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Development and Performance of A Temperature-Controlled Microwave Dryer
... After the rest period, when the temperature of the product falls below the determined drying temperature, MTCM automatically starts working again. During this period, if the product temperature does not fall below the drying temperature, MTCM waits for the specified rest period without operating again [36,48]. Purple cabbage puree was dried in MTCM at temperatures of 50, 60, and 70 °C until equilibrium moisture. ...
In this study, effective diffusion values were determined between 3.26–6.07 × 10–9 and 1.23–4.02 × 10–8 m²/s. SMER values were calculated between 3–4.1 × 10–3 and 9 × 10–3–1.6 × 10–2 kg/kWh. SEC values are calculated between 244.99–326.39 and 63.70–111.38 kWh/kg. Evaporating energy values were calculated between 4.83–4.86 and 4.81–4.89 kWh. Carr indexes of the powders were determined between 28.06 and 65.38 and Hausner indexes between 1.77 and 2.90. Minimum color change CD was detected at 70 °C and MTCM at 50 °C drying processes. The total phenolic substance has ranged from 3584.5 to 5774.5 μg GAE g–1 dw, total flavonoid substance from 499.3 to 1514.1 mg KE L–1 dw, total monomeric anthocyanin from 162.5 to 955.2 μg cy–3-O-glu g–1 dw, vitamin C from 500 to 820 mg L–1, and total antioxidant capacity ranged from 21.37 to 33.78 μmol TE g–1 dw.
... Equation number 5 was used to calculate the amount of moisture removed (SMER) in response to the unit energy value in the drying processes of waste tomato puree [45]. ...
In this study, the effect of carrier agent added at the rates of 5% and 10% to tomatoes with physical defects that have no market value and the effects of convective (CD), vacuum (VD), hybrid (HD), temperature controlled microwave (MTCM) methods on the energy parameters of powder production processes were investigated. The products reached their final moisture values in the shortest time with the MTCM method and in the longest time with the CD method. Effective moisture diffusion varied between 8.01 × 10− 8-1.97 × 10− 6 m²/s. It has been determined that MTCM has the lowest energy consumption. SMER values of drying processes varied between 0.0018329 and 0.007384 kg/kWh. SEC values ranged between 546.76 and 135.42 kWh/kg. Exin, Exout, Exevap, Ex-Vdryer, Ex-Vdrying, SI and IP values of drying processes are 3.65–4.54 J/s, 3.13–3.43 J/s, 10.91–14.17 kJ/kg, 2.94–3.72, 0.72–0.90, respectively. The values varied between 3.60 and 9.99 and 0.34–0.91. It has been observed that the VD method is more advantageous than other drying methods in terms of exergy energy values. The MTCM method was found to be more advantageous in terms of drying time and energy consumption parameters.
... Equation 4 was used to calculate effective diffusion values [8].Polaxtor brand PLX-15366 model power meter (± 0.02 kWh) was used to measure the energy values consumed in the drying processes of waste tomato puree.2.8. Speci c moisture extraction rate (SMER)Equation number 5 was used to calculate the amount of moisture removed (SMER) in response to the unit energy value in the drying processes of waste tomato puree[45].Here: SMER; speci c moisture removal rate (kg/kWh). ...
In this study, the effect of carrier agent added at the rates of 5% and 10% to tomatoes with physical defects that have no market value and the effects of convective (CD), vacuum (VD), hybrid (HD), temperature controlled microwave (MTCM) methods on the energy parameters of powder production processes were investigated. The products reached their final moisture values in the shortest time with the MTCM method and in the longest time with the CD method. Effective moisture diffusion varied between 8.01x10 − 8 -1.97x10 − 6 m ² /s. It has been determined that MTCM has the lowest energy consumption. SMER values of drying processes varied between 0.0018329–0.007384 kg/kWh. SEC values ranged between 546.76-135.42 kWh/kg. Exin, Exout, Exevap, Ex-Vdryer, Ex-Vdrying, SI and IP values of drying processes are 3.65–4.54 J/s, 3.13–3.43 J/s, 10.91–14.17 kJ/kg, 2.94–3.72, 0.72–0.90, respectively. The values varied between 3.60–9.99 and 0.34–0.91. It has been observed that the VD method is more advantageous than other drying methods in terms of exergy energy values. The MTCM method was found to be more advantageous in terms of drying time and energy consumption parameters.
