First, the mechanism of action of acidifiers
1. Reduce the pH of the gastrointestinal tract and increase the activity of digestive enzymes:
Before weaning, piglets, because they contain a large amount of lactose, are converted into lactic acid by the action of Lactobacillus in the stomach, so that the pH value in the stomach can be maintained at 4 to digest the milk protein. In the early weaned pigs, the digestive system and immune organs were not well developed, and the secretion of enzymes and gastric acid in the digestive tract was insufficient. Because of the sudden change from breast milk to the intake of solid feed, it also caused a significant increase in the pH value in the stomach. High, can reach 5.5 or more (Manner et al. 1984), a large number of experiments that the optimal pH value required by pepsin is 2.0-3.5, and the activity is weakened or even inactivated when PH>4.0. It is generally believed that a 28-day-old weaned pig can only secrete gastric acid equivalent to 20 ml of 1 mmol hydrochloric acid per day, so the feed that can be digested on the first day after weaning does not exceed 57 g, and the feed that is eaten can not be used well by piglets. The undigested feed produces enterotoxin under the action of harmful bacteria after the chyme enters the digestive tract, which causes the piglets to squat; when the acidifying agent is added, the acidity of the feed is lowered, and the pH value in the stomach decreases. The activity of pepsin can be further increased, and the amount of digested feed is correspondingly increased. At the same time, the acidity of the intestinal contents also affects trypsin. chymotrypsin. Carboxypeptidase. Amylase. Lipase. Secretion and activity of maltase and lactase.
2. Improve the gastrointestinal microflora
1) The normal gastrointestinal microbial ecosystem of early weaned piglets has not been established, and the pH of the gastrointestinal tract is further increased due to the stress of weaning, which can reach more than 5.5 harmful bacteria in the digestive tract (E. coli. Salmonella. Grape Hill) Mass reproduction, such as bacteria, (see Table 1) may cause intestinal microbial imbalance and cause normal physiological dysfunction of the gastrointestinal tract (Miller et al. 1985).
Table 1 PH range of growth of several major pathogens
Barrow et al. (1977) found that on the second day after weaning, the number of lactic acid bacteria in the intestine decreased significantly; the number of E. coli increased sharply. Mathew et al. (1991) reported that the number of lactobacilli in the ileal contents decreased to near zero within 2 days after weaning, and the number of E. coli increased significantly and was positively correlated with the increase in pH of the ileal contents. When the acidifier is added to the feed, the pH of the feed and digestive tract can be lowered. When the pH is less than 4.0, the above pathogens are largely inactivated, and various acid-resistant beneficial bacteria such as lactic acid bacteria and bifidobacteria are multiplied, Fuller ( 1977) Studies have shown that acidic conditions are conducive to the growth and growth of lactic acid bacteria, and have an inhibitory effect on the growth and reproduction of harmful microorganisms such as Escherichia coli. And the lactic acid, a metabolite of lactobacillus, can prevent E. coli from binding to its receptor in the intestine and inhibit the growth of E. coli (danielson et al. 1989), forming a beneficial microflora;
2) The theoretical basis of the antibacterial effect of acidifier:
In addition to lowering the pH value of feed and gastrointestinal tract, the acidifier can also inhibit the growth and reproduction of various pathogenic bacteria to some extent, and the different bacteriostatic effects of different acids due to different structural forms and dissociation in solution are also different. In general, the main antibacterial mechanism of acidifiers is as follows:
a, the transport mechanism of pathogens nutrients: in the transport of nutrients, because the bacterial cells have a certain negative charge, non-ionic compounds can enter the cell, while the ionic type can not enter. In a neutral or alkaline environment, organic acids cannot enter bacterial cells. In an acidic environment, these compounds are non-ionic and can enter cells, affecting the ion information of the dissociation groups, thereby promoting the inactivation of enzymes on the cell membrane. , affecting the operation of the material;
b. Inhibition mechanism of inorganic acid: Inorganic acid can be fully understood as acid ion and H+ in the digestive tract of animals. Its effect on bacteria is basically through H+ production effect, which reduces the osmotic pressure system of pathogenic bacteria and prevents the normality of pathogenic bacteria. Breeding, thereby inhibiting the growth and development of pathogenic bacteria;
c. Antibacterial mechanism of organic acids: organic acids can be separated according to the acidity of the digestive tract. In addition to the H+ effect, there are also some undissociated acid molecules. The degree of entry of organic acids into the cells of the pathogen depends on the difference in pH concentration inside and outside the cell, and also on the molecular weight, solubility and polarity of the organic acid; generally, the relative molecular weight is small, the polarity is small, and the organic acid with high solubility is easy. Diffusion into bacterial cells. Its mechanism of action on bacteria is:
1 causes changes in bacterial cell membrane charge, affecting the absorption of nutrients by bacteria: H+ is located on the outer side of the bacterial cell membrane, and OH- is located on the inner side. The potential difference generated by this proton gradient allows the bacterial cells to transport nutrients such as amino acids. After the undissociated organic acid enters the cell, the intracellular pH value is lowered, and the potential difference between the inside and outside of the bacterial cell membrane is weakened, thereby affecting the absorption of nutrients such as amino acids.
