Food processing freeze drying

Shelf Devices Equipment having heated and/or cooled shelves is available but is httle used for divided-sohds heat processing. Most extensive use of stationaiy shelves is freezing of packaged solids for food industries and for freeze drying by sublimation (see Sec. 22). 

The general rule for all freeze drying processes applies also to food The method of freezing, the freezing rate and the final temperature of freezing largely determine the quality of... 

The freeze drying of coffee extracts has been, and still is, the most frequent application of this process in the food industry. For economic reasons it is best to start with 40 % solid content in the extract. The final product is judged by the following criteria ... 

Donsi, G., Ferrari, G. and Di Matteo, P, Utilization of combined processes in freeze-drying of shrimps. Food Bioprod. Proc., 79 (2001) 152-159. 

The cited observations suggest that it is possible to identify potato cultivars with low or high phenolic acid content for human use and to select processing conditions that minimize losses of phenolic compounds. In summary, the methods we developed and used to determine the content and distribution of phenolic compounds in potato plant flowers, leaves, and tubers, in the peel and flesh parts of potato tubers, and in freeze-dried and processed commercial potatoes merit application in numerous studies designed to assess the role of potato phenolic compounds in host-plant resistance, plant breeding, plant molecular biology, food chemistry, nutrition, and medicine. The described wide distribution of phenolic compounds in different commercial... 

Preservation of cell structure, food taste, and avoidance of thermal degradation are reasons for the removal of moisture from such materials by sublimation. The process is preceded by quick freezing which forms small crystals and thus minimum damage to cell walls, and is likely to destroy bacteria. Some of the materials that are being freeze dried commercially are listed in Table 19.9(b). 

Typical lengths of cycles for food stuffs are 5-10 hr, for bacterial pellets 2-20 hr, and for biological fluids 20-50 hr. A production unit with capacity of 500 L may have 75 kW for refrigeration and 50 kW for heating. Conditions for the preparation of freeze dried coffee are preparation of an extract with 20-25% solids, freezing at —25—43°C, sublimation at approx. 200Torr to a final final moisture content of 1-3%, total batch processing time of 6-8 hrs. 

For many years, in connection with certain food products, a barrier to freeze-drying has been the problems associated with the storage stability of foods that are susceptible to lipid oxidation. In order for such foods to have a reasonable shelf life and acceptable flavor characteristics, protective additives, which retard oxidation, are often added before dehydration. Such antioxidants must carry through the process and not be lost because of volatilization. For these applications, BHA, BHT, and tert-butylhydroquinone (TBI1Q) have been found quite effective. 

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