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Open Access Review of Some Thermal Methods in Drying and Roasting Processes
Abstract
Thermal processing is a routine procedure in food science, with two important methods being drying and roasting. During thermal processing, simultaneous heat and mass transfer occur, where the distribution of heat and humidity depends on effective diffusivity. Various methods exist for achieving this, each differing in efficiency and energy consumption. The conventional method of thermal processing involves hot air (HA) or convection, which typically requires significantly more energy and time (at least 25%). However, there are newer thermal processing methos based on radiation, each with their own advantages and disadvantages. Nevertheless, all radiation-based methods generally consume less time and energy compared to the HA method. Different thermal processing methods have been studied and reviewed with regard to their energy consumption and effective diffusivity. In summary, while HA remains the routine method in industries , it demands considerably more energy and time compared to radiation-based methods. Radiofrequency is a non-thermal method that can also be employed to enhance the efficiency of various processing techniques.
Pecan (Carya illinoinensis) nuts are rich in functional compounds (unsaturated fatty acids, phytosterols, polyphenols, and tocopherols) associated with various health benefits. Commercially, pecan nuts are roasted to enhance their physical, chemical, and sensory properties. In the present study, response surface methodology (RSM) was used to optimise the quality and nutritional and antioxidant attributes of ‘Wichita’ pecan nuts roasted by using a microwave process with a range of microwave power (96.45–803.55 W) and roasting time (1.37–5.62 min). The microwave-roasted pecan nuts were analysed for hardness, total colour difference (TCD), and radical scavenging activity and modelled using the central composite design. The results showed that microwave power and roasting time significantly (p < 0.05) influenced the quality attributes of the pecan. The quadratic model adequately described the changes in TCD and hardness, respectively, while the 2FI model adequately described the changes in DPPH radical scavenging activity. To obtain the desired pecan nuts quality attributes (TCD = 1863.391; hardness = 28.755 N and DPPH radical scavenging activity = 33.877 mmol Trolox/g), the determined conditions were 700 W and 2.24 min, with a desirability of 0.557. The primary unsaturated fatty acids, including cis-oleic, cis-linoleic, α-linolenic, and stearic acids, were not affected (p < 0.05) by microwave roasting the pecan nuts at determined conditions. Volatile compounds, such as alcohols, aldehydes, ketones, lactones, hydrocarbons, and carboxylic acids, were identified in both raw and microwave-roasted pecan nuts, with limonene, which possesses various health properties, being the major volatile compound. It can be concluded that microwave roasting may be optimised using response surface methodology to produce quality pecan nuts that can be used as snacks or as an ingredient in other snack products.
Moslae herba is considered to be a functional food ingredient or nutraceutical due to its rich bioactive components. The present research was carried out to investigate the effects of different temperatures (40 °C, 50 °C and 60 °C) on the drying characteristics, textural properties, bioactive compounds, flavor changes and final quality attributes of Moslae herba during the hot air-drying process. The results showed that the Midilli model could effectively simulate the drying process of Moslae herba. The effective moisture diffusivity ranged from 3.14 × 10⁻⁵ m²/s to 7.39 × 10⁻⁵ m²/s, and the activation energy was estimated to be 37.29 kJ/mol. Additionally, scanning electron microscopy (SEM) images of Moslae herba samples showed the shrinkage of the underlying epidermal layers and glandular trichomes. In total, 23 volatile compounds were detected in Moslae herba. Among them, the content of thymol increased from 28.29% in fresh samples to 56.75%, 55.86% and 55.62% in samples dried at temperatures of 40 °C, 50 °C and 60 °C, respectively, while the other two components, p-cymene and γ-terpinene, decreased with an increase in the temperature. Furthermore, both radar fingerprinting and principal component analysis (PCA) of the electronic nose (E-nose) showed that the flavor substances significantly altered during the drying process. Eventually, drying Moslae herba at 60 °C positively affected the retention of total phenolics, total flavonoids and the antioxidant capacity as compared with drying at 40 °C and 50 °C. The overall results elucidated that drying Moslae herba at the temperature of 60 °C efficiently enhanced the final quality by significantly reducing the drying time and maintaining the bioactive compounds.
Fruits, vegetable, and root (FVR) crops are vital to achieve food and nutrition security (FNS), especially in Sub-Saharan Africa (SSA). However, their perishable nature results in losses across the value chain. The review discusses the application of dehydration technologies: solar, infrared (IR), microwave (MW), and radiofrequency (RF) to produce shelf-stable dried agricultural produce. Drying technologies for example IR, MW, and RF use radiation for heat transfer and are more energy efficient compared to traditional hot air drying. Due to shorter processing times and lower thermal load, the nutritional quality and functional properties of dried materials from IR/MW/RF are often superior compared to hot air convection ovens or solar drying. Combination methods with hot air, vacuum and ultrasonication, and pre-treatments are of great interest for higher efficiency and quality. There are, however, limited studies available on the use of IR/MW/RF dehydration technologies for FVR crops in SSA, albeit these technologies have potential and further investigations are required for adoption.
It has been the heartbeat of researchers and food engineers to discover the appropriate and effective technology for microbial control to ensure the safety of food. This is because microorganisms such as bacteria, fungi, viruses, and protozoa can cause contamination in food products. Many studies have published reviews on the use of single novel technology such as radio frequency (RF), pulsed light (PL), and cold plasma (CP) to process food to ensure food safety. However, no review has provided a comprehensive, detailed overview of the RF, PL, and CP treatments for pasteurizing food products in one publication. This study aims to review and present the future directions of RF, PL, and CP applications in light of various aspects of these technologies. The review concluded that although no novel technique (thermal or nonthermal) could adequately meet all the requirements for food safety, traditional thermal processes can be extremely useful in reducing and eliminating microbe contamination. However, they cannot be applied to temperature‐sensitive foods. Besides inactivating microbial spores, traditional thermal processes can denature proteins as well as organoleptic properties, such as taste, nutritional value, and sensory characteristics, illustrating the importance of these novel technologies as an alternative to heat‐based techniques. As a result of this study, hurdle technology is recommended since it has an additive effect and achieves better germicidal results.