In this study, banana slices were dried in convective (CD) and modified temperature-controlled microwave drying (MTCM) methods and at 40, 50, 60, and 70 ºC to produce banana chips. The kinetics, energy analysis, and physicochemical properties of the drying processes were investigated. Drying rates varied between 0.0039 and 0.10 g moisture g dry matter minute−1 for drying processes. Effective diffusion values were determined between 8.22 × 10–8–1.13 × 10–5 m2s−1 for drying processes. Rehydration capacities were determined between 49.10 and 65.88% for dry methods. Hardness values were determined between 38.00 and 66.30 N for drying processes. Specific moisture absorption rate values were determined to vary between 0.0015 and 0.0099 kgkWh−1. The energy efficiency values were determined to vary between 0.96 and 6.48% for processes, and the latent heat evaporating values ranged between 0.64 and 2.05 kWh. The most suitable (p < 0.05) color values for CD and MTCM were determined at 70 ºC. In this study, the MTCM 70 ºC method is recommended for drying kinetics and physical quality properties. In future studies, a more comprehensive analysis can be conducted by taking into account its bioactive properties.
Drying characteristics and quality of apple slices were studied using microwave intermittent drying at different microwave power density (2.2, 3.5, 4.8, and 6.1 W/g), sample thickness (3, 5, 7, and 9 mm) and heating time in each cycle (4, 5, 6, and 7 s). Results showed that the approximate constant rate drying periods and falling rate drying periods were found in all the curves. The drying time decreased with increasing microwave power density and heating time in each cycle, while increased significantly with the growth of sample thickness. Weibull distribution model could fit the drying curves well in all conditions (R ² > 0.99) with the scale parameter values changed from 45.88 to 206.08 min and the shape parameters values varied from 1.514 to 1.69. The moisture effective diffusivity calculated by Weibull model ranged from 0.3217 × 10⁻⁹ to 1.435 × 10⁻⁹ m²·s⁻¹. The flavor, texture and overall acceptability values displayed a decreasing tendency with the increase of microwave power density, while increased with the growth of thickness. And the shrinkage rates of apple slices showed a decline tendency with the increase of above parameters.
Practical applications
Apple is a kind of popular and important fruits with high nutrient content, and it has been widely planted in China. Due to its high moisture content, fresh apples will subject to various deteriorative reactions during storage and thus lead to serious post‐harvest losses. Microwave drying technology has been widely used in dehydration of agricultural products in China because of the significant heating efficiency. However, the continuous microwave heating may easily cause the overheating of raw materials and undesirable changes in quality. Microwave intermittent drying (MID) was proposed on the basis of conventional microwave drying and regarded as an efficient processing method by solving the above issues. Currently this technology has been successfully applied to ginkgo fruits, yam and banana slices. Analyzing the effects of drying parameters on MID kinetics and quality of apple slices will have important significance for optimizing the processing efficiency and consumer acceptability of dried products.
The effect of microwave‐vacuum drying on drying characteristics, protein, mineral content, and color of the apple was investigated in this study. Eight different drying treatments were applied with using levels of microwave power (200 and 300 W), vacuum conditions (200 and 400 mmHg), and pulse ratios (PR = 1 [Continuous] and PR = 2 [30 s ton/30 s toff]). Based on the drying settings used, increasing microwave power, vacuum treatments, and decreasing pulse ratio provided shorter drying time. The Page model gave a high fit of statistical values at all data points used to describe the drying kinetics of dried apple samples. At the 300 W–200 mmHg–PR = 2 application, the quantity of potassium was found to be the highest compared with that of the other drying conditions. The protein rate of dried apples was higher than the fresh samples because of water loss. The lowest effect on L* (lightness [78.27]), a* (green/red [10.70]), and b* (blue/yellow [32.72]) values were observed in samples dried at the 200 W–400 mmHg–PR = 2, 300 W–200 mmHg–PR = 1, and 200 W–200 mmHg–PR = 2, respectively. As a conclusion, the treatment of 300 W–400 mmHg–PR = 1 was found most effective with a shorter drying time (30 min) and high protein content (2.5%).
