Apple volatiles monitoring and control in microwave drying

Apple volatiles were online monitored with a zNose™ during microwave drying. The samples were first dried at three fixed temperatures of 60 °C, 70 °C, and 80 °C. Four typical volatile peaks were recorded every 4 min for all the three replicates. Apple samples could be dried in a shorter time at high temperatures but were more susceptible to burning. A fuzzy logic controller was then designed and employed to control the drying process. The controller used online detected apple volatiles and burning signals as variables and tried to retain apple volatiles, avoid burning, shorten drying time, and save energy. Based on the temperature curve achieved under fuzzy logic control, a simple linear control method was further developed where zNose™ online assistance could be omitted. Color and sensory evaluation by an untrained taste panel were performed after rehydration. With the new control strategies, apple color and overall appearance were intact and less time and energy were consumed.     

Optimal power control strategies in microwave drying

A microwave drying system, which can automatically and continuously adjust microwave power, control sample temperature and measure sample mass, was developed and used in apple drying. At three drying temperatures, i.e., 75, 65, 55 °C, applied microwave power was recorded and analyzed, and a relationship of the power with respect to drying rates was built. With this relationship, apples were further dried at the same three temperatures but using variable power during drying process based on the online obtained drying rates. A better temperature control was achieved and less energy was consumed with this method. Depending on the resulted power profiles, a linear and a three-step power control methods were further developed to simplify the control system. With the developed methods, mass measurements were no longer required and power control could be easily implemented, while temperature control and energy consumption were still acceptable.     

Temperature and power control in microwave drying

A microwave drying system with the ability of automatic temperature and power control was developed for apple drying. The samples’ mass and the moisture content could be obtained online. Four different drying modes were attempted with different combination of temperature and power controls. Feedback temperature control with predefined variable power profiles resulted in the best temperature control and product quality. All other modes proved their own pros and cons. The study also verified that it was difficult to maintain a constant temperature during microwave drying process when a feedback temperature control was not included. The results can be used to determine microwave power levels in microwave drying practices both with and without temperature control.     

Effects of microwave and infrared drying on the quality of carrot and garlic

The effects of microwave and infrared drying on the quality of carrot and garlic were studied and compared with the effects of conventional hot air (tray drier for carrot and fluid bed drier for garlic) drying. The quality of carrot and garlic were evaluated by instrumental and sensory analysis. Rehydration, moisture content, water activity, particle density, bulk density, porosity and colour values were obtained for microwave, infrared and hot-air dried vegetables. In addition, total moisture content versus time was represented by drying rate curves of carrot and garlic samples. Finally, free moisture content versus drying rate were compared for the three different drying methods.     

Drying rate control in the middle stage of microwave drying

Microwave drying of apples at constant temperatures follows typical drying curves. The middle stage of the drying process shows a faster drying rate and accelerated moisture evaporation. Meanwhile, more flavors are lost, surface color is degraded, and charring often occurs in this stage. To improve the drying effects, drying curves were controlled and changed in this study. The drying curve was linearized by automatically varying drying temperatures in the middle stages. The controlled drying process therefore led to an optimized temperature profile. To simplify the drying methods, apples were further dried with the obtained temperature profile, while drying curves were online monitored but not controlled. It was proved that slowing down the drying rate in the middle stage could improve the product quality in terms of color, flavor, and overall appearance, while the drying time and energy consumption were still acceptable (within 200 min and 22 kJ/g, respectively).     

Infrared thermography assisted control for apples microwave drying

A microwave based system was developed for drying of apple slices while on-line controlling their temperatures. An infrared thermography assisted control system allowed to realize temperature control by detecting the instantaneous maximum temperature among the samples under test instead of a chosen slice. The effectiveness of the system at hand in realizing temperature control both in space and in time was proven for three temperatures: 55, 65 and 75 °C. Temperature fluctuations became larger both with time increasing and with rising temperature levels thus causing a negative impact on dried fruit quality. In fact apples slices dried at 75 °C by microwave showed L^∗ (lightness) and white index (WI) values significantly lower (p < 0.05), when compared to the fresh samples and to the apples dried by hot air. To overcome such behaviour a mode stirrer was introduced resulting in halving the temperature spatial oscillations of sample slices.     

