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Longan (Dimocarpus longan Lour.) is delicious and sweet. It has a unique flavor and is rich in various nutrients . It is very popular among consumers. Longan is rich in high temperature and high humidity season, and its respiratory metabolism is strong. When it is picked at room temperature for 3~4 days, browning, rot, deterioration and loss of its edible value will occur. Therefore, in addition to a small amount of fresh food, longan depends on dry preservation . Longan dry processing methods mainly include hot air drying , vacuum drying , vacuum freeze drying , meat microwave drying , and microwave vacuum drying . Among them, hot air drying time is long, nutrient component loss is large ; vacuum drying and freeze drying, although the dry product quality is good, but the former has low drying efficiency, the latter has high investment cost; microwave drying is efficient, but it is easy to local temperature High and there are phenomena such as charring and gelatinization .
The microwave vacuum drying technology is a combination of the microwave drying technology and the vacuum drying technology. The vacuum drying utilizes the principle that the boiling point of water decreases in a vacuum environment, so that the article can more easily reach the temperature at which the water evaporates, thereby accelerating the drying process. Microwave drying uses the principle of microwave electromagnetic field to change the polarity direction of water molecules. In addition, the micro-electromagnetic field can cause intense molecular motion inside the object to be dried, thereby causing the object to dissipate heat from the inside to the outside, changing the tradition of heat conduction from the outside to the inside, so that the dried object can be dried more quickly. Reduce drying time.
Microwave vacuum drying is a new type of food drying technology. Meat microwave drying machine combines the advantages of microwave and vacuum drying technology: vacuum pushes internal moisture to mass transfer, moisture absorbs microwave energy and rapidly vaporizes, and porous structure inside the material to prevent tissue collapse ; At the same time, vacuum can inhibit material oxidation , to maintain the original color of food . At present, the technology has been successfully applied to the processing of fruits and vegetables such as mushrooms , carrots, silver ears , and potatoes. Microwave drying machine is widely used in the food industry.
1 Advantages of microwave vacuum drying technology
1) Efficient and uniform heating. Microwave vacuum drying technology combines the advantages of vacuum drying technology and microwave drying technology, which not only reduces the evaporation temperature of dried articles, but also speeds up the drying of articles. Moreover, the heating method of the microwave from the inside to the outside also makes the article more uniform in heat and better in quality.
2) Dry products are safe and hygienic. In the conventional drying techniques of sun drying, hot air drying, charcoal fire baking, etc., the drying time required is relatively long, so that the dried food in this process tends to breed bacteria or face a large unsanitary factor. The microwave vacuum drying technology requires a very short time, which can effectively avoid the occurrence of unsanitary factors.
3) Environmental protection and energy conservation. Since the microwave vacuum drying technology does not generate harmful gases and harmful substances, it has obvious environmental characteristics compared to drying techniques such as baking. In addition, the required drying time is short, so it can also have significant savings in energy consumption.
2 Effect on polysaccharide content of longan meat
In the microwave vacuum drying technology, there are three variables of microwave power, vacuum degree, and loading amount. Therefore, when analyzing the influence of microwave vacuum drying technology on the quality characteristics of longan, these three aspects should be combined for analysis. According to relevant research and literature, in the microwave vacuum drying technology, when the microwave power reaches 3000W, the longan meat is characterized by browning, and the caramel loss is more serious. When it is higher than 3000W, the phenomenon of burnt black will occur, and below 3000W It will show a fading golden yellow color as the power is reduced, so proper microwave power is beneficial to the polysaccharide retention. The effect of vacuum on the polysaccharide content is proportional to the trend. The higher the degree of vacuum, the higher the polysaccharide content. The effect of loading on the polysaccharide content is the same as the microwave power, which tends to increase first and then decrease.
3 Effect on the color change of longan meat
The longan color change is still analyzed from the perspective of three variables. To a certain extent, the change in the color of the longan meat is inextricably linked to the degree of retention of the polysaccharide. When the microwave power is at a low value, the color is yellowish and tends to be blackened as the microwave power increases. Therefore, in order to make the longan meat golden, it is also necessary to control the appropriate microwave power. The increase in vacuum is conducive to the improvement of the color of the longan meat. The higher the vacuum, the more golden the color. As far as the loading is concerned, it has a certain difference with the change trend of polysaccharide content. With the increase of the loading amount, the color of the longan meat is fuller and does not show a trend of increasing first and then decreasing.
4 Effect on SOD activity content of longan meat
The SOD active substance in longan is the main component of longan anti-aging and anti-oxidation effect. Therefore, whether the content of SOD activity in longan meat is properly related to the quality of longan meat and the retention of related effects. The effects of various variables on the SOD activity content of longan meat in microwave vacuum drying technology were basically consistent with the effects on polysaccharide content. However, the SOD content decreases as the microwave power increases, and there is no fluctuating process. The effect of vacuum on the activity of SOD also has a positive effect. The higher the degree of vacuum, the higher the SOD activity content. In addition, the SOD activity content is increased first and then decreased due to the increase in loading.
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