Quality assessment of electrohydrodynamic and microwave dehydrated banana slices

In contrast to the inherently energy-efficient electrohydrodynamic (EHD) methods, conventional drying methods are energy intensive. In the present study, comparisons were made between banana slice samples dried by high-voltage-DC powered multiple-point-to-plate electrodes and those dried by microwave (MW). For this purpose, banana slices were dried by an EHD drying system at electric field strengths of 6, 8, and 10 kV/cm and by an MW drying system at specific power levels of 9 and 18 W/g. No constant rate period was observed in the EHD; hence, the entire drying process occurred in the falling rate period, whereas the drying rates of MW-dried samples occurred at a constant rate and during the falling rate periods. Analysis of variance showed that drying method had a significant effect on drying time, rehydration ability, shrinkage, color parameters (L^∗, a^∗, b^∗, and ΔE), and consumed energy. The mean values of specific consumed energy for EHD and MW drying methods were 0.34 and 9.66 kJ/g, respectively. In terms of drying time, EHD was not fast; however, its advantages of less energy requirement and great product quality including lower shrinkage, great rehydration ability, and better appearance make it a good choice for drying banana slices.     

Evaluation of Drying Methods with Respect to Drying Kinetics,Mineral Content,and Color Characteristics of Savory Leaves

Sun, oven (50 °C), and microwave oven (700 W) drying of savory leaves (Satureja thymbra L.) were carried out to monitor the drying kinetics, changes in mineral content, and color degradation of the product. Oven and microwave oven drying shortened the drying time over than approximately 70% and 99% when compared to the sun and oven drying methods, respectively. Fresh and dried savory leaves had high amounts of K (8875.2–28468.0 mg/kg), Ca (3681.6–9852.03 mg/kg), Mg (1388.0–3102.0 mg/kg), and P (2313.2–5045.8 mg/kg) minerals. K, Ca, P, and Mg were the most abundant elements in savory samples. Mineral content of oven-dried savory were higher than the sun and microwave dried samples. Midilli and Küçük model was shown to give a good fit to the sun and oven drying. The Midilli and Küçük, modified page and page models exhibited high coefficient of determination (R ² ) values ranging between 0.9995 and 0.9997, to the experimental microwave oven drying data of savory. Microwave oven drying revealed optimum color values. Oven drying resulted in a considerable decrease in color quality of savory.     

Drying kinetics and quality of beetroots dehydrated by combination of convective and vacuum-microwave methods

Beetroot cubes were dehydrated by convective drying in hot air at 60 °C and by the combination of convective pre-drying (CPD) until moisture content 1.6, 0.6 or 0.27 kg/kg db and vacuum-microwave finish drying (VMFD) at 240, 360 or 480 W. The control samples were obtained by freeze-drying (FD). The drying kinetics of beetroot cubes was described with an exponential function. VMFD significantly reduced the total time of drying and decreased drying shrinkage in comparison with convective method. A critical moisture content divided the temperature profile of samples during VMFD into increasing and falling periods. At the falling temperature period a significant increase in the colour parameters L^∗, a^∗ and b^∗ was found. VM treated samples as well as FD ones exhibited lower compressive strength, better rehydration potential and higher antioxidant activity than those dehydrated in convection. Increasing the microwave wattage and decreasing the time of CPD improved the quality of beetroot cubes dried by the combined method.     

Electrohydrodynamic (EHD) drying of tomato slices (Lycopersicon esculentum)

The drying rate of tomato slices (Early Urbana L.) treated by a high voltage electrostatic field (HVEF) was investigated with an oven-drying and open-room drying sample, under identical conditions. ANOVA showed that the drying methods had a significant effect (P ? 0.01) on decreasing moisture content, surface temperature, and apparent color of tomato slices, but no difference was observed in shrinkage. Average drying rate for 3, 4, and 5 kV cm⁻¹ electrostatic fields over a period of 780 min increased by 1.3, 1.43, and 2 times, respectively, compared to that of the air-dried. Tomato slices dried with EHD⁺, which consumed only 16.5 mJ g⁻¹ of electric power, exhibited a better appearance (lightness and red color) with lower surface temperature than the air-dried and oven drying samples.     

