Rapid freezing is one of the most important food preservation technologies. It can preserve the sense and nutrition of food to the greatest extent, and is conducive to storage and prolongation of shelf life without adding any preservatives. It meets the three major pursuits of green, convenient and healthy food, and gradually becomes one of the most popular food varieties. Quick-frozen food is a kind of low-temperature product which uses fresh raw materials and is processed properly and frozen rapidly to reach consumption point at low temperature of - 18 ~20%.

With the increasing demand for quick-frozen food, it is necessary to study the factors affecting its quality change. References used different freezing methods to treat the same raw materials, studied their quality and shelf life after freezing. The results showed that the quality of quick-frozen food depends on the freezing rate. The freezing rate affects the size and distribution of ice crystals, which is inversely proportional to the size of ice crystals. Studies such as Fennema have also confirmed that freezing rate affects food quality. Alhamdan et al. treated fresh food and its shelf life by liquid nitrogen cryogenic freezing, monomer rapid freezing and traditional slow freezing methods respectively. It was found that liquid nitrogen frozen food had the best quality in the same storage period. That is, the faster the freezing rate was, the faster the nucleation rate of ice crystals was than the growth rate. At this time, the size, quantity and distribution of crystals were small, and the damage to internal tissues and cells of food was small. The speed of freezing depends largely on the performance of freezing equipment. Therefore, the performance of the equipment will directly affect the quality of frozen food, and the initial investment and energy consumption of the quick-frozen equipment in the frozen food processing production line is the largest, so it is very important for the research, design and optimization of the quick-frozen equipment. According to the current situation of domestic quick-freezing equipment, the factors affecting the performance of quick-freezing equipment are summarized from three aspects: energy consumption, air distribution and freezing time. 622434849871289423.jpg

1. Current status of quick-freezing equipment

Background of research on quick-freezing equipment

In the 1920s, the development of quick-freezing equipment originated in the United States, marked by the first quick-freezing machine in the world. Up to now, there are more than 20 kinds of quick-freezing equipment in developed countries such as the United States, Japan and Western Europe, and the equipment specifications are complete, the production per unit time is large and the automation level is high. However, the industry of quick-freezing equipment in China started relatively late. In 1973, China's food factories first introduced Japan's quick-frozen equipment for the production and processing of quick-frozen food. Later, with the increasing demand for quick-frozen food, attention began to be paid to the research and development of domestic quick-frozen equipment. Since 1985, more than ten types of quick-freezing equipment with different specifications have been developed independently. Asia has a large population base and a large demand for quick-frozen food. At present, Asia has become the largest market of quick-frozen equipment in the world, accounting for 39% of the world. The industry of quick-frozen equipment has broad prospects for development. At present, the quick-freezing equipment developed in our country is still far from the similar equipment abroad in terms of performance indicators, structural design, cleanliness and convenience (such as circular chamfer and inclined surface in freezing zone, which is not easy to grow bacteria, and all aspects of the unit are in a fully visual range, which is conducive to observation, timely cleaning, etc.) and automatic control performance.

Classification of quick-freezing equipment

Quick-freezing equipment can be classified according to different angles, such as fast freezing (5-20cm/h), medium-speed freezing (1-5cm/h) and slow-speed freezing (0.1-1cm/h). The size, shape and distribution of ice crystals in food after freezing are different due to three kinds of freezing speed. According to the cooling medium, there are four types, namely, air circulation freezing, spray freezing, contact freezing and immersion freezing. Freezing and air freezing are widely used in tunnel, spiral and fluidized freezing devices.

There are many kinds of quick-frozen equipment, but the advantages and disadvantages of different quick-frozen equipment are different. The performance of quick-frozen equipment directly affects the quality of quick-frozen food. Therefore, reasonable selection of equipment is very important to ensure the quality of food. Luyun et al. treated the same batch of silver pomfret with liquid nitrogen quick-freezing, plate quick-freezing and freezing in freezer respectively. After storage under the same storage conditions for a period of time, it was found that the volatile basic nitrogen (TVB-N) and K values of silver pomfret were the smallest and the concentration of salt-soluble protein decreased the slowest, which indicated that liquid nitrogen freezing was the most effective method to keep the same batch of silver pomfret. Quality of silver pomfret. Boonsumrej et al. used blast and ultra-low temperature freezing methods to freeze tiger shrimp respectively, and the same thawing method was used. It was found that the tiger shrimp thawed at ultra-low temperature had less loss and good quality.

In conclusion, different quick-freezing equipment will have different quality for the same food after freezing and thawing, so we should choose the quick-freezing equipment reasonably according to the characteristics and requirements of frozen food. Among these commonly used freezing methods, direct contact and fluidized bed freezing equipment have requirements on the shape of food, and liquid nitrogen spray freezing equipment is not widely used for the sake of operation economy. Therefore, air quick-freezing equipment is still the most widely used equipment, especially tunnel and screw quick-freezing equipment.

2. Performance optimization of quick-freezing equipment

Factors Influencing Energy Consumption of Quick-freezing Equipment

In recent years, the annual output of frozen food in China has increased rapidly, accounting for one third of the total global frozen food. Quick-freezing equipment in China is mainly used in frozen aquatic products, meat and poultry, fruit and vegetable food. The energy consumption of quick-freezing equipment is higher than other types of freezing equipment with the same volume. Therefore, the energy saving of quick-freezing equipment needs special attention. The factors affecting the energy consumption of quick-freezing equipment will be summarized from compressor selection, evaporator heat transfer and other aspects.

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