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Food is usually dried by heated air or vacuum contact dehydration methods during industrial processing. Products with low market values that are used in huge quantities as additives for convenient and half-processed food - like potatoes, carrots or parsley - are usually air or spray dried. Foods with a high value like broccoli or cauliflower, or fruits like strawberries, bananas or pineapples, or meat and plant extracts are vacuum or vacuum freeze dried. The freeze drying of frozen material in the low vacuum area around 1 mbar is linked with the highest investment and energy costs. However, excellent qualities of dried product are produced with regard to water uptake, taste, colour and shape.
At the end of the 1970s microwave vacuum dehydration was introduced, closing the gap between air drying and freeze drying. Products dried with microwave vacuum process distinguish themselves when compared to air-dried by an improved preservation of flavour, colour and shape in the case of fruits or of pungency in the case of onions. There is also a higher capacity for water uptake. In comparison to freeze-dried samples, a lower demand in energy and investment costs can be observed. Microwave vacuum dried fruits show a higher mechanical stability against rubbing down and breaking. In the case of juice concentrates of passion fruit, a higher preservation of volatile flavours than in the case of freeze-dried samples could be detected. Similar tasteful qualities to freeze-dried samples could be investigated for beef balls, dried by a microwave assisted vacuum convective dehydration process.
Figure 1 demonstrates the water uptake capacity of dried carrots. Samples dried under vacuum with microwaves show a faster and higher water uptake in comparison to air dried samples, whereas the rehydration quality of the freeze-dried product is still the best.
Table 1 shows the specific costs of investment and demands of energy of different drying processes. The higher values of the freeze drying process in comparison to all other processes can be clearly seen.
FUNDAMENTALS OF MICROWAVE VACUUM DEHYDRATION
Microwave vacuum driers usually operate in...