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.     

甘薯片真空微波干燥工艺的优化

为了获得高品质的膨化甘薯脆片,对真空微波干燥甘薯片工艺进行优化研究.在预干燥甘薯片水分含量、真空度、微波功率和加热时间对真空微波干燥甘薯片品质影响的单因素试验基础上,采用二次回归正交旋转组合设计优化真空微波干燥甘薯片的工艺条件.研究结果表明,适宜的水分含量、微波功率和加热时间可以提高甘薯片的膨化率和脆度,高真空度较利于甘薯片膨化;最佳真空微波干燥甘薯片的工艺参数组合为:预干燥甘薯片水分含量31.23％,微波功率771.38 W,微波加热时间39 s,真空度0.085 MPa.     

真空油炸甘薯脆片的研究

以甘薯为原料,对真空油炸甘薯脆片加工工艺进行优化实验,为获得优质甘薯脆片产品的开发提供了参考依据。     

响应面法优化微波膨化紫心甘薯片的工艺

将水分含量为20%的紫心甘薯片微波膨化,可得色泽、口味、膨化酥脆度适中的脆片。在单因素试验基础上,采用Plackett Burman试验,筛选出具有显著性影响因素,通过响应曲面法进一步分析这些因素的交互作用对微波膨化紫心甘薯脆片的影响,得到最佳膨化工艺参数,薯片厚度为1 mm,水分含量20%,均湿时间2 d,微波功率430 W,膨化时间40 s。     

Estimation of crispness and consumer acceptance of fried potato chips by mechanical and acoustic measurements

Fried potato chips retaining various moisture contents (MCs) (2.21–9.20%) were analysed to estimate the intensity of crispness and consumer acceptance by texture and acoustic measurements. The MC of the chips was highly correlated with the mechanical maximum force (MMF) in the texture measurement, total area (MTA) and number of sound peaks (NSP) in the acoustic measurement. The intensities of crispness and consumer acceptance decreased as the MC of potato chips increased. For the predictive models established, the combined use of mechanical and acoustic parameters was shown to better predict sensory crispness intensity [R² = 0.975, root mean square error of prediction (RMSEP) = 0.138] and consumer overall liking (R² = 0.966, RMSEP = 0.111) than either parameter alone. Based on the instrumental‐sensory crispness equivalent table established, the estimated values of the MTA were below 71.24, while the NSP should be above 22.81 to meet ‘slightly like’ category of consumer acceptance.     

Development of fortified low-fat potato chips through Vacuum Impregnation and Microwave Vacuum Drying

Potato chips, traditionally prepared by conventional frying, are the most widely consumed snack in the world; however, it has some health concerns associated. This work aims to produce potato chips fortified with calcium and vitamins C and E using Vacuum Impregnation and Microwave Vacuum Drying. The drying process conditions were optimized considering the power density and absolute pressure as independent variables, and some textural and nutritional properties, as dependent variables. The chips were evaluated for microstructure, sensory, and storage studies. The fortification process through Vacuum Impregnation significantly enhanced the content of fortifiers. A power density of 1.7 W·g⁻¹ and an absolute pressure of 4.0 kPa were found to be optimum microwave vacuum drying conditions yielding an average retention higher than 90% of calcium, 53% of vitamin C and 72% of vitamin E. It is concluded that the combination of vacuum impregnation and vacuum microwave drying is an alternative to produce value-added low-fat snack with acceptable sensory quality.Industrial relevancePotato is a promising food for the development of innovative food products with health benefits, due to its high consumption worldwide. Potato chips could be used as an ideal strategy for targeted nutrient delivery e.g. vitamins and minerals. The findings of this study showed that the use of vacuum impregnation as a fortification technique, with the subsequent stabilization by microwave vacuum drying, are promising ways to develop fortified low-fat potato chips.     

Improving the drying kinetics and microstructure of vacuum-fried ripened durian chips

This research evaluated the effects of process conditions on the physiochemical properties of vacuum-fried durian chips. Ripened durians were sliced and fried at 90, 100, 110 and 120°C under 8-kPa absolute pressure. From this study, microwave heating (MWH) during vacuum frying (VF) significantly reduced the frying time by 20% compared to VF (P < 0.05) with the effective moisture diffusivity for VF, pre-treatment before vacuum frying (P+VF) and VF+MWH at 110°C determined to be 3.63 × 10⁻⁰⁹, 4.30 × 10⁻⁰⁹ and 4.84 × 10⁻⁰⁹ m² s⁻¹, respectively. VF+MWH produced chips with significantly lower shrinkage, more yellowness (P < 0.05) and higher crispiness (consistent with the microstructure from scanning electron microscopy (SEM)). In addition, VF+MWH drastically decreased oil absorption by 10%, while P+VF yielded the lowest specific energy consumption (SEC). From sensory evaluation, the overall acceptability of VF+MWH chips rose from 5.83 to 6.07 (seven-point hedonic scale), thus making the product fall into the ‘Good’ to ‘Very Good’ range.     

