Comparison of four drying methods for re-structured mixed potato with apple chips

The effects of microwave-freeze drying (MFD), freeze drying (FD), microwave vacuum drying (MVD) and vacuum drying (VD) on texture, color, rehydration, sensory, microstructure and other quality parameters of re-structured chips made by blending apple with potato were studied. MFD chips had the best quality and these products were preferred by consumers. The drying time for MFD was lower than that for FD. The drying time for MVD was the shortest and the energy consumption was also the lowest. Microwave drying not only reduces the drying time but also improves the quality of dried products. Both MFD and MVD techniques are found to be suitable for production of re-structured chips of varying grades.     

Changes of microwave structure/dielectric properties during microwave freeze‐drying process banana chips

This experimental study investigated the trend of structure and dielectric properties during microwave freeze‐drying process banana chips. The mass of banana samples was 160 g, the microwave power set as 2 W g^?1 and the highest drying temperature set as 55 °C. The whole drying process can be finished within 6 h. A network analyser and light microscope were used to determine the dielectric properties and structure. The dielectric properties, ε′ (from 20.80 to 1.20) and ε′′ (from 7.74 to 0.15), and the size of cell get smaller as the drying process continues, especially during the 3–4 h drying, which is the end of primary drying stage and the beginning of secondary drying stage. The trend of dielectric properties and microstructure of samples during drying can be an exact indication of drying stage of MFD.     

Studies on different combined microwave drying of carrot pieces

Three different combined microwave (MW) drying methods were compared, namely microwave‐assisted vacuum drying (MWVD), microwave‐assisted freeze drying (MWFD), microwave‐enhanced spouted bed drying (MWSD), in terms of drying rate, drying uniformity, product colour, rehydration ratio, retention of β‐carotene and vitamin C, and energy consumption. The drying rate of MWVD and MWSD were much faster than that of MWFD. The largest drying rate was obtained in MWSD with 3.5 W g^?1. In general, the colour of MWSD products was very uniform. Rehydration ratio of MWFD carrot pieces was almost the same as the freeze‐dried (FD) products and better than MWVD and MWSD products. In addition, the highest retention of carotene and vitamin C was observed in MWFD carrot pieces. No significant differences were observed in carotene and vitamin C between MWVD and MWSD products. However, the energy consumption in MWFD was the highest.     

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 kinetics and effective moisture diffusivity of bamboo shoot slices undergoing microwave drying

Effect of microwave power on moisture content, moisture ratio, drying rate, drying time and effective moisture diffusivity (D_eff) of bamboo shoot was investigated using microwave drying. To study the effect of microwave power on drying, bamboo shoot samples (250 g) were dried at different power levels ranging from 140 to 350 W. To determine the kinetic parameters, drying data were fitted to various models based on the ratios of differences between initial and final moisture contents and equilibrium moisture content. Among the models proposed, Wang and Singh model gave a better fit for all drying conditions used. By increasing microwave output power, the D_eff values increased from 4.153 × 10^?10 to 22.835 × 10^?10 m² s^?1. A third‐order polynomial relationship was found to correlate the D_eff with moisture content. Further scope of this research work would include the effect of certain factors (shrinkage, case hardening, distortion of product and shape of bamboo shoot samples as an infinite slab) of practical significance to improve the model.     

Effect of microwave vacuum drying and hot air drying on the physicochemical properties of durian flour

Unripe durian cv. Monthong was subjected to microwave vacuum drying (MVD) at 1200, 1600 and 2200 W and hot air drying (HAD) at 40, 50 and 60 °C to produce durian flour. Drying rate of MVD and HAD was 0.34–0.58 kg water kg dry solid^?1 min^?1 and 0.02–0.06 kg water kg dry solid^?1 min^?1, respectively. An increase in drying rate by either increasing hot air temperature or increasing microwave power decreased the degree of crystallinity from 21.95% to 2.31% and from 7.72% to 4.05%, respectively. Moreover, the increased drying rate caused a decrease in endothermic enthalpy (ΔH_gel) and pasting properties. Starch content of the durian flour was 41.40–47.03%. The starch granule morphology of durian flour was disrupted which indicated gelatinisation of flour during drying. Due to a short drying process, the MVD flour had less a*‐value (P ≤ 0.05) than the HAD flour.     

