Feed acidifier, how much do you know?

In recent years, acidifiers, as feed additives with high efficiency, no pollution and no residue, are juxtaposed with prebiotics, enzymes, flavors, etc. as new green feed additives, and the application of feed acidifiers is becoming more and more common. At present, acidifiers have been widely used in piglet feed, poultry feed, silage feed and other fields, and the effect is remarkable, but there are still some problems, such as the use effect of some acidifiers is not stable, the amount of additives is large, the cost is high, and it is made into a premix. Inconvenient, some alkaline substances in the feed often neutralize part of the added acid, absorb too fast in the stomach, inhibit gastric acid secretion and the normal development of gastric function, easily absorb moisture and agglomerate, corrode processing machinery, transportation equipment, etc.

At present, there are many acidifier products on the market at home and abroad, which can be divided into: organic acid and inorganic acid, single acid and complex acid according to different classification methods. The complex acid may be a compound organic acid or an organic and inorganic compound acid. Organic acids can be divided into macromolecular organic acids (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 molecular organic acids (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). 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 process. How to choose a good acidifier from many congeneric products is a headache for feed production and animal producers.

The author believes that in order to choose a good acidifier product, we must first have a clearer understanding of the functions that acidifiers should have, and also have a clearer and objective understanding of various products, so as to be targeted.

1. The function that the acidifier should have

1) Reduce feed pH and acid binding capacity, and promote gastric proenzyme activation

The acid binding capacity of feed and feed is defined as the number of millimoles of hydrochloric acid required to bring the pH value of 100g of feed (feed) down to 4.0, and it has also been defined as the amount of hydrochloric acid required to bring the pH value of 1kg of feed down to 3.0. For all offspring, the pH value of 100g feed is reduced to the milligram equivalent of hydrochloric acid in the optimum range in the animal's stomach. The higher the acid binding capacity of the feed, the more free acids bound in the stomach, and the more pH rise in the stomach caused. It is well known that some important enzymes in the stomach, such as pepsinogen, require gastric acid to activate before they are catalytically active. Feeds with high acid binding capacity will affect the activation of proenzyme, resulting in low digestive enzyme activity in the stomach, which affects the digestion of nutrients, especially protein, in the feed. In addition, when the pH in the stomach is high, the acidity in the stomach cannot meet the sterilization requirements, and the 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 medium conditions for the growth of Escherichia coli.

Studies have shown that, in addition to mineral raw materials, the acid binding force of feed raw materials 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 force. This has a great impact on young animals and high-performance animals, because these animals must have higher protein in their feed to meet the needs of the body. Young animals often have insufficient acid binding capacity in their stomachs, and are easily impacted by high-protein feeds with high acid binding capacity, which affects the digestive physiological functions of animals, resulting in a syndrome characterized by low feed intake and diarrhea, which seriously affects animal production performance. In addition, the acid binding capacity of mineral raw materials, especially calcium and phosphorus source raw materials, is very high. The acid binding capacity of laying hens due to the large-scale use of calcium-containing raw materials has a high acid binding capacity, which seriously affects the digestion of feed by laying hens.

One of the main functions that feed acidifiers must have is to reduce the pH value and acid binding force of the feed, in order to improve the activity of digestive enzymes or activate some important enzymes. The ability of acidifiers to reduce pH value and acid binding force 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 value of the feed and the stomach. The dissociation degree of organic acids is relatively low, so the ability to reduce 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 lower cost of addition, research and practical applications have shown that the effect of inorganic acids is not as good as that of organic acids. On the one hand, a sharp reduction in gastric pH value can destroy the function of gastric mucosa and even burn the gastric mucosa, inhibiting 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 destroy the electrolyte balance of the diet, causing a decrease in feed intake, which seriously affects feed compensation and animal growth.

Macromolecular organic acids such as citric acid, lactic acid, fumaric acid, etc., because of their large molecular weight, less hydrogen ions can be dissociated per unit weight of acid molecules. Therefore, they are less effective than small molecular organic acids in reducing pH value and binding force of feed acids. Some macromolecular organic acids are so-called polyacids, that is, theoretically, one acid molecule can dissociate several hydrogen ions, but the dissociation of polymers above the second stage is very difficult, usually the degree of dissociation is very small, so in fact, organic acids in feed can only be dissociated at the first stage.

