The technological process of raisin production is: raw materials → deslagging → cleaning → drying → grading → packaging. [1]

Common problems in production

Backward drying method

Natural drying

raisins are almost naturally dried in China. Because the natural drying method is inefficient and time-consuming, drying promoters are widely used in China. The main chemical component of the drying promoter is potassium carbonate, which can improve the drying speed of grapes. However, after using the drying promoter, because the peel becomes thinner, it is easy to be mechanically damaged during stem removal, and it is easy to absorb water and deteriorate in a humid environment. In recent years, more and more new drying technologies have been developed and achieved good results: solar dryer can make full use of sunlight, shorten drying time and improve the sanitary quality of products, but the development of this method is seriously restricted by high production cost, large energy consumption and small production capacity; Hot air drying is widely used, simple and easy to operate, which can shorten the drying time and keep the color of seedless white grapes at 3℃. Microwave vacuum drying can remove the oxygen in the air, quickly reduce the water content of fruits and vegetables, control the activity of enzymes and reduce enzymatic browning; The gas jet impact technology does not add chemical reagents and additives, which is safe and hygienic and preserves the nutritional value of materials. [1]

Browning

Studies show that enzymatic browning is the main factor leading to browning of raisins. Squeezing, rubbing or bruising grape fruit particles during processing, as well as microbial pollution will accelerate the occurrence of enzymatic browning. The common anti-browning measures are: selecting fresh green raw materials, treating them with drying promoter, drying them thinly and evenly, and keeping the ambient temperature not too high. The raisins should be quickly cleaned and dehydrated, packed in vacuum or filled with inert gas, and stored and transported at low temperature (about 5℃).

1. Selection of raw materials: Dry grapes should choose varieties with thin skin, plump and soft flesh, beautiful appearance and high sugar content. Generally, "seedless white", "seedless dew" and "rose fragrance" and "milk" with seeds are used as raw materials. The fruit should be fully ripe and not too ripe.

2. Pruning: After harvesting, prune the fruit grains that are too small and damaged; if the fruit clusters are too large, cut them into several small clusters, and put a layer on the drying tray.

3. Alkali soaking treatment: In order to speed up drying and shorten the evaporation time of water, alkali treatment can be adopted. Soak in 1.5-4% sodium hydroxide solution for 1-5 minutes, and thin-skinned seeds can also be treated with .5% sodium carbonate or mixed solution of sodium carbonate and sodium hydroxide for 3-6 seconds. After the raw materials are treated with alkali, they are immediately rinsed in clean water. The drying time of fruit after alkali soaking can be shortened by 8 ~ 1 days. When drying white raisins, it takes 3 ~ 5 hours to fumigate sulfur.

4. Exposure: put the grapes into a drying tray and expose them to the sun for about 1 days. When a part of the surface is dry, you can turn it all over, put the wet ones on the surface and continue drying until two-thirds of the fruits are dry. When there is no grape juice oozing out by hand, you can fold the drying tray and dry it in the shade for a week. In sunny weather, the total drying time is about 2 ~ 25 days.

5. Softening: Stack the fruit skewers for 15-2 days, dry them evenly, and remove the fruit stalks at the same time, and get the finished product. 6. Packaging: raisins are packaged in cloth bags and wooden cases, with 5 kg or 25 kg in cloth bags and 25 kg in wooden cases.