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Three precautions for freeze-drying onions
2026-07-27
Q1: What are the key considerations for raw material selection and pretreatment of onions before freeze-drying?
A1: Not all states of onions are suitable for freeze-drying. Firstly, it is necessary to select onions with high freshness, no pests or diseases, and no mechanical damage, as stale or damaged raw materials are prone to produce unpleasant odors during the freeze-drying process, which can affect the final quality. Secondly, during the cleaning and sorting stage, it is necessary to thoroughly remove sediment and inedible roots and stems, and cut them into evenly sized and moderately thick sheets or cubes according to the characteristics of the freeze-drying equipment. Cutting too large or fibers too coarse can make it difficult for internal moisture to sublime, thereby seriously affecting drying efficiency and finished product quality.
Q2: How to avoid damage to onion cell structure during the pre freezing stage?
A2: Pre freezing is the most crucial pre step in the freeze-drying process, which directly affects the shape of onions after dehydration. Cut onions must be subjected to quick freezing treatment to rapidly reduce their temperature to below freezing point. The purpose of quick freezing is to allow the moisture inside onions to form small and uniform ice crystals, rather than large ice crystals. If the freezing speed is too slow to form large ice crystals, these ice crystals will puncture the cell wall of the onion, causing the cell structure to collapse during the subsequent vacuum drying process, and the resulting freeze-dried onion will lose its original plump shape and crispy taste.
Q3: What are the parameters and conditions that must be strictly controlled during vacuum freeze-drying and subsequent packaging stages?
A3: After entering the freeze-drying chamber, a high vacuum environment (usually between 10-50 Pa) must be established and maintained to lower the boiling point of water, allowing ice crystals to sublime directly into a gaseous state at low temperatures, avoiding discoloration or flavor loss of onions caused by high temperatures. At the same time, the heating temperature should be strictly controlled between 30-100 ℃ to ensure that the moisture is completely removed to below 5%. Due to the porous sponge like appearance of freeze-dried onions, they are highly susceptible to moisture absorption and oxidation. Therefore, they must be immediately sealed and packaged (such as using aluminum foil composite bags or filling with nitrogen) after being taken out of the oven, and stored in a dark environment below room temperature and relative humidity of 60% or below. They should also be kept away from odorous objects to prevent cross contamination.
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