Freezing outer layer product

Figure 2.3 illustrates a process in which the outer layer of a product is quickly frozen as a congealed crust. These CRUSToFREEZE plants have a capacity between 1500 and 5000 kg/h and require 0.5 to 0.8 kg LN2 pro kg of product, which has to be frozen totally on a conveyor belt. Figure 2.4 shows the product exit of the plant in Fig. 2.3. The freeze drying of coffee and tea extracts, fruit pulps or small pieces of meat require a multi- stage pretreatment. The granulated end product from coffee and tea extracts should have a defined grain size, a desired color, and a predetermined density. Fruit pulps should become granulated, with the appearance of fruit pieces, while meat pieces should not stick together like a small meat ball, but be recognized as single pieces when presented in a meal.

The kinetics of atmospheric freeze-drying can he investigated hy means of mathematical modeling, and the receding ice front model is perhaps the most common type of model used for this purpose. In this case, the product is considered to be divided into two layers, namely the frozen inner core and the dried outer layer. A receding interface is assumed to exist between the frozen and the dried layer. Subsequently, heat transferred from the surface of the product to the interface is used for sublimation of the ice, which diffuses from the interface to the surface and to the drying chamber at atmospheric pressure. In the case of a... 

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