Sustainable dehydration of onion slices through novel microwave hydro-diffusion gravity technique

Onion is a semi-perishable commodity having prominent nutritional value. The deterioration of onion during storage can lead to huge amount of post-harvest losses. Shelf life can be increased through drying, which also facilitates transportation, storage, and packaging due to reduced weight and volume. In the present study, a novel microwave-assisted drying technique called microwave hydro-diffusion gravity was investigated for process optimization and compared with conventional drying techniques. Results indicated that 400 W and 14 min of process were the best combination for drying that removed 80% moisture present in slices. The overall drying time of onion slices was significantly reduced (about six times) compared to hot air oven and freeze-drying methods. MHG dehydration prevented the burning of onions and maintained their sensorial attributes especially color and texture. Similarly, MHG in combination with hot air oven consumed only 0.5 MJ energy as compared to 3.24 and 3 MJ energy used by hot air oven and freeze drying, respectively. In short, results proved that MHG technique is much better than conventional techniques in terms of end product quality and process efficiency.Industrial relevanceMicrowave hydro-diffusion technology is optimized for onion drying. This technology will help in reducing the use of energy and will avoid the loss of water soluble components. These bioactive components have great importance in the field of pharmaceutical. Thus, this technology simultaneously dries the product and extracts the valuable components. It will increase the process efficiency and reduce the processing cost.     

Microwave assisted drying of short-cut (ditalini) macaroni: Drying characteristics and effect of drying processes on starch properties

Macaroni samples were dried by conventional hot air, microwave alone and hot air followed by microwave drying methods. The drying of macaroni samples took place in the falling rate period. Higher drying rates were observed with a higher microwave power level. A diffusion model was employed to fit the experimental drying data in order to characterize the drying rate of samples. Drying only with microwave energy (70 and 210 W) or hot air–microwave energy (70 and 210 W) resulted in substantial shortening of the drying time. The reduction was 61.8%, 87.3%, 61% and 78%, respectively, of the time to dry with hot air. All starch granules of semolina and hot air dried macaroni displayed a clear “Maltese cross” under polarized light. Starch granules of macaroni samples dried with microwave (70–210 W) and combined hot air–microwave (70–210 W) had irregular shapes but retained birefringence in about half or whole of each granule. The starch gelatinization enthalpies of macaroni samples dried with hot air, microwave alone (70 and 210 W) and combined hot air and microwave (70 and 210 W) drying were lower than that of semolina sample. This confirms that starch is not completely gelatinized during drying.     

Drying of carrots in microwave and halogen lamp–microwave combination ovens

Carrot slices were dried by using microwave, halogen lamp–microwave combination and hot-air drying. Microwave and halogen lamp–microwave combination drying were applied after carrots were dried to 0.47 kg moisture/kg dry solid by hot-air drying. Drying time, rehydration capacity and colour of the carrots dried by different methods were compared. The increase in microwave oven power level decreased the drying time. Microwave drying at the highest power and halogen lamp–microwave combination drying reduced the drying time to an extend of 98% in comparison to conventional hot-air drying and a high-quality dried product was obtained. Moreover, in the case of halogen lamp–microwave combination drying, moisture level was reduced to a level, which is lower than the one achieved by other methods. Less colour change occurred when microwave and halogen lamp–microwave combination drying were applied. Carrots dried in microwave and halogen lamp–microwave combination oven had lower L*, higher a* and b* values and had higher rehydration capacity as compared to hot-air drying.     

Infrared thermography assisted control for apples microwave drying

A microwave based system was developed for drying of apple slices while on-line controlling their temperatures. An infrared thermography assisted control system allowed to realize temperature control by detecting the instantaneous maximum temperature among the samples under test instead of a chosen slice. The effectiveness of the system at hand in realizing temperature control both in space and in time was proven for three temperatures: 55, 65 and 75 °C. Temperature fluctuations became larger both with time increasing and with rising temperature levels thus causing a negative impact on dried fruit quality. In fact apples slices dried at 75 °C by microwave showed L^∗ (lightness) and white index (WI) values significantly lower (p < 0.05), when compared to the fresh samples and to the apples dried by hot air. To overcome such behaviour a mode stirrer was introduced resulting in halving the temperature spatial oscillations of sample slices.     

