Dehydration of Garlic Slices by Combined Microwave-Vacuum and Air Drying

Abstract

Combination of microwave-vacuum drying and air drying was investigated as a potential mean for drying garlic slices. The sample was dried by microwave-vacuum until the moisture content reached 10% (wet basis), and then by conventional hot-air drying at the temperature of 45°C to final moisture content less than 5% (wet basis). Pungency, color, texture, and rehydration ratio of garlic slices dried by this method were evaluated and compared with those dried by freeze drying and conventional hot-air drying. The comparison showed that the quality of garlic slices dried by the current method was close to that of freeze dried garlic slices and much better than that of conventional hot-air dried ones. The lab microwave-vacuum dryer which the materials to be dried could be rotated in the cavity was developed by the authors.     

Effect of temperature and velocity of superheated steam on initial condensation of distillers’ spent grain pellets during drying

Initial condensation on the sample surface during superheated steam (SS) drying leads to increased sample moisture affecting its mechanical and thermal properties. A study was conducted to understand the effect of temperature and velocity of SS on the amount of initial condensation on distillers’ spent grain pellets with an initial moisture content of 25% (wet basis). These pellets were dried using SS at 120, 150, and 180°C with velocities 0.5, 1.0, 1.2, and 1.4 m/s. Separate experiments were conducted for recording mass and surface temperature of the pellets during SS drying. Mass recorded over the drying period was then compared with the predicted mass obtained by solving the standard heat balance and film condensation equations. The predicted values of mass flux due to initial condensation were in close agreement with directly measured values with a maximum mean square error of 0.20. There was a 60–64% decrease in the amount of initial condensation as the temperature of SS was increased from 120 to 180°C. The results indicate that the initial condensation can be minimal when the temperature of SS is equal or above 180°C with SS velocity equal or above 1 m/s using a preheated drying chamber.     

Dehydration of Garlic Slices by Combined Microwave-Vacuum and Air Drying

Combination of microwave-vacuum drying and air drying was investigated as a potential mean for drying garlic slices. The sample was dried by microwave-vacuum until the moisture content reached 10% (wet basis), and then by conventional hot-air drying at the temperature of 45°C to final moisture content less than 5% (wet basis). Pungency, color, texture, and rehydration ratio of garlic slices dried by this method were evaluated and compared with those dried by freeze drying and conventional hot-air drying. The comparison showed that the quality of garlic slices dried by the current method was close to that of freeze dried garlic slices and much better than that of conventional hot-air dried ones. The lab microwave-vacuum dryer which the materials to be dried could be rotated in the cavity was developed by the authors.     

Drying Kinetics and Physicochemical Characteristics of Laboratory-Prepared Corn/Wheat Distillers Grains and Solubles Dried with Superheated Steam and Hot Air

The effect of superheated steam (SS) drying and hot air (HA) drying on drying kinetics and changes in the color, crude protein, and amino acid concentrations (in particular, lysine content) of corn/wheat wet distillers grains (WDG) and centrifuged solubles (CS) was evaluated. An inversion temperature was reached at 139°C for WDG and 132°C for CS, above which moisture evaporation rate and qualitative changes under SS drying conditions exceeded the values noted in HA, and below which the reverse was observed. A significant decrease (from 8 to 50%) in the lysine content of WDG and CS was reported during SS and HA. The overall changes in the color (ΔE*) of corn/wheat WDG and CS ranged from 7.9 ± 2.6 to 27.2 ± 1.9 during SS drying and from 11.9 ± 3.7 to 32.0 ± 0.5 during HA drying. The observed deterioration in color was attributed mainly to changes in lightness (L*) and yellowness (b*) of dried samples. The values of L* and b* were reliable predictors of the lysine content of corn/wheat distillers co-products.     

Effect of Microwave-Vacuum Drying on the Carotenoids Retention of Carrot Slices and Chlorophyll Retention of Chinese Chive Leaves

Microwave-vacuum drying, and combined microwave-vacuum with air or vacuum drying were investigated as potential methods for drying green and high-carotenoid vegetables of Chinese chive leaves and carrot slices. The samples were dried by microwave-vacuum until the moisture content reached about 20% (wet basis), and then by conventional air drying at the temperature of 45-50°C or conventional vacuum drying at the temperature of 55-60°C or by continued microwave-vacuum drying at lower power level to final moisture content about 6% (wet basis). The carotenoid retention of carrot slices and chlorophyll retention of Chinese chive leaves, dried by these methods, was evaluated and compared with those dried by freeze-drying and conventional hot-air drying. The comparison showed that the carotenoid retention of carrot slices and chlorophyll retention of chive leaves, dried by the current methods, was very close to or even as much as those dried by freeze-drying and much better than those dried by conventional hot-air. Blanching was not necessary when using microwave-vacuum drying or combined microwave-vacuum with conventional hot-air drying or vacuum drying, because the activity of enzymes which were responsible for the degradation of color was greatly reduced with extremely rapid decreasing of moisture, and because oxygen was absent in microwave-vacuum drying.     

