What's fishmeal for? Why is there an odor during processing?

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Fish meal is an important protein feed for livestock and poultry, and also an important protein raw material for compound feed industry. Therefore, the quality of fish meal is closely related to the quality of feed products. It is very important to understand the factors affecting the quality of fish meal and evaluate the quality of fish meal reasonably. Factors affecting the quality of 1 fish meal 1. 1 Raw fish meal is generally divided into white fish meal and red fish meal. White fish meal is generally processed from cold-water fish such as eels. The crude protein content can be as high as 68% ~ 70%, which is expensive and mainly used for special aquatic feed. The feed for livestock and poultry is red fish meal. The main raw materials of red fish meal include silver carp, sardines, wind tail fish, herring and other small miscellaneous fish, fish and shrimp food processing leftovers. The crude protein content of red fish meal is generally above 62%, and the high protein content can reach above 68%. In China, fish meal is mostly made of various small miscellaneous fish and fish and shrimp processing scraps, and the protein content is below 50%. Choose different kinds of fish raw materials, and the quality of fish meal will be different [1]. 1.2 preservation before processing The time from fishing ashore to processing into fish meal and the preservation method during this period directly affect the freshness of fish meal. The freshness of fish meal raw materials has a great influence on the quality of fish meal [1]. Because of the long storage time and unreasonable storage method, the freshness of fish is reduced. On the one hand, the nutrients are destroyed, and the content of protein is reduced, which affects the smell of fish meal; On the other hand, it affects the processing of fish meal, and fish meal cannot be born, resulting in high fat content. Fish should be processed immediately after fishing to ensure the freshness of fish meal. The freshness of shore processing is worse than that of ship processing. Frozen fish on board is fresher than fish without ice. Generally speaking, the time from fish landing to processing into fish meal should not exceed 24 h, and it should not exceed 6 h in summer. 1.3 processing technology Due to different processing technologies, fish meal can be divided into defatted fish meal and undeleted fish meal. After cooking, squeezing, degreasing, drying, crushing and packaging, the crude protein content is above 60% and the crude fat content is about 10%. Non-defatted fish meal is divided into steamed fish meal and cooked fish meal. Steaming fish meal means steaming the fish and crushing it. Cooking fish meal means adding water to salted fish and dried fish, cooking, squeezing, desalting, desanding, drying and crushing. The quality of non-defatted fish meal is far worse than that of defatted fish meal [1] 1.4, because the freshness of fish meat is poor, the nutritional components are destroyed, or the fish meal is smoked or burnt at high temperature, and the quality of fish meal is seriously deteriorated. Because of the high fat content of fish meal, if it is stored for a long time, fish oil will be oxidized and rancid, and the quality of fish meal will be obviously reduced [2]. Fish meal adulteration 1.5 fish meal adulteration has become the most important factor leading to quality deterioration by human factors. The main feature is the use of cheap nitrogen-containing substances, such as urea, double shrinkage fertilizer, feather powder and so on. Skin powder, blood powder, etc. It is mixed into fish meal to increase the total nitrogen content. The true protein content of fish meal is reduced after adulteration, and the quality of fish meal is seriously affected. 2. Simple detection of fishmeal quality 2. 1 Identification of fishmeal adulteration is an important human factor affecting fishmeal quality. Seriously troubled the normal sales and circulation of fishmeal. It is very important to judge fish meal adulteration quickly and effectively. According to different adulteration purposes and methods, fish meal adulteration can be effectively and correctly identified. 2. 1.l Soil mixing and gravel mixing for the purpose of increasing the weight of fish meal: put a small amount of samples into a clean beaker, add 10 times the volume of water, stir vigorously, and after standing, soil can be observed at the bottom of the cup. Sandstone can be interpreted as the mixture of these substances. [3] Adding shell powder, crab shell powder and iron powder: first, take a few samples and put them in a beaker, and add a proper amount of dilute hydrochloric acid. If a large number of bubbles are generated and squeaks are made, shell powder is added. Then microscopic examination can be carried out to further confirm whether there is adulteration according to their respective microscopic characteristics [3]. 2. 1.2 Add vegetable protein, take a small amount of fish meal and burn it. If there is a smell of fried grain, it means that vegetable proteins such as rapeseed meal, cotton meal and soybean meal are mixed. In addition, microscopic examination can be carried out according to the morphological characteristics of various plant proteins to determine whether the cake-like substance [3] 2. 1.3 is mixed with animal protein [4] feather powder: a small amount of samples are put into the mixed solution of carbon tetrachloride and petroleum ether, stirred and left standing. If mixed with feather powder, you can see feather powder floating on the wave surface. Blood powder: slowly inject the filtrate of the detected fish powder into the solution prepared by aniline, glacial acetic acid and hydrogen peroxide. If there are green or blue rings or dots on the contact surface of the two liquids, it means that fish meal contains blood powder. Skin powder: take a little fish meal sample in a Petri dish, add a few drops of cupric acid glue and let it stand for a while. If there is no color change, it means leather powder. 2. 1.4 adding water-soluble non-protein nitrogen urea for the purpose of increasing total nitrogen: put a small amount of samples on the drip plate and add 2 ~ 3 drops of cresol red and urease solution. If urea exists, it will appear purple after a period of time. The higher the urea content, the shorter the time for purple. Biurea: Take fishmeal filtrate in a test tube, add a small amount of sodium hydroxide solution and copper sulfate solution in turn, and observe immediately after shaking. If it is purple, it means that the fish meal is mixed with biuret. 2.2 Microscopic examination of fish meal With the development of modern analytical instruments, microscopic examination has become an independent detection method. As a simple fish meal detection method, microscopic examination has the advantages of less investment, intuition, accurate judgment, accurate data and easy mastery of technology, and is increasingly favored by people. At present, this method has not been well popularized in China, but it needs to be strongly advocated [6] to identify fish meal according to the microscopic characteristics of fish meat, fish bones and fish scales. Fish meal is a kind of small particles with dull surface. Under the microscope, the surface of fish is rough, with fiber structure, similar to meat powder, and slightly lighter in color than meat powder. The fishbone is a translucent to opaque silver body, some fishbone pieces are amber, and the gap is a dark streamlined wavy line segment, like a whip grape branch, extending outward from the root along the whole edge. Different sizes and shapes of fishbone chips have different characteristics of fishbone chips in different parts of the fish. Fish scales are flat or curved transparent objects, which are concentric circles with alternating shades. There is a slight cross on the surface of the fish scale. Fish skin is a kind of crystalline convex lens-like object, translucent, and its surface is broken, forming ivory glass beads [6]. Just like fish meal has its own microscopic characteristics, all kinds of feed raw materials have their own color, hardness, tissue morphology, cell morphology and crystal morphology. If you are familiar with the characteristics of fish meal and various possible dopants, you can judge the quality of fish meal through microscopic examination. 2.3 Simple determination of salt content of fish meal Salt content of fish meal is one of the important indicators of fish meal quality. Its content affects the food consumption and water consumption of animals, thus affecting the health of animals. It is necessary to determine the salt content in fish meal quickly and simply. Weigh a certain amount of fish meal samples, add water and stir, remove a certain amount of supernatant, add potassium chromate solution to develop color, and titrate with a certain concentration of silver nitrate solution until the solution appears brick red fading. According to the amount of silver nitrate consumed by titration, the salt content in fish meal can be judged [6] 3. Judging the quality of fish meal from its appearance mainly depends on its color, smell, fever and caking. High-quality fishmeal, light brown with a little blue, has a special fishmeal flavor, and has no fever, caking, mildew, sour taste and ammonia smell [1, 6]. According to the laboratory index, the crude protein of high-quality fish meal should be above 62%, the crude fat should be below 10%, the moisture should be below 12%, and the salt content and sand content should not exceed 1%. Judging from the amino acid composition, the true protein should reach more than 95% of the total crude protein, in which methionine should not be less than 1.7% and lysine should not be less than 4.5% [6]. Judging from whether fish meal is adulterated, high-quality fish meal contains fish scales and fishbones, but the impurities should not be too much. Whether fishmeal is adulterated can be judged by adulteration identification. Quality reference index Quality index of domestic fishmeal (professional standard of fishmeal SCll883) (see table 1). Specific strength of imported fish meal quality index: moisture ≤12.0%; 62% ~ 70% of crude protein; Crude fat <12.0%; Salt content < 4.0%. Table 1 First, Second and Third Quality Standards of Domestic Fish Meal