In this research, Ginkgo biloba L. seeds (GBS) were freeze‐dried, infrared‐dried, hot‐air dried, and pulsed‐vacuum dried. A detailed analysis of their drying kinetics (drying curve, drying rate curve, effective moisture diffusivity, mathematical modeling), shrinkage and energy efficiency (specific moisture extraction, specific energy consumption), and physicochemical properties (soluble solids, total sugar, reducing sugar, ascorbic acid, phenolics, and tannins) were performed. Results showed drying methods had various effects on different indexes and were confirmed by principal component analysis. The drying process occurred in the falling‐rate period. The effective moisture diffusivity values amongst various drying methods ranged from 3.5 × 10⁻⁸ to 7.6 × 10⁻⁸ m²/s, which were within the normally expected range for food materials. The exponential model best fitted the experimental results for drying with coefficient of determination (R² ) > .981 and root mean square error, sum of square error, and reduced chi‐square closer to zero than the parabolic and inverse‐logarithmic (Log3p1) models. The freeze‐dried GBS had the highest nutritional (statistically the same with the fresh GBS) and lower shrinkage; nevertheless, it had the lowest moisture extraction rate and the highest energy consumption. Hot‐air dried GBS had the highest shrinkage and lowest nutritional retention. Infrared‐dried GBS was superior with the highest non‐tannic phenolic, proanthocyanidins content, effective moisture diffusivity, drying rates, and energy efficiency. The current findings indicate that infrared drying is a promising drying technology for GBS and has the potential to be applied on a commercial scale.
Practical application
Ginkgo seeds (GBS) are utilized as functional food across Asia. In China, ginkgo trees' extensive cultivation has surpassed market requirements, as GBS production could reach 60,000 tons per year. Therefore, there is an urgent need to find new opportunities to take advantage of the availability and usage of GBS regardless of the season and overcome the situation that GBS usage only remains at incipient phases of home‐cooked recipes. Drying is a common approach to preserve foods'. Nonetheless, there are some drawbacks to conventional drying method, such as low energy efficiency, lengthy process periods, and a decline in quality attributes. The infrared dryer provided a reduced drying time, highest drying rate, energy efficiency, and good quality; thus, it could be applied in the food industry to hot air and freeze‐drying, allowing the drying method for GBS faster. The research provided a predictive mathematical model for the parameter control GBS drying process.
Roasting is a food processing technique that employs the principle of heating to cook the product evenly and enhance the digestibility, palatability and sensory aspects of foods with desirable structural modifications of the food matrix. With the burgeoning demand for fortified roasted products along with the concern for food hygiene and the effects of harmful compounds, novel roasting techniques, and equipment to overcome the limitations of conventional operations are indispensable. Roasting techniques employing microwave, infrared hot-air, superheated steam, Revtech roaster, and Forced Convection Continuous Tumble (FCCT) roasting have been figuratively emerging to prominence for
effectively roasting different foods without compromising the nutritional quality. The present review critically appraises various conventional and emerging roasting techniques, their advantages and limitations, and their effect on different food matrix components, functional properties, structural attributes, and sensory aspects for a wide range of products. It was seen that thermal processing at high temperatures for increased durations affected both the physicochemical and structural properties of food. Nevertheless, novel techniques caused minimum destructive impacts as compared to the
traditional processes. However, further studies applying novel roasting techniques with a wide range of operating conditions on different types of products are crucial to establish the potential of these techniques in obtaining safe, quality foods.
In radio frequency (RF) assisted heat treatment of food products, heating behavior depends on the composition and geometry of a product. In this study, the effect of salt added to a food simulant on its dielectric properties and penetration depth (dp) were determined across a temperature range from 5 to 80 °C. Three primitive geometries (cube, cylinder and sphere) of a food simulant having a standardised volume (3.98 × 10⁻⁴ m³) heated in a custom-built 50 Ω RF system to investigate the impact of sample geometry and orientation on power absorption and heating uniformity. Experimental results revealed vertical oriented cylinder samples followed by cubes showed better temperature uniformity. The temperature at the core of food simulant samples after 6 min RF heating using 500 W power was 44 ± 7.1 °C, 45.3 ± 0.9 °C and 52.9 ± 3.5 °C in cube, cylinder, and sphere sample, respectively. However, the sphere sample was characterized by having the hottest spot at the bottom section. RF power level also played a significant role in heating uniformity with lower power input resulting not only in a slower heating rate but also in less uniform temperature distribution. This study provided a potential insight on the use of food simulants for evaluating RF heating design and performances.
Drying characteristics of combined radio frequency (RF) and hot-air (HA) were investigated for inshell hazelnuts (Corylus avellana L.), which have air-gap between kernel and shell (shell, kernel, and air-gap between them). Inshell hazelnuts were subjected to HA, HA followed by RF (HAFRF), RF followed by HA (RFHA), and HA assisted RF (HARF) drying (simultaneous process). Fiber optic sensors were used to monitor temperature profiles for both hazelnut kernels and shells at three representative locations (center, edge, and corner) in a tray in a 6-kW 27.12-MHz RF system with or without HA (~75 ˚C). RF heating resulted in higher kernel temperature than shell and was effective to remove kernel moisture. When RF heating was applied during or before HA drying, energy efficiency, drying efficacy, and nut quality were enhanced. This study provided a better understanding of the drying characteristics of the combined RF and HA process and suggested efficient RF-implemented drying strategies for inshell hazelnuts.
