| Atmospheric freeze drying is a new drying technology; many scholars at home and abroad have carried on the research report. Using freeze drying technology of fruits and vegetables can be dried and dehydrated products of high quality, but the drying time required length and high cost. In order to achieve more economic freeze drying and access to high-quality products, this paper introduces the atmospheric freeze drying technique to dry fruits and vegetables, and the effects of the loading scheme of different air temperature to the quality of the products and the drying process in the atmospheric freeze drying process. The results show that, according to the variation of the glass transition temperature of fruits and vegetables, the loading scheme the rise in atmospheric freeze drying process, the drying time can be reduced almost half and can obtain the lyophilized product is similar with that of ordinary freeze-drying product quality. In order to study the glass transition behavior of atmospheric freeze drying process, using DSC(differential scanning calorimeter) under different water content of glass transition temperature of apple slices was determined, and more accurately predict apple piece of glass transition temperature and the relationship between the moisture content. The main conclusions of the study are as follows:1. The glass transition temperature of the freeze drying strategy research based on atmospheric pressure. In the atmospheric freeze drying process, the inlet temperature has a significant effect on the product quality and drying efficiency. Low temperature can ensure good product quality, but the long drying time. In apple’s moisture content is reduced to 35%(w.b.) below, the glass transition temperature increased rapidly. Therefore, in the atmospheric freeze drying stage can be used in high temperature. In the atmospheric freeze drying process, under the premise of ensuring product quality, drying time drying strategy gradually warming can shorten the nearly half.2. Study on the relationship between the change of temperature and moisture content on the glass transition. Under the conditions of different water cut the glass transition temperature of apple slices were determined by using DSC, Gordon-Taylor equation can accurately predict apple piece of glass transition temperature and the relationship between the moisture content. In the activity of water and glass transition temperature of apple slices ring construction under the guidance of the state plan, according to the storage temperature conditions, can accurately select key storage water activity and moisture content. Study on the effect of the glass transition temperature of the freezing process, the glass transition temperature, the optimal guaranteed quality of apple slices.3. Study on atmospheric freeze drying garlic slices based on freezing point depression. Garlic is difficult to dry due to browning and degradation of heat-sensitive component. To achieve faster drying along with a high quality product, atmospheric freeze drying(AFD) were developed to dry garlic. The effect of different inlet air temperature programs on product quality and drying process was studied during AFD. It was found that freeze point depression could lead to part of water thawing resulting in product quality decrease when the air temperature was not low enough. Based on this, a step-down-up temperature program was developed to reduce the drying time and provide a similar good product quality. With the drying proceeding, the freeze point of garlic depressed gradually. Especially when the moisture content becomes low, the water in the samples is difficult to freeze. The inlet air temperature has significant effect on product quality and drying efficiency during AFD process. Fixed air temperature(-5℃,-10℃) can not ensure good product quality and high drying rate. Based on the freeze point depression, a step-down-up air temperature program is a better choice to obtain good product quality and low energy consumption. |
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