Characterisation of osmotically dehydrated papaya with further hot air drying and microwave vacuum drying

Papaya was subjected to osmotic dehydration (OD) prior to hot air drying (HA; 70 °C) and microwave vacuum drying (MVD; 3.75 W g^?1 and 13.3 kPa). An increase in immersion time in a sucrose solution [65/100 g (w/w)] at 40 ± 2 °C from 0 to 4 h decreased moisture content from 7.5802 to 1.2215 kg water kg dry solid^?1. During HA and MVD, effective moisture diffusivity was in the range 7.09 × 10^?8 to 9.13 × 10^?8 m² s^?1 and 2.85 × 10^?6 to 3.50 × 10^?6 m² s^?1, respectively, depending on immersion time. The hue angle of HA samples was 0.83–0.91, whereas that of MVD samples was 48.55–50.32. Both drying methods decreased springiness. Porosity was clearly observed in MVD samples. Rehydration rate was enhanced by increased immersion time and MVD (P ≤ 0.05). From preference mapping, MVD samples were more preferable than HA samples. Moreover, MVD samples with 1–3 h OD were more preferable than those with 4 h OD.     

Effect of combined microwave‐hot air drying and superheated steam drying on physical and chemical properties of rice

Jasmine rice (Oryza sativa L.) was subjected to two drying operations: combined microwave‐hot air drying (MHA) at initial power intensity of 3, 4 and 6 W g^?1 and superheated steam drying (SHS) at 300 °C and 400 °C. During drying, kinetic rate constants of SHS were significantly higher than those of MHA. Both drying operations could decrease enthalpy of starch gelatinisation from 9.28 J g^?1 to 1.64–6.17 J g^?1, increase gelatinisation extent to 33.51–82.33%, decrease crystallinity from 28.87% to 18.15–21.33%, improve scavenging ability of 1,1‐diphenyl‐2‐picrylhydrazyl, increase ferric reducing antioxidant power and increase hardness of cooked rice from 5.66 N to 5.83–6.55 N, depending on microwave power and drying medium temperature. However, taste profiles and liking scores were comparable to the regular brown rice. Therefore, MHA and SHS operations could be potentially used for reducing drying process and promoting antioxidant activity.     

Effect of osmotic dehydration on dielectric properties,microwave vacuum drying kinetics and quality of mangosteen

Mangosteen (Garcinia mangostana Linn.) with and without osmotic dehydration (OD) in sucrose solution was dried by microwave vacuum drying at 1200, 1440 and 1680 W. Because of water loss (49.12–49.98 g 100 g^?1) and solid gain (9.31–11.62 g 100 g^?1) during OD, dielectric constant, loss factor and loss tangent of mangosteen were significantly increased (P ≤ 0.05) to 24.82–25.12, 11.52–14.18 and 0.47–0.50, respectively. With the decreased initial moisture content and the modified dielectric properties, drying time of osmotically dehydrated mangosteen was shorter than that of mangosteen without OD. Moreover, an increase in microwave power enhanced drying kinetics. With OD, Tg of dried mangosteen was increased from ?7.01, ?3.00 to 11.11–25.96 °C. Hardness and lightness (seedless part) were significantly increased (P ≤ 0.05). Structure of dried seedless mangosteen was well protected, resulting in the improved rehydration ability (P ≤ 0.05). Nonetheless, rehydration ability of the mangosteen containing seed was not improved (P > 0.05).     

Shrinkage and quality characteristics of whole fruit of Chinese jujube (Zizyphus jujuba Miller) in microwave drying

Drying characteristics of whole fruit Chinese jujube was studied as distinct from fruit slices. Shrinkage, density, vitamin C, colour and rehydration of Chinese jujube were measured in microwave drying at power level of 1.2, 1.67 and 2.5 W/g (dry base, db). Unlike fruit slices or lumps where, no matter at the beginning or at the end of drying, shrinkage volume typically equals the volume of removed water, ratio of the shrinkage volume to the volume of removed water of Chinese jujube could range from 2 to 2.6 at the beginning of drying, and at the end of drying, it could be 1.15 for higher microwave level (2.5 W/g, db), whereas, for lower power level (1.2 W/g, db), it was about 1. Unlike shrinkage curves in hot air drying that could be described with a three‐segment linear model, shrinkage curves in microwave drying could be well described with a cubic polynomial (R² > 0.99). When compared with the low power level (1.2 W/g, db), higher microwave power level(2.5 W/g, db) increased by 87.9% of the retention of vitamin C, 14.6% of the density and 8.7% of the extent of browning, decreased by 12.9% of the soluble solids content.     

Microwave vacuum drying of foods with temperature control by power modulation

Microwave power modulation during drying was evaluated to guarantee mild temperatures, reducing burning spots when producing sweet potatoes snacks. Samples were peeled, sliced, blanched, and microwave vacuum dried with (MWVDc) and without (MWVD) controlling the temperature (60 °C) with a PID system. MWVD used constant power, while MWVDc adjusted the power input based on samples' temperature target. Moisture content was reduced from 4.5 g g⁻¹ (db) to 0.4 g g⁻¹ (db), resulting in water activity lower than 0.55. MWVD processing time was about 80, 15, and 10 min at 300, 1500, and 2700 W, respectively, resulting in dried samples with dark areas due to hot spots. The power modulation in MWVDc resulted in intermediary drying time (50 min) and uniform color products without burnings. This study demonstrated that automated temperature control leads to high-quality sweet potato chips by microwave processing.     

