The three key points of onion hot air drying are: controlling the drying temperature to avoid browning and hardening, doing well in slicing and pretreatment, controlling the final moisture content, and sealing and moisture-proof in a timely manner.
1. Temperature should be controlled in sections to avoid being too high
Onion hot air drying should use medium and low temperatures. Onion is a material that is prone to browning, loss of flavor, and rehydration. If the drying temperature is too high, the surface will quickly harden, the color will turn yellow and brown, and the flavor will also deteriorate.
A more prudent approach is to:
In the early stage, it can be controlled at around 55-65 ℃, and the surface free water should be drained first;
Reduce to 50-55 ℃ in the middle and later stages to avoid surface coking;
It is generally not recommended to exceed 70 ℃ for a long time, otherwise browning and flavor loss will significantly worsen.
If the equipment has a dehumidification and circulating air system, it should be used in conjunction to avoid "high temperature, high humidity, and stuffiness".
2. Slices should be evenly sliced, and color protection pre-treatment should be performed if necessary
The thickness of onion slices directly affects the drying speed and consistency of the finished product. The film is too thick, making it difficult to drain the internal moisture; The film is too thin and is prone to over drying, discoloration, or breakage. It is generally recommended to cut into relatively uniform thin slices or cubes, lay them on a single-layer tray, and not stack them.
If there is a high requirement for color, pre-treatment can be considered:
Short term blanching: quickly treating with boiling water and then cooling can inhibit enzyme activity and reduce browning;
Color protection treatment: Food grade potassium metabisulfite, sulfite or saline solution can be used for short-term soaking, but it must be strictly used according to GB 2760. The residual sulfur dioxide content in dried vegetables should usually not exceed 0.2g/kg.
If sulfur treatment is not used, more attention should be paid to low-temperature drying, rapid dehumidification, and light shielded packaging.
3. The final moisture content should meet the standard and be sealed as soon as possible after cooling
The moisture content of dried onion products is generally controlled at around 5% -7%, and can be further reduced to 3% -5% when storage requirements are high.
The judgment method is to break it by hand after cooling, and it should be brittle, not soft, and not bent. If it is still tough, it indicates that the internal moisture is too high, continue drying; If the color has noticeably darkened, it indicates that the temperature or time is too long.
After discharging, it should be cooled first and then sealed. Dried onions have strong moisture absorption properties. It is recommended to use aluminum foil bags, vacuum bags, or nitrogen filled packaging to avoid moisture regain, clumping, and loss of aroma.

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