Acerola is a tropical fruit whose residues from its processing contain amounts vitamin C, phenolic and other bioactive compounds at the same level of the fruit pulp and juice. In this work we used a novel system for acerola waste drying, which is composed by a new rotary dryer (developed by our research group), named roto-aerated dryer, equipped with a new pre-drying system using infrared radiation. In addition, a pretreatment with ethanol was performed to improve the process performance. The ascorbic acid content, total phenolic content, total flavonoid contents, and antioxidant activity were quantified before and after drying. The main phenolic acids and flavonoids were identified by high performance liquid chromatography. Operating conditions that promoted high moisture removal (91.2%) and that yield to a product with high antioxidant activity (IC50 = 11.6 ± 0.3 µg/mL) and low water activity (0.374) were identified. The pretreatment with ethanol led to an increase of up to 72% of the moisture reduction. Therefore, the drying results revealed that this novel dryer system combined with the ethanol pre-treatment is a good alternative for dehydration, reducing the process time and maintaining a high antioxidant activity.
Graphic Abstract
Roasting is a key process in production of nuts. Improving the flavor and crispiness of texture in nuts is considered as a purpose of roasting, which increases the overall acceptance of the product. This review aims to introduce the infrared method as a new technique of roasting and evaluate the quality characteristics of some nuts after infrared roasting. Usually, the traditional roasting methods are time-consuming with high energy consumption and low production efficiency. One of the best ways to decrease roasting time and energy consumption is to provide heat by infrared (IR) radiation. However, the low penetration power of infrared radiation is one of the limitations of this method. The combination of infrared with other thermal methods can overcome this limitation. Studies have been done on roasting of nuts and other foods by different IR roasting methods such as IR, IR-hot air, and IR-microwave roasting methods. This paper reviews the effect of different IR roasting methods on the quality characteristics of roasted pistachio, peanut, hazelnut, almond, sunflower, soybean, and other food products. IR heating has been applied successfully to the roasting of some nuts. The use of infrared roasting has several advantages in comparison with traditional convective roasting methods. According to the results of most of these studies, the combination of infrared with other thermal methods to roast nuts has distinctly improved the potential of the technology as compared to the IR roasting alone.
Food processing plays a crucial role in coping up with the challenges against food security by reducing wastage and preventing spoilage. The ultrasound technology has revolutionized the food processing industry with its wide application in various processes, serving as a sustainable and low-cost alternative. This non-destructive technology offers several advantages such as rapid processes, enhanced process efficiency, elimination of process steps, better quality product and retention of product characteristics (texture, nutrition value, organoleptic properties), improved shelf life. This review paper summarizes the various applications of ultrasound in different unit operations (filtration, freezing, thawing, brining, sterilization/pasteurization, cutting, etc.) and specific food divisions (meat, fruits and vegetables, cereals, dairy, etc.) along with, the advantages and drawbacks of the technology. The further scope of industrial implementation of ultrasound has also been discussed.
The growing concerns over product quality have increased demand for high quality dried food products and encouraged researchers to explore and producers of such products to implement novel microwave–assisted drying methods. This paper presents a critical review of the key principles and drawbacks of microwave–assisted (MW) drying as well as needs for future research. In this article, recent research into application of microwaves as an alternative heat source, applications and progress in hybrid MW–assisted drying that rely on various drying media and combined two or three stages of MW–assisted drying for the preservation of food products is reviewed critically. The effect of different MW–assisted drying methods, conditions and initial pretreatments on the thermophysical properties, color, nutritional value and rehydration potential of dried food products is discussed in detail along with the discussion on how the material properties evolve and change in structure, color, and composition during MW-assisted drying and recent attempts at mathematical modeling of these changes made for different fruits and vegetables. It should be noted that most of the published results were obtained in laboratory-scale dryers. Pilot–scale testing is needed to bridge the gap between laboratory research and industrial applications to fulfill the potential for novel hybrid and combined MW–assisted drying methods and to expand their role in food processing.
The main purpose of this study was to develop and apply an adaptive neuro-fuzzy inference system (ANFIS) and Artificial Neural Networks (ANNs) model for predicting the drying characteristics of potato, garlic and cantaloupe at convective hot air dryer. Drying experiments were conducted at the air temperatures of 40, 50, 60 and 70 °C and the air speeds of 0.5, 1 and l.5 m/s. Drying properties were including kinetic drying, effective moisture diffusivity (Deff) and specific energy consumption (SEC). The highest value of Deff obtained 9.76 × 10⁻⁹, 0.13 × 10⁻⁹ and 9.97 × 10⁻¹⁰ m²/s for potato, garlic, and cantaloupe, respectively. The lowest value of SEC for potato, garlic, and cantaloupe were calculated 1.94 × 10⁵, 4.52 × 10⁵ and 2.12 × 10⁵ kJ/kg, respectively. Results revealed that the ANFIS model had the high ability to predict the Deff (R² = 0.9900), SEC (R² = 0.9917), moisture ratio (R² = 0.9974) and drying rate (R² = 0.9901) during drying. So ANFIS method had the high ability to evaluate all output as compared to ANNs method.
We report on the analysis of volatile compounds by SPME-GC-MS for individual roasted coffee beans. The aim was to understand the relative abundance and variability of volatile compounds between individual roasted coffee beans at constant roasting conditions. Twenty-five batches of Arabica and robusta species were sampled from 13 countries, and 10 single coffee beans randomly selected from each batch were individually roasted in a fluidised bed roaster at 210 °C for 3 min. High variability (CV = 14.0–53.3%) of 50 volatile compounds in roasted coffee was obtained within batches (10 beans per batch). Phenols and heterocyclic nitrogen compounds generally had higher intra-batch variation, while ketones were the most uniform compounds (CV < 20%). The variation between batches was much higher, with the CV ranging from 15.6 to 179.3%. The highest variation was observed for 2,3-butanediol, 3-ethylpyridine and hexanal. It was also possible to build classification models based on geographical origin, obtaining 99.5% and 90.8% accuracy using LDA or MLR classifiers respectively, and classification between Arabica and robusta beans. These results give further insight into natural variation of coffee aroma and could be used to obtain higher quality and more consistent final products. Our results suggest that coffee volatile concentration is also influenced by other factors than simply the roasting degree, especially green coffee composition, which is in turn influenced by the coffee species, geographical origin, ripening stage and pre- and post-harvest processing
As a means of adding value to pineapple production and minimising post-harvest losses, sliced pineapples were dried using a Solar Conduction Dryer (SCD) and appropriate thin layer drying models to predict drying were developed whilst the performance of the SCD was also investigated. For the period of the experiment, ambient temperature and temperature in the dryer ranged from 24 to 37 °C and 25 to 46 ℃ respectively. The performance of the dryer was compared to open sun drying using pineapple slices of 3-5 mm in thickness where the slices were reduced from an average moisture content of 85.42 % (w.b.) to 12.23 % (w.b.) by the SCD and to 51.51 % (w.b.) by the open sun drying in 8 hours effective drying time. Pineapple slices of thicknesses 3 mm, 5 mm, 7 mm and 10 mm were simultaneously dried in the four drying chambers of the SCD and their drying curves simulated with twelve thin layer drying models. The Middilli model was found as the best fitted thin layer drying model for sliced pineapples. The optimum fraction of drying tray area that should be loaded with pineapples was also investigated by simultaneously loading 7 mm slices of pineapples at 50, 75, and 100 percent of drying tray area. Loading the slices at 50, 75 and 100 percent of drying tray area gave overall thermal efficiencies of 23, 32 and 44 percent, respectively, hence loading pineapple slices at 100 percent drying tray area was recommended as the best.
