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.     

The effect of microwave and conventional drying on antioxidant activity,phenolic compounds and mineral profile of date fruit (<Emphasis Type="Italic">Phoenix dactylifera</Emphasis> L.) flesh

In this study, the effect of microwave and conventional drying on antioxidant activity, phenolic compounds, mineral and total phenolic contents of date fruit meat (Silifke and Anamur) were determined. Both microwave (360, 540 and 720 W) and conventional (70, 90 and 110 °C) drying caused the reduction of phenolic compounds although there can not considerably variation in antioxidant activity of date flesh. Total phenolic contents of date flesh changed between 114.640 mgGAE/100 g (Silifke 540 W) and 133.484 mg GAE/100 g (Silifke 720 W) compared with control sample (156.128 mg GAE/100 g). While P contents of date fruit change between 86.00 mg/Kg (control) and 704.420 mg/Kg (360 W), P contents of Anamur samples ranged from 79.430 mg/Kg (Control) to 1288.780 mg/Kg (360 W). Mineral contents of date fruits dried in microwave oven were found higher than those of observed in conventional dried fruits (especially Ca and Mg contents of date fruits).     

Microwave‐Osmotic/Microwave‐Vacuum Drying of Whole Cranberries: Comparison with Other Methods

A novel drying method for frozen‐thawed whole cranberries was developed by combining microwave osmotic dehydration under continuous flow medium spray (MWODS) conditions with microwave vacuum finish‐drying. A central composite rotatable design was used to vary temperature (33 to 67 °C), osmotic solution concentration (33 to 67 °B), contact time (5 to 55 min), and flow rate (2.1 to 4.1 L/min) in order to the determine the effects of MWODS input parameters on quality of the dried berry. Quality indices monitored included colorimetric and textural data in addition to anthocyanin retention and cellular structure. Overall it was found that the MWODS‐MWV process was able to produce dried cranberries with quality comparable to freeze dried samples in much shorter time. Additionally, cranberries dried via the novel process exhibited much higher quality than those dried via either vacuum or convective air drying in terms of color, anthocyanin content, and cellular structure.     

Optimization of microwave-vacuum drying of pomegranate arils

Pomegranate arils (Punica granatum L.) were dried in a microwave-vacuum drier up to a final moisture content of around 5–6 % (d.b.). The effect of microwave power level (25–95 W), vacuum pressure (25–195 mm Hg) and sample mass (65–235 g) on drying efficiency and some quality attributes (color, texture, rehydration ratio and sensory score) of dried pomegranate arils were analyzed by means of response surface methodology. A rotatable central composite design was used to develop models for the responses. Analysis of variance showed that a second order polynomial model predicted well the experimented data. All three process parameters microwave power, vacuum pressure and sample mass strongly affected quality attributes of dried pomegranate arils and drying efficiency. A lower vacuum pressure during drying resulted in better quality products. Optimum drying conditions of microwave power level of 80 W, vacuum pressure of 60 mm Hg and sample mass of 193.7 g were established for microwave-vacuum drying of pomegranate arils. Separate validation experiment was conducted at the derived optimum conditions to verify the predictions and adequacy of the models.     

Effects of different drying methods on phenolic contents,antioxidant, and tyrosinase inhibitory activity of peach blossoms

Fresh peach blossoms of two varieties were dried by using different methods, which include shade drying, freeze drying, microwave drying and hot air drying (at 30, 60, 90, 120 °C). The effects of different drying methods on phenolic contents, antioxidant and tyrosinase inhibitory activity were evaluated. Among the seven drying treatments, microwave drying yielded the highest contents of total phenolics, flavonoids, anthocyanins, proanthocyanidins and individual phenolic compounds in dried peach blossoms, while hot air drying at 30 °C gave the lowest retention of total phenolics, flavonoids, anthocyanins. As compared with shade drying and freeze drying, hot air drying at 60, 90, 120 °C resulted in higher retention of phenolic content. The highest antioxidant activity in dried peach blossoms was obtained by microwave drying, while hot air drying at 30 °C gave the lowest. The effect of different drying methods on antioxidant activity of dried peach blossoms was consistent with that of phenolic content. The highest tyrosinase inhibitory activity was achieved by hot air drying at 120 °C for variety ‘yingchun’ and 90 °C for variety ‘huangjinmeili’, but there was no significant difference among hot air drying at 60, 90, 120 °C for both varieties. The tyrosinase inhibitory activities of peach blossoms by microwave drying and freeze drying were comparable and much lower than that of shade drying. The results indicated that microwave drying was a desirable method for the preservation of phenolic compounds and antioxidant activity in peach blossoms, while shade drying and hot air drying at high temperature were favorable for tyrosinase inhibitory activity.     

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.     