Influence of ultrasound pretreatments on diffusion coefficients,texture and colour of osmodehydrated sweet potato (Ipomea batatas)

The moisture and solute diffusivities, texture and colour of sweet potato pretreated in distilled water with ultrasound (UD), osmotic dehydration without ultrasound (OD) and osmotic dehydration with ultrasound (UOD) were investigated to determine the effects of ultrasound on osmodehydrated sweet potato. The moisture and solute diffusivities of all the pretreatments ranged from 2.89 to 4.95 × 10^?10 and 1.17 to 1.67 × 10^?10 m² s^?1, respectively. The moisture diffusivity of samples treated in UOD was significantly (P < 0.05) higher than that of OD and UD by 30.82% and 41.62%, respectively. Compared to the fresh sample, the firmness of samples treated in OD and UOD was reduced by 83.53% and 92.47%, respectively. UD increased the brightness (L*) and reduced the redness (a*) of sweet potato, while UOD enhanced the yellowness (b*) and chroma (C*) of sweet potato. The sweet potato samples treated in UOD had the highest total colour difference (ΔE).     

马铃薯的几种腌制技术

为了充分利用马铃薯,介绍了咸马铃薯、咸甜芥末马铃薯片、糖醋马铃薯片、辣油马铃薯条、酱马铃薯、酱辣麻马铃薯丝和泡马铃薯等7种马铃薯产品的原料要求、生产配方、工艺流程及操作要点,同时给出了7种产品特点:咸马铃薯色泽乳白,质脆味咸;咸甜芥末马铃薯片色泽浅黄,质地脆嫩,味甜辣咸;糖醋马铃薯片具有色泽金黄、质地脆嫩、酸甜可口之特点;辣油马铃薯条色泽红黄,香辣可口;酱马铃薯色泽酱红色或金黄色,香甜脆嫩;酱辣麻马铃薯丝色泽红褐,麻辣香脆;泡马铃薯色白微黄,嫩脆咸香,略带辣味。     

Effects of processing conditions on the quality of vacuum‐fried carrot chips

The effects of pretreatment and vacuum frying conditions on the quality of fried carrot chips were studied. The moisture and oil contents of fried carrot chips were significantly (p < 0.05) reduced when blanched carrot slices were pretreated by immersion in fructose solution and freezing prior to vacuum frying. Furthermore, more uniform porosity was observed on the vertical cross‐section of carrot chips when examined by scanning electron microscopy. During vacuum frying, the moisture content, colour and breaking force of carrot chips decreased while the oil content increased with increasing frying temperature and time. However, there was no apparent change in Hunter ΔE with time when the frying temperature was below 100 °C and the frying time was below 25 min. Results of this study suggest that vacuum frying at moderate temperature (90–100 °C) for 20 min can produce carrot chips with lower moisture and oil contents as well as good colour and crispy texture. Copyright © 2005 Society of Chemical Industry     

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.     

Characterization of microwave vacuum-dried durian chips

Durian CV. Monthong was subjected to microwave vacuum drying (at 13.33 kPa) to produce durian chips. Various levels of microwave power (3.88 W g⁻¹, 5.49 W g⁻¹ and 7.23 W g⁻¹) were used. Prior to the microwave vacuum drying, the sliced durian was either chilled at 4 °C or frozen at −18 °C. Both pretreatments yielded non-significant difference in dissipation factor (p > 0.05). Among several thin layer models, the Page model was found to be the best for explaining the drying characteristics of durian chips. An increase in the microwave power intensity produced a clear increase in the drying rate and did not affect lightness and yellowness of the durian chips (p > 0.05). The structure and hardness of the dried durian chips were comparable to that of conventionally fried durian chips. In addition, microwave vacuum drying reduced the fat content of the durian chips by at least 90%, compared with conventionally deep fried durian chips.     

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.     

真空微波加工马铃薯脆片的工艺特性

研究了真空度、微波功率、冷冻处理以及不同初始含水量对真空微波加工马铃薯脆片的影响。马铃薯片在真空度 0 0 8MPa时有较好的脆度 ;真空微波处理时间相对于普通微波干燥短 ,而其疏松程度更大一些。冷冻处理可以得到表面平整 ,变形小 ,表面颜色均匀的脆片 ,其断裂力较未冷冻处理脆片小。对由热风干燥得到的不同初始含水量的薯片进行真空微波加工 ,在初始含水量 3 6%左右有较高的膨化率。     

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).     

Storage stability and characterization of biochemical,rehydration and colour characteristics of dehydrated sweet corn kernels

The degradation kinetics of dehydrated sweet corn for total sugar, ascorbic acid, TSS, pH, rehydration parameters, colour and moisture content using low density polyethylene pouches (LDPE) and laminated aluminium foil pouches (LAFP) with or without vacuum were studied. The samples were kept for 3 months and were analysed at every 15 days. During the storage, total sugar, ascorbic acid and total soluble solids reduced from 10.50 to 9.73 g/100 g, 9.36 to 8.86 mg/100 g, 83 to 51°Brix respectively. Rehydration coefficient and rehydration ratio reduced from 90.28 to 75.27, 2.64 to 1.56. Colour L1 value, a1 value. b1 value changed from 61.00 to 70.13, 42.93 to 53.27, 55.58 to 64.24 respectively resulting in increase in ΔE value and chroma value up to 16.29, 70.23 to 83.45 respectively. The moisture content increased from 2.12 to 3.92% in packages. The average sensory score of 7.86 even after 90 days of storage in laminated aluminium foil pouch with vacuum indicates that the product may be stored for 90 days satisfactorily. Zero and first order models fitted well and can be used for predicting various properties of dehydrated sweet corn during storage.     

