1) Reduce feed pH and acid binding, promote gastric zymogen activation
The acid binding power of feed and raw materials is defined as the millimoles of hydrochloric acid required to reduce the pH value of 100g feed (raw material) to 4.0, and some people have defined it as the milligram amount of hydrochloric acid required to reduce the pH value of 1kg feed to 3.0. . For all young animals, the pH value of 100 g of feed reduced to the milligram equivalent of hydrochloric acid in the animal's stomach. The higher the acid binding power of the feed, the more free acid is bound in the stomach, and the more the gastric pH is increased. It is well known that some important zymogens in the stomach, such as pepsinogen, require gastric acid to be activated before they have catalytic activity. Feeds with high acid binding ability will affect the activation of zymogen, resulting in low digestive enzyme activity in the stomach, affecting the nutrition, especially protein digestion in the feed. In addition, when the pH in the stomach is high, the acidity in the stomach cannot meet the sterilization requirements, and microorganisms such as E. coli attached to the feed enter the small intestine and can reproduce in the small intestine (Sissons et al. 1989). In addition, undigested gastric contents provide good media conditions for E. coli growth.
Research shows that in addition to mineral raw materials, the acid binding capacity of feed ingredients has a high positive correlation with its protein content (R2> 0.8), that is, the raw materials with high protein content generally have high acid binding capacity. This has a great impact on young animals and high-performance animals, because these animals must contain higher protein in the feed to meet the needs of the body. The gastric acidity of young animals is often insufficient, and they are easily affected by high protein feeds with high acid binding capacity, which affects the animal's digestive and physiological functions, and leads to a combination of low feed intake and diarrhea. Disease, which seriously affects animal performance. In addition, mineral raw materials, especially calcium and phosphorus raw materials, have very high acid binding power. Due to the large amount of calcium-containing raw materials in the laying hen feed, it has a high acid binding ability, which seriously affects the digestion of the feed by the laying hens.
One of the main functions that feed acidifiers must have is to reduce feed pH and acid binding to increase digestive enzyme activity or activate some important enzymes. The ability of an acidulant to reduce pH and acid binding depends on the dissociation and molecular weight of the acid used. Inorganic acids have a high degree of dissociation and are fast, so they can quickly lower the pH value in feed and stomach. The dissociation degree of organic acids is relatively low, so the ability to lower the pH value is lower than that of inorganic acids, but the dissociation rate is slow, so the action time is longer.
Although inorganic acids have strong acidity and low added cost, research and practical applications show that the effects of inorganic acids are not as good as organic acids. On the one hand, it is worthwhile to reduce the pH of the stomach sharply, which will damage the gastric mucosa and even burn the gastric mucosa, inhibit the gastric acid secretion and the normal development of gastric function; on the other hand, inorganic acids may bring bad taste to the feed and reduce the palatability of the feed; In addition, inorganic acids may also disrupt the electrolyte balance of the diet and cause a decline in feed intake, which seriously affects feed compensation and animal growth.
Macromolecular organic acids such as citric acid, lactic acid, fumaric acid, etc., because the molecular weight is relatively large, there are fewer hydrogen ions that can be dissociated per unit weight of acid molecules. Therefore, they are less effective at reducing pH and binding power of feed acids than small molecule organic acids. Some macromolecular organic acids are so-called polyacids, that is, in theory, an acid molecule can dissociate several hydrogen ions, but the dissociation of polyacids above the second level is very difficult, and the degree of dissociation is usually very small, so the actual Organic acids in the feed can only undergo first-order dissociation.
Therefore, for the purpose of reducing feed pH and acid binding power, small molecule organic acids work better than inorganic and macromolecular organic acids.
2) Acidifier should promote intestinal microecological balance and prevent intestinal pathogenic microbial diseases in animals.
