Freeze drying machine is composed of refrigeration system, vacuum system, heating system, electrical instrument control system.The main components of freeze dryer are drying box, condenser, freezer unit, vacuum pump, heating/cooling device, etc.
Operation points of food lyophilizer:
Generally, pretreatment refers to all treatments before sublimation drying, so freezing before drying also belongs to pretreatment.The raw material pretreatment is the same as the routine fruit and vegetable drying and quick-frozen products. If it is necessary to select, clean, peel, slice, blanch and cool, please refer to the relevant content of this book for specific operation, which will not be detailed here.When freeze-drying fruit or vegetable juices, they are concentrated in advance using a less expensive processing method and then grainy during pre-freezing.
Pre-freezing is to freeze the raw materials after the pretreatment, it is an important process of freeze-drying.As a series of complex biochemical and physicochemical changes occur in the process of freezing, the quality of pre-freezing will directly affect the quality of freeze-dried fruits and vegetables.The effect of freezing rate of frozen material on its quality and drying time is considered in the freezing process.The differences between quick freezing and slow freezing are as follows: The ice crystal produced by quick freezing is smaller, while the ice crystal produced by slow freezing is larger;Large ice crystals are good for sublimation, while small ones are bad for sublimation.The smaller the ice crystal is, the more it can reflect the original structure and properties of the product after drying.But the freezing rate is high, so is the energy consumption.It is necessary to choose a good freezing rate to ensure the quality of freeze-dried food and the lowest freezing energy consumption.
2. Sublimation drying
It is the core process in the freeze-dried food production process, and the process conditions should be controlled well:
During drying, the wet load of the machine is the quality of being dried on the drying board per unit area, which is an important factor to determine the drying time.The thickness of the dried food is also a factor affecting the drying time.
In freeze-drying, the material is dried from the outer layer to the inner layer. Therefore, when the material is thicker, a longer drying time is required.In the actual drying, the dried materials are cut into a uniform thickness of 15 ~ 30mm.The amount of materials to be loaded by the drying board per unit area shall be determined according to the heating mode and the types of dried food.If the drying period is 6 ~ 8h, the load of drying plate material is 5 ~ 15kg/m2.
In order to shorten the drying time, the heat needed by sublimation of ice crystals must be supplied effectively during freeze-drying, so various practical heating methods are designed.The drying temperature must be controlled in such a way as not to cause melting of ice crystals in the dried material and not to cause thermal denaturation due to overheating of the dried part.Therefore, in a single heating mode, the temperature of the drying plate should be controlled at 70 ~ 80℃ in the initial stage of sublimation vigorous drying, 60℃ in the middle stage of drying, and 40 ~ 50℃ in the later stage of drying.
(3) Judgment of drying end point
The drying end point can be determined by the following indicators: the temperature of the material and the heating plate are basically consistent and maintained for a period of time;Pump set (or cold trap) vacuum gauge and drying chamber vacuum gauge tend to be consistent, and keep for a period of time;The temperature of the vacuum trap in the drying chamber basically returns to the index of no-load equipment and maintains for a period of time.The lyophilizer with large butterfly valve can close the large butterfly valve, the vacuum machine basically does not drop or drop very little.Above 4 judgement basis, can use alone, also can combine or use jointly.
Post-processing includes unloading, semi-finished product selection, packaging and other processes.
After freeze-drying, nitrogen or dry air is injected into the drying room to break the vacuum, and then the material is immediately removed in a sealed environment with a relative humidity of less than 50%, a temperature of 22 ~ 25℃, and less dust, and the semi-finished products are selected and packaged in the same environment.Since the lyophilized material has a large surface area and is highly hygroscopic, these processes need to be completed in a relatively dry environment.
4. Packaging and storage
After freeze-dried food has a huge surface area, some ingredients in the food are directly exposed to the air, easy to come into contact with the oxygen in the air and adsorbed water, resulting in the gradual deterioration of freeze-dried food.Secondly, most of the freeze-dried food has a natural color, these natural pigments are easy to be resolved in the light.Temperature is also an important factor affecting such chemical reactions during oxidation and pigment degradation.Therefore, the packaging of freeze-dried food mainly considers how to prevent or reduce the impact of the above four factors.