Drying characteristics of mango slices using the Refractance Window™ technique

Mango slices (1 and 2 mm thickness) were dried by using the Refractance Window™ (RW) technique with the water bath temperature set at 92 °C. Drying kinetics, water activity, and color change were determined and compared with those obtained by drying the slices in a tray dryer at 62 °C and an air velocity of 0.52 m/s. At 1 h, moisture contents of 0.013 (for 1 mm sample) and 0.048 (for 2 mm sample) kg water/kg dry solid for RW, and 0.966 (for 1 mm sample) and 3.614 (for 2 mm sample) kg water/kg dry solid for tray drying were obtained and water activity was less than 0.5 for RW and close to 0.97 for tray drying. The tray dryer took 4 h to reach results similar to those of RW at 1 h. The samples dried by RW exhibited diffusivities of 4.40 × 10⁻¹⁰ (for 1 mm sample) and 1.56 × 10⁻⁹ (for 2 mm sample) m²/s, which are larger than those exhibited by the air-dried samples, 2.08 × 10⁻¹¹ (for 1 mm sample) and 6.83 × 10⁻¹¹ (for 2 mm sample) m²/s.     

Use of dried pumpkins in wheat bread production

Pumpkins belong to the family of Cucurbitaceae. They are classified to Cucurbita pepo, Cucurbita moschata, Cucurbita maxima and Cucurbita mixta, according to the texture and shape of their stems. The nutritional value of pumpkin fruits is high, varies from one species or cultivar to another. Pumpkins are reach in vitamins C, B₁, B₆, K, and in mineral substances. There are no data found about vacuum microwave dried pumpkin application in wheat bread production. The main purpose of the research was to evaluate quality parameters of microwave vacuum dried pumpkins and to verify its application in wheat bread production. Following quality parameters of pumpkins and bread made with pumpkin additive were evaluated: moisture content (oven – drying method), vitamin C content (iodometric), carotenoid content (spectrophotometric), reducing sugars (LVS 252:2000), colour changes (ColorTec- PCM), total fat content (ISO 6492:1999), degree of bread liking (ISO 4121:2003), bread baking loss and dry off. Quality parameters of non dried pumpkins were: content of reducing sugars − 2.40 g⋅100 g^-1, vitamin C – 0.26 g⋅100 g^-1, carotenoids – 0.50 mg % (in dry matter). The technological parameters for pumpkins drying in vacuum microwave drier were used: pressure (70–50 mmHg), speed of tumbler – 6 rpm, one working cycle and three stages. During the pumpkins drying process a decrease in the following parameters was observed: moisture content – 10.5 times, vitamin C content – 2.0 times; increase of yellowness (b*) value – 1.5 times. The optimal dried pumpkin additive to the wheat dough was 10% of the total flour amount. The wheat bread sample with dried pumpkins additive is richer in carotenoids and reducing sugars comparing to control wheat bread sample. The results of sensory analyses using hedonic rating demonstrated that a higher degree of liking was attributed to the bread sample with dried pumpkins additive (7.3) comparing to control wheat bread sample (6.7).     

Kinetics and modeling of whole longan with combined infrared and hot air

The aim of this research was to evaluate the effects of operating variables on the drying behavior of whole longan undergoing a combined infrared and hot air drying process, to determine its kinetic parameters, and to develop drying kinetic models. The single-layer drying experiments were carried out at infrared powers of 300, 500 and 700 W, drying air temperatures of 40, 60 and 80 °C, and air velocities of 0.5, 1.0 and 1.5 m/s. The samples were dried until attaining a final moisture content of 0.2 kg water/kg dry solid. The results show that the drying had a short constant rate period followed by a falling rate period in all cases. The drying rate and product temperature were significantly influenced by infrared power, temperature and velocity of ambient air. In the constant rate period, the mass transfer coefficient varied from 3.646 × 10⁻³ to 1.914 × 10⁻² m/s. It increased with increasing infrared power, but decreased as air velocity and air temperature increased. In the falling rate period, theoretical and semi-empirical drying kinetic equations were used to describe the drying kinetics of the product. It was found that the overall effective diffusion coefficient and drying constant varied from 7.012 × 10⁻¹¹ to 6.681 × 10⁻¹⁰ m²/s and 0.026 to 0.234 h⁻¹, respectively. Both parameters increased with increasing infrared power and air temperature, but decreased with increasing air velocity. Combined regression equations developed to predict the drying kinetic parameters (h_D, D_eff and k) for all three models gave a fairly good fit.     