Total Phenolics, Flavonoids, and Antioxidant Activity of Sage (Salvia officinalis L.) Plants as Affected by Different Drying Methods

In the current study, we determined the effects of seven drying methods on total phenolics, flavonoids, individual phenolics, and antioxidant activity of the methanol extract of Salvia officinalis L. As compared with total phenolic content (TPC) of fresh plants, results showed that the highest TPC was recorded in plants dried by microwave (MW) at a power of 800 W/30 g of fresh plant and was 4.2 times higher than that of fresh plants whereas the lowest content was found in the case of plants dried by far-infrared (FIR) at 45 °C. The analysis of the different extracts by RP-HPLC showed a predominance of phenolic acids particularly in fresh plants and those dried by MW (600 W/30 g of fresh plant) whereas flavonoids predominate in the case of plants dried by FIR (65 °C). The assessment of the radical scavenging activity (RSA) against the stable radical 1,1-diphenyl-1-picrylhydrazyl (DPPH) showed an increase in the scavenging effect particularly in MW (800 W/30 g of fresh plant) dried plants with an IC50?=?13.49 μg ml^?1 (IC50 is the concentration required to cause 50 % DPPH inhibition). The complementary assessment of the RSA using the β-carotene/linoleic acid system showed an increase of this activity for all extracts and particularly for the extract derived from MW (600 W/30 g of fresh plant) dried plants as compared to fresh ones. Finally, all the plant extracts showed moderate reducing power as assessed by the ferric-reducing antioxidant potential. These results suggested that MW drying could be applied to retain phenolic contents and to enhance antioxidant activity of sage plant materials.     

Effects of infrared heating on drying kinetics,antioxidant activity,phenolic content,and color of jujube fruit

This study investigated the effects of different infrared power (IP) levels (62, 88, and 125 W) and a pretreatment (soaking in a solution of 5 % potassium carbonate and 0.5 % olive oil) on the drying kinetics and some quality parameters of jujube fruit. The drying characteristics of jujube were greatly influenced by the pretreatment and IP level. The models of Lewis, Logarithmic, Page, and Aghbashlo et al. were fitted to the obtained experimental data using nonlinear regression analysis. The Page model showed a better fit to the experimental drying data when compared to the other models. The effective moisture diffusivity, calculated using Fick’s second law, ranged from 4.75 × 10^?10 to 4.17 × 10^?9 m²/s. Significantly, higher total phenolic content (TPC) and antioxidant capacity values were obtained for the pretreated samples subjected to higher IP levels. The total color change (ΔE) of the dried samples significantly increased with increasing IP level. Jujube fruits should be pretreated and then dried at 88 W IP to reduce phenolic degradation and undesired color changes and to increase the quality of the dried product.     

食品微波真空干燥技术研究进展

微波真空干燥是用微波辐射作为加热源在真空条件下进行加热而使物料脱水的过程,具有快速、高效、低温等优点。本文阐述了微波真空干燥的作用机理,介绍了微波真空干燥的特性及研究现状,旨在为微波真空干燥技术在食品加工中的研究和应用提供参考。     

Polydextrose as Wall Material for Microencapsulation of Yacon Juice by Spray Drying

The juice from yacon roots was encapsulated by spray drying using polydextrose and gum Arabic as wall materials. The effects of the concentration of the encapsulating agents and drying temperature on total phenolics, antioxidant activity, fructooligosaccharides, moisture content, water activity, solubility, hygroscopicity, color, and morphology of the microparticles were investigated to assess the potential use of polydextrose as wall material. The microparticles produced with polydextrose showed retention of bioactive compounds and physicochemical characteristics similar to those produced with gum Arabic. The phenolic retention ranged from 73.67 to 85.49 %, and the antioxidant activity by DPPH varied from 80.78 to 90.58 %. The fructooligosaccharides have undergone little depolymerization into simple sugars even at high temperatures. With respect to the physicochemical characteristics, high stability (low moisture and water activity), low hygroscopicity, and high solubility were observed in the microparticles. The spray dried samples showed a hue angle close to 100, indicating yellow color of the particles. Regarding the microstructure, particle agglomeration was observed in both treatments, probably due to the hygroscopic characteristic of the spray dried powders.     

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.     

