In recent years, acidifiers have been used as high-efficiency, non-polluting, residue-free feed additives, along with probiotics, enzyme preparations, and flavoring agents, as new types of green feed additives, and the application of feed acidifiers has become increasingly common. At present, acidifiers have been widely used in piglet feed, poultry feed, silage and other fields, and the effect is remarkable, but there are still some problems, such as some acidifiers are not stable in use, large in additive amount, high in cost, and premixed. Inconvenient, some alkaline substances in the feed often neutralize a part of the added acid, absorb too fast in the stomach, inhibit gastric acid secretion and normal development of gastric function, easy to absorb moisture and agglomerate, corrosion processing machinery, transportation equipment and the like.
At present, there are many acidifier products on the domestic and international markets, which can be divided into: organic acids and inorganic acids, single acids and complex acids according to different classification methods. The complex acid may be a composite organic acid or an organic and inorganic complex acid. The organic acid can be divided into macromolecular organic acid (lactic acid CH3CH(OH)COOH, molecular weight 90; citric acid C3H4-OH-(COOH)3, molecular weight 192; fumaric acid COOHCH=CHCOOH, molecular weight 116; sorbic acid CH3CH=CHCH=CHCOOH, Molecular weight 112) and small molecule organic acid (formic acid CH2O2, molecular weight 46; acetic acid C2H4O2, molecular weight 60; propionic acid C3H6O2, molecular weight 74; butyric acid CH3(CH2)2COOH, molecular weight 88). The inorganic acid is mainly orthophosphoric acid. The quality of acidifier products is not only closely related to its active ingredients and content, but also greatly affected by factors such as production processes. How to choose a good acidifier from many similar products is a problem for feed production and animal producers.
The author believes that in order to choose a good acidifier product, we must first have a clear understanding of the functions that the acidifier should have. In addition, we must have a clear and objective understanding of various products, so that we can be targeted.
1. The function of acidifier should be
1) Reduce feed pH and acid binding, promote gastric zymogen activation
The acid binding capacity of feed and raw materials is defined as the millimoles of hydrochloric acid required to reduce the pH of 100 g of feed (raw material) to 4.0. It is also defined as the amount of hydrochloric acid required to reduce the pH of 1 kg of feed to 3.0. . For all young animals, the pH of the 100 g feed was reduced to the optimal range of milligrams of hydrochloric acid in the stomach of the animal. The higher the acid binding capacity, the more free acid is bound in the stomach, and the higher the pH in the stomach is. It is well known that some important zymogens in the stomach, such as pepsinogen, require gastric acid to activate before they are catalytically active. Feeds with high acid binding affect the activation of the zymogen, resulting in low activity of digestive enzymes in the stomach, affecting the digestion of nutrients, especially proteins, in the feed. In addition, when the pH in the stomach is high, the acidity in the stomach does not reach the sterilization requirement, and microorganisms such as Escherichia coli attached to the feed enter the small intestine and can multiply in the small intestine (Sissons et al. 1989). Moreover, the undigested stomach contents provide good media conditions for E. coli growth.
Studies have shown 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 to say, 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 levels of protein to meet the body's needs. Young animals have insufficient gastric acid secretion and are easily attacked by high-protein feeds with high acid binding capacity, which affects the digestive physiology of animals, leading to a combination of low feed intake and diarrhea. Disease, seriously affecting animal performance. In addition, the acid binding capacity of mineral raw materials, especially calcium sources and phosphorus source materials, is very high. In the laying hens, because of the large amount of calcium-containing raw materials, it has a high acid binding capacity, which seriously affects the digestion of the feed.
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 the acidulant to lower the pH and acid binding depends on the degree of dissociation and molecular weight of the acid used. Inorganic acids have a high degree of dissociation and are fast, so they can quickly reduce the pH of the feed and stomach. However, the degree of dissociation of organic acids is relatively low, so the ability to lower the pH 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 cost of addition, research and practical applications have shown that inorganic acids do not act as organic acids. On the one hand, the sharp decrease in pH in the stomach may destroy the function of the gastric mucosa and even burn the gastric mucosa, inhibit the normal development of gastric acid secretion and gastric function; on the other hand, the inorganic acid 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, causing a decrease in feed intake, which seriously affects feed remuneration and animal growth.