Practical applications
Apple (Malus domestica L.) is one of the most important fruit due to its nutritional composition and the presence of biologically active compounds that provide positive effects on human health. Therefore, apple is grown and farmed extensively in most agricultural areas around the world. Drying is one of the oldest methods of food preservation. Dried apple can be not only stored longer, but also used as packaged snacks and integral breakfast foods. Combining microwave and vacuum drying offers rapid and uniform drying, which can be used to dry food products on a commercial scale. In addition, application of intermittent microwave‐vacuum technology in the drying of apple can result in significant improvement in quality.
Background:
Blueberry juice powder was developed via foam-mat freeze-drying (FMFD) and spray-drying (SD) via addition of maltodextrin (MD) and whey protein isolate (WPI) at weight ratios of MD/WPI 0.4 to 3.2 (with a fixed solids content of 5 wt.% for FMFD and 10 wt.% for SD). Feed rates of 180 and 360 mL h(-1) were tested in SD. The objective was to evaluate the effect of the drying methods and carrier agents on the physical properties of the corresponding blueberry powders and reconstituted products.
Results:
Ratios of MD/WPI = 0.4, 1.0 and 1.6 produced highly stable foams most suitable for FMFD. FMFD gave high yields and low bulk density powders with flake-like particles of large size that were also dark purple with high red values. SD gave low powder recoveries. The powders had higher bulk density and faster rehydration times, consisting of smooth, spherical and smaller particles than in FMFD powders. The SD powders were bright purple but less red than FMFD powders. Solubility was greater than 95% for both FMFD and SD powders.
Conclusion:
The FMFD method is a feasible method of producing blueberry juice powder and gives products retaining more characteristics of the original juice than SD.
Drying kinetics, modeling, temperature profile and energy indices were investigated in apple slices during drying by a specially designed microwave-hot air domestic hybrid oven at the following conditions: 120, 150 and 180 W microwave powers coupled with 50, 60 and 70 °C air temperatures. Both sources of energy were applied simultaneously during the whole drying processes. The drying process continued until the moisture content of apple slices reached to 20% from 86.3% (wet basis, w.b). Drying times ranged from 330 to 800 min and decreased with increasing microwave power and air temperatures. The constant rate period was only observed at low microwave powers and air temperatures. Two falling rate periods were observed. Temperature of apple slices sharply increased within the first 60 min, then reached equilibrium with drying medium and finally increased at the end of the drying process. In order to describe drying behavior of apple slices nine empirical models were applied. The Modified Logistic Model fitted the best our experimental data (R² = 0.9955–0.9998; χ² = 3.46 × 10⁻⁵-7.85 × 10⁻⁴ and RMSE = 0.0052–0.0221). The effective moisture and thermal diffusivities were calculated by Fick’s second law and ranged from 1.42 × 10⁻⁹ to 3.31 × 10⁻⁹ m²/s and 7.70 × 10⁻⁹ to 12.54 × 10⁻⁹ m²/s, respectively. The activation energy (Ea) values were calculated from effective moisture diffusivity (Deff), thermal diffusivity (α) and the rate constant of the best model (k). The Ea values found from these three terms were similar and varied from 13.04 to 33.52 kJ/mol. Energy consumption and specific energy requirement of the hybrid drying of apple slices decreased and energy efficiency of the drying system increased with increasing microwave power and air temperature. Apples can be dried rapidly and effectively by use of the hybrid technique.
This research inspected the effects of freeze, microwave (120 and 350 W) and hot air (60, 70 and 80 °C) drying techniques on the color, drying characteristics, antioxidant capacity and total phenolic content of mango slices. Midilli et al., Two-term and Page models which exemplify drying characteristics are superior than alternative models. All of the color values (a, b, L, C, α and ΔE) were altered notably based on the used drying technique and colors nearest to the fresh sample were attained with freeze drying. In comparison to the fresh sample, the dried samples showed a decrease of 18.4-54.6% in antioxidant capacity. The total phenolic content value was notably highest one for the microwave dried sample at 350 W and the lowest one for a hot air dried sample at 80 °C (P<0.05). This research showed that microwave drying at 350 W is able to yield high-quality mango slices with the extra advantage of shortened drying time in relation to hot air and freeze drying.
