In modern pig production, in order to improve the performance of sows, most of them are weaned at 3 to 5 weeks old, but the digestive system and immune organs of early weaned pigs are not well developed, and the secretion of enzymes and gastric acid in the digestive tract is insufficient. The normal intestinal tract Micro-ecological systems have not yet been established (Yu Xuhua et al., 2002). Weaned piglets suddenly changed from sucking breast milk to eating solids and diet. Due to insufficient secretion in the stomach, the acidity in the stomach decreased significantly, which led to a decrease in feed digestibility, which led to low feed intake and weak disease resistance. Weaning syndrome, which is characterized by diarrhea and growth retardation (Tzipori et al., 1980; Barnett et al., 1989), seriously affects the growth performance of weaned piglets and is one of the major problems in pig production. Because acidifiers have the effect of lowering gastrointestinal pH, activating pepsin, improving the gastrointestinal microflora and improving the performance of early weaned piglets (Sciopioni et al., 1978; Kidder and Manners, 1978; Johnson, 1991; Gabert and Sauer) , 1994), at the same time, it has the advantages that antibiotics and other drugs do not have, and it has received people's attention. However, some scholars (Burnell et al., 1988; Risley et al., 1992) reported different results, which may be many factors affecting the action of acidifiers. At present, the addition of acidifiers in the feed of early weaned piglets has been widely used, but the effects of these acidifiers have been reported differently. The effects of different types of acidifiers are also different. The mechanism of action of various acidifiers is not the same. There are many factors affecting the action of acidifiers, so the mechanism of action needs further research. This paper reviews the mechanism of action of acidifiers and the influencing factors.
1 Mechanism of action of acidifier
The application of acidifiers has been around for more than 30 years, but its mechanism of action is still not very clear. The mechanisms currently widely accepted are as follows:
1.1 Acidification effect
The acidifying agent can reduce the pH value of the young animal's stomach, provide a reasonable animal digestion environment, activate the pepsin and maintain high activity, help the softening of the feed, the decomposition of nutrients, and prevent the pathogenic microorganisms from entering the body through the digestive tract from the external environment. . Stimulates the duodenum to secrete more trypsin, allowing the protein to be completely absorbed.
1.2 Participate in the metabolism, anti-stress in the body
Some organic acids are directly involved in the body's energy, structure and enzymatic reactions, and can provide the energy needed for animal stress. For example, fumaric acid and citric acid are indispensable components in the tricarboxylic acid cycle, and are easy to use energy (Wang Yanyan, 2002) ). In particular, fumaric acid is an essential component of the cyclic aerobic process of tricarboxylic acid. The molecular energy is consistent with glucose, but its energy generation pathway is shorter than glucose. It can be used for emergency generation of ATP under stress and in livestock production. Stress agent.
1.3 Promote the absorption of nutrients
The addition of KDF (potassium diformate) to the feed increases the digestibility and retention of some nutrients (including dry matter, energy, fat, nitrogen, some essential amino acids and phosphorus) (Roth et al., 1998a, b). Organic acids can form readily chelating chelate with some mineral elements. For example, fumaric acid and citric acid can form a bioavailable chelate with Ca, Cu, Zn, Fe and Mg to promote their absorption and retention. (Shi Baoming, 1999). Many scholars have confirmed that the addition effect of high copper and acidifier has an additive effect, that is, fumaric acid, citric acid or phosphoric acid forms a bioavailable complex with copper, which promotes the absorption and retention of copper and reduces it. Oxidation catalytic activity of copper. Some organic acids such as fumaric acid have antioxidant effects, and citric acid is a synergist for antioxidants. The addition of fumaric acid to the premix was preserved for 6 months, and the stability of VA and VC was improved compared to the absence of fumaric acid. At the same time, the acidic environment of the small intestine is beneficial to the absorption of VA and VD.
