Five Essential Facts About Feed for Eel Farming
I. Main binder -- α-starch: Binders were the earliest additives used in eel feed. In Japan in the 1950s, when raw fish was used as bait to farm yellowtail, antibiotics, wheat gluten and α-starch were added. Thanks to the use of binders such as α-starch, not only were antibiotics and nutrients prevented from leaching away and water pollution reduced, but the feeding rate was also raised, thereby accelerating the commercialization of feed.
The most important binder in eel feed is α-starch (also called pregelatinized starch). It is made from raw cassava or potato starch as the raw material, which after mixing with water, gelatinization, drying, grinding and cooling becomes a product with a pure white color, good viscoelasticity and excellent extensibility. Potato starch has the best palatability, corn starch has good palatability but poor binding, and cassava starch falls between the two but is lower in price. Cassava or potato starch, or a combination of both, can be selected according to the needs of the feed. The digestibility of α-starch by eels is about 80%, and its inclusion level in eel feed is 20-23%; adding more will affect its nutritional value. Besides having good viscosity itself, the binding performance of α-starch is also related to its fineness and to other substances in the feed.
1. Freshness of fish meal: Indicators such as the acid value and volatile basic nitrogen of fish meal should not be too high. Using fish meal of poor freshness will destroy the viscoelasticity of the feed.
2. Trace inorganic salts: The ratio should be reasonable and should not be excessive, especially since the concentration of divalent ions among them has a relatively large effect on the binding power of α-starch.
3. Grinding fineness of raw materials: It is best to reach 98% passing through an 80-mesh sieve, so that because of the large contact surface, binding is easy, and after the feed is mixed, the surface gloss is good and the product image is excellent.
In addition, after purchasing α-starch, it should be used up within 4 months of the production date; otherwise, under the influence of heat, light, air and other factors, it will be degraded, its molecular weight will decrease, and its binding power will decline.
II. Auxiliary binders
1. Sodium caseinate: It contains more than 88% crude protein. It can not only provide the protein source needed by eels, but is also a good binder for producing specially made feed for the black fry and white fry stages. Adding it can play a role in balancing amino acids. It is recommended to add 0.5-2% in the specially made black fry and white fry stages.
2. Sodium alginate: Also known as algin, it is a white or light yellow, non-toxic, odorless colloid, soluble in water to form a gel solution, and relatively stable at PH 6-9. For ease of use, it is mostly dried and ground into powder. Because it carries carboxyl groups -COO- on the polymer chains of poly-β-D-mannuronic acid and poly-α-L-guluronic acid, it can form water-insoluble calcium salts with Ca+1 (calcium ions) in the base material, so its viscosity is 5 times that of flour, and it is especially more suitable for use in water with high salinity. In feed, the recommended addition level is 0.1-0.3%.
3. Wheat gluten: It is made by washing starch out of flour with water, then drying and grinding. Its crude protein content is above 72%, its fineness is above 90% passing through an 80-mesh sieve, and its water absorption is about 80%. Adding 1-3% to feed helps the feed's stability in water. However, its lysine content is relatively low, and its amino acid composition is relatively unbalanced compared with the eel body.
In addition, other commonly used binders are sodium polyacrylate, resin compounds and carboxymethyl cellulose and other substances. Their molecular weights are as large as about one million, and they carry carboxymethyl, carboxyl and hydroxyl groups on their molecular chains. Relying on hydrogen bonds and ionic electrical effects, they have strong binding power with the base materials of the feed. The use effects of the above binders are related to the base materials used, the variety and quantity, water quality, water temperature and the degree of feed kneading.
III. Attractant substances
The feeding of eels living in natural waters is accomplished by the coordinated action of organs such as smell, vision and taste. In this process, eels rely on olfactory organs to perceive the position of the bait and determine the direction of the food. When vision reaches the range where it takes effect, vision then distinguishes whether it can be eaten. Therefore, the sense of smell of eels is very sensitive. Its olfactory sensitivity to phenylethyl alcohol far exceeds that of humans, with a detection threshold as low as 3.5×10-19, similar to that of dogs. Scholars such as Carr, Mackie and Fuke, through separation and analysis of raw materials, found that the feeding attractant components of fish feed are mainly nitrogen-containing compounds, especially amino acids, nucleotides, quaternary ammonium bases and organic acids. Certain short-chain fatty acids, pteridines, alloxazine derivatives and others also have good feeding attractant effects on some fish. Non-polar α-amino acids have the best feeding attractant effect.
In 1982, Carr proposed that attractants extracted from animal tissues have four characteristics: 1 low molecular weight, generally less than 1000; 2 nitrogen-containing; 3 non-volatile, water-soluble; 4 having amphoteric H+ and OH- characteristics. In addition, the feeding attractant components of eel feed also have a synergistic effect. That is to say, when some components are used alone, their activity is low or ordinary, but when two or more coexist, they show stronger activity, for example a, amino acids and betaine, b, amino acids and nucleotides, c, between two amino acids.