Therefore, for the purpose of reducing feed pH value and acid binding capacity, the effect of small molecular organic acids is better than that of inorganic acids and macromolecular organic acids.

2) Acidifiers should promote the balance of intestinal microecology and prevent intestinal pathogenic microbial diseases in animals.

A balanced and stable microecological environment is important to prevent animal diseases. The suitable pH value for the growth of several pathogenic bacteria is neutral and alkali. For example, the suitable pH value of Escherichia 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 Lactobacillus is suitable for reproduction in acidic environments. Therefore, acidifiers can inhibit the reproduction of harmful microorganisms by lowering the pH value of the gastrointestinal tract, reduce the consumption of nutrients and the production of bacterial toxins, and at the same time promote the proliferation of beneficial bacteria. In addition to lowering the pH value of the intestinal tract, organic acids have another mechanism of action to kill pathogens. After organic acids enter the gastrointestinal tract of animals, some dissociate to produce hydrogen ions, thereby reducing the pH value, while the other part does not dissociate but exists in molecular form. Only this part of the organic acids in molecular form can enter the interior of bacteria through the cell membrane of bacteria. The pH value in bacterial cells is neutral, so organic acid molecules will dissociate here to produce hydrogen cations and carboxyl negative ions. Hydrogen cations can reduce the pH value in bacterial cells, and bacteria must maintain the pH value in cells around 7.0 in order to maintain normal life. Therefore, bacterial cells need to expel hydrogen cations outside the cell through the H + -ATP pump, which consumes a lot of energy (ATP), thereby making bacteria lose vitality. On the other hand, carboxylic negative ions can inhibit the synthesis process of DNA and proteins in the bacterial nucleus, making bacteria unable to reproduce the next generation (Stratford & Anslow, 1998; Russel & Diez-Gonzales, 1998; Roeetal., 1998). Therefore, the direct germicidal effect of acidifiers depends on the degree of dissociation of the acid. The lower the degree of dissociation, the stronger the germicidal effect of the acid, and the worse the direct germicidal effect of the acid with the higher degree of dissociation. Generally speaking, the degree of dissociation of inorganic acids (such as phosphoric acid) is very high, so its direct germicidal effect is poor; the degree of dissociation of organic acids is low, and the germicidal effect is relatively strong. Among the organic acids, there are also differences in the germicidal effects of different organic acids. Citric acid and lactic acid have poorer germicidal effects due to their higher degree of dissociation than other organic acids, and formic acid, acetic acid, and propionic acid have better germicidal effects. Therefore, formic acid, acetic acid, and propionic acid are ideal organic acids with germicidal effects. In addition, the effect of acidifier also depends on the concentration and number of molecules of the acid. The higher the concentration, the more the number of molecules, and the better the effect, so the sterilization effect of macromolecular organic acids is relatively poor.

Inorganic acids can inhibit the growth of some bacteria by lowering the pH of the intestinal tract, but because of the fast dissociation speed of inorganic acids, it is difficult for them to reach the posterior segment of the intestinal tract, so the actual bacteriostatic effect of inorganic acids is not good. Because of the large molecules, it is difficult for macromolecular organic acids to enter the interior of bacterial cells, so the bacteriostatic effect of macromolecular organic acids is not as good as that of small molecular organic acids.

In addition, studies have shown that composite organic acids composed of small molecular organic acids have stronger bacteriostatic effects than single small molecular organic acids. R Pan (1997) tested the minimum inhibitory concentrations of formic acid, acetic acid, propionic acid, fumaric acid, citric acid and lactic acid on Escherichia coli and found that formic acid had the lowest effective inhibitory concentration, that is, the bacteriostatic effect was the best, followed by propionic acid, acetic acid and fumaric acid, while lactic acid and citric acid had the worst bacteriostatic effect. They also found that the bacteriostatic ability of the compound of formic acid and propionic acid was 2-4 times that of formic acid or propionic acid mono-acid.

3) Promote nutrient digestion

As mentioned above, feed acidifiers can promote the activation of proenzyme, especially pepsinogen, thereby improving the digestion of protein. In addition, acidifiers can combine with some mineral elements into complexes that are easily absorbed and utilized, promoting the absorption and retention of these elements in the body. At the same time, it also prevents minerals from forming insoluble salts that are not easily absorbed in alkaline environments, which affects the absorption of minerals and is also conducive to 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 tarsal bones, and the bone calcium content is positively correlated with the amount of propionic acid added to the diet.