Macromolecular organic acids such as citric acid, lactic acid, fumaric acid, etc., because of the relatively large molecular weight, the number of hydrogen ions per unit weight of acid molecules can be dissociated. Therefore, they are less effective at lowering pH and feed acid binding than small molecule organic acids. Some macromolecular organic acids are so-called polybasic acids, that is, an acid molecule can theoretically dissociate several hydrogen ions, but the dissociation of the polybasic acid above the second-order is very difficult, usually the degree of dissociation is very small, so the actual The organic acid in the feed can only be dissociated at a level.
Therefore, from the purpose of lowering the pH value of the feed and the acid binding force, the small molecule organic acid has a better effect than the inorganic acid and the macromolecular organic acid.
2) Acidifier should promote intestinal micro-ecological balance and prevent intestinal microbial diseases in animals.
A balanced and stable micro-ecological environment is important to prevent animal diseases. The optimum pH values ​​for the growth of several pathogenic bacteria are neutral alkaline, such as Escherichia coli suitable pH value of 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, the acidifying agent can inhibit the proliferation of harmful microorganisms, reduce the consumption of nutrients and the production of bacterial toxins by lowering the pH of the gastrointestinal tract, and at the same time promote the proliferation of beneficial bacteria. In addition to lowering the intestinal pH, organic acids have another mechanism of action to kill pathogens. After the organic acid enters the gastrointestinal tract of the animal, a part of it dissociates to produce hydrogen ions, thereby lowering the pH value, while the other part is not dissociated but exists in a molecular form. Only this part of the organic acid present in the form of molecules can enter the interior of the bacteria through the cell membrane of the bacteria. The pH in the bacterial cells is neutral, so the organic acid molecules will dissociate here to produce hydrogen cations and carboxy root anions. Hydrogen cations reduce the pH value of the bacteria cells, and the bacteria must maintain the pH of the cells at around 7.0 to maintain normal life. Therefore, the bacteria cells need to discharge the hydrogen cations out of the cells through the H+-ATP pump. Energy (ATP), which causes the bacteria to lose their vitality. On the other hand, carboxylate anions inhibit the synthesis of DNA and proteins in the nucleus of bacteria, making it impossible for bacteria 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 degree of dissociation of the acid, and the lower the degree of dissociation, the stronger the bactericidal effect of the acid, and the higher the degree of dissociation, the worse the direct bactericidal effect of the acid. In general, inorganic acids (such as phosphoric acid) have a high degree of dissociation, so their direct bactericidal effect is poor; the degree of dissociation of organic acids is low, and the bactericidal effect is relatively strong. The bactericidal effects of different organic acids in organic acids are also different. Among them, citric acid and lactic acid are more dissociated than other organic acids, and the bactericidal effect is poor. Formic acid, acetic acid and propionic acid have better bactericidal effects. Therefore, formic acid, acetic acid and propionic acid are ideal organic acids with bactericidal action. In addition, the role of the acidifier depends on the concentration of the acid and the number of molecules. The higher the concentration, the more the number of molecules, the better the effect, so the bactericidal effect of the macromolecular organic acid is relatively poor.
Inorganic acid can inhibit the growth of some bacteria by lowering the intestinal pH, but because the inorganic acid dissociation rate is fast, its effect is difficult to reach the posterior segment of the intestine, so the actual antibacterial effect of the inorganic acid is not good. The macromolecular organic acid is difficult to enter the inside of the bacterial cell because of its large molecular size, so the antibacterial effect of the macromolecular organic acid is not as good as that of the small molecule organic acid.
In addition, studies have shown that complex organic acids composed of small organic acids have stronger bacteriostatic action than single small organic acids. R猀猀氀 climbing (1997) tested the minimum inhibitory concentration of formic acid, acetic acid, propionic acid, fumaric acid, citric acid and lactic acid against Escherichia coli, and found that formic acid had the lowest effective inhibitory concentration, ie, the best antibacterial effect. Followed by propionic acid, acetic acid and fumaric acid, while lactic acid and citric acid have the worst bacteriostatic effect. They also found that the complex of formic acid and propionic acid has a bacteriostatic ability of 2-4 times that of formic acid or propionic acid monoacid.
3) Promote the digestion of nutrients
As mentioned above, feed acidifiers promote the activation of zymogens, particularly pepsinogen, thereby increasing protein digestion. In addition, the acidifying agent can combine with some mineral elements to form a complex that is 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, affecting the absorption of minerals, and also beneficial for the absorption of vitamins (such as VA, VD). Adding propionic acid to the broiler diet can increase the elasticity, compressive strength and bone calcium content of the tibia, and the bone calcium content is positively correlated with the amount of propionic acid added in the diet.
