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Precautions for fruit vacuum freeze-drying equipment
2026-09-02
Q1: Why do some fruits experience severe volume reduction, sticky surface, or even "collapse" after freeze-drying?
A: This is usually caused by insufficient pre freezing or rapid heating during the sublimation drying stage. The water inside the fruit must be completely frozen into tiny ice crystals (at a temperature lower than its eutectic point). If vacuum pumping is started before the ice crystals are completely frozen, or if the heating plate temperature is too high during the sublimation stage, it will cause the ice crystals to melt into water in advance, thereby damaging the original porous support structure of the fruit. To avoid collapse, it is necessary to ensure that the pre freezing temperature is sufficiently low (usually below -30 ℃), and adopt a strategy of "slow heating and stable pressure" during the drying stage, strictly controlling the material temperature not to exceed its eutectic point.
Q2: Freeze dried fruits are prone to moisture absorption, softening, and even local oxidation and blackening. What are some details that are easily overlooked during operation and packaging?
A: Freeze dried fruits have a highly porous sponge like interior and are extremely sensitive to moisture and oxygen in the air. In the discharge process, it is important to avoid opening the cabin door directly to allow external air to enter. Dry and sterile nitrogen or dry air must be filled first to break the vacuum. After removal, sorting must be quickly carried out in a low humidity (RH<20%) environment, and nitrogen sealed packaging must be carried out using aluminum foil bags or aluminum plated films. In addition, for fruits that are prone to browning such as apples and bananas, color protection treatment (such as soaking in vitamin C or citric acid solution) must be carried out immediately after cutting, otherwise the finished product is prone to blackening and discoloration.
Q3: Why does the freeze-drying time of the equipment become longer and even incomplete after running for a period of time?
A: This is often related to the water capture efficiency of the cold trap (condenser) and the sealing of the vacuum system. The cold trap is responsible for capturing the sublimated water vapor. If the defrosting is not thorough or the cooling capacity is insufficient, the temperature of the cold trap cannot drop below -45 ℃, and the water capture efficiency will be greatly reduced. The water vapor may even flow back into the vacuum pump. In addition, aging of the sealing ring or valve leakage can cause the vacuum degree to fail to meet the standard (usually maintained at 10-50Pa). Therefore, it is necessary to thoroughly defrost and clean the cold trap after each use, regularly check the oil quality of the vacuum pump, and conduct a vacuum leakage rate test before starting up to ensure that the equipment is in the best working condition.
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