ABSTRACTA fluorometric method and a rapid paper test method were developed for differentiating fresh from frozen‐thawed fish. The methods were based on measurement of the activity of neutral β‐N‐acetylglucosa‐minidase. This enzyme, found in fish red blood cells, was inactive in intact cells but was activated when cells were disrupted by freezing and thawing. Both methods were applicable for testing most common edible fish prior to filleting and required about 20 min using a
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