4) Other
The addition of organic or inorganic acids slows the emptying of food in the stomach, thereby increasing the residence time of the protein in the stomach and increasing the digestibility of the protein. In addition, the sour taste of some organic acidifiers is one of the animal's favorite tastes, which can induce the taste buds to excite and increase the feed intake. At the same time, it can mask some bad taste reactions in the feed and improve the palatability of the feed.
Some believe that organic acids can directly participate in the metabolism of the body and provide energy for animals. It is true that some organic acids are important intermediates in the energy conversion process, and in theory they can participate in the tricarboxylic acid cycle and ultimately provide energy to the animal. However, because the total amount of exogenous acidifiers is small and expensive, it is economically uneconomical to use organic acids as an energy source.
2. The type of acidifier and its advantages and disadvantages
1) Inorganic acidifier
Inorganic acidifying agents include hydrochloric acid, sulfuric acid, and phosphoric acid, and phosphoric acid is mainly used in production. The advantage is that it has strong acidity and low added cost. The dissociation rate of the inorganic acid is fast, which causes the pH of the animal's esophagus and stomach to drop sharply, which may burn the esophagus and stomach, inhibit the gastric acid secretion and the normal development of gastric function. Moreover, the pH-reducing effect of the inorganic acid can only reach the stomach and cannot reach the posterior segment of the intestine, so the action of the inorganic acid to suppress the harmful bacteria by reducing the pH is very limited. In addition, inorganic acids may also disrupt the electrolyte balance of the diet, causing a decrease in feed intake, which seriously affects feed remuneration and animal growth.
2) Organic acidifier
The organic acidifier has a higher price, but has a good flavor and a strong antibacterial effect, and has a good effect in promoting the growth performance of piglets. After in-depth discussion on the mechanism of action of organic acids, organic acids are divided into two categories: 1 can only reduce the number of harmful bacteria by reducing the pH of the gastrointestinal environment, such as fumaric acid, citric acid, Macromolecular organic acids such as malic acid and lactic acid. This kind of organic acid can only play its role in the stomach, can not reduce the pH value in the small intestine; and because the molecular weight is relatively large, the acid molecules per unit weight release less hydrogen ions, so their pH lowering effect is also smaller than that of small molecules. Poor acid. 2 not only can reduce the pH value in the environment, but also inhibit Gram-negative bacteria, because they can destroy the bacterial cell membrane to interfere with the synthesis of bacterial enzymes, thereby affecting the replication of the DNA of the bacteria, and finally produce the anti-Gram-negative bacteria. . Such organic acids are small organic acids such as formic acid, acetic acid, and propionic acid.
3) Composite acidifier
The composite acidifying agent is a composite of various specific organic acids and inorganic acids. The composite acidifier overcomes the shortcomings of single-type acidifiers such as single function, large amount of addition, and high corrosivity. The high-quality composite acidifier can play a synergistic effect with each other, which can greatly improve the function of the product, not only can lower the pH value of the gastrointestinal tract, but also has a good antibacterial and bactericidal effect, so the composite type The function of the acidifier is generally superior to that of a single type. Nowadays, a single type of acidifier has been gradually replaced, which has become a trend in the development of feed acidifiers.
3. Use organic acid salts and carriers to protect feed nutrients, equipment and workers.
Although the acidifier has an acidification effect on the feed, the directly added acid may corrode the feed production equipment, destroy the nutrients in the feed, especially vitamins and minerals, and damage the animal's esophagus and stomach. Personal injury can also result if not handled properly. Moreover, the directly added acid begins to dissociate rapidly in the esophagus and stomach, and the action time is very short, and may also inhibit gastric acid secretion and normal development of gastric function. The pathogenic microorganisms are mainly located in the posterior segment of the intestine, so the directly added acidifying agent does not play a bacteriostatic role.
The use of weak base salts of organic acids can greatly reduce these disadvantages. Studies have shown that organic acid weak base salts have comparable bacteriostasis to organic acids. In addition, the use of a carrier with a sustained release function not only prevents the acid from reacting with the nutrients in the equipment and feed, but also allows the acid to slowly release in the stomach and intestines, so that its action continues until the intestines of the intestine, which is good. Antibacterial effect.
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