Food usually has a moisture content of 30% to 97%, and the use of microwave drying or microwave enhanced drying of food has broad application prospects.
(1) The microwave fast drying of food, such as using microwave heating to dry vegetables into "dried vegetables" with a moisture content below 20%, can be more than ten times more efficient than traditional methods. This method is not only suitable for drying vegetables, but also for drying seaweed food. Observing vegetables that have been microwave-dried under a microscope, it can be found that their tissue is second only to that of fresh vegetables, but their drying time is shorter than that of freeze-dried products. Microwave can also be used for final drying of fried products, which not only saves oil but also produces light and delicious food with low oil content.
Hot air drying of dough takes 5-8 hours to avoid crusting or cracking. Microwave drying only takes 1-2 hours, drying from the initial moisture content to 20%, and using hot air to remove moisture can save 25% energy. Microwave drying can also reduce bacterial contamination, such as using a combination of hot air and microwave to dry garlic. The hot air dries the moisture from 80% to 10% (wet basis), and then uses microwave drying to 5%, reducing the total number of bacteria by 90% and saving 30% energy.
Microwaves can handle materials that deteriorate or degrade when the temperature exceeds 40 ℃ (sometimes even 15 ℃) in a vacuum state. Vacuum drying boxes can only rely on heat conduction and usually do not have convective heating, resulting in long drying times. Microwave vacuum drying products include concentrated fruit juice, tea powder, enzymes, vegetables (mushrooms, garlic, soybeans, etc.), and the operating cost is between spray and freeze drying. In these systems, the material is usually a paste like material, laid on a conveyor belt, and passed through a specially manufactured tunnel (vacuum degree of 133.3-2666Pa) to form a foam like substance with good rehydration performance.