Drying and Quality Characteristics of Shredded Squid in an Infrared-Assisted Convective Dryer

Drying is one of the most common methods for processing and preserving squids. A novel forced convective dryer based on infrared heating was developed with an online temperature control. By setting the drying medium temperature of 50°C, we studied the effects of infrared wavelength and air velocity on drying characteristics of the shredded squid and qualities of dried squid products. We also compared it with the conventional hot-air drying (HAD) and advanced microwave vacuum drying (MVD). The infrared heating rate increase was faster than that of HAD. The heating and drying at the wavelength of 2.5–3.0 µm were more effective than those at the infrared wavelength of 5.0–6.0 µm. Specific energy consumption linearly increased with the air velocity. Microstructure observation showed that the infrared-dried rehydrated sample displayed a muscle fiber structure similar to the fresh sample. The infrared-dried squids had less drying shrinkage, brighter color, and better rehydration capacity than HAD products. Their sensory qualities were better than HAD and MVD products. Above all, infrared drying with wavelength of 2.5–3.0 µm and air velocity of 0.5 m/s was suggested as the best drying condition for squids in this study.     

A New Model and Quality of Unfrozen and Frozen Cooked Rice Dried in a Microwave Vibro-Fluidized Bed Dryer

This study aimed to develop a suitable drying model for microwave vibro-fluidized bed drying in a single-mode applicator (MVFB-SMA drying) of cooked rice with and without prefreezing treatment and to investigate the effects of prefreezing treatment and drying temperature (110–185°C) on quality of dried cooked rice. During the process of drying cooked rice from 60 to 10% (wet basis), results indicated that drying rate increased, whereas drying time decreased with prefreezing treatment and increased drying temperature. The drying rate and drying time of unfrozen and frozen cooked rice ranged from 0.196 to 0.497 g water/g dry matter/min and 0.228 to 0.554 g water/g dry matter/min; and from 7 to 2.5 min and 5.5 to 2 min, respectively. A new model was proposed in this study (MR = exp(?k t ⁿ ) + bt + c) to compare with 11 commonly used drying models. The new model describes the MVFB-SMA drying data most satisfactorily. The values of effective diffusivity were between 1.70 × 10^?7 and 5.72 × 10^?7 m²/s for the unfrozen sample and between 1.99 × 10^?7 and 5.86 × 10^?7 m²/s for the frozen sample. Their activation energy values were 23.66 and 21.19 kJ/mol, respectively. Prefreezing treatment provided a whiter product with a less uniform porous structure and higher bulk density. Slower ability to rehydrate was also observed for the frozen cooked rice dried at 160 and 185°C. An increase in drying temperature resulted in changes in whiteness, microstructure, bulk density, and rehydration capability. No prefreezing treatment and drying at 160°C seemed to be the optimal process condition for cooked rice, ensuring whiteness, a porous structure, low bulk density, and high rehydration capability.     

Effect of Osmotic Dehydration with Pulsed Vacuum on Hot-Air Drying Kinetics and Quality Attributes of Cherry Tomatoes

The effects of osmotic dehydration (OD) with or without pulsed vacuum (PV) on hot-air drying kinetics and quality attributes of cherry tomatoes were investigated. Both OD and PVOD pre-treatments were performed for 3 h at 50°C in 50 and 70^o Brix sucrose solutions with a solution-to-fruit mass ratio of 4:1, and PVOD was applied for 15 min before OD at atmospheric pressure. Samples were further dried at air temperature of 70°C. Effective moisture diffusivity (D _eff) of osmotically dehydrated samples increased gradually while the D_eff curve of fresh samples had a plateau stage during hot air drying. Lower glass transition temperature, T_g, values of osmotically dehydrated samples indicated that they needed a lower storage temperature. Both OD and PVOD pre-treatments had advantages in shortening drying cycles and improving quality of products. Compared with air drying, osmo-air drying decreased the total drying time, color change, and hardness of dried samples by 32.26%, 18.11%, and 88.21%, respectively, and increased volume ratio and vitamin C retention rate by 72.31% and 125.82%. As compared with OD, PVOD decreased color change and hardness by 28.48% and 45.17%, increased volume ratio and vitamin C retention rate by 27.41% and 17.77%, but there was no significant difference shown in drying time. Therefore, osmotic pre-treatment can shorten the total dehydration time, and improve the general quality of dried cherry tomatoes.     