Far-infrared and microwave drying of peach

Little detailed information is available for the far-infrared and microwave drying characteristics on peach and far-infrared combined with microwave drying on other food products. Experiments were conducted to study microwave and far-infrared dehydration characteristics and two-stage drying process involving far-infrared following microwave drying on peach. As microwave drying power and infrared drying power increased, dehydration rate of peach increased and whole drying energy consumption decreased. Peach experienced two falling rate periods when dried with microwave drying or far-infrared drying, and the first falling rate period under moisture content of peach more than 1.7 (dry basic, d. b.), the second falling rate period under less than moisture content 1.7 (d. b.). The same water loss will consume more energy and the steeper curve of energy versus moisture content were obtained when the moisture content is less than 1.7 (d. b.). However, differed from microwave drying, an accelerating dehydration rate period existed in the initial period of far-infrared drying. The effects of infrared drying power, microwave drying power and exchanging moisture content at former far-infrared drying converting into latter microwave drying (three factors) on energy consumption rate and sensory quality (two indices) are significant. The interaction effect of infrared drying power and exchanging moisture content on two indices is significant. The effects of second-order of microwave drying power and of interaction between infrared drying power and microwave drying power on energy consumption rate were not significant. The effects of second-order of exchanging moisture content and of interaction between exchanging moisture content and microwave drying power on sensory quality were not significant.     

Combined convective and microwave assisted drying: Experiments and modeling

The drying process is largely used in many different industrial applications, such as treatment of foods, production of cosmetics and pharmaceuticals, manufacturing of paper, wood and building materials, polymers and so on.     

干制状态气体对胡萝卜干燥过程特性及制品品质的影响

分别以空气及氮气、二氧化碳气体置换部分空气等3种状态的气体作为干燥介质，采用QTM干制实验设备对胡萝卜粒进行对比干燥试验，探讨干制状态气体对胡萝卜粒的干燥过程特性及制品品质的影响。结果表明，干制状态气体与胡萝卜粒的干燥过程特性及制品的品质关系密切。降低干燥室内气氛了体的氧气含量，可维持较佳的糖酸比及显著提高Vc保存率，制品的质量较好；在气体的氧气含量相同状态下，充人氮气置换部分空气的制品质量最好，且具有较高的干燥速度和复水比。     

均匀平铺载荷下微波干燥室内的能量分布

对均匀平铺的典型载荷条件下微波干燥室的能量分布特性进行了研究。根据实验数据分析可知当具有较大的介电常数的物料均匀平铺在干燥床上时 ,微波干燥室内的能量分布可根据天线的辐射能分布瓣型来确定。理论上 ,在微波干燥床上的XZ面内的θ=± 2 4°50′50″范围内及YZ面内的θ =± 36°2′40″范围内的微波功率密度大于该面内最大微波功率密度的二分之一     

Power density control in microwave assisted air drying to improve quality of food

Currently, the new trend is the development of microwave hot air drying control in order to solve the problem of hot spots on the product. So in this study, a microwave/hot air drying system with the ability of automatic power density control was developed for tomato drying. Three power densities (3, 5 and 7 W/g) at 40 °C air temperature were applied and the output, reflected microwave power and the mass of the sample were registered online. Two different drying strategies were implemented. The first consisted of a microwave drying without specific power control. The second one introduced power control. A feedback control based on specific power related to the actual sample mass resulted in the best power control and the improvement of product quality compared to the dry sample without power control.     

