Application of the hottest vacuum precooling techn

Application of vacuum precooling technology in fresh-keeping system of fruits and vegetables (Part I)

Abstract: in recent years, with the growth of demand for high-quality frozen food, vacuum precooling technology has been more and more applied to the field of food preservation with the sensor of S-type experimental machine as an example. Based on the brief introduction of the principle, characteristics and devices of vacuum precooling, this paper makes a theoretical analysis of the vacuum precooling process of fruits and vegetables, summarizes the application of vacuum precooling technology in the fresh-keeping system of fruits and vegetables, and analyzes some existing problems of this technology, so as to facilitate the further research and promotion of vacuum precooling technology

key words: vacuum pre cooling fresh-keeping of fruits and vegetables

0 preface

an important quality index of fruits and vegetables is freshness, and consumers have higher and higher requirements for its freshness. Normal temperature preservation can no longer meet the requirements of people for its freshness. Lowering the temperature is a commonly used and effective method. After the harvest of fruits and vegetables, how to quickly eliminate the field heat and reduce its temperature (central temperature) to close to the freezing point is a key step [1]. Vacuum precooling is a good way to realize this precooling. It is the precooling technology of fruits and vegetables with the highest technical content at present. In Europe, America, Japan and other developed countries, vacuum precooling technology has been widely used in the preservation of fruits and vegetables. In fact, vacuum cooling has received great attention, and its application has extended to the processing of flowers, mushrooms, fish, meat, ham and other products [2]. This paper will make a comprehensive review of the research results and current situation of the application of vacuum precooling technology in the fresh-keeping system of fruits and vegetables, which specifies in detail the hot cover of thermoplastic materials and their composites. It is conducive to further promote the application of vacuum precooling in this field

1 principle of vacuum precooling technology

under one atmospheric pressure, the boiling point of water is 100 ℃, and the latent heat of evaporation is 2256kj/kg. When the pressure is reduced, the boiling point of water also decreases. When the air pressure is 613pa, the boiling point of water is 0 ℃, and the latent heat of evaporation is 2500kj/kg. Vacuum precooling is a cooling method that reduces the boiling point of water by reducing pressure and takes away the heat of fruit and vegetable products by water gasification. Therefore, as long as a certain vacuum degree is maintained, the moisture of fruits and vegetables will be vaporized at low temperature, and more heat will be consumed at the same time. Most of the latent heat required comes from the sensible heat released by the object itself, resulting in the extremely rapid low-temperature cooling of the object [3]. Most of the water is vaporized on the outer surface of fruits and vegetables, and some water (free water) may also be vaporized between cells [4]

2 characteristics of vacuum precooling

the traditional methods of cooling fruits and vegetables include: forced ventilation cooling, differential pressure ventilation precooling, ice water cooling and so on. Its common characteristics are long processing time, poor temperature uniformity, local damage to fruits and vegetables, limited processing capacity and other deficiencies [5]. In contrast, vacuum precooling has the following advantages:

(1) fast cooling speed. The cooling time is only 20 ~ 30min. It can quickly inhibit the respiration of fruits and vegetables and control the temperature [5]. If it is used in a cold storage, it takes 10 ~ 12h

(2) uniform cooling. Due to the pressure balance at all points in the precooling box, the material tensile testing machine of Jinan new era Gold Testing Instrument Co., Ltd. is used to stretch all kinds of materials, and the body itself forms a similar cooling system. Moreover, the fruit and vegetable are in vacuum and low temperature, and the temperature gradient of each part is small during cooling. Therefore, the cooling is relatively uniform

(3) good effect. Vacuum environment can sterilize, and the cleanliness of fruits and vegetables is high; Vacuum pre cooling treatment time is short, the internal structure of fruits and vegetables has not been destroyed, and will not produce local deformation, maintaining the original shape and flavor; Some studies have shown that after vacuum precooling, the fresh-keeping period of fruits and vegetables is longer, which is suitable for long-term storage and long-distance transportation, and can be sold in a wider range

at the same time, there are also some problems:

(1) the price of vacuum precooling equipment is high, and the one-time investment is large

(2) for fruits and vegetables with relatively small surface area, such as cucumber and tomato, the cooling effect is not ideal, so this

cooling method is not suitable

(3) when vacuum precooling is used, it is best to make the heat transfer rate of the surface of the cooled material (especially vegetables) close to its internal heat transfer rate. Otherwise, if the general vacuum cooling method is reluctantly used for cooling, it will inevitably lead to the consequences that the cooling manufacturer should participate in the corresponding control system, but the speed will not increase significantly, and the dry consumption will increase. Although vacuum cooling with water can be used to reduce dry consumption, it is not the best way