Freeze drying includes two processes: freezing and sublimation. The purpose of freezing is to give food a suitable shape and structure to facilitate the sublimation process. Sublimation is a process in which the moisture in food absorbs heat and sublimates into water vapor, which is removed through the condensation system.
Food freeze dryer
There are two freezing methods: self-freezing method and pre-freezing method.
The self-freezing method is a method that uses flash food in a vacuum to absorb the latent heat of evaporation, causing the temperature of the food to drop below the freezing point and self-freeze. If a high vacuum can be created quickly, a large amount of water will evaporate immediately and a large amount of heat will be absorbed, helping the food to complete the freezing process quickly. However, the self-freezing method often causes deformation or foaming of the food, so it is not suitable for foods with high requirements for form and shape. It is generally only used to freeze dry powder products.
Pre-freezing is a method of pre-freezing food into a certain shape. This method can better control the shape of the food and the state of the ice crystals, so it is suitable for freezing most foods.
After freezing, the food will be sublimated and dried in the drying chamber. When the ice crystals sublimate, they must absorb the sublimation heat. Therefore, there is a heating device in the freeze dryer chamber to provide this heat. The heating methods include plate heating, infrared heating and microwave heating.
The plate heating method is to place the pre-frozen food between two heating plates and carry out contact heat exchange to achieve drying. In order to enhance the heat exchange efficiency, there is usually a hydraulic actuator to press the heating plate tightly against the food. The temperature of the heating plate is maintained and regulated by the internal circulating hot steam or liquid medium to ensure that it not only meets the heat required for the sublimation of ice crystals in the food, but also prevents its temperature from rising to a level that may cause the temperature of the heating plate to rise. causing melting, usually at a temperature of 38 to 66°C. In addition, the heating plate is in close contact with the food, although it can accelerate the heat transfer process, the escape of water vapor after the sublimation of ice crystals is hindered. but also causes the internal pressure of the food to increase, even exceeding the triple point pressure, causing the ice crystals to melt. Therefore, an expandable metal mesh is often placed between the heating plate and the food to keep the way for the water vapor to escape. This can not only improve the heat transfer efficiency but also speed up the removal of water vapor, thereby accelerating the freeze-drying process. When using the plate heating method, the foam layer filled with non-condensable gases such as water vapor and air will gradually grow due to the continuous sublimation of ice crystals, which will create an increasingly high resistance to heat transfer and evaporation, causing it to dry. the main limiting factor in speed. This defect can be overcome by using infrared heating and microwave heating. These two heating methods are often used in combination with plate heating to perform drying in the middle and late stages of freeze drying. This can not only overcome the heat transfer resistance between the foam layers, increase the speed of freeze drying, but also reduce the cost. freeze drying.