Color measurements have traditionally been linked to expensive and difficult to handle equipment. The set of mathematical transformations that are needed to transfer a color that we observe in any object that doesn't emit its own light (which is usually called a color-object) so that it can be displayed on a computer screen or printed on paper is not at all trivial. This usually requires a thorough knowledge of color spaces, colorimetric transformations and color management systems. The TCS3414CS color sensor (I2C Sensor Color Grove), a system for capturing, processing and color management that allows the colors of any non-self-luminous object using a low-cost hardware based on Arduino, is presented in this paper. Specific software has been developed in Matlab and a study of the linearity of chromatic channels and accuracy of color measurements for this device has been undertaken. All used scripts (Arduino and Matlab) are attached as supplementary material. The results show acceptable accuracy values that, although obviously do not reach the levels obtained with the other scientific instruments, for the price difference they present a good low cost option.
Microwave has gained widespread popularity in coal drying. In the current electromagnetic models, coal is always assumed as a solid continuum. The thermodynamic evolution and the fluid migration in coal cannot be accurately predicted. In this study, a fully coupled electromagnetic, heat transfer and multiphase porous media model was developed to investigate the drying kinetics of coal under microwave irradiation. Results show that the temperature rise of coal during microwave irradiation is characterized by “fast-slow-fast”, and water evaporation is the key factor in thermal distribution. The water in the hot spot decreases, while the vapour increases and migrates towards the cold spot. The saturation time of the water always lags behind that of the vapour. In addition, the higher the microwave power, the quicker the fluid reaches the ultimate value. The nonuniform microwave heating enhances the thermal gradient and gas pressure gradient in coal, leading to irreversible damage. Outcomes of this study can be used to identify the drying behaviors of coal during microwave irradiation and thereby help to optimize the microwave applicators.
The effect of drying temperature and air velocity on apple quality parameters, such as color difference (CD), volume ratio (VR) and water absorption capacity (WAC) in convective drying was experimentally studied. Optimization of drying conditions was carried out in the range of air temperatures from 50 to 70 °C and air velocity from 0.01 to 6 m s⁻¹. A novel algorithm of multi-objective optimization, based on artificial neural network (ANN), genetic algorithm (GA) and Pareto optimization was developed. Three optimization objectives included simultaneous minimization of CD, maximization of VR and maximization of WAC. Objective functions for CD, VR and WAC were developed by using ANN training on the experimental dataset of apple drying at 50, 60 and 70 °C. Pareto optimal set was developed with elitist non-dominated sorting genetic algorithm (NSGA II). Unique Pareto optimal solution within specified constraints was found at air temperature 65 °C and velocity 1 m s⁻¹. This mode of apple drying resulted in CD = 5.24, VR = 49.66% and WAC = 0.488. Experimental verification showed that maximum error of modelling did not exceed 3.24%.
An efficient drying process for high-value products such as bacterial cultures is characterized by high retention of viability or activity, but low energy demand. Microwave-vacuum drying was tested in this work as a new technique for the preservation of probiotics and starter cultures, which has the potential to fulfill all of these requirements. The impact of chamber pressure (7, 15, 30 mbar), microwave power (1–5 W g⁻¹) and maximum product temperature (30–50 °C) on drying time, drying rate, efficiency of microwave input and survival rate of the model strain Lactobacillus paracasei ssp. paracasei F19 (L. paracasei F19) was studied. Microwave drying time ranged from 50 to 240 min with microwave power as main influencing factor on drying behavior and efficiency. The maximum product temperature and vacuum pressure did not affect the drying time, absorbed energy and thus microwave efficiency significantly, but both parameters had a great impact on survival rate. The combination of 3 W g⁻¹ microwave power input and 35 °C maximum temperature at a low chamber pressure of 7 mbar resulted in a very short drying time of 64 min, a high level of energy efficiency (27%) and also the best product quality in terms of survival rate (33%). Thus, a microwave-vacuum drying process is a potential alternative to conventional freeze or vacuum drying.
