Two-Stage Intermittent Microwave Coupled with Hot-Air Drying of Carrot Slices: Drying Kinetics and Physical Quality

To display the advantages of two-stage intermittent microwave coupled with hot-air (60 °C) drying (IM&AD), different drying methods were compared. The activation power density of samples dried by IM&AD increased slightly and then rapidly as moisture content decreased. Drying kinetics, specific energy consumption and dried product quality, such as colour, rehydration ratio and α- and β-carotene contents, of carrot dried by IM&AD under the optimum conditions were assessed and compared with those of carrot dried by hot-air (60 °C) drying, hot-air (60 °C) drying followed by low-power microwave (145 W) drying, high-power microwave (175 W) drying followed by hot-air (60 °C) drying and high-power microwave (175 W) drying followed by low-power microwave (145 W) drying. The effective diffusivity increased gradually and then rapidly as moisture content decreased in all five drying processes. The IM&AD is a promising way for industrial application because it showed the lowest drying time with relatively low energy consumption and provided the best quality of final products with the best colour appearance, highest rehydration ratio and highest α- and β-carotene contents.     

Drying Characteristics and Quality Parameters of Microwave-Dried Grated Carrots

The effectiveness of various microwave-convective drying treatments was compared with convective drying to determine the most favourable treatments in terms of drying kinetics, specific energy consumption and dried product quality of grated carrots. The quality of the dried products differed among drying treatments depending on the microwave power level, pulse ratio and drying temperature. The intermittent microwave-convective drying at ambient temperature with a pulse ratio of 3.0 at 2.23 W?g^?1 provided considerable savings in drying time and specific energy consumption when compared to convective drying and produced high-quality product with better physical (colour and texture) and sensory attributes for carrots. The intermittent microwave-convective drying conducted at lower drying temperature with relatively long power-off time resulted in a more stable and gentle drying process and could be preferred to produce high-quality products.     

Microwave drying of pine and spruce

Drying rates and power densities are determined for pine-and sprucewood when dried from green to 8% moisture content by microwave power. The process is controlled by measurements of internal wood temperature, internal vapour pressure and rate of moisture evaporation. Microwave power densities ranged from 25 to 78 kW/m³, microwave energy consumption from 365 to 760 kWh/m³. Internal wood temperatures up to 140 °C were used. Internal vapour pressure in the wood could rise to about 20 kPa without checking. Maximal drying rates of 0.20 to 0.45% moisture content per minute are possible to obtain when drying above fiber saturation (fsp). Below fsp the feasible drying rates ranged from 0.10 to 0.20% moisture content per minute. Spruce dried approximately 1.6 times faster than pine. No conditioning of the wood was necessary since the wood was free of stresses. The wood was free of checks but colour changes occured in the interior of some specimens.     

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.     

Drying Kinetics and Energy Consumption in the Dehydration of Pomegranate (Punica granatum L.) Arils and Rind

The objective of this study was to evaluate the drying kinetics and energy requirements during convective drying (CD) (50, 60 or 70 °C), vacuum-microwave drying (VMD) (240, 360 or 480 W) and a combined method of convective pre-drying and vacuum-microwave finishing drying (CPD-VMFD) while processing pomegranate arils and rind. Drying kinetics of CD and VMD was described using seven basic drying models; however, VMFD was modeled only by the Henderson and Pabis equation. Pomegranate rind required less drying time (26–460 min) than arils (51–1,395 min) due to its higher porosity and lower sugar content. The drying time and the energy consumption were reduced when the air temperature and microwave wattage were increased. VMD required less energy than CD within the entire range of moisture content assayed. CD energy consumption can be reduced by its combination with VMD (CPD-VMFD), and this reduction was more important when the VMFD was applied earlier. VMD and CPD-VMFD are a good option for food drying industry in order to reduce both the drying time and the energy consumption while processing pomegranate arils and rind. Therefore, industries can optimise the drying process and consequently save their financial needs.     