Radio frequency (RF) heating is an advanced and emerging technology for food application because of its higher penetration depth, heat distribution and low energy consumption. Due to this reason, RF has entered the food industry to sterilize, pasteurize, disinfect food and agricultural commodity like fruits and dry nuts, post baking and in many other applications. The demand for safe, hygienic, tastier, no fat and preservative free food has widened up RF application in the modern industries. The paper aims to review the RF heating and its status of application in food processing. It also emphasizes on application of RF heating highlighting the impact on general quality aspects of food and describes the advantage and drawback of the RF system.
Effects of 480 and 640 W power levels and 2, 3 and 4 min for roasting of pistachios using microwaves (MW) on the physicochemical properties of pistachios in comparison with an Iranian traditional method of soaking in salty water and drying under hot-air at 135°C for 20 min were investigated. MW roasting of pistachios decreased water activity values, producing a softer nut. Twenty one volatile compounds were identified in roasted pistachios using GC-MS. The highest volatile concentration and sensory odor intensity scores were observed for hot air roasted pistachios and MW at 640 W for 4 min. Roasting reduced the force needed to cut pistachios. Unsaturated fatty acid contents in MW roasted pistachios were higher (82.0%) than in hot-air produced nuts (74.7%). Total phenolic content values increased with roasting time and with the MW power level. Pistachios can be successfully roasted using microwaves as a fast and economical method. © 2015, The Korean Society of Food Science and Technology and Springer Science+Business Media Dordrecht.
In this study, we investigated the genetic mapping of root biomass and root/shoot ratio. We utilized a large ( n = 345) bi‐parental recombinant inbred line (RIL) population from the ‘Penny’ × ‘Yecora‐Rojo’ cross to investigate the partitioning of biomass above‐ and belowground and to identify the quantitative trait loci (QTL) that influence root biomass and root/shoot ratio. Genotyping of 345 RILs by using genotyping by sequencing produced 2918 single‐nucleotide polymorphism markers by which a genome‐wide map of 3507 cM was constructed. Phenotyping was conducted in an augmented design with large pots in controlled environment. We identified two significant QTL regions, QRt.peye‐5A and QRt.peye‐5B, which control root biomass and the root/shoot ratio. QRt.peye‐5A, marking a 3.15 Mbp region on chromosome 5A, explained 11% of variations in root biomass and 9.5% of variations in root/shoot ratio, with the narrow region harbouring 28 genes. QRt.peye‐5B, marking a 12.2 Mbp region on chromosome 5B, explained 7% of variations in root/shoot ratio and harbours 104 genes. The root/shoot ratio enhancing alleles at QRt.peye‐5A and QRt.peye‐5B come from ‘Penny’ and ‘Yecora‐Rojo’ respectively. These QTL regions contains genes such as the two MADS box transcription factors on the 5A QTL that are candidate genes for Vrn1 locus, and other genes previously postulated for root traits such as a COBRA‐like COBL2 and landmark hormonal responses genes such as IAA16 , IAA4 and BRI1 , DREB2A‐INTERACTING PROTEIN2 (DRIP2) and bHLH92 which has a role in amelioration of stress conditions.
This study demonstrates a technical scale set-up for microwave-assisted extrusion of high moisture, texturized food systems. The set-up has been evaluated in a stationary study regarding the correlation of microwave tuner position and gap with the reflected microwave power and the product temperature, respectively. On that basis, a casein-based model food system has been plasticized by continuous microwave extrusion. The product outlet temperature and chemical composition have been investigated with varying screw speeds and power inputs.
The stationary study resulted in specific tuner positions, giving an area of desired process parameters. This area was successfully transferred to a continuous product application, where desired gel-sol transition was achieved. With increasing power input, the product temperature increased, while it decreased with increasing screw speed and constant power input. The fat, protein, and dry matter content of the casein-based model system did not differ significantly throughout all trials.
Industrial relevance
Microwave heating is a promising alternative heating method for food applications. Within microwave extrusion it shows high potential for upscaling applications whilst maintaining a homogenous temperature distribution throughout the product. Moreover, microwave technology offers high efficiencies and the possibility to use green energy sources. Furthermore, the space saving design and easy application provide high potential for scale-up.
Background
Radio frequency (RF), a dielectric heating technique with some advantages such as rapid, uniform, and volumetric heating and high energy efficiency, can act as an alternative method to conventional thermal treatment. Food texture is a parameter of great significance in the food industry and plays a crucial role in determining consumer satisfaction with thermally processed food products.
Scope and approach
This review provides an overview of applications of RF in food processing links such as drying, blanching, pasteurization, disinfestation, and thawing. The mechanisms behind the variations in food texture (e.g., hardness, firmness, tenderness) from cell microstructure, tissue integrity, and corresponding chemical composition changes were elucidated during the RF process compared with the conventional heating method.