1.4 Impact on microflora
Lin Yingcai et al (2001) added complex acidifier to early weaned piglets at 14 days of age. The results showed that the addition of complex acidifier 1 500g/t increased the number of lactobacilli, E. coli and total bacteria in the small intestine of piglets, and the total number of bacteria in the small intestine. The proportion of lactobacilli in the middle increased, the proportion of E. coli decreased, and the ratio of the number of lactobacilli to the number of E. coli also increased. This indicates that the complex acidifier not only directly affects the growth of bacteria in the intestinal tract of piglets, but also affects the balance of microflora in the intestinal tract.
1.5 Antibacterial effect
Cherring et al (1991) and Russell (1992) summarize the different views on the mechanism of antibacterial action of organic acids. The antibacterial activity of organic acids is related to the decrease in pH, and the antibacterial activity increases with the decrease of pH. Non-dissociated organic acids are fat-soluble and thus able to enter microbial cells, but the carrier mechanism also appears to be involved in membrane transport. Once in the cell, the acid releases protons in a relatively alkaline environment, resulting in a decrease in intracellular pH. The microbial mechanism inhibits the action of important enzymes of the microbe and forces the bacterial cells to dissipate protons, causing the accumulation of anions in the cells, which is dependent on the pH gradient of the membrane. Acid anions are important for the antibacterial action of organic acids and their salts. Nuria Canibre etal (2002) reported that some studies have shown that organic acids have a strong bactericidal effect without significantly lowering the pH of the gastrointestinal tract. In general, lactic acid bacteria can survive at lower pH values, which means that they are more tolerant to organic acids than other bacteria such as E. coli. One explanation is that the concentration of potassium in the cells of Gram-positive bacteria is higher and neutralizes the anion of the acid (Russell & Diez-Gonzalez, 1998).
1.6 Other functions
The acidifying agent lowers the pH value of the stomach contents, and the acidic chyme enters the small intestine to chemically and mechanically stimulate the small intestine wall, so that it secretes enterostatin, reflexively inhibits gastric peristalsis, slows the gastric emptying speed, and makes the protein more Time digests in the stomach, reducing the burden on the small intestine (Kirchgessner and Roth, 1982 Donald et al, 1991). The acidifying agent can also enhance immunity, improve the palatability of the diet, and reduce the metabolism of intestinal microbes such as ammonia and polyamines, and serve as a mold preservation preservative for the preservation of the feed.
2 Influencing factors
At present, domestic acidifier products are single, and most of them belong to a single acid preparation. A small number of complex acidifier products rarely consider animal physiological characteristics and feeding environment, and there are many factors that affect the effect of acidifiers. The effect of adding acidifiers to a wide variety of materials varies widely.
2.1 Adding species and dosage
Different acidifiers have different physical and chemical properties and biological properties, and their effects are different. For example, fumaric acid has strong inactivation activity against Staphylococcus, Streptococcus, Escherichia coli, etc., and has no inhibitory effect on lactic acid bacteria; formic acid, propionic acid against Salmonella, fungi, Clostridium, Bacillus, Gram-positive bacteria, etc. It has a strong inhibitory effect; sorbic acid can effectively inhibit the growth of yeast and mold, and it is good for the growth of beneficial microorganisms (Guo Fangbin, 1996). The addition amount is insufficient to achieve the desired effect, such as the pH cannot be lowered to an appropriate level to more fully increase the activity of the digestive enzyme. Excessive addition will affect palatability and destroy acid-base balance, which will lead to serum alkaline storage, increased HCO3-concentration, and increased pH of stomach and small intestine contents. To some extent, it causes metabolic acidosis.
2.2 The composition of the diet is different.
Diet is the main exogenous factor affecting the acidity of digestive tract in piglets. The acidity of the diet is an important factor, and the acidity of the diet of piglets is closely related to the selection of raw materials and the determination of nutrient levels. The acidity of different feed ingredients is different, among which the mineral materials have the highest acidity, the animal raw materials are the second, and the vegetable feed ingredients have lower acidity. Bolduan et al. (1988) and Zhang Hongfu et al. (2001) respectively determined the acidity of some feed ingredients as shown in Table 1. R. Blank et al. (1999) reported that the addition of fumarate to low-buffered diets increased the digestibility of ileum CP, GE, and major amino acids. The best response was 2% FA, and the apparent digestibility of ileal amino acids increased by 4.9. ~12.8 units, while the addition of FA to high buffer capacity diets only increased the digestibility of amino acids other than CP, GE and histidine. High buffer capacity diets reduce amino acid digestibility by 1 to 10 percentage points compared to low buffer capacity diets.
