Search

The present work is about the production of oil-free potato snacks using successive cycles of microwave heating-vacuum pulses and was therefore called microwave multiflash drying (MWMFD). Drying kinetics and product physical properties were assessed, as well as the influence of blanching (BL) and vacuum impregnation (VI) as pre-treatments before drying. The drying curves presented three periods: a heating period, a high drying rate period promoted by the vacuum pulses, and a falling rate period. The drying time of 400 g of potatoes was approximately 60 min, reducing gradually the microwave power from 800 to 200 W. The vacuum-pulse step changed the material, resulting in a porous and fragile microstructure, characteristic of crispy products, as well resulting in a product surface with a golden yellow color. BL and VI combined was the best pre-treatment for obtaining very crispy oil-free potato snacks. The results of this study indicated MWMFD as a suitable process to produce crispy potato snacks, free of oil, in only 60 min.

The impact of the processing conditions of the edible Undaria pinnatifida brown seaweed on the dehydration and bioactive features was determined using the green microwave hydrodiffusion and gravity technology. For the alga solid phase, the corresponding dehydration and rehydration kinetics, their mathematical modelling, volumetric shrinkage, energy consumption, microscopical and colorimetric properties, phenolic content and antioxidant capacity were analyzed. Results indicated that an increase in microwave power involved a notable drop in the seaweed dehydration time. The shortest dehydration treatment to reach an adequate final moisture content (

Microwave vacuum heating of blackberries was carried out to investigate the effects of microwave power and vacuum degree on the temperature of blackberries during the heating process, and the distribution of the temperature field was observed. Using numerical simulations, a combined electromagnetic and heat transfer model was established. The simulations can be used to observe the locations of hot spots and cold spots during heating. The appropriate microwave power and vacuum could also be selected using the simulation to reduce the temperature difference of the locations of hot spots and ensure uniform heating. The results show that, for blackberries heated for 2 min at a microwave power of 400 W and vacuum degree of −80 kPa, the temperature of the hot spot was maintained at about 60 °C and the temperature difference was about 0.27; thus, the heating was uniform, meeting the industrial drying requirements for blackberries.

This research aimed to develop a closed-loop control (CLC) system to improve the microwave freeze-drying (MFD) process and to examine the effects of a dynamic microwave logic control (DMLC) on the drying characteristics of MFD. The development process consisted of two parts: (1) the MFD experiment to develop the DMLC, and (2) the implementation of the CLC with the DMLC on the MFD process. In the first part, the MFD process was examined to obtain the strategy for drying the carrot slices using microwave powers of 100 W, 200 W, and 300 W, with a temperature profile of the sample from −15 °C to 40 °C, and the final moisture content of 6% (wet basis). In the second part, the DMLC was strategically developed and integrated into the CLC system. The results showed that in the MFD process, the DMLC was developed based on a drying-phase configuration and dynamic control between the microwave power and real-time moisture content sensing to provide feedback to the CLC system. After applying the DMLC into the CLC system, the efficiency of the MFD process was improved by up to 62.4% by shortening the drying time, as compared with the freeze-dry (FD) process. The MFD-DMLC also resulted in the quality of carrot equivalent to that of a traditional FD process. Since the DMLC exhibited great potential to improve the MFD process, it could be developed for future industrial use for a high-performance MFD process in terms of product quality and process efficiency.

The dielectric properties of potatoes in the moisture content range of 8.3%–75.4% (w.b.) and the temperature range of 25 °C–85 °C were measured with a coaxial probe network analyzer. The regression equation of loss factor of potatoes with its moisture content and temperature was developed first and then introduced into an established LabVIEW control program to make dielectric loss factor as feedback signal for guiding the power output of the microwave drying system. Results show that the dielectric loss factor of potato chips increased with the increase in moisture content. When the moisture content of potato chips was 35%–75.4%, the loss factor decreased with the increase in temperature. The opposite trend was observed when the moisture content was lower than 35%. The proposed microwave power control scheme based on dielectric loss factor feedback and selected probable procedure on/off ratio can prevent the temperature rise of the material in the middle and last drying stages. This condition ensures the quality of the product.

Drying of pomegranate arils was done using microwave-vacuum technique, using microwave power of 25 to 95 W, vacuum pressure of 25 to 195 mm Hg and sample mass of 65 to 235 g. The effective moisture diffusivity varied from 5.18 × 10 −11 to 6.58 × 10 −10 m 2 /s. Effective moisture diffusivity ( D eff ) values were found to increase as microwave power increases or sample mass decreases for constant values of remaining variables while vacuum pressure had negligible effect. A third order polynomial relationship was found to correlate the effective moisture diffusivity ( D eff ) with moisture content. Multivariate polynomial models were also developed for estimating the effective moisture diffusivity as a function of the microwave-vacuum drying process parameters.