Key findings and conclusions
A decrease in turgor pressure due to moisture loss and damage to cell structure integrity, as well as changes in pectin fractions, starch, and protein molecules, all are impacting the texture of plant-based foods during drying, blanching, pasteurization, and disinfestation. The mechanical damage to muscle fiber tissue caused by ice crystals is the primary reason for the variations in meat texture during thawing. Additionally, future research regarding continuous industrial-scale production and innovative applications is required.
Solar drying is a cost‐effective method for preserving fruits and vegetables and most suitable for rural farmers in developing countries. The spatial and temporal distribution of temperature in a solar conduction dryer (SCD) and its drying performance were investigated. The quality of the dried product was also evaluated by nine‐points hedonic scale test. Eight products were selected and dried in SCD and open sun drying (OSD) with three trials. Experimental data were fitted to three thin layer drying models. Results revealed that the drying air temperature was 1‐14 °C higher than the ambient air temperature depending on the solar radiation and drying position. The dryer saved 16‐35% drying time compared to OSD with an overall efficiency of 6.04‐19.8%. The overall acceptability of SCD‐dried products was higher than that of OSD‐dried products. Kinetic modelling showed Page model to be the best model in describing the drying characteristics of fruits and vegetables.
Electromagnetic waves are frequently used for food processing with commercial or domestic type microwave ovens at present. Microwaves cause molecular movement by the migration of ionic particles or rotation of dipolar particles. Considering the potential applications of microwave technique in food industry, it is seen that microwaves have many advantages such as saving time, better final product quality (more taste, color and nutritional value) and rapid heat generation. Although microwave treatment used for food processing with developing technologies have a positive effect in terms of time, energy, or nutrient value, it is also very important to what extent they affect the textural properties of the food that they apply to. For this purpose, in this study, it has been investigated that the effects of commonly used microwave treatments such as drying, heating, baking, cooking, thawing, toasting, blanching, frying, and sterilization on the textural properties of food. In addition, this study has also covered the challenges of microwave treatments and future work. In conclusion, microwave treatments cause energy saving due to a short processing time. Therefore, it can be said that it affects the textural properties positively. However, it is important that the microwave processing conditions used are chosen appropriately for each food material.
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Food loss and waste represent an increasing concern under social, economic and environmental perspective, either in developed or developing realities. It is estimated that more than 1.3 billion tons of food waste are generated along the whole food supply chain, from agricultural to final consumption stages, with associated environmental impacts estimated in approximately 3.3 Gigatons of CO2 equivalent per year (6% of total anthropogenic greenhouse gases emissions). Indeed, food waste issue has been accounted among the 17 Sustainable Development Goals in terms of responsible production and consumption, with the aim of halving per capita global food waste and reduce food losses by 2030. The present paper, through a systematic, analytical and configurative review on food waste global warming potential, focuses on the role of Life Cycle Assessment and its related opportunities and challenges along upstream, core and downstream stages, considering at the same time the challenges embedded within alternative disposal technologies. Through the choice of 16 different research string and the selection of 33 papers out of more than 2000 articles between 2011 and 2021, the authors highlight the environmental impacts associated to food waste with regards to: (a) entire food baskets; (b) specific food commodities; (c) food service and households' experiences; (d) diverse disposal alternatives (e.g., anaerobic digestion, incineration, landfill), addressing future research and suitable opportunities to reach national and international sustainable goals.
Radio frequency-assisted hot air drying (RF-HAD) was carried out to produce carrot powders. Effect of electrode gap, sample thickness and sample weight on RF-HAD efficiency was investigated. 90 mm of electrode gap, 2.5 cm of sample thickness and 600 g of sample weight were selected as RF-HAD parameters in order to obtain acceptable drying rate and avoid excessive deformation. RF-HAD kinetics were studied and compared with hot air drying (HAD). RF-HAD increased drying rate and reduced drying time by 45%. Also, the quality of carrot powders obtained after RF-HAD was evaluated and compared with those produced by air and freeze drying. The results showed that the samples dried with RF-HAD had greater contents of β-carotene, ascorbic acid, and total phenols, higher antioxidant activity and improved rehydration capacity compared to HAD. The better retention of cellular microstructure after RF-HAD also resulted in better rehydration capacity. Beta-carotene, ascorbic acid, total phenolic content and antioxidant activity retention rates were 80.26%, 45.33%, 72.42% and 67.49%, respectively, for RF-HAD. These values were 89.36%, 47.02%, 79.98% and 70.61% for freeze dried samples. The retention of bioactive compounds after RF-HAD was comparable to those achieved by freeze drying.
This research investigated the application of combined hot air and radio frequency drying technology on the drying efficiency and the quality of black carrot pomace (BCP). Electrode gap (8, 9 and 10 cm), thickness (2, 3 and 4 cm), weight (500, 750 and 1000 g) and compaction density (0.44, 0.66 and 0.88 g/cm³) were the parameters studied to determine the drying characteristics of the pomace by hot air-assisted radio frequency (HARF) drying system. Electrode gap of 8 cm and thickness of 2 cm, weight of 500 g and compaction density of 0.44 g/cm³ were identified as the ideal HARF drying conditions. HARF drying shortened the drying time by 39% compared to conventional hot air (HA) drying. Six different mathematical models were used for describing the drying characteristic of BCP. The effective moisture diffusivities were estimated by two approaches: the diffusion equation method and the method of slope for HARF drying and HA drying methods. HARF drying provided the dried products with better color, phenolic content, anthocyanin content and antioxidant capacity.
Industrial relevance
Black carrot pomace is an industrial waste left after the juice extraction from black carrot. The most common utilization way of this valuable by-product is to use it as animal feed. The reason behind this fact is the difficulties in drying of a high moisture bulk material in huge quantities produced in winter season. If it is not dried on time, it gets fermented and become useless for the separation of valuable components such as anthocyanin, pectin and fiber. So, a fast and efficient drying method and its characterization is essential for the utilization of this valuable by-product.