A balanced and stable micro-ecological environment is important to prevent animal diseases. The suitable pH for the growth of several pathogenic bacteria is neutral alkaline, such as the appropriate pH of E. coli is 6.0-8.0, streptococcus is 6.0-7.5, staphylococcus is 6.8-7.5, clostridium is 6.0-7.5, and lactic acid Bacillus and the like are suitable for reproduction in an acidic environment. Therefore, by reducing the pH of the gastrointestinal tract, acidifiers can inhibit the reproduction of harmful microorganisms, reduce the consumption of nutrients and the production of bacterial toxins, and promote the proliferation of beneficial bacteria. In addition to lowering intestinal pH, organic acids have another mechanism of action to kill pathogens. After entering the animal's gastrointestinal tract, some of the organic acids dissociate to produce hydrogen ions, which lowers the pH value, while other parts do not dissociate but exist in molecular form. Only this part of the organic acids in molecular form can enter the bacteria through the cell membrane of the bacteria. In bacterial cells, the pH is neutral, so organic acid molecules will dissociate here to produce hydrogen cations and carboxylate anions. Hydrogen cations will reduce the pH value of bacterial cells. To maintain normal life, bacteria must maintain the pH value of the cells at about 7.0. Therefore, bacterial cells need to discharge hydrogen cations to the outside of the cells through the H + -ATP pump. This process consumes a lot of Energy (ATP), which inactivates bacteria. On the other hand, carboxylate anions can inhibit the synthesis of DNA and proteins in the nucleus of bacteria, making bacteria unable to reproduce the next generation (Stratford & Anslow, 1998; Russel & Diez-Gonzales, 1998; Roeetal., 1998). Therefore, the direct bactericidal effect of the acidifying agent depends on the dissociation degree of the acid. The lower the dissociation degree is, the stronger the acid sterilization effect is, and the higher the dissociation degree is, the worse the direct bactericidal effect is. In general, inorganic acids (such as phosphoric acid) have a high degree of dissociation, so their direct bactericidal effect is poor; organic acids have a low degree of dissociation, and their bactericidal effect is relatively strong. The bactericidal effect of different organic acids among organic acids is also different. Among them, citric acid and lactic acid have a lower sterilization effect because of higher dissociation than other organic acids, and formic acid, acetic acid, and propionic acid have better sterilization effects. Therefore, formic acid, acetic acid and propionic acid are ideal organic acids with bactericidal effect. In addition, the role of acidifiers also depends on the concentration of the acid and the number of molecules. The higher the concentration, the more the number of molecules, and the better the effect, so the bactericidal effect of macromolecular organic acids is relatively poor.
Inorganic acids can inhibit the growth of some bacteria by lowering the intestinal pH. However, because of the rapid dissociation of inorganic acids, it is difficult to reach the posterior intestinal tract, so the actual antibacterial effect of inorganic acids is not good. The large-molecule organic acid is difficult to enter the bacterial cell because of its large molecule, so the antibacterial effect of the large-molecule organic acid is not as good as that of the small-molecule organic acid.
In addition, research shows that composite organic acids composed of small-molecule organic acids have stronger antibacterial effects than single-molecule organic acids. Panyu (1997) tested the lowest bacteriostatic concentration of formic acid, acetic acid, propionic acid, fumaric acid, citric acid, and lactic acid against E. coli, and found that the lowest effective bacteriostatic concentration of formic acid, that is, the best bacteriostatic effect. Followed by propionic acid, acetic acid and fumaric acid, lactic acid and citric acid had the worst antibacterial effect. They also found that the complex of formic acid and propionic acid was 2-4 times more bacteriostatic than formic acid or propionic acid.
3) Promote digestion of nutrients
As mentioned above, feed acidifiers can promote the activation of zymogens, especially pepsinogen, thereby improving protein digestion. In addition, acidifying agents can be combined with some mineral elements into complexes that can be easily absorbed and utilized, and promote the absorption and retention of these elements in the body. At the same time, it can prevent minerals from forming insoluble salts that are not easily absorbed in an alkaline environment, which affects the absorption of minerals and also facilitates the absorption of vitamins (such as VA, VD). Adding propionic acid to the diet of broilers can increase the elasticity, compressive strength, and bone calcium content of the cheekbones, and the bone calcium content is positively correlated with the amount of propionic acid added to the diet.
4) Other
The addition of organic or inorganic acids can slow down the emptying rate of food in the stomach, thereby increasing the residence time of the protein in the stomach and improving the digestibility of the protein. In addition, the acidity of some organic acidifiers is one of the favorite tastes of animals, which can induce the excitement of taste buds and increase the feed intake. At the same time, it can also mask some bad taste reactions in the feed and improve the palatability of the feed.
Some opinions suggest that organic acids can directly participate in body metabolism and provide energy for animals. It is true that some organic acids are important intermediate products in the process of energy conversion. In theory, they can participate in the tricarboxylic acid cycle and ultimately provide energy for animals. However, because the total amount of external acidifiers is small and expensive, it is obviously not economically advantageous to use organic acids as energy sources.