Chemical Composition, Antioxidant Capacity, and Sensory Quality of Pomegranate (Punica granatum L.) Arils and Rind as Affected by Drying Method

The objective of this study was to evaluate the application of: (1) freeze drying, (2) convective drying (50, 60, or 70 °C), (3) vacuum–microwave drying (240, 360, or 480 W), and (4) a combined method of convective pre-drying and vacuum–microwave finish drying in the processing of pomegranate arils and rind. The quality parameters under study included sugars and organic acids, punicalagins and ellagic acid, total polyphenols, total antioxidant activity, and sensory quality. In general, drying led to a reduction in all studied parameters; however, the behavior of arils and rind was different. Vacuum–microwave drying at 240 or 360 W was the best drying treatment for arils, while rind required freeze drying or soft conditions of convective drying (50 °C). Further research is needed to obtain proper results with convective pre-drying and vacuum–microwave finish drying of arils and rind. With proper selection of the drying protocol, high-quality dried arils will be available for consumers; these arils will be characterized by high contents of fructose (25 g 100 g^?1), phytic acid (2.2 g 100 g^?1), punicalagins (0.57 mg g^?1), total polyphenols (1.6 mg eq gallic acid g^?1), high antioxidant capacity (0.6 mg eq Trolox g^?1), and high intensities of garnet color, sweetness, sourness, and fresh pomegranate aroma. Besides, dried rind with very high contents of active compounds (123 mg g^?1 of punicalagins and 108 mg eq gallic acid g^?1) and high antioxidant capacity (26 mg eq Trolox g^?1) will be also available as functional material.     

微波辅助热风干燥预处理对油炸紫薯片品质的影响

为研究微波辅助热风干燥预处理对油炸紫薯片品质的影响,以厚度为3 mm的新鲜紫薯片为对象,首先采用不同微波功率(259、280、358 W)辅助热风(50、60、70 ℃)干燥方式对紫薯片进行预干燥,对不同微波功率(259、280、358 W)干燥后的紫薯片油炸8、3.5、2.5 min,研究紫薯片预处理过程的干燥特性及花青素含量,以及油炸紫薯片产品的色泽、脆度、硬度和脂肪含量等。结果表明:随着微波预处理功率的升高,紫薯片达到干燥终点的时间缩短(90 min以上),平均干燥速率显著提高;并且热风干燥温度越高,微波预处理对干燥效率的促进作用也越明显。而低功率(259 W)的微波辅助50 ℃热风干燥联用更有利于干燥紫薯片花青素的留存;较低功率(259、289 W)的微波预处理不仅在保护产品颜色上具有优势,还可以使得油炸紫薯片更高的硬度和更好的脆性。在不同微波预处理功率下,油炸紫薯片的脂肪含量最低值基本一致。本研究可为微波辅助热风技术在干燥紫薯及其他农产品干燥中的应用提供参考。     

响应面法优化青椒微波-真空冷冻联合干燥工艺及品质分析

该研究采用微波-真空冷冻联合干燥方式对青椒进行干制,研究微波功率、中间转换点含水率及真空冷冻干燥时间对青椒干燥产品维生素C含量、感官评分、复水比及a*的影响。通过单因素试验和响应面试验优化微波-真空冷冻联合干燥工艺,并对青椒联合干燥产品的营养成分、风味进行了比较分析。最后研究了微波-真空冷冻联合干燥、微波干燥、真空冷冻干燥对青椒干燥品质的影响。结果表明,联合干燥的最佳工艺条件为微波功率381.17 W,中间转换点含水率61.81%,真空冷冻干燥时间12.04 h。3种青椒联合干燥产品在营养成分和风味上分别具有一定差异性。微波-真空冷冻联合干燥和真空冷冻干燥的青椒产品在质构、青椒组织微观结构、部分理化指标方面均优于微波干燥的青椒产品。     

Color,Mechanical, and Microstructural Properties of Vacuum Assisted Microwave Dried Saskatoon Berries

Saskatoon berries were dried using a vacuum assisted microwave technique. Central composite rotate design and response surface methodology were used to develop the regression models and to study the influence of drying processing variables (microwave power, drying time, and fruit load) on color, mechanical properties, and microstructure of dried berries. All the three variables had significant effects on the above mentioned properties (p< 0.05). Drying affected the color at variable levels and increased the lightness of dried berries compared to frozen berries. Hardness, gumminess, and chewiness of dried berries increased and springiness and cohesiveness of dried berries decreased with microwave power and drying time. The porosity and the destruction level in the microstructure of dried berries increased with the microwave power. The findings of this study will be useful to identify desirable microwave-vacuum drying operating conditions for targeted dried Saskatoon berry products.     

