Why it turns brown - Enzymes stayed active.*Fruit enzymes keep working until the core hits −18°C. Slow freezing or skipping pre-treatment (blanch, or dip in 1% citric acid) lets them brown the flesh during the long drying cycle. - Vacuum wasn't deep enough. Sublimation needs a chamber vacuum around 10–50 Pa. If the vacuum is too weak, ice melts instead of sublimating — sugars migrate to the surface and brown on contact with air. - Residual moisture too high. Above 4–5% moisture, water reactivates enzymes and triggers Maillard browning during storage. Why it collapses - Slow freezing = large ice crystals.Big crystals tear cell walls, so the structure sinks as it dries. - Shelf temperature too high.Heat pushed in too fast during primary drying melts the frozen matrix before it's dry — structure collapses. - Overloaded trays. Thick layers block vapor escape, slowing drying and trapping heat. The fix, in one checklist 1. Pre-treat to stop enzymes (citric acid dip or blanch). 2. Freeze fast — below −35°C for a solid core. 3. Hold vacuum at 10–50 Pa, keep fruit shelf temperature moderate. 4. Dry to -3–4% residual moisture, then package immediately with desiccant and nitrogen. Consistent color and structure aren't luck — they're process control. If your batches vary, start by checking your vacuum curve and heating profile, not your recipe.
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