Freezing and Freeze Drying

Only a small portion of common (Agaricus) mushrooms produced are frozen or freeze-dried, and a large part of the frozen mushrooms are sold to reprocessors, such as freeze-dryers, or used in specialized frozen food products.

Washing and Blanching. Lower microbial loads are the basic requirements for these products, and vigorous washing and utilization of some disinfectants have been found effective for this purpose. High levels of chlorine, up to 50 ppm in the wash water, have been found beneficial for reducing the microbial load.

Mushrooms contain a highly potent polyphenoloxidase. It is this enzyme which causes the undesirable discoloration that appears in bruised or cut mushrooms. During the washing operation, soluble oxidized phenolic compounds resulting from the enzymatic action are washed off, and in some cases the washing water may turn a distinct red. Blanching adequate to destroy the enzyme also produces an undesirable gray color. Therefore, mushrooms for freezing are usually given a short blanching in hot water, primarily to help reduce microbial contamination rather than for enzyme inactivation. It is necessary to follow such a blanching immediately with a cold-water quench. A short-time dip in sodium bisulfite solution containing 200 ppm S02 or in 1% salt and 0.09% sodium bisulfite solution is an effective alternative for prevention of this enzyme discoloration.

Freezing and Freeze-Drying. A rapid freezing rate is essential for mushroom freezing. It is desirable to freeze mushrooms to a center temperature of — 10°F (— 23°C) in 3 to 5 min for diced or sliced mushrooms and in about 20 min for button or whole mushrooms. I.Q.F. (individually quick frozen) freezing and refrigerant dip freezing are recommended.

Freeze-drying is carried out in a high-vacuum chamber in which the ice on or in the mushroom tissue disappears by sublimation. Uniform loading is essential, otherwise burning or incomplete drying of a part of the product may occur. When the ice on the surface of the mushrooms disappears, application of heat can be started. The end point of the freeze-drying is the point at which the moisture content of the product reaches about 3%. From 12 kg of raw mushrooms, 1 kg of freeze-dried product is obtained. After reconstitution, 1 kg of the freeze-dried product will yield 8 kg of reconstituted product.

Packaging. Vacuum packaging inhibits oxidative discoloration (13) and a sauce coating may also serve the same purpose. The freeze-dried product is usually packed in airtight and moistureproof containers under nitrogen gas.

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