Effect of Dehydration on Raspberries: Polyphenol and Anthocyanin Retention, Antioxidant Capacity, and Antiadipogenic Activity

ABSTRACT:  Fresh and dried raspberries prepared by freeze drying (FD), microwave-vacuum (MIVAC), hot-air drying (HAD), and a combination of hot-air drying and microwave-vacuum (HAD/MIVAC) drying methods were evaluated for polyphenol retention, total polyphenol and anthocyanin contents, total antioxidant capacity, and antiadipogenic activity (the inhibition of fat cell development). Ellagic acid and quercetin were present in the largest concentrations in fresh and dehydrated raspberries. Dehydration led to a loss of polyphenols and anthocyanins and antioxidant capacity. Polyphenols (aglycone form) were retained in the greatest amount: 20% (freeze dried) to 30% (HAD/MIVAC) (fresh = 100%). A total of 30% of polyphenols (glycoside form) were retained in raspberries dried by the HAD/MIVAC methods with 5% of retention observed for raspberries dried by FD, HAD, or MIVAC. FD and MIVAC resulted in higher retention of anthocyanins (aglycone form) than other drying methods. It was also observed that antioxidant activity was reduced by dehydration. Adipogenesis was inhibited by polyphenolic glycosides (30%) and aglycones (30% to 40%) in fresh and HAD/MIVAC raspberries. Extracts from dried raspberries by HAD/MIVAC methods were relatively more effective at inhibiting adipogenesis compared to HAD and FD dried raspberries.     

不同干燥方式对甘蓝理化性质和抗氧化活性的影响

为研究不同干燥方式对甘蓝理化性质和抗氧化活性的影响,改善干制甘蓝的品质,提高甘蓝附加值,本试验采用65 ℃热风干燥、1 kW微波干燥、热风-微波联合干燥处理甘蓝,测定干燥后甘蓝的色泽、V_C、游离酚、总酚含量和抗氧化活性,并对抗氧化相关指标进行相关性分析。结果表明:联合干燥的甘蓝色泽最佳,ΔE值为15.01±0.59;不同干燥方式甘蓝总酚和游离酚含量差异显著(P<0.05),而酚的存在状态变化较小;干燥后甘蓝抗氧化能力有一定程度的降低,微波干燥甘蓝V_C、游离酚、总酚含量均最高,分别为23.14 mg/100 g FW、80.22、124.85 mg GAE/100 g FW,保留率分别为67.3%、90%、86.6%,其中DPPH、ABTS自由基清除能力和Fe3+还原能力最强。相关性分析显示:不同干燥方式甘蓝V_C、酚类与抗氧化活性之间呈显著相关(P<0.05)。因此,甘蓝采用微波干燥方式优于热风干燥和热风-微波联合干燥,现有的干燥生产线可尝试结合微波干燥技术,以提高干制品的品质。     

板栗微波真空干燥特性及干燥工艺研究

研究微波真空干燥方式下,微波强度、腔体压力等参数对板栗干燥过程中质热传递的影响规律。采用Box-Behnken中心组合试验设计,以水分含量和白度值为评价指标,确定板栗微波真空干燥过程中微波功率、压力、干燥时间的最适工艺参数。结果显示:板栗微波真空干燥过程主要为加速和降速阶段,恒速阶段持续时间较短。微波功率和真空度均对干燥时间有显著影响,功率越大,真空度越高,干燥速率越快,干基含水率和水分比都随着干燥时间的延长而逐渐下降。由回归模型得出板栗微波真空干燥的最佳工艺参数为时间12min,压力-56kPa,功率3kW。微波真空干燥的微波功率、腔体压力、干燥时间均对板栗品质有影响,以模型得出的干燥参数进行干燥,可保证板栗干燥后的品质,且干燥效率高、能耗低。     

MICROWAVE and SPOUTED BED DRYING of FROZEN BLUEBERRIES: the EFFECT of DRYINGAND PRETREATMENT METHODS ON PHYSICAL PROPERTIES and RETENTION of FLAVOR VOLATILES

Frozen blueberries (Vaccinium corymbosum L cv. 'Elliott') were dried in a microwave and spouted bed combined dryer (MWSB) at 70C air temperature and 3.7 W/g microwave power (wet material). the effect of pretreatment using a 2.5% Ethyl Oleate & 0.2 NaOH dipping solution followed by sucrose osmotic treatment was investigated. the drying kinetics of MWSB drying was compared with spouted bed (SB) drying with dipping treatment, and with tray drying. the rehydration ratio, the color, and the bulk density of MWSB dried blueberries were compared with those of freeze, tray, and SB drying. the drying time needed to reduce blueberry moisture content from 82.5% to 15% (wet basis) using MWSB drying was 1/19 and 1/24 (with and without pretreatment) of the time for tray drying. the MWSB drying resulted in a low bulk density and more reddish and less blue color compared with other methods. MWSB dried frozen blueberries exhibited a higher rehydration ratio in short soaking times. Analysis of flavor volatiles by GC/MS identified ten heat-generated compounds. Microwave heating generated three unique flavor compounds (2-Butanone, 2-methyl butanal, and 3-methyl butanal). Freeze-dried frozen blueberries lost several flavor compounds including the typical blueberry aroma, the 1, 8-Cineole.     

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.