Effect of Convective and Vacuum–Microwave Drying on the Bioactive Compounds,Color, and Antioxidant Capacity of Sour Cherries

The aim of this study was to determine the effect of microwave power during the vacuum–microwave drying (VMD) on sour cherries in terms of drying kinetics, including the temperature profile of dried material, as well as on some quality factors of the finished product including phenolic compounds, antioxidant capacity, and color. The content of phenolic compounds, antioxidant activity, and color change were used as quality indicators of dried sour cherries. Sour cherries were dehydrated by convective drying (CD) at temperatures of 50 °C, 60 °C, and 70 °C and by VMD at the initial microwave power of 240 W, 360 W, and 480 W reduced to 240 W and 120 W in order to avoid a rapid increase in temperature at the critical moisture content of ca. 1 kg/kg dry mass. Control samples were prepared by freeze drying (FD). The increase in air temperature during CD as well as the increase in material temperature during VMD deteriorated dried product quality in terms of the content of phenolic compounds, antioxidant activity, and color, which was consistent with anthocyanins content. However, VMD turned out to be much better than CD and competitive to FD. The best quality of the dried product and its more attractive color were achieved at VMD at 480 W followed by drying at microwave power reduced to 120 W, which corresponds to anthocyanins content. In addition, the drying process had a positive impact on contents of quercetin and keampferol derivatives. Dried sour cherries have a long shelf life and therefore may be a fine alternative to fresh fruit all year round.     

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.     

A comparative study on brown-rot fungus decay and subterranean termite resistance of thermally-modified and ACQ-C-treated wood

The resistance of thermally-modified and Alkaline Copper Quaternary type C (ACQ-C) treated aspen (Populus tremuloides Michx), jack pine (Pinus banksiana Lamb.), yellow-poplar (Liriodendron tulipifera L.), and Scots pine (Pinus sylvestris L.) against the brown-rot fungus (Gloeophyllum trabeum) and Eastern U.S. subterranean termite (Reticulitermes flavipes) was studied. Wood materials were thermally-modified at a temperature of 210 °C for 15 min. ACQ-C was impregnated into yellow-poplar and jack pine wood at three different retention levels and at each level both leaching and non-leaching procedures were conducted. Results indicate that ACQ-C-treated yellow-poplar and jack pine became significantly more resistant to the brown-rot fungus compared to the thermally-modified wood and the untreated control. Thermally-modified yellow-poplar and jack pine were more resistant to this fungus than untreated wood. For aspen and Scots pine, the resistance to G. trabeum was improved after the thermal modification, but it remained susceptible to this brown-rot fungus decay. Termite susceptibility of thermally-modified aspen, jack pine, and yellow-poplar was comparable to that of untreated controls. Significantly greater termite attack occurred on thermally-modified Scots pine wood than it did on untreated wood. This likely is attributed to some compounds contained in Scots pine wood that inhibited termite attack.     

Effect of drying method on the surface wettability of wood strands

An experiment was conducted to investigate the effects of drying methods, namely rotary drum, lab oven, microwave and air-drying, on the surface wettability of wood strands by the Wilhelmy plate method. The measurements included the surface contact angle, free energy, and liquid sorption capacity. Thirty industrial strands including southern yellow pine (Pinus spp.) and poplar (Populus spp.) were randomly collected for each drying method. The results from this study showed that the surface contact angle of a strand depends on the liquid used as well as the drying method and wood species. Liquid sorption capacity of the pine strands increased with decreasing surface contact angle, but this trend was not observed with the poplar strands. The total surface energy was almost the same for strands within species, regardless of the drying method, but it was higher on poplar strands than on pines. The polar and disperse components of surface free energy varied with both drying method and wood species. The surface polar energies of oven-dried pine strands and rotating drum-dried poplar strands were higher than on strands dried by other methods. For lower temperature drying methods (air and microwave), the lowest polar component was observed on the radial face of the pine strands. For the higher-temperature drying methods (oven and rotary-drum), the lowest polar component was found to on the tangential face of the pine strands.     

Microwave-Vacuum Drying of Lactic Acid Bacteria: Influence of Process Parameters on Survival and Acidification Activity

Freeze drying is one important step in downstream processing of probiotics and starter cultures production and requires a long duration of the drying step, which, therefore, often is the overall production bottleneck. Microwave accelerated vacuum drying already showed promising results in terms of product quality for various food products. However, poor information on drying microorganisms by microwaves is available. The aim of this work was to set up a suitable microwave-vacuum drying process for the conservation of lactic acid bacteria and to investigate the optimum process parameters to achieve dried cultures with high survival and activity. The probiotic Lactobacillus paracasei ssp. paracasei F19 was used as model strain. The influence of the process parameters chamber pressure (7–30 mbar), specific microwave power input (1–5 W/g), and maximum product temperature (30–45 °C) on the main quality parameters survival rate, metabolic activity, and water activity of the dried samples were analyzed. Continuous energy input was shown to act detrimentally to the cells due to extreme temperature rise in the second drying stage. Proper temperature regulation could be obtained by pulsed microwave input according to a maximum set temperature. Intermediate microwave power between 3 and 4 W/g, the lowest tested pressure level of 7 mbar and low product temperatures of 30–35 °C, resulted in the highest survival and activity of the bacterial cells. With these results, it could be shown that microwave-vacuum drying appears as a promising alternative drying technique for the preservation of starter and probiotic cultures.     

