Hot Air Microwave Drying Research Articles

Effects of Four Different Drying Methods on the Quality Characteristics of Peeled Litchis (Litchi chinensis Sonn.)

Four different drying methods, namely, air drying (AD), microwave-assisted vacuum drying (MWVD), coupled microwave–hot air drying (MWAD), and freeze drying (FD), were studied, in terms of drying time, color, reducing sugar content, shrinkage, microstructure, and sensory evaluation. The drying rate of MWVD and MWAD were notably faster than that of AD and FD. The highest reducing sugar content was observed in MWAD, followed by MWVD. No significant differences were observed in reducing sugar between air-dried and freeze-dried products. FD had the best color and shrinkage among the other drying methods. The color and shrinkage of MWVD drying is close to the fresh litchis and much better than hot air and MWAD. Scanning electron microscopy (SEM) images showed that the MWVD and FD litchis have a porous structure, whereas AD and MWAD litchis have a compact structure. The sensory evaluation suggests that peeled litchis dried by the four studied drying methods are of a quality that is accepted by consumers.

Cassava slices drying by using a combined hot-air single-plane microwave dryer

Characteristics of cassava drying were investigated by using a microwave hot-air drying system. Two waveguides were installed on a single plane of the microwave cavity. The drying experiments were carried out at two levels of sample surface temperature set-points, 70℃ and 80℃respectively. Cassava (Rayong-9) with 2.5 kg weight and 61% moisture content on wet basis was dried in the dryer for about 300 - 340 minutes until the final values of moisture content of about 20% db were achieved. It was found that the drying time decreased with an increase in sample-surface temperature set point. Approximately 87% of the moisture was removed during the drying period. It was found that there was a rapid decrease in moisture ratio values followed by the gradual decline period in all experiments. With regard to drying kinetics, 5 commonly used mathematical models were examined with the experimental data. It was found that Page’s and diffusion models provided a good agreement between the experimental and predicted moisture ratio values for all temperature set-points. The regression results indicated that highest values of coefficient of determination and adjusted coefficient of determination as well as lowest value of standard error of estimation were reported for the case of Page’s model at 80℃ temperature set-point.

CASSAVA CHIP DRYING BY USING A SMALL-SCALE HOT-AIR MICROWAVE OVEN

Characteristics of cassava microwave hot-air drying were investigated in a 2-planes microwave hot-air dryer. The drying experiments were carried out at two levels of sample surface temperature set-points viz. 70 and 80°C respectively. Cassava (Rayong-9) with 2.5 kg weight and 61% moisture content on wet basis was dried in the dryer for about 5-5.3 h. It was found that the drying time decreased with an increase in sample-surface temperature set point. Approximately 89% of the moisture was removed during the drying period. The rapid decrease in moisture ratio values followed by the gradual decline period was found in all experiments. With regard to drying kinetics, 5 commonly used mathematical models were examined with the experimental data. It was found that Page’s and diffusion models provided a good agreement between the experimental and predicted moisture ratio values. The regression results indicated that high values of coefficient of determination and adjusted coefficient of determination as well as low value of standard error of estimation were reported for the case of these two models.

Modeling dehydration and rehydration of cooked soybeans subjected to combined microwave–hot-air drying

Abstract Pre-cooked soybeans were subjected to convective hot-air, microwave and combined microwave–hot-air dehydration. Three microwave levels (210, 300, 560 W) and three air temperatures (160, 180, 200 °C) were examined. Drying kinetics, rehydration kinetics and colour change were investigated relative to microwave level and air temperature. Combined microwave–hot-air drying decreased the drying time required when compared to drying with either hot-air or microwave energy alone. Predictive models were developed to describe dehydration and rehydration kinetics. Dehydration rate, rehydration rate and total colour change of rehydrated product generally increased with microwave level and air temperature. Within the studied range, optimal drying occurred for the lowest levels of both microwave and air temperature studied, i.e. microwave power = 210 W, air temperature = 160 °C. Industrial relevance Application of microwave energy simultaneously with convective hot-air drying to pre-cooked soybeans was up to four times faster when compared to convective drying alone, and up to twice as fast when compared to microwave drying alone. Rehydration time for soybeans subjected to combined microwave–hot-air drying was 50–60% less than for legumes subjected to either convective or microwave drying alone. Therefore, application of combined microwave–hot-air drying to pre-cooked soybeans resulted in significant saving in process time, while also producing a dehydrated product with fast rehydration properties.