微波干燥技术在果蔬干制中的应用

系统地介绍了果蔬干制的工艺流程、操作方法及在加工过程中的注意事项。以一些具体的果蔬为例,介绍了微波干燥在果蔬干制中的具体应用,通过与其他干制方法进行对比,来比较微波干燥在果蔬加工方面的优劣性,并在此基础上对今后微波干制技术做了预测,提出了一些建议。     

Comparison of volatiles of banana powder dehydrated by vacuum belt drying,freeze-drying and air-drying

Banana puree was dehydrated by using three different drying methods: vacuum belt drying (VBD), freeze-drying (FD) and air-drying (AD) to produce banana powder. Volatiles were extracted from dried banana powder with solid phase micro-extraction (SPME), and separated and identified by gas chromatography–mass spectrometry (GC–MS). SAS (statistical analysis system) was used to evaluate the contribution of the main volatiles. Esters play the most important role in banana powder aroma. The main components detected in banana powder which are responsible for its fruity odor were 3-methylbutanoic acid 3-methylbutyl ester, 3-methylbutyl acetate and butanoic acid 3-methylbutyl ester. Most of the alcohols identified in banana powder were enols and some were of the long chain type. Eugenol and elemicin which give the product its typical mellow aromas were also identified. Alkyls, alkene and alkyne constituted the minor components in banana powder. Basing on the principal component analysis of statistical analysis system, it can be inferred that the preferred method for producing banana powder with the optimum aroma is FD, followed by VBD and then AD.     

Effect of microwave drying on the phytochemical composition of volatiles of ginger

The influence of drying methods on the volatile components of Zingiber officinale rhizomes was studied. The drying methods employed were convection (CD) and microwave (MW) drying. The drying time reduced significantly from 720 min in case of CD to 25–48 min in MW drying depending on the microwave power level (PL 100, 80, 60 and 40 correspond to 800, 660, 525 and 385W, respectively). The MW‐dried (PL100, 800W) sample retained the maximum volatile fraction (3% v/w). Seventy‐four constituents accounting for more than 99% of the essential oil from fresh ginger were identified. The concentration of major compound (zingiberene), in essential oils, was marginally increased (7%) in MW dried (PL100), whereas it was decreased not only in CD (43%) but also at other MW PLs employed in drying (25–49%). Hence, it could be concluded that MW substantially reduced the drying time and the sample dried at PL 100 retained maximum quantity of volatiles.     

Production of bread crumbs by infrared-assisted microwave drying

This study is aimed to investigate the possibility of using infrared-assisted microwave drying for production of bread crumbs and to determine drying conditions in halogen lamp-microwave combination oven to produce bread crumbs with the highest quality. Bread crumb dough were dried from about 40.9 to 8% moisture content by conventional, microwave, infrared and infrared-assisted microwave drying separately and in combinations. Halogen lamp and microwave power levels of 30, 50 and 70% were used. Percent reduction in the drying time were 80.2–94.0 for infrared, 96.5–98.6 for microwave and 96.8–98.6 for infrared-assisted microwave drying. ΔE values were lower in microwave drying and higher in infrared drying. Infrared-assisted microwave dried crumbs generally had similar color values with the conventionally dried ones. Effect of power on color change was not observed. All drying methods were found to be effective in increasing water binding capacity, which increased with microwave power in microwave drying but the influence of halogen lamp power was not seen in infrared drying. At fixed microwave (halogen lamp) powers increase of halogen lamp (microwave) power did not affect the water binding capacity in infrared-assisted microwave drying.     

Volatile components of white Hypsizygus marmoreus detected by electronic nose and HS-SPME-GC-MS: Influence of four drying methods

Four drying methods, viz hot air drying (HD), vacuum drying (VD), microwave-vacuum drying (MVD), and vacuum freeze-drying (FD), were used to dry white Hypsizygus marmoreus (WHM). The volatile and taste components of the dried WHM were comparatively analysed by electronic nose technology (E-nose) and head-space solid-phase microextraction combined with gas chromatography-mass spectrometry (HS-SPME-GS-MS). Principal component analysis (PCA) showed that E-nose could distinguish fresh WHM and four dried products clearly. Dominant volatile components in fresh WHM were found to be 1-octen-3-ol, 3-octanone, and (E)-2-nonenal. However, it was found that the drying methods caused damage to the 8-carbon compounds (C8), especially to 8-carbon alcohol greatly. For instance, HD produced more ester materials, MVD yielded more aldehyde compounds, FD brought more hydrocarbon products, and VD produced more ketone substances.     