Combined effect of acetylation and heat treatment on the physical,mechanical and biological behavior of jack pine (<Emphasis Type="Italic">Pinus banksiana</Emphasis>) wood

A comparative study on the combined effect of heat treatment and acetylation on jack pine wood properties was undertaken and the results were compared with those of each treatment carried out individually. The dimensional stability and mechanical property of wood with different treatments were examined and statistically analyzed. The results demonstrated that combined acetylation of jack pine wood with acetic anhydride and heat treatment at 190 °C has a positive effect on the dimensional stability. Results also suggested that the dimensional stability was affected more than the modulus of rupture (MOR) and modulus of elasticity (MOE) by both heat and acetylation treatments under the experimental conditions used. In addition, the hardness increases after high temperature modification but decreases slightly after acetylation. A comprehensive investigation of the effects of heat treatment and acetylation separately and together (combination treatment) on the fungal durability of jack pine wood against a brown rot fungus, Poria placenta (pp), and a white rot fungus, Trametes versicolor (tv) has also been performed. The results indicated that the weight loss caused by fungi is reduced by both modifications. It was also found that combination of heat treatment and acetylation offers additional bioprotection. FTIR results indicated that the heat and acetylation treatments have a significant influence on the chemical properties, but less influence on their structures.     

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.     

The Effect of Microwave-Vacuum Pretreatment on the Drying Kinetics,Color and the Content of Bioactive Compounds in Osmo-Microwave-Vacuum Dried Cranberries (<Emphasis Type="Italic">Vaccinium macrocarpon</Emphasis>)

The aim of this study was to compare the effectiveness of microwave-vacuum pretreatment conducted at 100, 500 and 800 W on the drying kinetics of whole cranberries (Vaccinium macrocarpon) during hybrid osmotic and microwave-vacuum drying. Additionally, the effect of microwave-vacuum pretreatment and subsequent osmo-microwave-vacuum drying on selected quality indicators of dried cranberries, including phenolic content, antioxidant activity and color, was studied, and the quality of samples was compared with freeze dried, microwave-vacuum dried and osmo-microwave-vacuum dried samples. Irrespective of microwave power, the initial pretreatment accelerated mass transfer during osmotic dehydration of cranberries, and the Weibull model well fitted the experimental data. Final microwave-vacuum drying of cranberries was a two-stage process involving a relatively long phase with a constant drying rate, followed by a short period with a decreasing drying rate. Microwave-vacuum and osmo-microwave-vacuum drying resulted in similar retention of polyphenols and similar antioxidant activity, both of which were relatively higher than in freeze-dried cranberries. However, microwave-vacuum pretreatment at low microwave power (100 W) before dehydration also resulted in high retention of phenolic compounds, high antioxidant activity and attractive color, which were consistent with the high content of total anthocyanins and flavonoids. Microwave-vacuum, osmo-microwave-vacuum and osmo-microwave-vacuum drying combined with microwave-vacuum pretreatment at low microwave power (100 W) were the most suitable methods for the production of high-quality dried whole cranberries.     

Investigating shrinkage and moisture diffusivity of melon seed in a microwave assisted thin layer fluidized bed dryer

This work aimed to investigate the drying behavior of melon seed during combined fluidized bed-microwave drying system. Three drying air temperatures (40, 55 and 70 °C), three microwave powers (270, 450 and 630 W) and three air velocities (0.8, 1.5 and 2.3 m/s) were tested. Five mathematical models were selected to fit the experimental data for drying kinetics, and the results revealed that the Aghbashloo et al. model exhibited, in all cases, the best performance in fitting the experimental data (R² varying from 0.99088 to 0.99998; χ² from 0.00000 to 0.00185 and RMSE from 0.02289 to 0.82316). Calculated values of moisture diffusivity for dried melon seed varied from a minimum of 6.51 × 10?10 to a maximum of 6.59 × 10?9 m²/s under the tested drying conditions. Moisture diffusivity values increased as air temperature and microwave power was increased. Shrinkage values were calculated and found to vary in the range from 46.99 to 15.09 %.     

