Drying mechanism of goji berry
1. The drying technology of goji berry
The purpose on drying goji berry is to store nutrition for a long time without destroying. By drying out some water, the concentration of soluble matters will be increased to an extent that can’t be used by microorganisms. Drying is a way to remove the moisture from fresh fruit of goji berry by the heat energy. The drying process is a complex combination of thermal, diffusion, biological and chemical phenomena.
2. The drying mechanism of goji berry
Humidity gradient and temperature gradient are the inner motivation of drying goji berry. While drying, the surface moisture of the fruit is more than the equilibrium moisture and evaporated, and the moisture inside spread to the outside. The higher the humidity gradient and temperature gradient are, the faster the water spread. The water diffusion will cease if the internal and external water content of the fruit achieve balance.
During the drying process, the moisture diffusion inside is slower than the evaporation on the surface, which will affect the drying speed. Therefore, controlling factors of drying reasonably can accelerate the internal moisture diffusion and drying. However, if the temperature of medium is changed, the water on the surface will be evaporated quickly and the hard shell will be produced, which will delay the speed of drying, in addition, because of the big difference between inside and outside moisture, it will bring some internal stress and even cause a cracking to the fruit.
3. Influencing factors of drying
During the drying, speed determines the quality of goji berry, the drying speed is related to the drying method, and will be influenced by heating methods, temperature, medium temperature, humidity, air velocity, fruit and state. 
Drying methods of goji berry
We have accumulated valuable experience and summed up a lot of drying methods in actual production and scientific research. Here are some drying methods of goji berry.
1. Aired-drying is a natural drying method with using ventilation and sunshine. Ningxia old farmers often collected and laid the fresh goji berry which is about 2em thick in an open ventilated place with fruit racks for drying. Generally, summer fruit can be dried for 4-5 days, but for autumn fruit, maybe 7-9 days. Fresh fruit should not be exposed to strong light for the first two days. As for this drying method, it is the most economical one but with long time and losses. The fruit is easy to be putrid and polluted by dust, flies and rats.
2. Hot air drying is a method of drying with drying machine, heating air by hot-blast stove and sending the hot air to the fresh fruits to heat up. There are some data suggested that goji berries should be first heated at 40-45℃ with wind speed of O.15m/s for about 6 hours, and then at 6o℃ with wind speed of 0．35m／s for 6 hours, finally at 65℃ with wind speed of 0.25m／s for about 20 hours. Above all, this method has the advantages of low cost, large processing capacity and easy operation, but will also cause the losses and low qualities.
3. The method of drying goji berry whose equipment is vacuum freeze dryer has been widely used in Ningxia, Tianjin and other places. This machine freezes immediately the water in the fruit, and then makes the ice sublimate to steam and remove in vacuum. While drying, fruit should be frozen at -30℃ at first with the pressure of 30Pa, and then be heated at the temperature no more than 70℃ for 20 hours, finally it could be stored hermetically. Goji berries dried by this method will have advantages of bright red color, less loss of nutrients, high drying quality, low moisture content (less than 3%),convenience of saving and high value-added rate. But the equipment is expensive.