Degradation of Phytosterols During Near-Ambient Drying of Rapeseeds in a Thick Immobile Layer

The effect of the drying method applied and subsequent rapeseed storage on changes in phytosterols was determined. After harvest, rapeseeds were dried by the near-ambient method in a thick immobile layer of 2 m and using air heated to a temperature of 60, 80 and 100 °C. Analyses of phytosterol contents were performed immediately after drying and after 6 and 12 months of storage at a temperature of 10 ± 2 °C. Results showed a significant effect of drying conditions, cultivar-specific differences and storage time on the contents of phytosterols. Near-ambient drying of seeds resulted in a reduction in total sterol contents by 6–20 %, while for drying with hot air it was by 14–40 %. The level of sterols decreased by 13–18 % after a 1 year storage of seeds dried by the near-ambient methods. A reduction in 12–22 % in sterols for seeds dried by high temperature occurred after 1 year of storage.     

Sorption Properties of Vacuum-Dried Strawberries

This work demonstrates the influence of changes in parameters of vacuum drying (temperature and pressure) on the sorption properties of dried strawberries. Fruits were dried at 50 and 70°C under pressures of 4 and 16 kPa. Vacuum drying was also conducted during the first 4 h at 70°C and then the temperature was decreased to 50°C at a pressure of 4 kPa. The other combination included increasing the pressure after the first 4 h from 4 to 16 kPa at a drying temperature of 70°C. Sorption isotherms were determined in the dried strawberries. It was shown that with increasing drying temperatures, there was a notable deterioration in the capacity for absorbing water vapor by the vacuum-dried fruit. On the other hand, the pressure at which vacuum drying proceeded did not significantly affect water vapor absorption. Changing the parameters of vacuum drying—that is, temperature in the range of 50–70°C and pressure in the range of 4–16 kPa—affected the shape and structure of the resultant dried strawberries. The combination of vacuum drying with convective drying also influenced the shape and structure of the dried fruit.     

Effect of aging temperature on the porosity characteristics of subcritically dried silica aerogels

Silica aerogels were synthesised by subcritical drying technique which involves controlled solvent exchange and aging of the wet gel in silane solution followed by drying under controlled conditions. Effect of temperature of aging in silane solution on the porosity characteristics of silica aerogels and the thermal pore stability of the resultant gels were investigated. Aging in silane solution leads to an increased degree of condensation reactions, siloxane crosslinking and the dissolution and reprecipitation of silica monomers to the gel structure and enhances the total strength of the gel. Thermal aging of the wet gel have a pronounced effect on bulk density, linear drying shrinkage, surface area and pore volume. As the temperature of aging increases the bulk density decreases whereas the surface area and pore volume were found to increase. We could achieve a surface area of 1040 m²/g, pore volume 1.2 cc/g and an average pore size of 49 Å corresponding to an aging temperature of 70 °C. Thermal pore stability of the gel was found to be up to 700 °C above which densification of SiO₂ gel starts. The novel findings will help in tailoring the process parameters to prepare mesoporous oxides from sol–gel precursors with specific pore features.     

Effects of High-Temperature Fluidized Bed Drying and Tempering on Kernel Cracking and Milling Quality of Vietnamese Rice Varieties

Studies on the effects of high-temperature fluidized bed drying and tempering on physical properties and milling quality of two long-grain freshly harvested Vietnamese rice varieties, A10 (32±1% wet basis moisture) and OM2717 (24.5±0.5% wet basis moisture), were undertaken. Rice samples were fluidized bed dried at 80 and 90°C for 2.5 and 3.0 min, then tempered at 75 and 86°C for up to 1 h, followed by final drying to below 14% moisture (wet basis) at 35°C by thin-layer drying method. Head rice yield significantly improved with extended tempering time to 40 min. Head rice yield tended to increase with decreasing cracked (fissured) kernels. The hardness and stiffness of sound fluidized bed dried rice kernels (in the range of 30–55 N and 162–168 N/mm, respectively) were higher than that of conventionally dried ones (thin layer dried at 35°C). The color of milled rice was significantly (P < 0.05) affected by high-temperature fluidized bed drying, but the absolute change in the value was very small.     

