How to control slicing and spreading to avoid uneven dryness and wetness?
Mushrooms have high moisture content, soft tissue, uneven slice thickness, or excessively thick bedding, which increases resistance to hot air penetration and ice sublimation, making them prone to external dryness and internal wetness, slow rehydration, and darkening color.
Suggestion:
The slice thickness is generally controlled at 4-6mm, and thick mushrooms or whole mushroom pieces can be relaxed to 6-10mm;
The paving material should be single-layer, uniform, and non overlapping. The common paving amount is about 10kg/㎡, which can be adjusted according to the equipment capacity;
After cleaning, try to drain as much as possible to avoid water accumulation on the tray; Pre freeze as soon as possible after cutting to reduce browning;
Varieties that are prone to browning can be considered for short-term color protection with blanching or citric acid solution, but blanching may cause deformation and nutrient loss, which needs to be confirmed through small-scale testing.
How to set the freeze-drying curve so as not to collapse or regain moisture?
The two biggest concerns of mushroom freeze-drying are incomplete pre freezing and material temperature exceeding the eutectic point during the sublimation stage, which can cause ice crystal melting, tissue collapse, and decreased rehydration.
Reference control ideas:
Pre freezing: The center temperature generally needs to be lowered to below -25 ℃ to -35 ℃ and maintained for sufficient time to ensure complete freezing; The eutectic point of shiitake mushrooms is around -18 ℃ to -23 ℃, and different varieties need to be tested.
Sublimation drying: The vacuum degree is usually controlled at 10-60Pa, and the cold trap is kept below -60 ℃; The material temperature must be below the melting point, and the shelf should be slowly heated to avoid surface crust.
Analysis of drying: In the later stage, the temperature can be gradually raised to around 30-45 ℃ to remove bound water, and the moisture content of the finished product is usually controlled at 3% -5%.
In actual production, it is not enough to simply copy the temperature value. It is best to first measure the eutectic point/melting point of a small batch of materials, and then determine the pre freezing endpoint and sublimation upper limit.
How to control the discharge and packaging to maintain color and shelf life?
Freeze dried mushrooms are porous and highly hygroscopic. If exposed to a high humidity environment at room temperature after leaving the machine, they will quickly regain moisture, turn yellow, and lose their aroma.
Suggestion:
After discharge, enter a low-temperature and low humidity environment as soon as possible for tray removal and sorting, with the relative humidity preferably controlled below 40%;
Before packaging, sample and measure moisture content, rehydration rate, and sensory status, and remove undercooked and discolored products;
It is recommended to use vacuum packaging or nitrogen filled packaging for packaging, and conduct a sealing inspection;
Finished products should be stored in a cool, dry, and dark place to avoid repeated heating.

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