Color yellow brown yellow brown yellow brown.

Coarse protein /%> 55 50 45

Crude fat/%<10 <12 <14.

Moisture /% 12 12 12

Salinity /%< 4 < 4 < 5

Sand fraction /%< 4 < 4 < 5

4. Improvement of fish meal quality inspection index 4. 1 Limitation of conventional fish meal quality inspection index. Often the existing quality inspection indicators can't truly reflect the quality of fish meal. For example, the crude protein index can only reflect the total nitrogen content of fish meal, but not the true protein content. Some indicators are targeted quality inspection indicators in the case of known adulteration, such as iodine reaction, crude ash and other indicators. But in practical work, it is impossible to determine the conventional indicators one by one to prevent adulteration. The evaluation of conventional measurement indexes also has limitations. For example, eating low-quality fish meal. Protein of meat powder and other animals can not truly reflect the quality of fish meal only from the total amount of amino acids or the contents of lysine and methionine. 4.2 Selection and improvement of quality inspection indicators Sensory inspection and microscopic analysis can be used as the first step of quality inspection because they are intuitive and rapid. The reasonable evaluation of fish meal quality needs to be comprehensively judged by several chemical detection indexes. ① Moisture content: Moisture is the key index of fish meal quality, which affects the quality level and storage period of fish meal. ② Crude protein content: Crude protein reflects the protein level of fish meal, and its content can roughly judge the protein level of fish meal; ③ Water-soluble total nitrogen: The determination of water-soluble total nitrogen can exclude the incorporation of water-soluble non-protein nitrogen. If the water-soluble total nitrogen is too high, it can be inferred that the fish meal contains water-soluble non-protein nitrogen or the storage period is too long, which leads to deterioration. ② Acid value: The degree of oxidative rancidity of fish oil in fish meal can be identified by acid value; ⑤ In vitro digestibility: It can truly reflect the digestible protein content in fish meal; ⑤ Amino acid composition: establish the essential amino acid balance model of standard fish meal, and make correlation analysis with the essential amino acid balance model of fish meal samples to be tested. The fish meal with good correlation is regarded as high quality fish meal, otherwise it is low quality fish meal. Therefore, the authenticity and quality of fish meal can be identified according to the balance mode of essential amino acids in fish meal.

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