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Three important aspects of vacuum freeze-drying process for berries
2026-10-19
1. Before pre freezing, should we protect the color, cut it to a certain thickness, and freeze it to a certain degree?
Berries are prone to browning and juice flow, and pretreatment cannot be simply cleaned. Strawberries and raspberries can be cut into 5-10mm slices or half fruits. When blueberry jelly is dried, it is best to scratch/puncture the surface of the fruit to help remove moisture.
Color protection can be achieved by soaking in 0.5% -1% citric acid or citric acid+vitamin C for a short period of time, and if necessary, blanching to kill enzymes to prevent anthocyanin oxidation and discoloration.
The pre freezing temperature should be lower than the eutectic point of strawberries, which is about -15 ℃. Pre freezing is usually selected between -25 ℃ and -40 ℃, and the center is completely frozen before entering the warehouse.
Pre freezing is not the faster the better: quick freezing can protect color and cell structure, but if the grain size is too small, the sublimation resistance will increase, and it needs to be balanced with the thickness of the tray and drying time.
2. Can the shelf temperature be increased during the sublimation stage to meet production demands?
You can't rush in. Berries have high sugar content, and once the ice crystals melt, they will collapse, become sticky, and darken in color.
The vacuum degree during the sublimation stage should be controlled at 10-30Pa, and the shelf temperature usually starts from -20 ℃ to -10 ℃, gradually increasing in stages to ensure that the material temperature is always below the eutectic point/collapse temperature.
To determine whether sublimation has ended, it is not just about time, but also about whether the vacuum degree, material temperature, and water outlet rate have stabilized; After confirming that most of the free water has sublimated, it enters the analytical drying process and is generally heated to 30-40 ℃ to reduce the final moisture content to 3% -5%.
How to prevent moisture regain, fading, and loss of aroma after discharge?
Berry freeze-dried products have many pores and strong moisture absorption, and may soften after being exposed to air for a few minutes after discharge. It should be quickly packaged in a low humidity environment, with priority given to using aluminum foil bags/nitrogen filled packaging. Residual oxygen should be kept at a low level and stored away from light.
If the finished product appears dark or brown, it is likely due to insufficient pre-treatment enzyme inactivation, insufficient color protection, or excessively high resolution temperature; If the finished product collapses and the center becomes sticky, it is mostly due to insufficient pre freezing or rapid heating during the sublimation stage; If there is a resurgence of moisture and clumping in the later stage, it is usually due to poor packaging sealing or high environmental humidity.
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