1. Anti collapse: The pre freezing and drying curves should be matched
The collapse, flattening, and bubbling of strawberries are usually related to insufficient pre freezing, coarse ice crystals, and rapid heating during the sublimation stage.
The maturity of raw materials should be stable: overripe fruits have soft flesh and weak support, and are more prone to collapse after freeze-drying.
Pre freezing should be thorough: the eutectic point of strawberries is about -15 ℃, and pre freezing usually needs to be lowered to below -25 ℃, while ensuring that the center temperature is completely frozen.
Quick freezing is better than slow freezing: Quick freezing can form small ice crystals, reduce cell rupture, and make the pores of the finished product more uniform.
Sublimation stage temperature control: The shelf temperature should not be raised too quickly, and the freezing layer temperature should be lower than the melting point/collapse temperature to avoid surface crust and internal water vapor not being discharged.
Proper drying analysis: gradually increase the temperature in the later stage to remove bound water, but the maximum temperature should not be too high to avoid sugar softening and anthocyanin degradation.
2. Anti browning and color preservation: Color preservation and oxygen barrier are equally important
Strawberries have thin skin and are prone to oxidation. If they turn dark or brown after freeze-drying, their sales will significantly decrease.
Fruit selection and cleaning: Remove soft, rotten, and diseased fruits, and minimize mechanical damage during cleaning.
Color protection treatment: Compound color protection solutions such as citric acid and vitamin C can be used for short-term soaking to inhibit enzymatic browning.
Slice evenly: The entire jelly has a long drying cycle, and it is recommended to control the slice thickness at around 5-8mm for consistent drying.
Low temperature oxygen isolation: Freeze drying itself is a closed and low oxygen environment, but it should be packaged as soon as possible after discharge to avoid prolonged exposure to air.
Storage in the dark: Anthocyanins are sensitive to light and oxygen, and finished products should be stored in the dark and sealed.
3. Moisture absorption prevention and softening: Moisture, packaging, and microorganisms should be controlled together
Freeze dried strawberries are porous and sponge like, with strong moisture absorption; If the moisture content is too high or the packaging is not tight, it is easy to go from crispy to soft, sticky, and chewy.
Suggested approach for control points and purpose
Suggested approach for control points and purpose
The final moisture content of the finished product should be controlled below 3% to maintain crispness and reduce the risk of moisture regain
Low humidity clean workshop for packaging environment, with minimal relative humidity to reduce moisture absorption and clumping
Packaging material: high barrier aluminum foil bag, vacuum or nitrogen filled to isolate oxygen, moisture, and maintain brittleness
Microbial control raw materials are thoroughly cleaned, and if necessary, evaluate sterilization methods such as irradiation. Freeze drying cannot completely sterilize, and porous structures are prone to adsorbing microorganisms
Storage conditions: cool and dry, away from light, anti squeezing, preventing pore collapse and flavor deterioration
If it is necessary to go on the production line in the future, it is recommended to conduct a small-scale verification first: slice thickness, plate laying amount, pre freezing time, sublimation/analysis temperature curve, and endpoint moisture. Strawberry varieties, sugar content, and maturity are different, and the freeze-drying curve cannot be directly applied to parameters of apples or other fruits and vegetables.

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