Note 1: Adopt gradient annealing pre freezing to prevent "high sugar collapse"
The high sugar content of durian can significantly reduce the eutectic point and glass transition temperature of the material. If conventional rapid pre freezing is used, not only will coarse ice crystals form and damage the cell structure, but the porous skeleton of the fruit flesh will also soften and collapse due to insufficient support during subsequent sublimation.
Operation suggestion: It is necessary to abandon the "one-step" quick freezing method and adopt the optimized process of "slow gradient cooling+low-temperature annealing insulation". The pre freezing temperature should be strictly controlled below -45 ℃ to -50 ℃, and the ice crystals should be uniformly refined through gradient cooling, fully solidifying the sugar matrix structure inside the material, and improving the anti collapse ability of the material from the root.
Key technique: Ensure that the center temperature of the material is completely below its eutectic point before entering the drying stage. This is the first line of defense to ensure that durian powder ultimately presents a loose, porous, sponge like structure and does not shrink or deform.
⚙️  Note 2: Accurately control the segmented drying parameters to avoid "surface crust"
Durian powder is extremely sensitive to changes in temperature and vacuum during the freeze-drying process. If the temperature rise during the sublimation drying stage is too fast or the vacuum degree is unstable, it is very easy to cause the surface of the material to dry and form a "hard shell" first, resulting in the inability of internal water vapor to escape smoothly, further exacerbating deformation and causing residual water to exceed the standard.
Operation suggestion: Strictly follow the three-stage drying process. One drying (sublimation drying) requires a vacuum environment of 30-60Pa, slowly raising the temperature of the partition from -30 ℃ to around 30 ℃ for about 10 hours to allow the free moisture to sublime smoothly; For secondary drying (analytical drying), the vacuum degree is controlled at<35Pa, and the temperature is gradually increased to 60 ℃ to completely remove residual bound water.
Equipment debugging: Throughout the entire process, the equipment needs to maintain uniform shelf heating and stable cold trap ice capture capability (cold trap temperature usually needs to be ≤ -55 ℃), ensuring efficient condensation of water vapor without backflow to the material, and ultimately strictly reducing the moisture content of the finished product to below 5%.
🛡️  Attention 3: Strictly adhere to the quality of raw materials and moisture prevention during post-processing, and eliminate the "sour and rancid odor"
Freeze drying technology can only change the shape of fruits and cannot reverse the quality of the raw materials themselves. Recently, third-party evaluation agencies have detected extremely high levels of yeast and coliform bacteria in some commercially available freeze-dried durians, and even found that some products have a sour and bitter appearance. This is often due to the use of low maturity, uneven quality, and even spoiled scraps of raw materials for mud compaction and shaping. In addition, durian is rich in unsaturated fatty acids, which can easily absorb moisture and oxidize after freeze-drying.
Operation suggestion: The raw material must be high-quality mature durian with soil yellow flesh, obvious luster, moderate softness and tightness (such as varieties like Golden Pillow), and the use of bad fruit or inferior scraps must be strictly prohibited.
Core standard: After freeze-drying and discharging, sorting and sealing packaging must be completed in a very short time. The packaging material must be sealed with high barrier aluminum foil bags for vacuum or nitrogen filling, and food grade desiccants should be placed inside the packaging to prevent durian powder from absorbing moisture and softening or experiencing oil oxidation and rancid taste during storage and transportation.

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