2种方式干燥的天然橡胶干燥特性之研究

为探讨微波干燥天然橡胶新技术，分别在温度115℃下，采用电热烘箱产生热空气和微波干燥装置干燥湿天然橡胶胶粒，对二者的干燥特性及硫化胶性能进行对比分析研究。结果表明，微波干燥只需13．467min就可使胶料含水量降至0．791％，热空气干燥则需211min。天然橡胶微波干燥和热空气干燥主要包含失水率加速期、失水率减速期、失水率相对恒速期3个失水阶段。整个干燥过程中，微波干燥失水率较大，而热空气干燥失水率比较小。微波干燥天然橡胶硫化胶的耐热氧老化性能得到显著提高：其老化前后拉断伸长率变化率（-35％）和拉伸强度变化率（-59％）明显高于热风干燥天然橡胶硫化胶的拉断伸长率变化率（-62％）和拉伸强度变化率（-89％）。     

Mathematical Model of Fixed-Bed Drying and Strategies for Crumb Rubber Producing STR20

The objectives of this research were to investigate empirical and diffusion models for thin-layer crumb rubber drying for producing STR20 rubber using hot air temperatures of 110–130°C and to study the effect of drying parameters such as inlet drying temperature, volumetric flow rate, and initial moisture content on the quality of dried rubber. Finally, a mathematical drying model for predicting the drying kinetics of crumb rubber was developed using inlet air flow rates of 300–600 m³/min-m³ of crumb rubber (equivalent to 1.8–5.0 m/s) with the crumb rubber thickness fixed at 0.25 m. The average initial moisture content of samples was in the ranges of 40 and 50% dry basis while the desired final moisture content was below 5% dry basis. The results showed that the drying equation of crumb rubber was highly related to the inlet air temperature, while the drying constant value was not proportional to the initial moisture content. Consequently, the experimental data were formulated using nine empirical models and the analytical solution of moisture ratio equation was developed by Fick's law of diffusion. The result showed that the simulated data best fitted the logarithmic model and was in reasonable agreement to the experimental data. The effective diffusion coefficient of crumb rubber was in the range of 1.0 × 10^?9 to 2.15 × 10^?5 m²/s corresponding to drying temperatures between 40 and 150°C, respectively. The effects of air recirculation, inlet drying temperature, initial moisture contents, air flow rate, and drying strategies on specific energy consumption and quality of samples were reported. The experiments were conducted using two different drying strategies as follows: one-stage and two-stage drying conditions. The results showed that initial moisture content and air flow rates significantly affected the specific energy consumption and quality of rubber, while the volumetric air flow rate acted as dominant effect to the specific energy consumption. The simulated results concluded that the percentage of recycled air between 90 and 95% provided the lowest specific energy consumption as compared to the others.     

Effect of Drying Conditions and Level of Condensed Distillers Solubles on Protein Quality of Wheat Distillers Dried Grain with Solubles

Reduced protein quality is one of the concerns currently confronting the supply and utilization of wheat distillers dried grain with solubles (DDGS) as an animal feed ingredient. This study assessed the protein quality of wheat DDGS, expressed as acid detergent insoluble crude protein (ADICP) and lysine content, by blending wet distillers grain (WDG) with varying condensed distillers solubles (CDS) levels and drying using forced air convection, microwave, and microwave–convection methods. As the CDS level was increased, the protein content of wheat DDGS generated from the three drying methods increased. Interactions of CDS level with drying air temperature, microwave power, and microwave–convection settings had a significant effect (p < 0.05) on average ADICP and lysine contents. Higher ADICP and lower lysine contents were observed in samples dried at higher temperature, microwave power, and microwave convection settings. Further, the CDS level significantly affected the color parameters of microwave- and microwave–convection-dried samples and the drying air temperature–CDS level interaction significantly affected the color of forced air convection–dried samples. Significant lysine content–redness, ADICP–lightness color parameter, and ADICP–total color difference correlations were found in forced air convection–, microwave-, and microwave–convection-dried samples, respectively. Microwave and microwave–convection drying achieved desirable protein quality associated with low-temperature drying at much shorter times.     

Effect of Different Drying Methods on Rheological and Textural Properties of Balangu Seed Gum

The purpose of the present study was to investigate the effect of different drying methods on the rheological and textural properties and color changes in Balangu seed gum. Three drying methods, including air drying (40–80°C), freeze drying, and vacuum drying, were used. The apparent viscosity decreased from 0.161 to 0.056 Pa s with increasing temperature from 40 to 80°C (shear rate = 60 s^?1) and freeze-dried gum exhibited the highest viscosity among all dried gums (0.203 Pa s). Different time-independent rheological models (power law, Bingham, Herschel-Bulkley, Casson, and Vocadlo) were used to fit the experimental data and the results revealed that the Herschel-Bulkley model was the most suitable to describe the flow behavior of Balangu seed gum over the whole experimental range (r > 0.98). The hardness values of air dried Balangu seed gum gels varied from 33.1 to 40.4 g and were significantly lower compared to the freeze-dried and vacuum-dried gums (46.9 and 46.6, respectively). The consistency of samples decreased from 386.27 to 245.33 g · s when the drying air temperature increased from 40 to 80°C. The results indicated that the freeze-dried gum exhibited the highest hardness and consistency. The color of air-dried gum was darker (lower L* value) compared to the freeze- and vacuum-dried samples.     