Microwave drying of mango ginger (Curcuma amada Roxb): prediction of drying kinetics by mathematical modelling and artificial neural network

Curcuma amada (Mango ginger) was dried at four different power levels ranging 315–800 W to determine the effect of microwave power on moisture content, moisture ratio, drying rate, drying time and effective diffusivity. Among the fifteen thin layer drying models considered for evaluating the drying behaviour, the semi‐empirical Midilli et al., model described the drying kinetics very well with R² > 0.999. Drying rate and effective diffusivity increased as the microwave power output increased. Activation energy was estimated by a modified Arrhenius type equation and found to be 21.6 kW kg^?1. A feed‐forward artificial neural network using back‐propagation algorithm was also employed to predict the moisture content during MW drying and found adequate to predict the drying kinetics with R² of 0.985.     

微波干燥条件对杏脯干燥特性与品质的影响

目的:提高杏脯的干燥效率及产品品质。方法:分别用转盘式微波炉(RMD)和微波对流耦合干燥机(MCD)干燥杏脯,考察微波功率、微波发射方式、切分程度及物料是否转动对杏脯干燥特性、焦化率、色值、感官品质及复水特性的影响,并与传统热风干燥(HD)进行比较。结果:与HD的1 040 min(16块)和840 min(48块)相比,微波干燥显著缩短干燥时间,不同微波干燥条件下所需的干燥时间为40～400 min;脉冲比越大或功率越高或物料尺寸越大,干燥所用时间越短,在MCD中控温微波干燥耗时最长。无论是在RMD还是不控温MCD中静态干燥,杏脯均出现严重的烧焦现象,焦化率为17%～100%,物料转动时焦化率高于静态干燥的,而在MCD中控温静态干燥避免了物料的烧焦现象,且MCD中控温静态干燥的杏脯色值和感官评价最接近HD的,复水比与HD仅相差3.45%～5.17%,获得最高的感官评价分(87.2分)。结论:MCD中控温静态干燥可以作为杏脯的高效干燥方法。     

马蹄淀粉微波间歇干燥工艺研究

以马蹄为原料,采用微波间歇方式对马蹄淀粉进行干燥试验,考察其对马蹄淀粉的含水率、白度、酸度、碘蓝值及综合品质的影响,并采用正交试验优化干燥工艺。结果表明,马蹄淀粉微波间歇干燥最佳工艺条件为:微波功率210 W,装载量1.47kg/m2,加热时间14 min,间歇时间0.75min。在最佳工艺条件下,微波间歇干燥后马蹄淀粉含水率为10.12%,白度为97.5%,酸度为0.21 mL,碘蓝值为17.90。经微波间歇干燥后含水率达到淀粉安全含水率的标准,白度得到小幅提高,酸度下降,碘蓝值上升,综合品质得到提高。     

Microwave–hot air drying applied to selected cassava cultivars: drying kinetics and sensory acceptance

The effects of three cassava cultivars produced in Brazil were assessed (IAC Espeto, IAC 14 and IAC Caapora) in order to obtain chips by precooking and hot air–microwave drying (70 °C, 95 W). The difference in the composition of cultivars has not influenced the drying of cassava slices. The drying kinetics presented a constant drying rate period followed by a decreasing drying rate with effective diffusion coefficient of 2.8–3.0 × 10^?10 m²s^?1. The hedonic means of all attributes ranged from ‘neither like nor dislike’ to ‘like moderately’. Therefore, the choice of the more suitable cultivar can be based on plant productivity and product yield. The IAC 14 cultivar resulted in the highest root production per plant, product mass per roots and chips yield.     

Mass transfer characteristics during convective,microwave and combined microwave–convective drying of lemon slices

BACKGROUND: The investigation of drying kinetics and mass transfer phenomena is important for selecting optimum operating conditions, and obtaining a high quality dried product. Two analytical models, conventional solution of the diffusion equation and the Dincer and Dost model, were used to investigate mass transfer characteristics during combined microwave‐convective drying of lemon slices. Air temperatures of 50, 55 and 60 °C, and specific microwave powers of 0.97 and 2.04 W g^?1 were the process variables. RESULTS: Kinetics curves for drying indicated one constant rate period followed by one falling rate period in convective and microwave drying methods, and only one falling rate period with the exception of a very short accelerating period at the beginning of microwave‐convective treatments. Applying the conventional method, the effective moisture diffusivity varied from 2.4 × 10^?11 to 1.2 × 10^?9 m² s^?1. The Biot number, the moisture transfer coefficient, and the moisture diffusivity, respectively in the ranges of 0.2 to 3.0 (indicating simultaneous internal and external mass transfer control), 3.7 × 10^?8 to 4.3 × 10^?6 m s^?1, and 2.2 × 10^?10 to 4.2 × 10^?9 m² s^?1 were also determined using the Dincer and Dost model. CONCLUSIONS: The higher degree of prediction accuracy was achieved by using the Dincer and Dost model for all treatments. Therefore, this model could be applied as an effective tool for predicting mass transfer characteristics during the drying of lemon slices. © 2012 Society of Chemical Industry     