Mechanical properties of wood treated with PF-pyrolytic oil resins

Jack pine and sugar maple wood samples were treated in a two-step process that involved first a copper chloride or a copper chloride-sodium borate mixture and then a phenol-formaldehyde resin containing a certain percentage of softwood bark pyrolytic oil. Various controls were also prepared for comparison. The modulus of rupture (MOR) of jack pine samples was generally negatively affected by the treatment, the observed values being lower than those of the controls. These values were statistically lower or equal to those of untreated samples. On the other hand, the modulus of elasticity (MOE) of treated jack pine samples was not found to be statistically different from that of the untreated wood. Mechanical properties behaved in a similar way for sugar maple wood. Unlike the other treatments, copper chloride–sodium borate and the resin containing 85% of pyrolytic oil as phenol substitute appeared to slightly improve the mechanical properties of both wood species even if not significantly so. Treatment with PF-pyrolytic oil resin resulted in similar or slightly better mechanical properties when compared to CCA-treated wood. The amount of treating water soluble salts retained in samples after the first treatment had a significant impact on MOR of both wood species and on MOE of sugar maple.     

莴苣微波喷动均匀干燥工艺

通过单因素实验和正交实验确定莴苣微波喷动干燥的最佳工艺参数,并将产品和热风、喷动、真空冷冻、真空微波干燥的产品进行比较,旨在发现微波喷动干燥的优势所在。产品的质量主要通过复水率、叶绿素质量分数、色泽、能耗、感官评定等方面来衡量。实验结果表明,莴苣最佳微波喷动干燥参数为干燥前期(10min),微波功率为300W,热风温度为68～78℃,喷动风速为8m/s;干燥后期(45min)微波功率为200W,热风温度为68～78℃,喷动风速为6m/s。检测结果表明,微波喷动干燥产品的复水性、叶绿素质量分数、色差值和感官评定均优于热风干燥、喷动干燥和真空微波干燥产品,复水性和叶绿素质量分数略低于真空冷冻干燥产品,微波喷动干燥在节约能耗方面有显著效果。     

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.     

Quantity and chemical composition of essential oil of peppermint (Mentha × piperita L.) leaves under different drying methods

This work was aimed at quantitative and qualitative analyses of the essential oil of peppermint leaves under different drying methods. Thin layer drying experiments of the leaves were performed in shade, hot air dryer (at temperatures of 50°C, 60°C, and 70°C), and microwave oven (at power levels of 200, 400, and 800 W). Essential oils of the fresh and dried samples were extracted by hydrodistillation and analyzed using gas chromatography-mass spectrometry (GC/MS). The highest (22.24 g/kg _{dry matter}) and the lowest (1.33 g/kg _{dry matter}) oil yields were obtained from the hot air-dried leaves at temperature of 50°C and microwave-dried leaves at power of 800 W, respectively. In general, increasing drying temperature decreased the essential oil content. The GC/MS analysis of essential oils showed that the chemical compounds belonged mostly to oxygenated monoterpenes class (72.34–86.41%). The chemical compounds group was significantly (p < 0.01) decreased by microwave drying at power levels of 200 and 400 W. The assessed drying methods caused significant (p < 0.05 and/or p < 0.01) variations in the main constituents of the peppermint leaves essential oil including menthol, menthone, menthofuran, 1,8-cineole, and menthyl acetate. The minimum (35.01%) and maximum (47.50%) concentrations of menthol, as the major compound of the oil, were found in hot air-dried leaves at temperature of 50°C and microwave-dried leaves at power of 400 W, respectively. The percentage of menthone, as the second constituent in the essential oil, was significantly lost (p < 0.01) under microwave drying.     

发酵菜籽饼微波间歇干燥工艺研究

目的优化发酵菜籽饼微波间歇干燥工艺。方法采用Box-Benhnken响应面分析法,以质量干燥速率和平均干燥能耗为评价指标,研究微波功率、间歇时间和微波加热时间对发酵菜籽饼微波间歇干燥的影响。结果微波功率、微波加热时间、间歇时间之间存在交互作用,各因素对质量干燥速率的影响强度依次为:微波加热时间微波功率间歇时间,对发酵菜籽饼的平均干燥能耗影响强度依次为:微波功率间歇时间微波加热时间。发酵菜籽饼干燥的最佳工艺为:间歇时间60 s,加热时间9 s,微波功率595.49 W,共微波加热1 h,在该优化条件下,发酵菜籽饼的质量干燥速率为0.5168 kg/(h·kg),平均干燥能耗为8.53 kJ/g。结论微波功率、间歇时间、微波加热时间对发酵菜籽饼的微波间歇干燥都有显著影响,选择适当的微波间歇干燥工艺能达到提高干燥速率和减少干燥能耗的目的。     

Microwave drying of wet processed wood fibre insulating boards

A continuous microwave dryer was used for drying wet processed wood fibre based insulating boards. Drying characteristics and energy consumption were compared with convective drying methods. Microwave drying turned out to be faster and facilitates even economical drying of thick boards.