2 affect the activity of enzymes in the process of bacterial metabolism: organic acids can non-selectively interfere with the function of enzymes in bacterial cells and interfere with the permeability of bacterial cell membranes, which can block the separation of α-ketoglutarate and succinic acid in the Krebs cycle. The action of hydrogenase inhibits the function of phosphorylase in energy metabolism; it can be inactivated by binding to various sulfur-containing enzymes in cells, thereby inhibiting the growth of microorganisms.
d, antibacterial mechanism of volatile organic acid salts: the antibacterial effect of volatile organic acid salts is mainly dependent on their solubility and pH value of the digestive tract. Generally, the water-soluble bacteriostatic effect is greater than the water-insoluble, and The acidity of the digestive tract reaches a certain level, and it can be reduced to an organic acid in the stomach to achieve a bacteriostatic action.
Note: All of the above bacteriostatic effects are based on reducing the pH in the environment.
3. Slow down the emptying speed of the stomach, increase the residence time of the chyme in the stomach, and prolong the digestion time of the feed in the stomach: when the acidic chyme enters the small intestine, it produces chemical stimulation on the intestinal wall of the duodenum, so that it is secreted. Intestinal gastrin, feedback inhibits contraction of the stomach.
4, stimulate the development of digestive organs, promote the secretion of digestive glands, such as: can promote the development and secretion of the pancreas, promote the secretion and discharge of bile.
Second, the factors affecting the use of acidifiers
1. Varieties and quantities of added acid: different physicochemical properties and biological properties of different acids, the effect is not the same;
a. The molecular weight and dissociation constant of various acids (see Table 2) are different. Generally, the larger the ka, the smaller the PH value under the same concentration condition, the faster the H+ is given, and the greater the degree of dissociation; The organic acid with smaller molecular weight has better bacteriostatic effect than the organic acid with large molecular weight under the same PH value;
Table 2 Partial acid molecular weight and dissociation constant
b. The dissociation of various acids under different pH conditions (see Table 3) is different: according to the degree of dissociation of various acids under different pH conditions, the coordination of various acids can be carried out in a targeted manner, and the degree of general dissociation Larger acid can effectively reduce the PH value in the environment, and a considerable part of the acid with a small degree of dissociation is in a non-dissociated state; it can effectively inhibit the growth and development of various pathogens in the environment;
Table 3 Partial acid dissociation ratio under different PH conditions (%)
c. The effect of various acids on bacteria:
a) Formic acid, propionic acid: against Salmonella. Fungi. Clostridium and Gram-positive bacteria have inhibition and inactivation (Enthoven et al. 2002);
b) Fumaric acid: inhibition of Staphylococcus aureus. Escherichia coli, etc.;
c) The experiment concluded that under the condition of PH4, hydrochloric acid, lactic acid and formic acid showed certain antibacterial effects on Escherichia coli, among which hydrochloric acid was the weakest and lactic acid was stronger, while formic acid showed the strongest performance.
d. Different acids have different feed intake, daily gain and feed-to-weight ratio for weaned piglets, and the difference is large;
2. Feed composition: The pH value and acidity of different feed ingredients are different (see Table 4). The pH value and acidity of different feed diets are also different (see Table 5). The acidity and PH value of the feed. It greatly affects the PH value of the contents of the digestive tract of animals; Zhang Hongfu and other experiments believe that the feed intake, daily gain, feed utilization and feed utilization rate of weaned piglets are related to the acidity of the feed diet. When the diet's acidity declines, all the above indicators have improved, and when the diet's acidity level is 20, it is most ideal;
Table 4 PH value and acidity of feed ingredients
Table 5 PH value and acidity of different protein feeds
3, the absorption rate of acidifier: acidifier release or absorption rate in the stomach too fast, can inhibit the secretion of gastric acid and normal development of the stomach, and the added acidifier can not enter the small intestine, can not effectively reduce the back of the digestive tract PH value;
4, the addition time of acidifier: It is generally believed that the acidifier is mainly used in piglets, but the effect on growth and fattening pigs is not significant; the use of acidifier in sow feed can block the vertical transmission of E. coli in breast milk. For the growth of pigs, the use of acidifiers is not significant. It may be related to the choice of acid and acid treatment. If the organic acid is coated, most of it will not be released in the stomach. Direct access to the small intestine, in theory, should have a certain effect, but the actual results need to be demonstrated through a large number of experiments;
5. The type and amount of acidifier used are inconsistent, and there is a lack of an additive standard. The effect of adding less is not obvious, and metabolic acidosis may occur if more is added.
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