The aim of this study was to evaluate the influence of microwave assisted extraction on the isolation of anthocyanins and phenolic acids from sour cherry Marasca. A general factorial design was used to study the effect of temperature (from 50 to 70 °C), irradiation time (5–12 min) and microwave power (350–500 W) on individual, total anthocyanins and individual and total phenolic acids. Optimal microwave assisted extraction conditions differed for anthocyanins and phenolic acids, especially in terms of temperature and irradiation time, so lower temperature (60 °C) and shorter time (6–9 min) was more convenient for anthocyanins extraction, while phenolic acids gave higher extraction yield at higher temperatures (70 °C) and longer irradiation time (10 min). The optimal microwave power did not differ significantly for studied compounds, ranging about 400 W. Compared to conventional extraction, microwave assisted one showed higher efficiency for all studied compounds.

The effects of Gac oil extraction conditions including microwave power, microwave time, steaming time and hydraulic pressure on extraction efficiency (EE), and β-carotene and lycopene contents were studied. It was found that the EE, and β-carotene and lycopene contents could be enhanced by suitable extraction conditions. Microwave drying was found to be better than air drying for pretreatment. Moisture content after drying and steaming between 8% and 11% (wt/wt) were best for pressing. Results showed that the most suitable conditions for Gac oil extraction from 900 g samples were microwave power of 630 W, microwave time of 65 min, steaming time of 20 min and hydraulic pressure of 170 kg/cm 2 . Under these conditions, the highest EE of 93% was achieved while Gac oil contained the highest content of β-carotene and lycopene at 140 and 414 mg/100 mL, respectively.

The purpose of this research is to study the stabilization effect of microwave on wheat germ using continuous microwave processing equipment. Results from the study indicated that microwave technique had a significant stabilization effect on wheat germ. When the on-line processing capacity of wheat germ was 20 kg/h, the optimal conditions for wheat germ stabilization were microwave power of 4 kW, the processing time of 8 min and ventilation rate of 60 N m3/h. Under such conditions, the residual lipase activity decreased to 14.01% of that of the raw wheat germ. The lipoxygenase lost its activity completely under such intensity microwave processing. After 60 days’ accelerated storage, the acid value increased only 6.56% of that of the stabilized wheat germ before storage. In addition, microwave could cause the death of harmful microorganisms. It was clear that the continuous microwave stabilization process was practicable for wheat germ stabilization.

Durian CV. Monthong was subjected to microwave vacuum drying (at 13.33 kPa) to produce durian chips. Various levels of microwave power (3.88 W g −1 , 5.49 W g −1 and 7.23 W g −1 ) were used. Prior to the microwave vacuum drying, the sliced durian was either chilled at 4 °C or frozen at −18 °C. Both pretreatments yielded non-significant difference in dissipation factor ( p > 0.05). Among several thin layer models, the Page model was found to be the best for explaining the drying characteristics of durian chips. An increase in the microwave power intensity produced a clear increase in the drying rate and did not affect lightness and yellowness of the durian chips ( p > 0.05). The structure and hardness of the dried durian chips were comparable to that of conventionally fried durian chips. In addition, microwave vacuum drying reduced the fat content of the durian chips by at least 90%, compared with conventionally deep fried durian chips.

A microwave drying system with the ability of automatic temperature and power control was developed for apple drying. The samples’ mass and the moisture content could be obtained online. Four different drying modes were attempted with different combination of temperature and power controls. Feedback temperature control with predefined variable power profiles resulted in the best temperature control and product quality. All other modes proved their own pros and cons . The study also verified that it was difficult to maintain a constant temperature during microwave drying process when a feedback temperature control was not included. The results can be used to determine microwave power levels in microwave drying practices both with and without temperature control.

Microwave tempering and infrared assisted microwave tempering of frozen foods were simulated by using finite difference method. The effects of microwave power and infrared power on tempering were discussed. Three different microwave power levels (30%, 40%, and 50%) and three different infrared power levels (10%, 20%, and 30%) were used. The increase in microwave power level and infrared power level decreased tempering times. The change of heat capacity and the dielectric properties of frozen potato puree with respect to time were measured. The temperature distribution inside the sample was modeled, and the predicted results were compared with experimental results. The predicted temperatures showed good agreement with the experimental results.

The current study was conducted for evaluating the effects of two different strategies viz., phase control and cycle control, on the microwave/air drying process. The evaluation was carried out through combined microwave and convective drying of potato samples. Results showed that different power control methods had different impacts on drying kinetics and product quality. In both drying modes, the drying time increased with the decrease of microwave power density and the increase of air velocity. The drying rates of cycle-controlled drying are significantly higher than those of phase-controlled drying. In terms of rehydration capacity the phase-controlled drying mode produced better results. The product colour and sensory attributes were independent of the power control methods.

The objective of the study was to compare the quality of cakes during baking in microwave, infrared (IR) and IR-microwave combination ovens. Quality parameters evaluated were weight loss, specific volume, firmness and colour of cakes. As control, conventionally baked cakes were used. Cakes baked in a microwave oven had the lowest quality. It was possible to achieve surface colour formation in cakes by IR and IR-microwave combination baking. IR-microwave combination baking reduced weight loss and firmness while it increased the volume of microwave baked breads. Halogen lamp-microwave combination oven produced cakes having similar firmness and colour values with conventionally baked ones. The best conditions for baking of cakes in IR-microwave combination oven was 5 min at 70% halogen lamp and at 50% microwave power levels. When an IR-microwave combination oven was used at this condition conventional baking time of cakes was reduced by about 75%.