Effects of Different Drying Methods on the Quality of Squid Cubes

The effects of three different drying methods (e.g., intermediate infrared-assisted convection drying, low-frequency (915 MHz) microwave drying, and hot-air drying) on the quality of squid cubes were investigated. The quality parameters used to evaluate the drying efficiency were color, shrinkage, rehydration ratio, and hardness of the final dried products. The results showed that intermediate infrared-assisted convection drying and low-frequency microwave drying have the advantages over hot-air drying in terms of drying rate, shrinkage percentage, and rehydration ratio. Low-frequency microwave-dried samples displayed a puffed structure and their hardness was the highest. The intermediate-wave infrared-assisted convection drying, at 60°C, was identified as the best method for squid-cube drying in this study.     

Drying Characteristics of Carrots Immersed in a Fluidized Bed of Fluidizing Particles Under Reduced Pressure

The drying characteristics and properties (color and shrinkage) of carrots (as a representative agricultural product) were experimentally examined in a fluidized bed under reduced pressure. Dry hot air and superheated steam were used as the drying gases. Rice and carrot powders (0.125–0.355 mm in diameter) were used as the fluidizing particles, in addition to glass beads (0.12 mm in diameter).

It was confirmed that the drying rate using a fluidized bed was much higher than without a fluidized bed (hot-air drying), regardless of the type of fluidizing particles used. Under reduced pressure, both with and without a fluidized bed, the drying rate was higher than that at atmospheric pressure using hot air. The drying rate was sufficiently high for fluidized-bed drying with superheated steam, though the drying rate was higher with hot air than with superheated steam. As the drying temperature increased, the volume ratio (befor/after drying) of the sample increased. At high drying temperatures (373 and 423 K in the present study), the color of the sample changed; in other words, a heat-induced change in the properties of the carrot was observed. At a low drying temperature (333 K in the present study), the drying method did not affect the color of the carrot; i.e., the color of the dried material was maintained even in a fluidized bed under reduced pressure when the drying rate was higher.     

Effect of High Pressure Pretreatments on Structural and Dehydration Characteristics of Aloe Vera (Aloe barbadensis Miller) Cubes

The effect of high-pressure (HP) pretreatments (300–500 MPa for 5–15 min) on dehydration characteristics of aloe vera cubes (AVC), dried at 50–70°C and air velocity of 1–2 m/s, was studied. Pretreatments resulted in higher drying rates, leading to a reduction in drying time compared to untreated ones. The drying rate and drying time were most significantly affected (p < 0.05) by the drying temperature, followed by air velocity, pressure level, and dwell time. HPP enhanced the firmness of AVC with a maximum of up to 21% for the sample treated at 500 MPa for 15 min. Microstructural analysis using scanning electron microscopy (SEM) indicated non-uniform structures in the pretreated and dried AVC samples. Semi-quantitative elemental detection (EDS) confirmed the presence of a considerable amount (14%) of calcium in aloe vera.     

Effects of Germination Process and Drying Temperature on Gamma-Aminobutyric Acid (GABA) and Starch Digestibility of Germinated Brown Rice

In this work, germination method, germination time, and drying temperature were investigated for their effects on gamma-aminobutyric acid (GABA) and starch digestibility. The germination method and germination time influenced the GABA and dietary fiber contents as well as starch and glucose, but both factors did not provide a faster hydrolysis of starch for germinated brown rice because of the dietary fiber. When the germinated samples were dried by a hot-air fluidized bed dryer at 130 or 150°C, the GABA content was not decreased and the amylose-lipid complexes occurred. Dissociation temperature of the complexes was given in a range of 100–117°C, which was lower than that of complexes in the non-germinated paddy. Thus, the amylose-lipid complexes in the germinated samples obtained at high temperature lost some crystalline structure when cooked by the boiling method. The corresponding rate of starch hydrolysis or glycemic index of the germinated samples changed insignificantly from that of the shade-dried germinated sample or non-germinated brown rice which was dried in shade.