Comparison of three microwave‐assisted drying methods on the physiochemical,nutritional and sensory qualities of re‐structured purple‐fleshed sweet potato granules

Purple‐fleshed sweet potato (PFSP) with high anthocyanin level was attractive to health concern. Three drying methods, namely microwave‐assisted freeze‐drying (MWFD), microwave‐assisted vacuum drying (MWVD) and microwave‐assisted spouted bed drying (MWSBD), were compared in the drying of PFSP granules. Product’s texture, colour, anthocyanin level and energy consumption were investigated. Drying periods varied among drying methods. MWSBD needed the least time to finish the drying process. Different drying methods had a significant effect (P < 0.05) on texture and anthocyanin level. Maximum penetration force of MWSBD treated sample was lowest (14.99 N). Anthocyanin level of MWSBD products was not as high as that of MWFD products, but colour and appearance were fine. MWFD products owned good crispness through sensory evaluation. Energy consumption of MWFD was about two times higher than that of MWVD. Final results indicated that MWSBD may be an alternative way to MWFD with measurements to maintain the anthocyanin level.     

Quality Retention in Strawberry and Carrot Purees Dried with Refractance WindowTM System

ABSTRACT: The quality retention characteristics of strawberry and carrot purees dried using the Refractance Window^TM (RW) drying method were evaluated against freeze drying, drum drying, and spray drying methods. Ascorbic acid retention of the strawberry purees (94.0%) after RW drying was comparable to 93.6% in freeze-drying. The carotene losses for RW drying were 8.7% (total carotene), 7.4% (α-carotene), and 9.9% (β-carotene), which were comparable to losses of 4.0% (total carotene), 2.4% (α-carotene), and 5.4% (β-carotene) for freeze-dried carrot purees. The color of the RW-dried carrot purees was comparable to fresh puree. For RW-dried strawberry purees, the color retention was comparable to freeze-dried products. RW drying altered the overall perception of aroma in strawberries.     

不同干燥方式对黑毛豆仁品质的影响

研究热风干燥、微波干燥、热风-气流膨化干燥、微波-气流膨化干燥、冷冻干燥5种不同的干燥方式对黑毛豆仁品质(VC和花色苷保留率、色泽、硬度、脆度、感官质量和微观结构)的影响。结果表明：冷冻干燥黑毛豆仁的VC和花色苷保留率最高、色泽良好,但硬度过低;微波干燥黑毛豆仁在VC和花色苷保留率方面仅次于冷冻干燥,产品的色泽和风味差;热风干燥黑毛豆仁的品质最差;热风-气流膨化干燥黑毛豆仁的品质次于微波-气流膨化的;微波-气流膨化干燥黑毛豆仁在VC和花色苷保留率、色泽方面与微波干燥产品无显著性差异,硬度和脆度得到了明显的改善,产品的感官质量较好,可作为最佳干燥方式应用于生产。     

不同干燥方式对胡萝卜片吸湿性及品质的影响

分别对热风干燥、冷冻干燥、真空微波干燥、冷冻与真空微波联合干燥这4种干燥方式对胡萝卜片的品质变化和吸湿性方面的影响进行了讨论,以VC和胡萝卜素的保持、色泽的差异、膨化率,干燥时间等为质量参数,以及以吸湿率为吸湿性参数分别进行比较。联合干燥产品在VC和胡萝卜素的保持、色泽方面略差于冻干产品,但它的膨化率得到了提高,口感上也有改善,干燥时间大大缩短,并且抗吸湿性要比冷冻干燥好,这样就能很好地保持果蔬脆片特有硬脆性。采用冷冻与真空微波联合干燥方式能较好地改善胡萝卜片的品质,缩短干燥时间,降低吸湿性。     

Drying Kinetics and β-Carotene Degradation in Carrot Undergoing Different Drying Processes

Superheated steam drying, which is an airless drying technology, has recently received much attention as an alternative to conventional hot air drying, which is a relatively oxygen‐rich drying process and causes much product quality degradation. However, because most food products are damaged when subjected to superheated steam at atmospheric or higher pressures, lowering the dryer operating pressure is preferred. In this study, the effects of a low‐pressure superheated steam drying (LPSSD), vacuum drying, and hot air drying on the drying and degradation kinetics of β‐carotene in carrot were investigated experimentally. LPSSD and vacuum drying led to less degradation of β‐carotene in carrot than in the case of hot air drying. The empirical models, which can describe the experimental data of β‐carotene degradation in carrot undergoing different drying techniques, were also proposed. β‐Carotene degradation in carrot depended more on the carrot temperature than its moisture content in all cases.     

