Foods typically have a moisture content of 30% to 97%, and the application of microwave drying or microwave-enhanced drying of food has broad application prospects.
(1) Microwave rapid drying of food: For example, using microwave heating to dry vegetables to a moisture content below 20% ("dried vegetables") can be more than ten times more efficient than traditional methods. This method is not only suitable for drying vegetables but also more suitable for drying seaweed-based foods. Under a microscope, the tissue of microwave-dried vegetables is found to be closer to that of fresh vegetables than any other method except freeze-drying, but the drying time is shorter than freeze-drying. Microwaves can also be used for the final drying of fried products, saving oil and resulting in light, flavorful foods with low oil content.
Hot air drying of dough takes 5-8 hours to avoid crusting or cracking, while microwave drying only takes 1-2 hours to dry from the initial moisture content to 20%, with hot air removing the moisture, saving 25% energy. Microwave drying can also reduce bacterial contamination; for example, in the combined hot air and microwave drying of garlic, hot air dries the moisture from 80% to 10% (wet basis), and then microwave drying reduces it to 5%, resulting in a 90% reduction in total bacteria and a 30% energy saving.
Under vacuum conditions, microwaves can process materials that would spoil or degrade at temperatures above 40°C (sometimes even at 15°C). Vacuum drying ovens only use heat conduction, usually without convective heating, resulting in very long drying times. Products dried using microwave vacuum drying include fruit concentrates, tea powder, enzymes, and vegetables (mushrooms, garlic, soybeans, etc.). The operating cost is between spray drying and freeze-drying. In these systems, the material is usually a paste-like substance, spread on a conveyor belt, and passed through a specially manufactured tunnel (vacuum degree of 133.3-2666 Pa), forming a foamy substance with good rehydration properties.