How To Freeze Dry Flowers?
The flowers prepared by drying and color-preserving could have the same shape, color, and aroma as the original ones, and they will never wither. Dried flowers are very suitable for cafes with low light, hotels, and homes. Vacuum freeze-drying is a new method of drying flowers that has emerged in recent years. Using industrial freeze drying machine to process large fresh flower plants can basically maintain their characteristics. How to freeze dry flowers?
How to freeze dry flowers — overview
Floral freeze drying is divided into three processes: pre-freezing, sublimation drying, and vacuum drying. Because flowers are composed of stalks, leaves, stamens, and petals, the moisture content of each part is different. If the drying effect of each part of the plant is studied, their respective moisture contents should be measured first and their eutectic points should also be identified.
How to freeze dry flowers — pre-freezing
Pre-freezing means that before the plants are freeze-dried, the flowers are frozen into a certain shape in advance using a general freezing method as a separate operation. In floral freeze drying, the maximum temperature of material freezing greatly affects the quality of the target products and energy consumption. The pre-freezing temperature must be lower than the eutectic temperature, otherwise, it will cause blistering on the surface and shrinkage of the product. Under normal circumstances, the lowest temperature for freezing the product is 5 to 10°C lower than the eutectic temperature. But the freezing temperature can not be set too low, otherwise, there will be problems like high energy consumption.
In addition, floral freeze drying also has requirements on the pre-freezing rate. The sublimation rate is faster when slow pre-freezing, and slower when fast freezing. A slower cooling rate is preferred to ensure the quality of the product.
How to freeze dry flowers —sublimation drying
To maintain the continuous sublimation and drying, two basic conditions must be met during the process of foral freeze drying, namely the continuous supply of heat and the continuous elimination of vapor. The drying process is gradually drying from the surrounding to the inner center. The driving force for heat transfer in the drying process is the temperature difference between the heat source and the sublimation interface, while that for mass transfer is the vapor partial pressure difference between the sublimation interface and the cold trap. The larger the heat transfer rate, the faster the vapor partial pressure difference, and the faster the mass transfer rate.
In sublimation drying, the heating temperature should not exceed the eutectic temperature of the material. If the temperature is too high, the ice crystals will melt, which will affect the quality of the product. What’s more, a successful floral freeze drying also needs an appropriate heating method, which directly affects important parameters such as material temperature distribution, sublimation interface temperature, sublimation interface water vapor flux, and drying time.
How to freeze dry flowers — vacuum drying
Vacuum drying is the last procedure of floral freeze drying, which is to remove moisture existing in the form of bound water distributed in the matrix of the material after sublimation and drying. The temperature required for vacuum drying is much higher than the sublimation temperature.
High temperatures and low pressure are conducive to vacuum drying. For the materials that do not require much on this process, such as frozen tomato slices, of which 95% water can be eliminated during the sublimation process, their vacuum drying time is very short. At this time, the consideration for increasing the drying rate should be based on sublimation drying.