Bioactive components,ABTS radical scavenging capacity and physical stability of orange,yellow and purple sweet potato (Ipomoea batatas) powder processed by convection‐ or vacuum‐drying methods

This study aims to investigate the antioxidant properties and physical stability of convection‐ or vacuum‐dried orange, yellow and purple sweet potato (Ipomoea batatas) powder upon room temperature storage at different relative humidity (RH) of 33%, 43%, 54% and 75% for 25 days. Both convection and vacuum‐dried samples had lower total phenolic content (TPC), anthocyanin content and antioxidant activity compared to the fresh samples. Vacuum‐dried powder retained higher antioxidant activity than convection‐dried powder. Storage of the powder at high RH of 75% resulted in apparent clump formation, which was likely attributed to the decrease of glass transition temperature (T_g). In general, purple sweet potato powder contained the highest TPC (255.0 mg GAE 100 g DW^?1) and antioxidant activity (1924.0 μmol TEAC 100 g DW^?1), while orange powder had the highest beta‐carotene content (127.2 mg 100 g DW^?1). Vacuum‐dried sweet potato powder, which has relatively higher antioxidant activity, could be a potential functional ingredient or natural colourant for the food industry.     

熟化甘薯片微波干燥特性及其动力学模型

为了探索熟化甘薯片微波干燥特性,提高熟化甘薯片干制品质及干燥效率,研究不同微波功率、装载量与切片厚度对于熟化甘薯片微波干燥特性及能耗的影响,对熟化甘薯片进行了微波干燥试验。结果表明:熟化甘薯片的微波干燥可分为加速、恒速和降速三个阶段。微波功率与加载量对熟化甘薯片的干燥影响较大,微波功率越大,装载量越小,熟化甘薯片的干燥速率越快,干燥时间越短。采用4种常见的薄层干燥模型对微波干燥过程进行拟合,结果表明Page模型是最适合描述熟化甘薯片微波干燥过程中水分变化规律的薄层干燥模型。在微波功率200~600 W,装载量200~400 g,切片厚度6~10 mm范围内,熟化甘薯片的微波干燥能耗为2.8235~5.6289 kJ/g。研究结果可为熟化甘薯片微波干燥工艺提供参考。     

Effects of vacuum level and microwave power on rosemary volatile composition during vacuum-microwave drying

The influence of the vacuum level and the microwave power on aroma compounds and sensory quality of Rosmarinus officinalis dehydrated by vacuum-microwave (VM) method was evaluated. VM drying kinetics consisted of two periods: linear until a critical point and exponential beyond that point. The time needed to dry rosemary was shorter for high values of microwave power and vacuum intensity. Volatile compounds of rosemary samples were extracted by steam-hydrodistillation and analyzed by GC-MS. Thirty-one compounds were identified, with α-pinene, verbenone and 1,8-cineole being the major components; monoterpenoids were the predominant chemical family. The total quantity of volatiles of fresh rosemary (27.2 g kg⁻¹) decreased during drying, independently of treatments, to a mean of 14.8 g kg⁻¹. Soft conditions of VM drying (low vacuum level and microwave power) are recommended to get the highest concentrations of volatiles and the best sensory quality (e.g. 72-74 kPa and 360 W).     

Physical and thermal properties of potato chips during vacuum frying

Vacuum frying (1.33 kPa), with the aid of a de-oiling mechanism, was used to produce low-fat potato chips.The kinetics of oil absorption and oil distribution in the potato chips was studied so that effectiveness of the de-oiling system could be established. Non-linear regression was used to fit the experimental data to the models used to describe oil absorption in potato chips with time.Moisture content, oil content, microstructure, diameter and thickness expansion, bulk density, true density, and porosity of chips fried at different temperatures (120, 130, and 140 °C) was performed to evaluate the effect of process temperature on the product. The convective heat transfer coefficient at the oil-chip interface was determined for the same temperature range.The final oil content of the potato chips was 0.072 ± 0.004, 0.062 ± 0.003, and 0.059 ± 0.003 g/g solid for frying temperatures of 120, 130, and 140 °C, respectively. These values are lower (80-87% less) than those found in the not de-oiled potato chips, which indicates that the de-oiling mechanism is crucial in vacuum frying processing. A significant difference (P < 0.05) was observed in oil content and oil distribution within temperatures. The rate of change in product quality attributes was greatly affected by temperature; however, the final values of moisture content, bulk density, true density, porosity, diameter shrinkage, and thickness expansion were not affected by temperature.During vacuum frying, the convective heat transfer coefficient changed considerably as frying progressed; moreover, it increased with temperature reaching a maximum between 2200 and 2650 W/m² K depending on frying temperature.