Combined microwave-hot air drying of peeled longan

Objectives of this research were to develop and evaluate a drying process for peeled longan using combined microwave-hot air (MWHA). Each experiment employed the peeled ‘Dor’ longan to be dried to final moisture content of 18% (dry basis). Hot air temperature was regulated for three levels: 40, 50, and 60 °C, coupled with MW regulated for four levels: 100, 180, 300 and 450 W. Results show that MWHA drying process yielded a unique convex-shaped drying rate period, followed by a falling rate period. A step-wised drying process using 40 °C hot air with 450 W-MW power for 1.7 h, followed by 60 °C hot air with 300 W-MW power for 3.3 h provided the maximum drying efficiency. Compared to the 65 °C hot air drying to obtain the golden brown flesh, combined MWHA process reduced drying time by 64.3% and specific energy consumption by 48.2%.

Effect of microwave power, air velocity and temperature on the final drying of osmotically dehydrated bananas

This paper reports on a study of the final stage of microwave-hot air drying of osmotically dehydrated bananas, focusing on the effects of microwave power, air temperature and air velocity on drying kinetics and product quality, evaluated in terms of colour, apparent volume and porosity. The drying process was divided into three periods: phase I (760 W; 2 kg moisture/kg dry matter); phase II (380 W; 0.67 kg moisture/kg dry matter); and phase III (0 W, 76 W, 150 W or 230 W up to the final sample moisture of 0.17 kg water/kg d m). Three conditions for the hot air were tested: 50 °C and 3.3 m/s; 70 °C and 3.3 m/s; 70 °C and 5.7 m/s. The results show that increasing the microwave power in phase III increased the drying rate, thus making the drying time shorter. However, higher microwave power also caused temperature runaway leading to charring on the dried product. Air flow cools the product surface and improves product quality by reducing charring.

Optimisation of dehydration and rehydration properties of cooked chickpeas (Cicer arietinum L.) undergoing microwave–hot air combination drying

Combined microwave–hot air drying is an innovative technique that could dramatically reduce processing times for many foods. In this study, combination drying of whole, pre-cooked chickpeas was investigated for three microwave power (210, 300, 560 W) and three air temperature (23, 160, 250 °C) settings. Asymptotic models were proposed to describe both moisture ratio and weight gain on rehydration as functions of time, microwave power (MW) and air temperature (T). Combination drying with MW=210 W and T=160 °C was optimal in terms of drying time, rehydration time, texture and colour.

Drying of garlic ( Allium sativum) cloves by microwave–hot air combination

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.

Drying, shrinkage and rehydration characteristics of kiwifruits during hot air and microwave drying

Hot air, microwave and hot air-microwave drying characteristics of kiwifruits (5.03±0.236 mm thick) were investigated. Drying rates, shrinkage and rehydration capacities of these drying regimes were compared. The drying took place in the falling rate drying period regardless of the drying method. Drying with microwave energy or assisting hot air drying with microwave energy resulted in increased drying rates and substantial shortening of the drying time. Shrinkage of kiwifruits during microwave drying was greater than hot air drying. Less shrinkage was observed at hot air-microwave drying. Microwave dried kiwifruit slices exhibited lower rehydration capacity and faster water absorption rate than the other drying methods studied.

Drying of American ginseng ( Panax quinquefolium roots by microwave-hot air combination

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.