segments is developed. The shrinkage of packed beds is also studied. Changes in bed volume during drying are almost only a function of bed-averaged water content. A simple way of predicting porosity in the bed is applied and its results are compared with experimental data.
The method of volume averaging is used to present a general theoretical analysis of the problem of mass transfer during the drying of shrinking multiphase systems. The results are used to derive a complete theory for the drying of cellular materials in the stage during which the cellular structure prevails. Constitutive equations for the diffusive flux of water in each phase of the system and the assumption of local equilibrium are used to obtain the one-equation model of water transport. The method of closure for the spatial deviations is discussed in detail, and a closure scheme is developed that allows for theoretical prediction of the effective mass conductivity tensor.
A general physical-chemical model based on observations, is proposed for high-frequency dielectric behavior of solid foods based on mechanisms of interaction between water and the biochemical constituents of foods.
This chapter discusses tunnel dehydrators, as used to dehydrate certain fruits and vegetables. It provides a general introduction to the subject, primarily for the use of students, food technologists, and engineers. Although it goes into a number of matters concerned with the design and operation of this kind of equipment, it is, in no sense, a manual either of design or of operation. Emphasis has been laid on the discussion of underlying principles and the more recent advances in application of these principles. An effort has been made to bring together published information from many different sources, some of which are not widely available. Tunnel dehydrators, as a class, are frequently called tunnel-and-truck, truck-and-tray, or simply tunnel driers. Principles pertaining to their use have been discussed. All tunnel-and-truck dehydrators for fruits and vegetables have a common feature that distinguishes them from other kinds of drier. Numerous factors govern the selection of the drying system, or type of dehydrator, for a particular plant. Economic factors, such as initial and installation costs and operating labor costs, although important, are not the only criteria for proper selection. Some of the other important considerations have been listed in the chapter.
The quality of dehydrated foods is dependent in part on changes occurring during processing and storage. Some of these quality affecting changes involve modification of physical structure of foods. These modifications affect texture, rehydratability and appearance. Other changes are due to chemical reactions, but these are also affected by physical structure, primarily due to effects on diffusivities of reactants and of reaction products. Physical structure is greatly affected by phase transitions of food components, notably of starch and of sugars. Recent research in this field provides guidelines for optimization of quality of dehydrated products.
In this paper results from a study of the shrinkage of Mutsu apple slices during drying in a connective oven are presented. The results are expressed as relationships between volume and thickness reduction as well as between volume reduction and water content. No differences were observed with respect to slice size. Volume change was correlated to thickness change through an exponent of 1.50±0.24 for all samples measured. Volume change was linearly correlated with water content with r = 0.95.
A review of the heating principles of microwave and dielectric (RF) energies. Components of microwave and RF dryers and applications of each
The dissipation of electromagnetic energy inside a material creates a thermal imbalance state producing some reactions different from those observed through slow classic drying processes. Important drying rates obtained by microwave application can be understood by taking into account induced pressure gradients which greatly accelerate the thermomigration mechanism and thereby modify the physical properties of the product. The shrinkage of porous materials during the drying stage is very sensitive to the internal vapour pressure.The quality of such products depends on the shrinkage behaviour, and it is therefore interesting to study this phenomenon in order to control the characteristics of the product. This study is an experimental approach leading to a theoretical model describing the shrinkage mechanism. This model is developed from results obtained by a computer controlled measurement system allowing to regulate the drying kinetics.
Shrinkage of rectangular slabs of squid flesh during air drying at 70°C and 15% relative humidity was determined. A material balance equation considering dry fibre, water and air phases can predict the bulk shrinkage and apparent density of squid flesh.
Crisps adversely made from stored whole potatoes turn brown, whereas texture is adversely affected when crisps are made from dehydrated slices. Rehydration as a step prior to crisp-making has been investigated in two commercially grown potato varieties in Mauritius. The raw material was analysed for physical, chemical and processing characteristics including peeling losses, blanching time, drying rates and yield. Rehydration rates and ratios were determined. The sensory characteristics of the crisps were evaluated. Potatoes of the Exodus variety were smaller with more eyes and had higher peeling loss than those of the Spunta variety. The former had higher specific gravity and better drying characteristics. There were no significant differences between the varieties in respect of blanching time, chemical composition and crisp quality, and the drying curves were similar. Slight differences were observed in the rehydration characteristics. Crisps made from dehydrated slices were inferior in texture and taste to those of the control. However, crisps made from rehydrated slices were comparable to control crisps.
