Fresh fruits and vegetables are seasonal and difficult to store. Drying is one of the widely used techniques in fruit and vegetable preservation. With the rapid development of fruit and vegetable snack foods, the research on drying technology is also intensified. Relevant research at home and abroad mainly focuses on processing technology and its process parameters, drying kinetics, and the quality of dried fruits and vegetables. The use of low-energy processing technology can maximize the nutritional content of fresh fruits and vegetables in the shortest possible drying time. The retention of this, to obtain better sensory quality.

The freeze-drying process of the vacuum freeze dryer is carried out in a vacuum and low temperature environment, which will not damage heat-sensitive substances, and also inhibit the biological activity of enzymes and the growth and reproduction of microorganisms, so it can better retain the color, fragrance, and fragrance of fresh fruits and vegetables. Taste, shape and nutrient ingredients can prepare high-quality dried fruit and vegetable products. Therefore, freeze-dried fruits and vegetables can satisfy consumers' pursuit of nutrition, deliciousness, and health of fruits and vegetables. However, due to high cost, high energy consumption, and low yield, it is also one of the most expensive drying processes. In order to achieve large-scale industrial production, it is necessary to further optimize or adopt more effective combined drying technology to meet the requirements of high quality and low energy consumption.

Studies have shown that in addition to drying conditions, dryer types and material characteristics that affect the quality of freeze-dried fruits and vegetables and drying energy consumption, pre-drying treatment and pre-freezing conditions also play an important role in the freeze-drying process. In addition, compared to a single drying technology, the freeze-combined drying technology, as a composite technology of staged drying, combines the advantages of freeze-drying and other drying methods to achieve complementarity, and has better advantages in drying quality, cost and energy consumption. It has received widespread attention from domestic and foreign scholars and industry, and has broad application prospects.

According to different drying mechanisms, drying is divided into hot air drying, microwave drying, vacuum drying, FD (freeze drying) and infrared drying. Although the production cost of hot air and microwave is not high, and the industrial application is extensive, they have serious nutritional loss and poor heating uniformity, which seriously affect the product quality; the other three drying methods have obvious advantages in product quality, but because of the heat transfer of vacuum drying Difficulties, high energy consumption in FD production, and limited infrared drying penetration capabilities are difficult to apply on a large scale in production practice.

The vacuum freeze dryer FD (freeze drying) is based on sublimation dehydration. In the low temperature environment required for drying, it can greatly reduce the loss of flavor and aromatic compounds, produce a porous structure, and reduce volume shrinkage; in addition, because most of the enzymatic reactions are stopped, the fresh fruits and vegetables are kept in the initial stage. State, the product has good quality.

However, due to operations such as maintaining a low temperature state, providing sublimation heat, and reducing the total pressure of the drying chamber, the vacuum freeze dryer FD (freeze drying) is a high-energy, high-cost production technology. Therefore, improving the entire drying process, improving drying efficiency, and reducing energy consumption have become a hot topic in many researches.

Let us briefly mention the mathematical modeling and optimization of the drying process of the vacuum freeze dryer.

Drying kinetics is usually used to describe the macro and micro mechanisms of heat and mass transfer in the drying process. It is affected by many factors such as drying methods, drying conditions and material characteristics. It has the following aspects in terms of improving equipment design, optimizing the drying process, and maintaining quality and reducing consumption. important meaning. Thin-layer drying is a widely used method to determine the drying kinetic model of fruits and vegetables. The relationship between the moisture ratio MR and the time t is often used to describe the dynamic changes in the drying process, using the coefficient of determination R2, chi-square χ2, mean square error RMSE, etc. Statistical indicators verify the accuracy of the model. It can be used as an important tool to estimate the drying curve, improve the drying process, and achieve low energy consumption requirements. However, in practical applications, there is no thin-layer drying model that can effectively summarize all the drying kinetics of fruits and vegetables. In terms of improving quality and reducing consumption, in addition to the establishment of drying kinetic models and changing drying parameters to optimize the drying process, current research mainly focuses on controlling pre-treatment methods, pre-freezing methods, and using combined drying technology.