Freshness index of protein: VBN
The freshness of protein is determined by the level of volatile base nitrogen (VBN) and histamine.
The VBN of volatile salt-based nitrogen-fish meal has good freshness within 50 mg/100g. More than 150 mg/100g indicates that it has started to rot. Fish meal with VBN over 150 mg/100g is not suitable for general use.
The average histamine content of histamine-white fish meal is 28 ppm. The histamine content of red fish meal is much higher than that of white fish meal. The average imported steam drying is 267 ppm, the imported direct fire drying is 442 ppm, and the domestic fish meal is 321 ppm, which is lower than that of imported direct fire drying. It can be seen that the content of histamine in domestic steam fish meal is only lower than that in imported steam fish meal, but much better than that in imported direct fire dried red fish meal.
Freshness index of fat
The freshness of fat is generally measured by acid value, peroxide value, TBA or AOM. Feed enterprises generally use acid value (AV) to determine the freshness of fish meal fat. High acid value means that the hydrolysis of oil is serious and the freshness is not good. Free fatty acid will destroy the viscoelasticity of feed. High acid value can not be used as feed material for eel. According to the analysis and determination, the acid value of white fish meal on industrial ships is less than 1 mg KOH/g, the shore white fish meal is 1-1.5 mg KOH/g, the domestic red fish meal is 3-5 mg KOH/g, the imported steam-dried red fish meal is 2.61 mg KOH/g, and the direct-fire-dried red fish meal is 4-5 mg KOH/g. Therefore, white fish meal on industrial ships and shore is generally used as feed material for eels and turtles, and steam dried red fish meal. The acid value of dried imported and domestic red fish powder should be reduced to less than 1.5mgKOH/g.
Color: light yellow to light brown, excessive color indicates excessive heating, too shallow, indicating insufficient heating. The color of the whole batch of soybean meal should be basically the same.
Taste: Roasted soybean flavor, no rancidity, mildew, coking, and no smell of raw beans.
Texture: uniform fluidity, irregular fragments, powdery or granular, without excessive impurities.
Specific gravity: 0.515 - 0.65Kg/l
Soybean meal contains about 43% protein, 2.5%-3.0% lysine, 0.6%-0.7% tryptophan, 0.5%-0.7% methionine, 0.5%-0.8% cystine, less carotene, only 0.2-0.4 mg/Kg, 3-6 mg/Kg luciferin and riboflavin, 15-30 mg/Kg niacin, 2200-2800 mg/Kg choline. Soybean meal is lack of methionine, crude fibers are mainly from soybean skin, nitrogen-free extracts are mainly disaccharides, trisaccharides and tetrasaccharides, starch content is low, mineral content is low, calcium is low, phosphorus is high, vitamin A, B, B2 is low. Table 2 reflects the composition comparison of soybean meal and other kinds of oil dregs.