Moisture diffusivity of rough rice under infrared radiation drying

To design efficient infrared (IR) dryers for rough rice, it is important to understand the drying behavior of rough rice under IR heating. The objective of this study was to determine the moisture diffusivity of rough rice under IR heating followed by cooling. The effects of initial moisture content, rice temperature, drying bed thickness, tempering, and cooling methods on moisture diffusivity and moisture diffusivity coefficient were investigated. Samples of freshly harvested medium grain rice (M202 variety) with initial moisture content (MC) of 25.8, 31.2 and 33.8 g moisture/100 g dry solid were used. They were dried with IR radiation intensity of 5348 W/m², for six exposure times, 15, 30, 40, 60, 90 and 120 s. The tested drying bed thicknesses were single-layer, 5 mm and 10 mm. The unsteady diffusion equation based on Fick’s law and slope methods were used to describe moisture diffusivity. The results indicated that rough rice moisture diffusivities under IR heating and cooling were significantly affected by rice temperature and tempering treatment, respectively. High heating rate and moisture diffusivity were achieved with IR heating. It took only 60, 90 and 120 s to achieve about 60 °C rice temperature with corresponding moisture diffusivities of 4.8 × 10⁻⁹, 3.6 × 10 ⁻⁹ and 3.4 × 10⁻⁹ m²/s during heating for drying bed thicknesses of a single layer, 5 mm and 10 mm, respectively. The moisture diffusivity coefficients during heating and cooling of IR dried rice with tempering were much higher than those of convective drying, which reflected the high drying rate of the IR drying method.     

The effect of osmosis pretreatment on hot‐air drying and microwave drying characteristics of chili (Capsicum annuum L.) flesh

Chili flesh pretreated with or without osmotic dehydration (OD) was dried in the hot‐air drying (AD) oven at 50–80 °C or in the microwave drying (MD) oven at 60–180 W. Results showed that the samples osmotically treated in mixed solution (10% salt + 50% sucrose) had the best dehydration effect as compared with single salt or sugar solutions. During the drying process, osmotically treated samples had one falling‐rate period and their effective moisture diffusivities (D_eff) showed a rapidly linear increase with the decrease in moisture content, while directly drying samples showed a three‐phase falling‐rate period and their D_eff increased gradually at the initial period and then rapidly at the final period. When the moisture content decreased, the activation energy increased gradually; however, for AD after OD, it decreased. Among all the processes, MD at 60 W after OD presented the largest vitamin C retention rate and the best colour difference, needing less drying time.     

Effect of hot air drying and sun drying on color values and β-carotene content of apricot (Prunus armenica L.)

In this study, sulphurated and nonsulphurated Hacihaliloglu apricots (Prunus armenica L.) which is the most widely produced cultivar in Turkey were used to study the effects of different hot air drying temperatures (50, 60, 70, and 80 °C) and sun drying on color and β-carotene content of apricot. The time required to obtain the desired final dry matter in hot air drying was lower than sun drying. Sulphuration also decreased drying time at all drying conditions. Color values and β-carotene content of hot air dried samples were favorable in comparison to air drying. β-carotene content in dried apricots at 70 and 80 °C was 7.14, 7.17 mg 100 g⁻¹ dry matter and 6.12, 6.48 mg 100 g⁻¹ dry matter for sulphurated and nonsulphurated apricots, respectively. A good relationship was found between treatments (drying temperatures and drying times) and β-carotene content for sulphurated and nonsulphurated apricots (R²=0.9422 and 0.9129, respectively).     