Forced Air Drying of Partially Freeze-Dried Compressed Carrot Bars

The quality of compressed carrot bars produced by combining freeze drying with air drying was investigated. Quality parameters measured were color, texture, rehydration ratio, carotene, ascorbic acid, alphatocopherol, and sensory acceptance. It was found that a high quality compressed carrot bar could be obtained by freeze drying to 20–40% moisture, equilibrating with microwave energy, compressing, then air drying at 60°C. The scanning electron microscope proved useful in delineating reasons for differences in texture and rehydration.     

干燥方式对紫薯全粉水合特性及抗氧化能力的影响

以紫薯\     

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.     

Assessment of convection,hot‐air combined with microwave‐vacuum and freeze‐drying methods for mushrooms with regard to product quality

Combined drying of hot air and microwave‐vacuum has been proposed as an alternative method to improve the quality of dried mushrooms, especially the structural and textural properties. In the present study, the effect of different drying methods namely, convective hot‐air drying, hot air combined with microwave‐vacuum drying and freeze‐drying on qualitative attributes of pretreated mushrooms was investigated. The quality assessment was based on colour, texture, density, porosity and rehydration characteristics of the dried mushrooms. Combined drying of hot air and microwave‐vacuum resulted in a dried product of superior quality when compared to the slices dried completely by conventional hot air, exhibiting lower overall colour variation, higher porosity, greater rehydration ratio and softer texture. In a dry state, mushrooms with a puffy structure and unique crispy texture were created by the combined drying method, which might be considered as important characteristics for developing nonfat snack‐type food products.     

微波干制胡萝卜丝工艺参数的试验研究

以提高蔬菜干制品质为目的,采用自制的微波干燥试验装置,研究了不同干燥功率、物料铺放厚度及排湿风速对干制胡萝卜丝品质和能耗的影响。采用正交试验的极差和方差分析方法,进行了参数综合优化。结果表明:不同微波干燥参数对干制胡萝卜丝品质有显著的影响,物料脱水过程主要处于恒速阶段,微波干燥功率为1.125W/g,铺放厚度为1.5cm,风速为75m/min时,可确保胡萝卜丝干燥后的食用价值且便于储存,而且能耗较低。     

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.     

Tomato peel drying and carotenoids stability of the extracts

Tomato peels were firstly dried by different methods (hot air, freeze‐drying, and fluidized bed drying) to evaluate the recovery of lycopene, β‐carotene and DPPH radical scavenging activity. Comparison of the results showed that hot air drying at 50 °C was a suitable method and alternative to freeze‐drying to preserve carotenoids compounds and antioxidant activity in tomato peels. Then, ethanol/ethyl acetate (1:1) extracts from tomato peel, previously dried at 50 °C by hot air, were submitted to heat (100 °C) and light treatment (1000 lumen) to evaluate their stability as natural food dyes. Heating of the extracts caused a progressive reduction of total carotenoids, up to about 30% after 250 min of treatment, whereas the colour at the end of heat treatment showed small changes, with an overall colour difference (?E) equal to 7. Fluorescent lighting treatment showed an almost total degradation of carotenoids in the extracts after 48 h combined with a fading colour.     

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

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

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。     

Effects of vacuum and microwave freeze drying on microstructure and quality of potato slices

Potato slices immersed in 0.5% CaCl₂ solution for 10 min were examined using light microscopy for their microstructure in frozen state before drying, during the sublimation drying stage, in the desorption drying stage and in the final dried form. Additionally, the final dried product was also tested for vitamin C content, color, starch content, texture and sugar content. Experiments were carried out using conventional vacuum freeze dryer as well as a microwave freeze dryer. Results for both unblanched and blanched potato tissues, crystal growth during sublimation drying stage was observed to cause structural damage to the cell walls. Blanched tissue suffered more damage during the freezing process. Interestingly, microwave freeze drying yielded product similar in quality to that obtained in vacuum freeze drying with conductive heating.     

