The quality judgment and detection of fishmeal used in agricultural feed

Fishmeal is a high-quality protein feed, but fish meal on the market is often adulterated, and most of the fish meal blended is cheap, unfeasible feather powder, plant rhizome powder, and leather, which greatly reduces the quality of feed. Therefore, the quality of fishmeal must be monitored. The following describes the method of quality testing of fishmeal.

First, sensory characteristics

Pure fish meal is generally yellow-brown or yellow-brown, but there are also a small number of white fish meal, gray fish meal, etc. depending on the type of fish and there are differences. It has aroma of cooked fish and slightly fishy flavor. The smell of fish mixed with fish sauce is heavy, and there must be no rancidity, ammonia odor and other corrupt smells and excessive burnt smell. The fishmeal is powdered and contains scales, fish bones, etc. The well-treated fishmeal has visible fish threads.

The muscle fibers of fresh fish are sticky, so grab the fishmeal by hand and rub it with your fingers. The fish with good stickiness is fresher.

The particle fineness is at least 98% through a standard sieve having a mesh width of 2.80 mm.

Second, chemical testing

1. Crude Protein Crude protein is a general term for nitrogenous substances in fishmeal. From the detection data and production status of multiple samples, the crude protein of domestically produced fishmeal is generally 45% to 55%, and the crude protein of imported fishmeal is generally 60%. ~67%, too low may be attributed to high impurities in fishmeal raw materials, too high may be due to adulteration. Crude protein assays are generally performed using Ketzer flasks.

2. Pepsin Digestibility Pepsin digestibility refers to the ratio of protein digested by pepsin to crude protein. Qualified fish meal, its protease digestibility should not be less than 85%.

Method steps: Accurately weigh about 1 g of dehydrated and dehydrated fish meal, place it in a 300 ml conical flask, add 150 ml of preheated (42°C～45°C) 0.2% pepsin solution, cover and seal, and stir at 45°C. Digestion for 16 hours (a constant temperature oscillator is available). After digestion, it was filtered with filter paper, and the undigested material on the filter paper was washed with warm water. The undigested material was transferred to a Kevlar flask with the filter paper for digestion. The subsequent steps were followed by measurement of crude protein to measure the amount of undigested crude protein. Pepsin digestibility of fish meal = (meal of fish meal - amount of undigested crude protein) / crude protein 100%

In addition, the crude fat of fishmeal should generally not exceed 12%. The coarse fishmeal fiber should be almost zero, and too high with fiber raw materials. Such as: raw sugar, wood chips and so on.

Third, the detection of some common adulteration

1. Test Method for Feather Powder Mixed in Fish Meal Steps: Put about 1 gram of fish meal into a 500 ml conical flask, add 100 ml of a 1.5% sulfuric acid solution to a bottle, and boil for 30 minutes with a slight fire and continue to add distilled water to keep boiled sulfuric acid. Concentration, after the completion of the settling, take a small amount of bottle residues were placed on two slides. Observe under 50 to 100 times the microscope.

The residue treated with sulfuric acid, if light-colored feather powder, is colored yellow by hydrolysis with sulfuric acid. If there is dark feather powder, the color is dark brown or black after sulfuric acid hydrolysis. Under the microscope, the fully hydrolyzed feather powder is yellow, gray or brown translucent and granular, while the incompletely hydrolyzed feather powder can be seen in some tubes. These are feather stems or quill tubes, mostly yellow and brown. Some are similar to glass particles.

2. Incorporation of cottonseed cake in fishmeal The 20-mesh and 40-mesh sieves were placed together, and then the fishmeal was placed in a sample sieve and shaken for about 5 minutes. Observe the 40-mesh screen in the middle and observe whether there are cotton fibers on the screen. Most of the cotton fibers are dark yellow or brown-yellow. In addition, dark brown cotton shell fragments are also visible on the screen. It can be confirmed that fish meal is mixed with cotton seed cake.

3. Test method for incorporation of urea and other nitrogenous fertilizers in fishmeal: Take 1g to 2g of fishmeal in a beaker, add 50ml of distilled water and mix thoroughly. Filter with qualitative filter paper and wash 7-8 times with distilled water. The total volume of the washing solution is about 200 ml, and then all the fishmeal is placed on a piece of filter paper and placed in an oven at 65° C. for 2 hours. The crude protein content can be measured by Kjeldahl method. For example, fish meal is marked with 65.2% crude protein and 34.7% after washing, indicating that the latter is spiked with a certain amount of soluble nitrogen fertilizer.

4. Fish meal mixed with rapeseed cakes or cotton cakes The normal fishmeal is very low in crude fiber, generally 0.1% to 1%, and the crude ash content is generally not more than 23%. When mixed with rapeseed cake or cotton cake, The crude fiber is increased to more than 10%. Therefore, when the ash content is higher than 28% and the crude fiber content is higher than 2%, adulterants are present.

5. Incorporate vegetal material or fine sand into fishmeal. Take a small amount of sample, place it in a clean glass or beaker, add 10 times volume of water, stir it vigorously, and after standing, observe the surface floater and bottom sediment. If there is plant material such as rice hull flour, peanut shell flour, wheat bran, etc. on the surface of the water, it indicates that there is hydrolyzed feather powder or plant material mixed in. If there are sand and mineral substances at the bottom of the cup, it means that these substances are incorporated.

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