Microwave convective drying (MCD) is gaining increasing interest due to its unique volumetric heating capability and ability to significantly reduce drying time and improve food quality. The main objective of this paper is to discuss, critically analyze and evaluate the recent advances in MCD and suggest the future directions in this field. The main focus of this paper is the mathematical modeling and experimental investigations in microwave convective drying of food materials. Recent developments in mathematical modeling of MCD is discussed and existing experimental setup and their advantages and disadvantages are discussed and analysed. Long drying time is a concern in food industries. Reductions in drying time by applying MCD compared to convection drying are calculated and discussed. It was apparent that the proper integration of mathematical modeling and experimental technique is the best way to maximize the advantages of this drying method. Although a plethora of research is being carried out on this topic, there is still need for research to develop fundamental modeling to optimize the process parameters and scale up this technology for the industrial application. Overall, the review provides an in-depth insight into the latest development of MCD and its mathematical modeling approaches and will hopefully serve to inspire future work in the field.
The use of process additives was evaluated in the drum drying of commercial mango pulp, using corn starch, maltodextrin 10/20 DE and glyceryl monostearate. The mass flow rate and some powder properties were analyzed: moisture content, vitamin C, total phenolic compounds, carotenoids, β-carotene, glass transition temperature (Tg), hygroscopicity, solubility, color, rheological behavior and color of reconstituted pulp. The pulps presented non-Newtonian and pseudoplastic fluid behavior, while Tg values ranged from 29 to 38°C. The process carried out with 3% corn starch and 0.5% glyceryl monostearate (dry basis) resulted in greater mass flow rate (8.0 ± 0.2 kg/h m²) and vitamin C retention (61.0 ± 0.7%).
Due to the nonuniform energy distribution in a microwave (MW) cavity, MW-based heating applications remain a substantial barrier. A continuous power-controlled MW conveyor-belt drier using multiple 2.45-GHz MW sources improves heating uniformity. By controlling the input power of MW sources sequentially, the electric field and temperature variations, hot and cold spots in the MW cavity, have been significantly reduced. Experimental results show that the MW heating using the proposed continuous power-controlled method can achieve an improved heating uniformity of approximately 34% compared with a simultaneous multiple input method (conventional mode).
Energy consumption for drying of mushroom slices was evaluated using various drying methods including hot air, microwave, vacuum, infrared, microwave-vacuum and hot air-infrared. Results of data analysis showed that the lowest and highest energy consumption levels in drying mushroom slices were associated with microwave and vacuum dryers, respectively. The use of vacuum in conjunction with microwave drying increased energy consumption relative to microwave drying alone. Energy consumption in the hot air dryer showed a downward trend with increasing temperature and an upward trend with increasing air velocity. In drying mushroom using infrared radiation, it was observed that increased air velocity increases drying time and consequently the amount of consumed energy. Using a combination of hot air and infrared drying decreased energy consumption relative to infrared drying alone and increased it relative to hot air drying. In the combined microwave-vacuum dryer, drying time and consequently energy consumption decreased in comparison to the vacuum dryer. Hot air-infrared drying of mushroom slices proved to have the lowest energy consumption.
This paper presents a new empirical model for single layer drying process, which was verified with selected experimental data. The present model is also compared with other single layer drying models available in the literature, using laboratory as well as field data from literature.
The effects of blanching and high hydrostatic pressure (HHP) treatments on natural flora evolution, polyphenoloxidase (PPO) activity and color of banana puree adjusted to pH 3.4 and water activity (aw) of 0.97 were evaluated during 15 days storage at 25°C. Standard plate as well as yeast and mold counts of HHP treated purees were <10 CFU/g throughout storage. Blanching time was found to affect (P<0.05) puree color. HHP treatments retained the initial color of the banana purees. Longer browning induction times and slower browning rates were observed when a longer blanching time was combined with a 689 MPa pressure treatment. A residual PPO activity < 5% was observed in the puree when a 7 min blanch was followed by HHP treatment at 689 MPa for 10 min.
Apple cylinders (Granny Smith) were dried in a combined hot air–microwave system. Drying experiments were carried out at various air temperatures (25, 30, 40 and 50 °C) combined with different levels of microwave incident power (0, 3, 5, 7 and 10 W/g) until 0.11 (d.b.) moisture content was obtained. Vacuum impregnation with an isotonic solution was used as a pretreatment before drying. Sample weight changes were determined for different drying conditions. When drying with combined air–microwave system, the drying rate curve could be divided in four periods limited by four critical points. An empirical model was proposed to estimate the drying kinetic constants as a function of the air temperature and the microwave power level for both sorts of samples, fresh apples and impregnated apples. At the end of the process different tissue matrices were obtained depending on the different drying conditions.
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