Drying (hot air drying) is different from freeze-drying, as it mainly relies on higher temperatures (usually between 40 ℃ -80 ℃ or even higher) and flowing hot air to remove moisture from food. This high temperature and prolonged heat exposure make the following types of food unsuitable or unsuitable for conventional drying processes: 1 The drying process of food that is highly prone to oxidation and discoloration usually takes several hours or even longer, and food exposed to hot air for a long time is prone to oxidation reactions. For example, sliced apples, pears, potatoes, etc. will quickly undergo enzymatic browning during the drying process, resulting in a burnt black and unsightly color. Although it can be alleviated by soaking in citric acid or vitamin C solution in advance, if dried directly without treatment, the quality of the finished product will be extremely poor. two Food containing a large amount of volatile aromas and essential oils can accelerate the loss of volatile substances at high temperatures. Fresh herbs such as basil and mint, raw garlic, green onions, and some delicate tea leaves will lose their unique aroma after being dried in hot air, and even produce a peculiar smell similar to "cooked vegetables" or "hay", losing their original flavor value. three Food sugars with high sugar content and viscosity are prone to melting and undergo caramelization reaction at high temperatures. Fruits such as honey, jam, syrup, or those with high sugar content (such as ripe grapes and figs) not only have difficulty draining moisture during drying, but can also cause the surface to quickly crust and become sticky, even burnt, while the interior remains moist, making it highly susceptible to bacterial growth. four Fat rich foods are similar to freeze-drying, as fat does not evaporate during the drying process. High temperatures can accelerate the oxidation and rancidity of fats, leading to the production of a "halal flavor" in food. For example, fatty meat, pork belly, and nuts with high oil content not only have a greasy taste after drying, but also have a very short shelf life and are highly prone to spoilage. five Foods that are delicate, fragile, or have a waxy surface, such as ripe strawberries, peaches, etc., are easily deformed, cracked, or even turned into a paste during the shrinkage process of hot air drying. Foods with wax, such as whole blueberries, grapes, plums, etc., have a natural wax protective film on their skin, which can seriously hinder the evaporation of internal moisture. These types of food must undergo "blanching" (briefly blanching in boiling water) or alkaline treatment to break down the skin wax before drying, otherwise they cannot be dried at all. six High temperatures in foods containing thermosensitive nutrients can damage vitamin C, some B vitamins, and certain active enzymes in the food. If the purpose of drying is to preserve these nutrients to the greatest extent possible (such as certain traditional Chinese medicines or high-end health products), conventional hot air drying is not suitable. The most suitable foods for drying are low-fat, low sugar, structurally stable, and not easily oxidized foods, such as most root vegetables (radish, potato slices), lean meat, mushrooms, grains, etc. For the above unsuitable foods, it is usually necessary to use more moderate processes such as freeze drying, microwave drying or spray drying.

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