Effects of layer arrangement on heating uniformity and product quality after hot air assisted radio frequency drying of carrot

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.     

Characterization of microwave vacuum-dried durian chips

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.     

Study of banana dehydration using sequential infrared radiation heating and freeze-drying

The drying and quality characteristics of banana slices processed with a sequential infrared radiation and freeze-drying (SIRFD) method were investigated. Cavendish bananas slices with 5 mm thickness were predehydrated using IR heating at each one of three radiation intensities, 3000, 4000, and 5000 W/m² or hot air at 62.8 °C. The predehydrated samples with 20% and 40% weight reductions obtained using 4000 W/m² IR intensity were then further dried using freeze-drying for various times to determine the effect of predehydration on the drying rate during freeze-drying. To improve the quality of dried banana chips, the banana slices were also treated with a dipping solution containing 10 g/l ascorbic acid and 10 g/l citric acid before the IR predehydration. Control samples were produced using regular freeze-drying without the predehydration. The quality characteristics of dried banana chips, including color, thickness shrinkage and crispness, were evaluated. The predehydration results showed that the drying rate of IR heating was significantly higher than the hot air drying and increased with the increase of IR intensity. For example, it took 10 and 38 min to achieve 40% weigh reduction by using IR at 4000 W/m² and hot air drying, respectively. However, the banana slices with IR predehydration dried slower during freeze-drying compared to the samples without predehydration, which was due to texture changes that occurred during the predehydration. Acid dipping improved product color and also reduced freeze-drying time compared to non-dipped samples. It has been concluded that SIRFD can be used for producing high crispy banana chips and additional acid dipping improved product color and reduced required freeze-drying time.     

Characterization of hot air drying and microwave vacuum drying of fingerroot (Boesenbergia pandurata)

Fingerroot (Boesenbergia pandurata) was subjected to hot air drying and microwave vacuum drying. Effective moisture diffusion coefficient during the hot air drying at 60 and 70 °C were 0.2073 × 10^?10 and 0.4106 × 10^?10 m² s^?1 respectively. By using the microwave vacuum drying (13.3 kPa) at the power of 2880 and 3360 W, the effective moisture diffusion coefficient were increased to 5.7910 × 10^?10 and 6.8767 × 10^?10 m² s^?1 respectively. Based on Lewis model, drying rate constants were 0.0002, 0.0004, 0.0061 and 0.0072 s^?1 for the hot air drying at 60 and 70 °C and the microwave vacuum drying at 2880 and 3360 W respectively. Compared with the hot air drying, the microwave vacuum drying decreased drying time by 90%. Rehydration ability of the microwave vacuum dried samples was also significantly improved (P ≤ 0.05), because of porous structure. In addition, the rehydrating water of the microwave vacuum dried samples contained higher b*‐value (yellowness) than that of the hot‐air‐dried samples (P ≤ 0.05).     

Drying of Garlic Slices Using Convective Pre-drying and Vacuum-Microwave Finishing Drying: Kinetics, Energy Consumption, and Quality Studies

Garlic slices were dehydrated by convective drying (CD) and by a combined method consisting of convective pre-drying followed by vacuum-microwave finishing drying (CPD-VMFD). Drying of garlic using VMFD led to samples with high values of lightness and slightly lower values of the coordinates a* and b*. The treatment CPD-VMFD significantly decreased the drying time from 630 min for CD to 49 min and contributed to an important energy saving compared with CD (54.4–86.3 %). The total drying time was shorter when vacuum microwaves were introduced earlier and microwave power was higher. The highest temperature (114 °C) was found for samples pre-dried to a moisture content of 60 % and then dried at 360 W while showing the lowest energy consumption (98?±?1 kJ?g^?1). In general, the total antioxidant capacity increased after drying of garlic slices; however, the content of total phenolic compounds showed the opposite trend, and thus other compounds, such as allicin, should be involved in the antioxidant capacity of dried garlic. Garlic slices were CP dried until 30 % of moisture content followed by VMFD at 240 or 360 W showed the best results in terms of energy consumption and antioxidant capacity.     

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

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 relevanceApplication 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.