Microwave vacuum drying of osmotically dehydrated mandarin cv. (Sai‐Namphaung)

Mandarin [mandarin cv. (Sai‐Namphaung)] was subjected to osmotic dehydration prior to microwave vacuum drying. Osmotic solutions were varied using different ratios of sucrose to glycerol (9:1, 7:3 and 5:5 w/w). Because of the decreased moisture content and solid gain during osmotic dehydration, dielectric properties of mandarin were changed significantly (P ≤ 0.05). The osmotically dehydrated mandarin was then dried further using microwave vacuum drying at 4.8 and 6.4 W g^?1. Among thin layer models, page model was the best to describe the drying of osmotically dehydrated mandarin. An increase in the microwave power could increase drying rate without significant effect on hardness of dried samples. Nonetheless, the hardness was significantly (P ≤ 0.05) reduced by an increase in the glycerol ratio in the osmotic solution. The increase in microwave power and glycerol ratio significantly (P ≤ 0.05) decreased β‐carotene content and thereby affected colour of the dried mandarin.     

A multi-stage combined heat pump and microwave vacuum drying of green peas

The effect of multi-stage heat pump fluidized bed atmospheric freeze drying (HP FB AFD) and microwave vacuum drying (MVD) on the drying kinetics, moisture diffusivities, microstructure and physical parameters of green peas was evaluated. The results were compared with those obtained for microwave vacuum drying (MVD) and hot air convective drying (HACD). In case of combined method, the initial drying rate was about 0.04 l/min. The application of MVD increased the drying rate to the values 0.08 l/min. The drying rates of green peas dried by MVD and HACD were 0.59 and 0.20 l/min, respectively. MVD samples were characterized by a structure with minimal changes in respect to fresh samples. However, HP FB AFD and MVD satisfied important requirements, such as high product quality (due to low material temperature during AFD and low pressure during MVD), and increased drying rates in the final stage due to application of microwave heating.     

Evolution of the physicochemical properties of oil-free sweet potato chips during microwave vacuum drying

The demand for healthy and convenient foods is a worldwide trend. Sweet potato attracted great attention due to its carbohydrates with a low glycemic index. Dehydrated sweet potatoes can be an excellent alternative for using and adding value to this raw material. The objective of this study was to evaluate the physicochemical properties of sliced sweet potato during the microwave vacuum drying (MWVD) for producing crispy oil-free chips. Fresh sweet potato samples were selected, peeled, sliced, blanched, and then dehydrated using a microwave oven adapted with a vacuum chamber and a rotation system to operate under vacuum. It was measured the evolution of moisture, water activity, temperature, color, apparent specific mass, porosity, and acoustic/mechanical analysis of the texture during the MWVD. Crispy sweet potato chips were obtained in <30 min, presenting low moisture (0.028 g g⁻¹ db) and water activity (0.262). The dehydrated samples showed high porosity (67.5%) and a low apparent density (0.456 g cm⁻³). Optical micrographs and acoustic/mechanical properties showed an expanded (puffed) product structure with large pores, which resulted in irregular acoustic/mechanical signals, characteristics of a crispy food matrix. Colorimetric analyses indicated a little change between fresh and dried samples, with an absence of burnt spots. In conclusion, MWVD is a suitable process to produce highly porous sweet potato chips, adding value, and extending the vegetable's shelf life.     

Effective diffusivities and energy consumption of daylily in microwave drying

Drying kinetics of fresh daylily flower buds (Hemerocallis Citrine Baroni cv. Mengzihua) in a microwave oven at 60, 120, 180 and 240 W, respectively, were investigated. The results indicated that, drying rate increased at the initial time (a warm‐up period) and then decreased at the end time (a falling rate period) after reaching a plateau (a constant rate period). A third‐order polynomial model could well describe the relationships between effective diffusivity and moisture content, and the model's coefficients correlated linearly with microwave power level. A logistic model could well present the relationships between activation energy and moisture content. The curves of energy consumption vs. microwave power level indicated that in the initial phase of drying, the energy utilisation efficiency was almost the same at different microwave output power levels; however, at the end phase of drying, drying at lower microwave levels was worse in energy utilisation.