Background: Drying is one of the oldest preservation techniques used in the food industry. This technology has evolved and different drying systems have been applied, such as convection, conduction, or radiation, and combinations thereof.
Scope and approach: This review studies the VMD process whose principle is based on the electromagnetic radiation emitted by the magnetron on a food matrix in a reduced pressure environment and which is transformed into heat. VMD is a promising, fast, and efficient process for reducing moisture content. Both research and industrial use have grown by 303% in the last 20 yr. The objective of this review is to provide an overview of major developments in VMD research and its application in various foods and other materials. Based on the characteristics and parameters of the VMD equipment in the drying process, it was possible to classify the effect VMD has on different food matrices.
Key findings and conclusions: The review describes the phenomenological behavior of the drying process and how to optimize it using equipment modifications with computer simulations. The different VMD systems have been classified as static, rotary, continuous flow, and proprietary equipment. In addition to some characteristics of process efficiency and energy consumption, VMD exhibits energy consumption savings of approximately 50% compared with other drying systems. Finally, product quality is discussed under different conditions, such as equipment design and temperature uniformity. Thus, it is challenging to design and built new VMD systems with higher energy efficiency in food drying.
Drying temperature, air velocity, material thickness during hot air drying usually have a significant influence on drying kinetics and quality attributes of fruits and vegetables. However, under constant drying temperature, the influence of relative humidity (RH) of hot air on heat and mass transfer is often ignored in drying practice. On the other hand, the current literature about the influence of relative humidity of drying medium on drying kinetics and quality of fruits and vegetables are inconsistent or even contradictory. Usually, the relative humidity is roughly controlled based on experience. Therefore, it is necessary to summarize and reveal the influence mechanism of relative humidity of drying medium on drying kinetics and quality of fruits and vegetables. The RH effect mechanism on drying kinetics is analyzed in two aspects including heat transfer and mass transfer. Increasing RH intensifies heat transfer process while decreasing RH intensifies mass transfer process. Applied high RH in the initial drying stage and then decreasing RH can improve drying efficiency compared to the continuous dehumidification RH drying method. Additionally, step-down RH drying strategy method helps to mitigate the formation of casehardening on product surface and promote pore network, which is benefit for moisture transfer and diffusion. High RH during hot air has the potential to improve the pore network of materials, which is helpful for rehydration ratio. Additionally, low RH increases drying rate so that material surface generates a rigid crust or shell that fixes the volume. Low temperature but high RH drying medium can preheat the sample as soon as possible so as the energy which was used to heat the drying air can be saved. The applicable of step-down RH drying strategy is suitable for those food materials drying, whose β value is more than 1 in Weibull distribution model and Bi value in Bi-Di model is more than 0.1. In other circumstances, continuous dehumidification drying is suitable for food material drying. The auto RH control adjustment drying strategy has been investigated based on material temperature. Such RH control drying strategy provides theoretical foundation for enhancing drying efficiency and quality of fruits and vegetable materials.
Solar energy has long been utilised in crop conservation. The use of solar dryers as a drying method overcomes the drawbacks of the traditional approach to drying crops under the sun. Previous studies were coherently described and focused on the development and thermal convection of greenhouse dryers, structural analysis, crops cultivation for offering choices to reader about solar greenhouse dryers in agriculture. Based on the previous related studies, the current work proposes an evolve and detail review of solar-assisted greenhouse dryers, namely, passive dryers, active dryers and hybrid dryers, from the aspects of system performance, collector performance and the drying characteristics of various crops. Drying crops in a solar greenhouse dryer protects crops from insects, rain and dust. Thus, these dryers are preferred in natural and conventional drying. This review provides a satisfactory guide for researchers to compare, develop and improve solar greenhouse drying systems to achieve optimum crop drying rates based on thermal convection mode.
Drying plays a critical role in processing of various food and bioproducts as significant amount of moisture need to be removed for further processing. The rate of moisture removal could impact not only the drying kinetics and efficiency but also the product quality which eventually will decide the acceptance of the final dried products in the market. In an attempt to achieve better process efficiency and product quality as well as to make drying process a more sustainable operation, researchers have embarked to envisage and to improve current dryer design by combining the benefits of various drying mechanisms into a single or combined/sequential operations known as hybrid drying. Hybrid dryers could offer several advantages over conventional dryers such as better drying efficiency, higher energy efficiency, more efficient moisture removal process, better product quality and enhanced retention of nutritional/bioactive compounds. In this paper, hybrid drying literatures published mainly from the recent decade are reviewed to provide researchers and industrial practitioners an up-to-date reference material that could contribute to current research and knowledge transfer especially in the development of more sustainable and efficient drying processes.
The conventional drying technology prolongs the shelf life of carrots but may cause the serious loss of nutrients. The aim of this study was to investigate the effects of intermittently rearranging layers on the heating uniformity, drying characteristics and quality of carrot slices under the hot air assisted radio frequency (RF) heating. The carrot slices were dried for 270 min using hot air (60 °C) assisted RF heating at the electrode gap of 100 mm firstly, and then followed hot air drying to achieve the final moisture level (0.11 kg/kg (d.b.)). The results showed that the selected hot air assisted RF drying protocol for the carrot slices reduced 30% of the duration compared to the single hot air drying. The carrot slices dried by the combined drying method had the highest total carotenoid value (P ≤ 0.05) except for possessing accepted color and rehydration. Therefore, the combined drying method could improve the drying rate and maintain heat sensitive substances in carrot slices.