In factory production, liver meal powder, milk powder or fresh milk essence, betaine, earthworm powder and amino acid-type protein sources can generally be selected as attractant substances.
IV. Digestive aid substances
The digestion of feed by eels mainly takes place in the intestine. The intestine of the eel is relatively short, with a ratio of its length to body length of 0.52-0.82, but the pepsin activity of the eel is relatively strong, 4 times higher than that of rainbow trout, while its amylase activity is relatively weak. It generally takes 6 hours for the eel to complete the process of digesting and absorbing feed. Generally, in the production of feed, brewer's yeast, molasses yeast, single-cell bacterial protein and live bacterial preparations with digestive and intestinal-regulating effects are also added.
1. Brewer's yeast: It is composed of yeast cells from the wort culture of the beer industry and the residue of that culture medium, obtained after drying. Its crude protein is generally 40-50%, and it contains large amounts of B vitamins and unknown growth factors. In eel feed, 2-3% can be added.
2. Molasses yeast: It is rich in B vitamins and is obtained by low-temperature spray drying. It can promote the appetite of eels and has a relatively good attractant effect. The various nutrients it contains are well preserved, and it has a relatively strong aromatic fragrance.
3. Single-cell bacterial protein: It is obtained after bacteria are refined and dried during the fermentation of beet molasses. It has a sweet, fragrant taste like yeast and good palatability. It contains bacterial protein, minerals, vitamins and unknown growth factors that eels can easily absorb. Its crude protein content is above 65% and crude fat below 4.5%. It is recommended to add 1-2% in feed.
4. Live bacterial preparations: They are composed of Lactobacillus, Streptococcus faecalis, Pediococcus and other nutritional substrates, with a total effective live bacterial count of more than 120 million per gram. They have digestive aid and intestinal-regulating effects. The so-called intestinal-regulating effect means that after the speed of small intestine peristalsis becomes slower, nutrients can be fully absorbed, while large intestine peristalsis is accelerated, shortening the time that excrement stays in the animal body. In the intestine, these beneficial bacteria can compete with harmful bacteria for living space and nutrients, and can combine with favorable bacterial cells in the intestine to resist bad bacteria and even kill bad bacteria. In particular, Pediococcus acidilatici has especially strong antagonism, and its efficacy in inhibiting the growth of miscellaneous bacteria is 10 times that of general lactic acid bacteria. In addition, it contains large amounts of B vitamins, can produce high doses of lactic acid in the body, inhibit the growth of miscellaneous bacteria, stimulate the secretion of digestive enzymes, and increase protein utilization.
V. Liver-protecting and gallbladder-strengthening substances
The liver is an important gland in the eel's body. It can secrete bile, which is stored in the gallbladder. Besides helping to digest fat, it can also store excess glucose in the body in the form of glycogen, synthesize vitamin A in the liver, and also detoxify toxic substances produced by the abuse of antibiotics, insecticides and so on. It can be said that the liver is the “chemical plant” in the eel's body. In order to strengthen the normal absorption function of eels and eliminate the adverse effects caused by eels living long-term in high-density environments, deteriorating water quality, abuse of drugs and so on, it is necessary to add some Chinese medicines, bile acids and other liver-protecting and gallbladder-strengthening substances to eel feed.
1. Bupleurum: Bitter in nature, slightly cold, with the function of releasing the exterior and harmonizing the interior. It contains saponins, volatile fatty acids and other substances, and has antiviral and protozoa-killing effects, as well as resistance to liver damage and other functions.
2. Isatis root: Bitter in nature, cold, with indigo glycoside as the main component, and with the function of clearing heat and detoxifying. Chinese patent medicines are generally sold on the market at 10-15 grams added per kilogram of feed.
3. Bile acids: They can promote the digestion and absorption of fat, fat-soluble vitamins and cholesterol. Against the rancid substances produced by ingesting aquatic feed with high oil content, they have detoxifying and liver-strengthening effects. Generally, 100 grams are added per ton of feed.
4. Cholagogues and other liver-protecting and gallbladder-strengthening agents can strengthen liver function, promote bile secretion, increase cholesterol excretion, reduce fat deposition in the liver, promote the physiological metabolic effects of carbohydrates and saccharides, and regulate the endocrine system. These substances (including fatty acids, fermentation concentrates and dehydrocholic acid, etc.) can enable the eel's liver to eliminate toxins in the blood through hydrolysis, oxidation, acetylation and other effects, regenerate damaged liver cells, and enhance the secretory function of liver cells.