Domestic fish meal stereomicroscopy was used to detect the mixture of fish meal, fish bone, fish scales and fish viscera. The characteristics of the situation inspection are as follows:
Fish meat: large particles, rough surface, fibrous structure, yellow or brown, transparent, shaped broken hoof tendons, seems elastic.
Fish bones: including fish bones, fish skulls, translucent or opaque fragments, of various sizes and shapes, white to white and yellow. Some fish bones are amber-colored, smooth in surface, slender and pointed, spine-like. Careful observation shows that fish bones fragments have the characteristics of a large end or a small end. Translucent, with texture on the front, hard and inelastic skull.
Scale: Flat or curly algal sheet, nearly transparent, with concentric circular lines. Fish Eye: The surface is fragmented, with milky spherical particles, translucent, glossy and hard.
Detection of crude protein is the general term of nitrogen-containing substances in fish meal, including true protein and non-protein nitrogen-containing substances, the latter mainly includes free amino acids, nitrates, ammonia and so on. Domestic fish meal crude protein is generally 45-55%, imported fish meal crude protein is generally 60-67%. The method of determination is Kjeldahl flask method with specific steps.
Detection of crude protein of true protein reflects the content of all nitrogen-containing substances, but can not reflect the true protein part. Therefore, it is necessary to determine the true protein. Protein can precipitate by salting out with heavy metal salts under certain alkaline conditions. This precipitate is insoluble in hot water, while non-protein nitrogen is soluble in water. Wash the sediment with hot water and remove the water-soluble nitrogen. The remaining sediments were determined by Kjeldahl method, and the content of true protein was obtained. The ratio of true protein to crude protein could be used to determine whether water-soluble non-protein nitrogen-containing substances were incorporated into fish meal. Methods of determination:
Pretreatment of sample: Sample 1-2 g in 200 ml beaker, add distilled water 50 ml to boil, add 1% copper sulfate 20 ml, stir while adding, stir for one minute after adding, place for more than 1 hour or stay overnight, filter sediment with medium-speed qualitative filter paper, wash residue repeatedly with hot water above 70 C until filtrate. No SO2-4 (take 5% barium chloride test solution 5 drops in the surface dish, add 2 moles/liter hydrochloric acid 1 drop, drop into the filtrate, in the black background observation should be no white precipitation). The filter paper and residue are wrapped and put into the oven. The sample is dried for 2 hours at 65-75 C. The sample is digested in a flask together with the filter paper.
Then the crude protein was measured and the true protein content was determined. The ratio of true protein content to crude protein content is the ratio of fish meal to true protein. Crude protein should conform to product requirements, true protein ratio should conform to the following values: imported fish meal should not be less than 80%, domestic fish meal should not be less than 75%. When measured fish meal true protein ratio is less than the above value, the fish meal is mixed with water-soluble non-protein nitrogen substances.