Weight gain during soaking of apple cylinders dried at different levels of microwave incident power and different air temperatures was modeled according to two different approaches. The first one based on Fick’s equation of diffusion was not suitable to model the sorption data. The second proposed model was Peleg’s equation, which gave a good fit to experimental data. The weight gain and volume recovery were higher at higher levels of microwave power; however, higher levels of microwave incident power caused a lower water holding capacity.

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.

In this study, the design and plasma generation principle of a microwave induced plasma sterilization technique is described. Generation of low temperature argon plasma was investigated for different microwave power densities. Inactivation effects of the plasma were studied on bio-indicators carrying Escherichia coli as a function of microwave power density and particularly for different locations of the bio-indicator holder. The results showed that argon plasma having a temperature below 60 ° C are generated having less than 4.21 w / cm 3 of microwave power density at an ambient temperature of 34 ° C . Survival of E.coli was significantly reduced when the microwave power density of the plasma treatments increased. While, high inactivation was observed at the center, it was less effective at the two opposite edges of the bio-indicator holder.

Garlic cloves were dried with hot air and combined microwave–hot air drying methods in an experimental dryer. The combined microwave–hot air drying experiments were carried out with 100 g sample sizes at temperatures of 40°C, 50°C, 60°C and 70°C at air velocities of 1.0 and 2.0 m/s, using continuous microwave power of 40 W. For comparison of hot air drying, the same sample sizes were taken for experiments and the drying air temperatures and air velocity were 60°C and 70°C, and 2.0 m/s respectively. The total drying time, the color and flavor strength of dried garlic cloves were used to evaluate the performance of the combined microwave–hot air drying and the conventional hot air drying processes. Combined microwave–hot air drying resulted in a reduction in the drying time to an extent of 80–90% in comparison to conventional hot air drying and a superior quality final product.

Studies were undertaken to develop a coating system for popcorn in order to achieve maximum number of popped kernels during microwave heating. The effects of microwave power levels and different ingredients (hydrogenated oil, sodium chloride, butter, sodium bicarbonate) on popping behaviour of popcorn during microwave heating were studied using a central composite design. All the regression models were significant at the p R 2 value in the range of 89–92%. The models developed can be used as a tool to study popping behaviour of popcorn. Popcorn having more than 75% of popped kernels and lower than 0.05 g/cm 3 of bulk density can be achieved using 10% hydrogenated oil, 2% butter and 0.5% sodium chloride and heating popcorns at 70% power in a microwave oven with a magnetron having actual power output of 660 W and operating frequency of 2450 MHz.

American ginseng (Panax quinquefolium) roots were dried, respectively, with hot air and combined microwave-hot air methods in a modified experimental microwave oven. For hot air drying, the loading size, drying temperature and air flow rate were 100 g, 40 °C and 60 l/min, respectively. For combined microwavehot air drying, the additional microwave power of 60 W was used. Drying time to achieve the desired moisture level (10%) in the final product, and the colour of the product, were used to evaluate the combined microwave-hot air drying and the conventional hot air drying methods. Combined microwave-hot air drying resulted in a substantial decrease (28.7–55.2%) in the drying time and had little influence on the colour of the final product.

Microwave vacuum drying of banana slices was investigated experimentally. This type of drying procedure is preferable to conventional drying techniques in order to avoid product degradation due to high temperatures encountered in convective drying. The drying process was examined by introducing pulsegenerated microwave power in banana samples. The material temperature was monitored. Temperature peaks in the last stages of drying indicated that drying could be favoured if temperature was maintained below a maximum level, so that the final product should not be burned by hot spots during microwave drying. This procedure produced dehydrated products of excellent quality as examined by taste, aroma, smell and rehydration tests.

The effect of a progressive increase in the bound strength of moisture in soybeans — a porous hygroscopic solid — during microwave heating is investigated. Thin layers of soybeans were dried with a single mode (TE 10 ) microwave cavity operating at 2450 MHz. The absorbed microwave power, temperature, and mass loss of the seeds were monitored continuously. Under a constant absorbed microwave power of 0.57 W/g, the temperature of a soybean seed increased rapidly at the initial stages of drying, reached a maximum of about 75 °C in approximately 50 min, and decreased gradually during the latter stages of drying to about 58 °C in 150 min. To maintain a constant drying temperature the absorbed microwave power had to be progressively increased as required. The results indicate that drying increases the average bound strength of the remaining moisture in a hygroscopic material and the energy needed to evaporate a unit mass of moisture increases accordingly. Consequently, progressively higher absorbed microwave powers are needed to maintain the temperature of low moisture solid foods during microwave heating processes that are accompanied by significant moisture loss.