Drying as an effective food preservation method, extends the shelf-life of the food and
reduces storage, packaging, and transportation costs. Each of the common drying
methods imposes limitations on the end products' quality and energy efficiency. Novel
approaches such as electro-based drying methods have recently emerged to improve
the drying process. They can be defined as a drying process in which electrical energy
is employed during the pretreatment or during the drying treatment, namely
electrohydrodynamic (EHD), pulsed electric field (PEF), ultrasound-assisted, and
dielectric drying (microwave and radiofrequency). This review aims to provide an
overview regarding the promising common electro-based drying methods used for food
and agriculture products. Besides, the mechanism of action and potential applications
of electro-based drying methods are discussed. Existing literature data demonstrates
that electro-based water removal technologies can enhance drying kinetics without any
negative impact on food quality
The present study aims at developing a low-temperature energy-efficient sequential drying approach, using radiofrequency (RF) waves and low humidity air (LHA) for heat-sensitive foods like apple. Efficacy of employing RF at primary (RF + LHA) and secondary (LHA + RF) drying stage with LHA was studied in comparison with LHA and HA (hot air) and RF (2 A) drying, individually at 40 °C. LHA + RF reduced the drying time by 37% and energy consumption by 52% compared to LHA. Apple slices dried with sequential LHA + RF approach well preserved the polyphenols (98%), flavonoids (87%), and ascorbic acid (77%) compared to FD. Sequential drying did not affect the color (ΔE = 7.4 ± 0.7) and also improved the rehydration ratio (3.9 ± 0.1). The principal component analysis demonstrated that LHA + RF had higher retention of volatile compounds (aldehydes, esters, acids, alcohols) than other studied methods. The study suggests that the application of RF at the secondary drying stage with LHA accelerates the drying process and reduces energy requirement with better retention of product quality attributes.
Pure titanium (Ti) often exhibits biologically inert performance in clinical medicine due to the occurrence of surface passivation. Surface modification is crucial to improve the biological activity and long-term stability of Ti. In this research, Induction heating + acid etching + alkali heat (IHT + AET + AHT) treatment methods were used to rapidly fabricate a biologically activated structure on the Ti surface. The effects of IHT + AET + AHT on the microstructure, phase composition, properties, and formation mechanisms of the coatings were investigated. The results showed that a fine and uniform porous network composite coating of sodium titanate and rutile TiO2 was formed by IHT + AET + AHT methods, which showed good adhesion and corrosion resistance. Compared with AHT, IHT + AET + AHT can significantly reduce the pore size of sodium titanate from 300 to 350 nm to 80–100 nm. The IHT + AET + AHT coating has a good ability to induce hydroxyapatite (HA) formation in simulated body fluid (SBF). Moreover, the HA formed on the IHT + AET + AHT coating exhibits excellent long-term stability.
This study presents a state-of-the-art overview on the application of ultrasound technology in the drying of food products, including the ultrasound pre-treatment and ultrasound assisted drying. The effect of main parameters and ultrasound technology on the drying kinetics and food quality were discussed. Inconsistencies were pointed out and analyzed in detail. Results showed that for ultrasound pre-treatment, the food products may lose or gain water and increase of ultrasonic parameters (sonication time, amplitude and ultrasound power) promoted the water loss or water gain. When ultrasound technology was applied prior to drying, an increase in drying kinetics was always observed, though some different results were also presented. For ultrasound assisted drying, the ultrasound power always gave a positive effect on the drying process, however, the magnitude of ultrasound improvement was largely dependent on the process variables, such as air velocity, air temperature, microwave power and vacuum pressure, etc. The application of ultrasound technology will somehow affect the food quality, including the physical and chemical ones. Generally, the ultrasound application can decrease the water activity, improve the product color and reduce the nutrient loss.
Hot air-assisted radio frequency (HA-RF) heating has great potential in roasting of cashew nut kernels with high roasting efficacy. This study further investigated HA-RF roasting effects on nutritional quality, malondialdehyde (MDA) and aroma compounds of cashew nut kernels. After HA-RF roasting, crude fiber content of kernels increased from 2.49 g/100 g to 3.18 g/100 g while ash and protein content did not change significantly (p > 0.05). In raw and roasted kernels, 14 fatty acids were detected including 7 saturated fatty acids and 7 unsaturated fatty acids. Fatty acid composition and individual amino acid content were not affected by HA-RF or conventional hot air (HA) roasting. MDA content increased from 0.36 mg/kg to 0.48 mg/kg and 0.64 mg/kg after HA-RF roasting and HA roasting. The types of aroma compounds of cashew nut kernels roasted by HA-RF and HA were similar, but HA-RFK had lower furan content and higher ketone and terpene contents. This study indicated HA-RF roasting was comparable or even better than HA roasting in improving nutritional quality and flavor.
Background
Microwave processing techniques have been extensively used in the food industry due to its significant reduction in cooking time and energy consumption. Microwave processing technologies such as microwave drying, heating and sterilizing play a significant role in food quality and safety control. However, few reviews have been published in recent years summarizing the latest developments in the application of microwave technology in the food industry.
Scope and approach
This review focuses on recent applications of microwave processing technologies including microwave drying, heating, and sterilizing in fruit (banana, apple, olive, sour cherries, pomegranate arils, blueberries, kiwifruit, aronia, strawberry, and grape tomato), vegetables (potato, bamboo shoot, purslane leaves, onion, green bean, pumpkin, eggplant, edamame, sea tangle, garlic, kale, red cabbage, tomato, cassava, lentils, chickpea, broccoli, Brussels sprouts, cauliflower, jalapeño peppers, and coriander foliage), and meat products (sardine fish, restructured silver carp slices, sea cucumber, beef semitendinosus muscle, bovine supraspinatus muscle, camel longissimus dorsi muscle, foal meat, bovine gluteus medium muscle, chicken steak, mature cows semimembranosus and semitendinosus muscles, kavurma (a ready-to-eat meat product), salmon, cod, drumettes, and beef slices), changes in product quality as affected with microwave processing are discussed in details, and future directions of research are presented.
Key findings and conclusions
Microwave drying has the advantages of low energy consumption and high efficiency as compared to conventional drying, while producing more porous structure of foods. Microwave drying usually combines with other conventional drying to enhance the quality of a food product. Compared with the traditional method, microwave heating or cooking can generally retain higher levels of bioactive components, antioxidant activity and attractive color of vegetables, while microwave cooking with water can cause a serious drop in nutrients due to leaching and thermal liability. Microwave sterilization has the capacity to completely inactivate microorganisms and effectively destroy enzyme activity, and less effect on antioxidant activity, texture and color of food products compared with conventional pasteurization.
