Tea Powder Freeze-drying Machine

Tea powder freeze-drying machine

The principle of freeze-drying instant tea powder is: first, the concentrated tea that needs to be dried is completely frozen at low temperature, and then the ice crystal (water) is directly removed by sublimation under high vacuum state, so as to get freeze-dried tea powder.

Because the whole drying process is completed under the conditions of vacuum and low temperature, avoiding the effect of oxidation and high temperature on the biological active substances, the biological active substances in the tea powder are preserved to the maximum extent, and the dried tea powder is porous spongy structure, easily soluble in water, and can be quickly recovered. The quality requirements of freeze-dried instant tea are as follows: unchanged biological activity, uniform color and lustre, full shape, stable structure, fast dissolution and low residual moisture.

1. Pre-treatment

Pretreatment mainly includes extraction, purification, concentration and other processes, the purpose of which is to extract the active components of tea leaves and concentrate them to a certain volume after purification, so as to reduce the workload in the process of freeze-drying and save energy. The results of pretreatment have a decisive influence on the color and taste of freeze-dried instant tea, so enough attention must be paid to the pretreatment.

2. Prefreeze

The concentrated tea soup before pre-freezing is an aqueous solution containing tea polyphenols and their polymer mixtures. Its molecular weight ranges from several hundred to tens of thousands. Its solubility varies greatly with temperature, and it is easy to form post-cold muddiness. The solution has a strong viscosity and can adhere to the Teflon coated lyophilized tray and is difficult to break off. This property must be taken into account when prefreezing.

It can improve the freeze-drying rate, and the gap left by fine ice crystals after sublimation is small, so that the sublimation of the lower layer is blocked. The quick-freezing finished products have fine particles, uniform appearance, large specific surface area, good porous structure and fast dissolution rate, but the moisture absorption of the finished products is relatively strong.

There are two main pre-freezing methods of concentrated tea soup. One is to put it on the plate first, and then to cool and freeze it in the quick-frozen storage. The other is to use the rapid ice machine in small batches (each batch is about 1~2kg) to make thin ice about -30 degree , and then scrape into broken ice and then spread the dish. There are frozen spray granulation equipment abroad, in the process of spray blowing strong cold air, directly freeze the fog droplets, its advantage is very uniform particle size.

3. Conditions and speed of sublimation

In the freeze-drying process of instant tea, the temperature of the product and the cold trap are usually -25 degree and -30 degree , and the saturated vapor pressure of the ice at this temperature is 64 Pa and 38 Pa, respectively. Therefore, a certain pressure difference is formed between the sublimation surface and the condensation surface, which can be compared to the sublimation power. It will promote the sublimation of water vapor products, with a certain flow rate to reach the surface of the cold trap, thereby condensing into frost. The heat of condensation of ice is about 2.812kJ/g. If no heat is supplied during the sublimation process, the product has to reduce the internal energy to obtain the heat of condensation until the temperature is balanced with the cold trap temperature, and the sublimation stops. In order to maintain the temperature difference between the sublimation surface and the cold setting surface, sufficient heat must be provided to the product.

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