Onion vacuum freeze-drying is much more expensive than ordinary hot air drying, so pre-treatment, pre freezing, sublimation, and final water control need to be more precise. Following the self questioning and self answering process, it is suitable for conducting a production line process review.
Should raw materials be graded?
I need it. The differences in onion variety, age, and sugar content directly affect the eutectic point, drying time, and finished product flavor. Rotten, moldy, sprouted, hollow, and severely mechanically damaged items should be removed. If you need to process onion powder, seasoning blocks, or instant crisps in the future, it is recommended to grade them according to variety, particle size, and dry matter content.
2. Do I need to blanch after cutting?
Look at the product positioning.
Making powders, seasoning blocks, and rehydration materials: Short term blanching or heat treatment can be considered to inhibit enzyme activity, reduce browning, and improve rehydration stability. nine hundred and twelve
Make ready to eat crisps or emphasize freshness and spiciness: be cautious when blanching, as excessive blanching can lose the sulfide flavor.
After blanching, it must be quickly cooled and the surface free water drained.
How to control the cutting thickness?
Onion is usually diced, sliced, or shredded before freeze-drying. The thickness or particle size should be as uniform as possible, with a common experience of around 3-8mm, depending on the equipment loading and finished product form.
Excessive thickness will prolong the freeze-drying time; Over crushing is prone to clumping and flying, and the subsequent crushing and packaging losses will also increase.
Can pre freezing be omitted?
No. Pre freezing is a prerequisite for the success or failure of freeze-drying, and the center temperature of onions must be kept below their eutectic point, generally 5-10 ℃, to completely freeze the moisture.
Onions belong to the vegetable category, and the pre freezing temperature can usually refer to the range of -20 ℃ to -35 ℃, but the optimal value needs to be confirmed through small batch experiments or eutectic point determination.
Insufficient pre freezing can lead to foaming, collapse, and decreased rehydration after vacuuming.
How to choose between quick freezing and in box pre freezing?
Pre freezing inside the box: The equipment is simple and suitable for tray freeze-drying, but the cycle is long.
After quick freezing, enter the freeze dryer: it can shorten the pre freezing time, make the ice crystals finer, and have better rehydration properties, but it is necessary to control surface frosting and increase loading temperature. one
If a continuous production line is used, quick freezing can be used as a pre-treatment, but the final point of pre freezing must be confirmed to reach the center temperature standard, not just the warehouse temperature.
How to determine the thickness of the paving material?
It is recommended to lay the tray thinly during freeze-drying, usually controlled at around 10mm, and not too thick at most.
Excessive thickness can cause the outer layer to be dry while the inner layer still contains ice, resulting in increased sublimation resistance and significant differences in batch water content. The loading of mesh belts or pallets should be uniform, avoiding excessive thickness at the edges and corners.
What are you most afraid of during the sublimation stage?
I am most afraid of rapid heating, unstable vacuum, and thick plating.
Food grade onions are freeze-dried, and the vacuum degree during the sublimation stage is usually controlled at around 10-30 Pa. The shelf temperature slowly increases, and the material temperature is always lower than the disintegration temperature.
Once the surface crust forms, the internal moisture cannot be released, resulting in the finished product being "brittle on the outside and soft on the inside", slow rehydration, and increased energy consumption.
How to determine the end of drying analysis?
During the analysis stage, the temperature can be moderately raised to around 30-40 ℃, and high vacuum can be maintained to remove bound water
There are three points to consider when judging the endpoint:
The finished product is light, brittle, and fragile;
The moisture content meets the standard, and freeze-dried onion powder or chips usually need to be controlled to a lower moisture content;
The vacuum degree and cold trap temperature tend to stabilize, with no significant formation of new frost.

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