4) Other

Adding organic or inorganic acids can slow down the emptying speed of food in the stomach, thus increasing the residence time of protein in the stomach and improving the digestibility of protein. In addition, the sour taste of some organic acidifiers is one of the favorite tastes of animals, which can induce taste bud excitement and increase feed intake. At the same time, it can also cover up some adverse taste reactions in the feed and improve the palatability of the feed.

Some views believe that organic acids can directly participate in the metabolism of the body to provide energy for animals. It is true that some organic acids are important intermediates in the energy conversion process, and theoretically they can participate in the tricarboxylic cycle and eventually provide energy for animals. However, because the total amount of exogenous acidifiers is small and expensive, it is obviously not economical to use organic acids as an energy source.

2. Types of acidifiers and their 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. Its advantages are strong acidity and low addition cost. The dissociation speed of inorganic acids is fast, which makes the pH value of the esophagus and stomach of animals drop sharply, which may burn the esophagus and stomach, inhibit the secretion of gastric acid and the normal development of gastric function. Moreover, the pH-lowering effect of inorganic acids can only reach the stomach, but not the posterior part of the intestine, so the effect of inorganic acids on inhibiting harmful bacteria by lowering the pH value is very limited. In addition, inorganic acids may also destroy the electrolyte balance of the diet and cause a decrease in feed intake, thereby seriously affecting feed compensation and animal growth.

2) Organic acidifier

Organic acidifiers are expensive, but have good flavor and strong bacteriostatic effect, and have a good effect on promoting the growth performance of piglets. After in-depth discussion of the mechanism of action of organic acids, people divided organic acids into two categories: ① The effect of indirectly reducing the number of harmful bacteria can only be achieved by reducing the pH value of the gastrointestinal environment, such as fumaric acid, citric acid, malic acid, lactic acid and other macromolecular organic acids. This type of organic acid can only play its role in the stomach and cannot reduce the pH value in the small intestine; and because the molecular weight is relatively large, the hydrogen ions released by the acid molecule per unit weight are relatively small, so their pH-lowering effect is also worse than that of small-molecule organic acids. ② Not only can it reduce the pH value in the environment, but it also has an inhibitory effect on Gram-negative bacteria, because they can destroy bacterial cell membranes and interfere with the synthesis of bacterial enzymes, thereby affecting the replication of bacterial DNA, and finally produce the effect of anti-Gram-negative bacteria. Such organic acids include small molecules such as formic acid, acetic acid, and propionic acid.

3) Composite acidifier

The compound acidifier is composed of various specific organic acids and inorganic acids. The composite acidifier overcomes the shortcomings of a single type of acidifier, such as single function, large addition amount and strong corrosiveness. The components of the high-quality composite acidifier can play a good synergy with each other, which can greatly improve the function of the product. It can not only reduce the pH value of the gastrointestinal tract, but also has a good bacteriostatic and bactericidal effect, so the function of the composite acidifier is generally superior to that of the single type. Now it has gradually replaced the single type of acidifier and has become the development trend of feed acidifier.

3. Use organic acids and carriers to protect feed nutrients, equipment and workers

Although acidifiers have acidifying effects on feed, directly added acids may corrode feed production equipment, destroy nutrients in feed, especially vitamins and minerals, and damage the esophagus and stomach of animals. If not handled properly, it may also cause personal injury. Moreover, the directly added acid begins to dissociate rapidly in the esophagus and stomach, and the action time is very short, and it may also inhibit the secretion of gastric acid and the normal development of gastric function. The pathogenic microorganisms are mainly located in the posterior section of the intestine, so the directly added acidifiers cannot play a bacteriostatic effect.

The use of weak alkali salts of organic acids can greatly reduce these unfavorable factors. Studies have shown that weak alkali salts of organic acids have the same bacteriostatic effect as organic acids. In addition, the use of a carrier with a slow-release function not only prevents the acid from reacting with the nutrients in the equipment and feed, but also allows the acid to be released slowly in the stomach and intestines, so that its effect continues to the back of the intestine, which has a good bacteriostatic effect.

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