Effect of food ingredient on microwave freeze drying of instant vegetable soup

The objective of this work was to study the effect of three different food ingredients viz. NaCl content (1-15 g/100 g water), sucrose content (5-25 g/100 g water) and sodium glutamate content (0.5-10 g/100 g water) on microwave freeze drying (MFD) of instant vegetable soup. Response surface methodology (RSM) was employed to optimize their addition levels in instant vegetable soup for MFD. Finally, a comparison of MFD for instant vegetable soup with and without added ingredients was carried out. The results showed that NaCl content and sucrose content had significant influence on drying time and sensory quality, while sodium glutamate content had insignificant effect. The optimum ingredients of the instant vegetable soup for MFD were: NaCl content of 3.2-5.3 g/100 g water, sucrose content of 2-6.8 g/100 g water and sodium glutamate content of below 4.5 g/100 g water. Comparing with MFD vegetable soup without ingredients, condiments addition could increase drying rate and shorten drying time.     

Microwave drying characteristics of potato and the effect of different microwave powers on the dried quality of potato

Little detailed information is available on the microwave drying characteristics of potato and the use of different microwave powers to dry food products. Experiments were conducted to study the microwave drying characteristics and the dried quality of potato. The study focuses on describing the microwave drying characteristics of potato and discussing the effect of sample thickness, drying power and mass. The results show that if the power level increases, the mass load decreases and the thickness of the sample decreases, the dehydration rate increases and the drying energy consumption decreases. There are two falling rate periods during microwave drying of potato: the first falling rate period is for a moisture content of more then 1.1 (dry basis, DB); the second falling rate period is for a moisture content of less than 1.1 (DB). The same water loss will consume more energy when the moisture content is less than 1.1 (DB). A two-stage drying process was employed during microwave drying of potato. The microwave power of the first drying stage differed form that of the second drying stage. The moisture content of the breakpoint for the conversion of the first drying stage into the second drying stage is a moisture content of 1.1 (DB). A quadratic orthogonal regression experiment was conducted, and the effects of slice thickness, the first drying load power and the second drying load power on sensory quality, the rehydration ratio and the energy consumption rate were established. The slice thickness, the first drying load power and the second drying load power linearly affected the three indices. The effect of the product of the second drying load power and the potato slice thickness on the rehydration ratio and the energy consumption rate is significant. The optimum drying parameter combination for the three indices was obtained. The rehydration ratio of the dried products decreased with an increase of the second drying load power and the slice thickness. The concave curves of the sensory quality versus the three factors were shown.     

Shrinkage, vitamin C degradation and aroma losses during infra-red drying of apple slices

The goal of this study was to determine the effect of drying temperature on the shrinkage, ascorbic acid (vitamin C) degradation and aroma retention of apples. Apple samples were found to shrink continuously until a water content value of 2 kg/kg d.b. The degradation of ascorbic acid followed a pseudo-first-order reaction and the degradation rate constant increased when temperature increased from 40 to 70 °C. The loss of aroma volatiles increased with temperature and drying time.     

Characterisation of osmotically dehydrated papaya with further hot air drying and microwave vacuum drying

Papaya was subjected to osmotic dehydration (OD) prior to hot air drying (HA; 70 °C) and microwave vacuum drying (MVD; 3.75 W g^?1 and 13.3 kPa). An increase in immersion time in a sucrose solution [65/100 g (w/w)] at 40 ± 2 °C from 0 to 4 h decreased moisture content from 7.5802 to 1.2215 kg water kg dry solid^?1. During HA and MVD, effective moisture diffusivity was in the range 7.09 × 10^?8 to 9.13 × 10^?8 m² s^?1 and 2.85 × 10^?6 to 3.50 × 10^?6 m² s^?1, respectively, depending on immersion time. The hue angle of HA samples was 0.83–0.91, whereas that of MVD samples was 48.55–50.32. Both drying methods decreased springiness. Porosity was clearly observed in MVD samples. Rehydration rate was enhanced by increased immersion time and MVD (P ≤ 0.05). From preference mapping, MVD samples were more preferable than HA samples. Moreover, MVD samples with 1–3 h OD were more preferable than those with 4 h OD.