Dependable data on bulk density, volumetric shrinkage due to water loss and porosity are needed to model processes such as drying, packaging and storing. Experimental data are presented for all three properties. It is possible to model the water-loss-based bulk shrinkage coefficient to obtain a predictive equation based on composition of the foodstuff. From this, a generalized correlation is obtained which predicts bulk shrinkage coefficient knowing only the initial moisture content of the food. Porosities for the foodstuffs considered can be predicted through suitable correlations, but there is no generalized equation spanning all foods.
The shrinkage in dehydration of root vegetables such as carrots, potatoes, sweet potatoes and radishes was investigated. The samples were dried in flowing hot air. The surface areas were measured by photographing the samples. Three drying models were postulated for the formulation of the relation between the changes of the surface area and the moisture contents. The uniform drying model, which in the case of drying is that the shrinkage in volume equals the volume of water lost by the evaporation, agreed with the measured values during the early stages of the drying. The core drying model by assuming the formation of the dried layer at outer side of material was better in agreement with the experimental results. The semicore drying model, which is the intermediate model between the uniform and the core drying model, and the empirical equations of the shrinkage were also considered.
The apparent density of garlic cloves (with a moisture content range of 10–65% wet basis) was evaluated by weighing the product in air and determining the corresponding buoyancy force in toluene. An analysis of variance (ANOVA) revealed a significant effect (P < 0·001) of moisture on apparent density. The true density of dry garlic powder was investigated using density.bottles. The data were fitted to the general density model for fruit and vegetables and to a second order polynomial equation. Internal porosity varied during heated air drying in a non-linear (quadratic) form. Shrinkage during drying was measured by observing dimensional changes in garlic slabs (1·8 × 1·5 × 0·8 cm) dried in a convection oven, and a pilot cross-flow dryer, with drying air at 70°C and 14·5% relative humidity. Shrinkage was found to be fibre-oriented and different from the reported isotropic shrinkage of fruits and vegetables.
To minimize quality losses occurring during processing and storage and to predict shelf-life, quantitative kinetic models, expressing the functional relationship between composition and environmental factors on food quality, are required. The applicability of these models is based on the accuracy of the model and its parameters. In this paper, the calculation of the Arrhenius parameters and the accuracy of the derived model were compared, using three statistical methods, namely: linear least squares, nonlinear least squares and weighted nonlinear least squares. Results indicated that the traditional two-step linear method, was the least accurate and the derived energy of activation and the pre-exponential factor had the largest confidence interval. The latter was shown to have a profound effect on the precision of the calculated rate constant and the predicted shelf life. Based on previous reports that indexes of deterioration
Literature data on storage stability of dehydrated foods were compiled and critically evaluated. Stability of nine different groups of dehydrated products is reported in terms of time required to produce earliest quality defects, and in terms of specific quality measurements at time of failure, where these were available. Methods of drying, storage and pretreatment are also reported for each product.
The paper also discusses the contribution of various factors to stability of dehydrated foods, as well as research needs in this area.
The particle density of granular and gelatinized corn starches was determined in the moisture range 0 to 1 kg water/kg dry solids, using a gas stereopycnometer. The bulk porosity (void fraction) of spherical starch samples at various moisture contents was estimated from the bulk and particle densities during air drying at 60°C. The particle density data fitted a polynomial function of moisture content (X), passing through a maximum value of 1500 kg/m3 at X = 0.15. The bulk porosity of the starch samples increased linearly during drying, reaching a value of 0.45 near dryness. Differences in structure between granular and gelatinized starches during drying were observed by stereomicroscopy. The changes in porosity could be related to variations of the effective moisture and thermal diffusivities of starch materials.
Structural changes in potato during drying were studied by light microscopy. It was observed that the degree of shrinkage of potato during low-temperature drying is greater than at high-temperature drying. Shrinkage also affects the physical properties of materials, such as the density and porosity. In the early stage of drying the density increased as the moisture content decreased, reaching a peak and then decreased with further decrease in moisture content. The density at a given moisture content decreased with increasing drying temperature. A model was developed to represent the relationship between the density and moisture content. The total porosity increased steadily as moisture content decreased in the early stages of drying and then increased sharply towards the end of drying. The percentage changes in thickness, length and width of the potato samples during drying increased linearly with decreasing moisture content.
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Microwave -Hot Air Drying Of Pasta, Onions And Bacon Microwave Enera AI
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Mathematical Model Of Simultaneous Heat And Mass Transfer In Food With Dimensional Changes And Variable Transport Parameters
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Prediction Of Dielectric Propenies In Solid Foods Of High Moisture Content At Ultrahigh And Microwave Frequencies
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Dehydration And Rehydration Of Potatoes
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A Mass Transfer Theory. Chemical Engn. Science, 43,2919-2928.
Microwave - Hot Air Drying Of Pasta
- F G Smith
Drying Methods And Phenomenon
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