Convective drying is a popular technique for drying of food materials. A plethora of empirical models have been used to fit the convective drying kinetics of food materials in general and pumpkin in particular. In literature, there exists a gap in the study of convective drying of pumpkin using first principle based model. In this article, a first principle based model for heat and mass transfer with appropriate convective boundary condition have been formulated. The model resulted in the set of partial differential equations, which have been solved using method of lines (MOL). The simulated model demonstrates good qualitative agreement with the reported literatures and experimental data. The Arrhenius parameters (activation energy and the pre exponential factor) for effective diffusivity have been estimated by fitting experimental data. Using the estimated values, the drying kinetics under various drying conditions have been simulated and found to be in good agreement with the experimental data. The different drying regimes exhibited by the pumpkin were explained using the temperature and moisture profiles predicted by the model.
Roasting is an important step in the processing of peanut kernels. In this study the combination of infrared (IR) and hot-air was explored for roasting of peanut kernels and the effects of processing conditions including hot air temperature (100 and 120 °C), infrared power (130 W, 165 W and 200 W) and roasting time (10, 20, 30 min) on different characteristics of kernels (moisture content, color, textural characteristics, chemical properties, specific energy consumption and sensory properties) were investigated. The results showed that increasing the roasting air temperature, infrared power and roasting time decreased the moisture content, fracture force, hardness, apparent modulus of elasticity, compressive energy, pH and solid soluble content and increased the browning index, total phenolic compounds, and total acceptance of peanuts kernels. The total color difference (ΔE) varied from 2.87 to 19.33. After roasting, hardness of the peanut kernels reduced from initial 91.31 to 33.86 N and total phenolic compounds value were 2.71 mg GAE/g, all in the acceptable limit for good quality of roasted peanuts. The lowest value of specific energy consumed in roasting peanut kernels was observed at 100 °C and 200 W for 10 min (5.06 kWh/kg) and the highest value was at 120 °C and 130 W for 30 min (23.20 kWh/kg). This study demonstrated that combination of infrared and hot-air roasting can produce high-quality roasted peanuts with lower energy cost and time roasting; therefore it could be considered as a new technology for the peanut roasting industry.
Industrial relevance
Infrared radiation could be considered as a promising technique for roasting of soybeans due to lower energy cost.
Dried kiwis are highly needed in food industries such as cereals, ice-cream, beverages and supplemental products. In this paper, drying characteristics and product quality of hot air dried kiwi slices were studied. Hot air drying of kiwi slices was investigated at drying temperature ranged from 40°C to 60°C and slice thickness of 0.3 cm and 0.6 cm. Results showed that drying of kiwi slices at higher drying temperature stimulates the drying rate, which leads to shorter total drying time required. The drying kinetics of kiwi slices was best fitted by approximation diffusion model. Increased in drying temperatures and slice thickness of kiwi enhanced the effective moisture diffusivity (Deff). The highest Deff of the kiwi slices was recorded as 1.5681 x 10-8 m2/min at slice thickness of 0.6 cm. In terms of quality analysis, kiwi slices dried at temperature of 60°C with fastest drying rate retained most of the Total Phenolic Content (TPC) in the dried sample. However, drying of kiwi slices at high drying temperature deteriorated the vitamin C content of kiwi slices due to thermal degradation. Thinner kiwi slices could preserve higher amount of TPC and vitamin C during the drying process, yet the best hot air drying temperature for drying of kiwi slices could be relied on the consumers' preference based on the dried product quality as reported in the current work.
This article aims to review and analyze the aspects and characteristics related to infrared food drying. Indeed, with a review of 100 relevant publications all dealing with infrared food drying, this article notes that infrared drying has several advantages over other common food drying methods. Shorter drying time, a better final dried product quality, and more energy savings in the process are revealed as the most important advantages of infrared drying over convective heat drying. Infrared dryers can also be easily combined with other drying methods such as hot air, microwave, vibration, and vacuum. This article clearly shows that using infrared heating for food drying purposes has become more popular in the last decade and its application in the industrial drying of different foodstuffs has been employed widely.
Background
The efficiency of hand drying is important in preventing pathogen spread, but knowledge surrounding which drying methods contribute least towards contamination of the environment and users is limited.
Aim
To compare the propensity of three common hand-drying methods (jet air, warm air dryers, and paper towels) to contaminate the environment, users, and bystanders.
Methods
Hands were coated in lactobacilli to simulate poorly washed, contaminated hands, and dried. The investigation comprised 120 air-sampling tests (60 tests and 60 controls), divided into close and 1 m proximity from the drying process. Separate tests used hands coated in paint to visualize droplet dispersal.
Findings
Air bacterial counts in close proximity to hand drying were 4.5-fold higher for the jet air dryer (70.7 cfu) compared with the warm air dryer (15.7 cfu) (P = 0.001), and 27-fold higher compared with use of paper towels (2.6 cfu) (P < 0.001). Airborne counts were also significantly different during use of towel drying versus warm air dryer (P = 0.001). A similar pattern was seen for bacterial counts at 1 m away. Visualization experiments demonstrated that the jet air dryer caused the most droplet dispersal.
Conclusion
Jet air and warm air dryers result in increased bacterial aerosolization when drying hands. These results suggest that air dryers may be unsuitable for use in healthcare settings, as they may facilitate microbial cross-contamination via airborne dissemination to the environment or bathroom visitors.
Through three editions, this has been the must-have resource on food properties and their variations. The book defines food properties and the necessary theoretical background for each. It also evaluates the usefulness of each property in the design and operation of important food processing equipment. This new edition addresses advanced knowledge of food properties by providing the latest developments in the field. It offers three new chapters covering glass transition temperature pertaining to food, kinetics related to non-thermal processing, and micro-structural properties of foods.