微波真空冷冻干燥对芒果干制品品质特性的影响

为获得较优的芒果干燥方法,以复水性、感官为评价指标,比较微波真空冷冻干燥、热板真空冷冻干燥和热风干燥3种不同干燥方法对芒果干制品品质的影响。结果表明：真空冷冻干燥法的产品各项指标均优于热风干燥法。微波真空冷冻干燥的产品复水性最好,25℃与100℃最大复水比分别为3.363、3.674;其次为热板真空冷冻干燥,两温度条件下与微波真空冷冻干燥产品复水比相差较小;热风干燥产品复水性最差,分别为2.140、3.028。感官指标中色泽、香气和口味3方面均为：微波真空冷冻干燥＞热板真空冷冻干燥＞热风干燥。     

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.     

Fruit snacks from raspberries: influence of drying parameters on colour degradation and bioactive potential

Raspberries were dehydrated using air and freeze‐drying with wet and dry sugar infusion pretreatments. Product quality factors such as colour, bioactive compounds, antioxidant capacity and sensorial characteristics were analysed. Special emphasis was placed on the analysis of anthocyanin degradation and its relationship with colour deterioration and with polymeric compounds development and browning. Freeze‐dried raspberries presented a higher retention of bioactive compounds and a lower content of polymeric compounds than air‐dried ones. Dried samples without pretreatment (control) showed the highest retention of total phenolic content (freeze‐dried ≈82% and air‐dried ≈37% retention), but the lowest sensory acceptability. Although sugar infusion pretreatments caused an important loss of bioactive compounds (9–18% of TPC retention), a higher sensorial acceptability was obtained. Pretreatments with bisulphite and acid allowed obtaining the best quality attributes in terms of anthocyanin and polyphenol content, antiradical activity and colour retention. Polyphenol intake through pretreated dried raspberries (115–299 mg gallic ac./100 g intake) would be higher in some cases than that of usually consumed foods as vegetables, cereals and several fresh fruits.     

Comparison of the effect of microwave freeze drying and microwave vacuum drying upon the process and quality characteristics of potato/banana re‐structured chips

This study investigated the drying characteristics of microwave freeze‐drying (MFD)/microwave vacuum drying (MVD) of banana/potato restructured chips of varying proportion and microwave power. The results showed the MFD drying time had a maximum one‐hour time difference between the samples treated with 2 W g^?1 and 3 W g^?1 microwave power; and the higher potato content samples have about 30 min predominance than low potato content samples in drying time and the total drying time of MVD samples was less than 60 min. For the rehydration ratio, MFD samples were far superior to the MVD samples. The MFD samples had more than 4.5 rehydration ratio. The data of the colour difference metre showed that 3 W g^?1 microwave power would make samples slightly charred. The biggest differences between the MFD and MVD chips lie in their texture and shape; the hardness of MVD samples was 30.86 N, thrice higher than MFD samples.     

Effects of microwave and hot‐air drying methods on colour, β‐carotene and radical scavenging activity of apricots

The effects of drying by microwave and convective heating at 60 and 70 °C on colour change, degradation of β‐carotene and the 2,2‐diphenyl‐1‐picrylhydrazyl radical (DPPH) scavenging activity of apricots were evaluated. Microwave heating reduced significantly the drying time (up to 25%), if compared with convective one, also owing to the higher temperature reached during the last phase of the process, as monitored by infrared thermography. Colour changes of apricot surface, described with lightness and hue angle, in both drying methods followed a first‐order reaction (0.927 ≤ R² ≤ 0.996). The apricots dried by microwave were less affected by the darkening phenomena. The evolution of β‐carotene in fresh apricots (61.2 ± 5.6 mg kg^?1 d.w.) during the drying highlighted a wider decrease (about 50%) when microwave heating was employed for both the temperatures used. Radical scavenging activity increased (P < 0.05) in all dried samples except for hot‐air dried apricots at 60 °C.