2.3 Protein Types and Levels
The acidity required for digestion of different types of proteins in the gastrointestinal tract is different. For example, a suitable pH value for milk products is 4, and soy protein and fish meal require an acidity of pH 2.5. Dietary protein levels are also important factors influencing the effectiveness of acidifiers. It is reported that 2% of fumarate is added to both diets with 16% and 20% crude protein levels. The average daily gain is 281 g and the feed efficiency is 0.49. They are 329g and 0.62 respectively.
2.4 absorption rate in the stomach is too fast
Inhibition of gastric acid secretion and normal development of gastric function, the added acid can not reach the small intestine, can not effectively reduce the pH value in the small intestine, inhibit the growth of harmful bacteria, and promote the growth of beneficial bacteria. A study on the metabolism of broiler diets with propionic acid showed that only a small amount of propionic acid reached the lower digestive tract and the cecum (Hume et al., 1993). A mixture of formic acid and propionic acid was added, and higher concentrations were found only in the sac and the sand sac (Thompson and Hinton, 1997).
2.5 Adding time
The best time is 4 to 5 weeks old (Ou Xiuqiong, 1999). Donald et al. (1991) Lin Yingcai et al. (2001) reported that the acidifier had better effect on piglets within 2 weeks after weaning than piglets after 2 weeks of weaning.
2.6 Adaptability
The tolerance of prokaryotic or eukaryotic microorganisms to the acidic environment in the gastrointestinal tract has begun to receive attention. Recently, in the development of explanations including the regulation of recognition, such as structural genes have a special tolerance mechanism. Inducible acid tolerance has been demonstrated in many Gram-negative and Gram-positive bacteria. A consistent view is that microbes in an acidic environment can experience a deteriorating environment and undergo a process of molecular reaction through special stress protein synthesis. This protein is presumed to prevent or repair damage to macromolecules caused by stress. Some stress proteins are induced by a range of stress conditions, while others are induced by specific stress-producing responses (Bearson et al., 1997). In addition, different microorganisms, even those that evolved, have developed different ways of living in acid (Lin et al., 1995). According to a study by Bearson et al. (1997), there is a correlation between the response of enteric bacteria to acid and pathogenicity. Kwon and Ricke (1998) suggest that short-chain fatty acids in the gastrointestinal tract or in feed ingredients increase the toxicity of S. typhimurium by increasing its tolerance to acids. A study by Quivey et al. (2000) on the adaptive mechanism of Streptococcus mutans showed a significant change in the composition of fatty acids at pH 5 compared to growth at pH 7. According to many scholars, the ratio of saturated or unsaturated growth at low pH indicates that the change in cell membrane fatty acid composition may be an adaptive response to acid. This change can be achieved directly by changes in the permeability of the protons by the basic bimolecular fat or indirectly increasing the ability of the F1F0ATPase to liberate protons to affect the permeability of the cell membrane to protons. (Saklani-Jusforgues et al., 2000) studied the effect of acid adaptation on survival and mobility after infusion of L. monocytogenes in the stomach of mice. L. monocytogenes (grown at pH = 5.5) adapted to acid were found to have higher survival rates in the cecal and colonic cavities of mice than non-adaptive L. monocytogenes (growth at pH = 7.2). In addition, acid-adapted bacteria migrate more in mesangial lymph nodes than non-acid-adapted bacteria. Although the bacteria can adapt to the acid under in vitro conditions, it is not certain whether this adaptability is the same in the intestine when the organic acid is fed to pigs or chickens.
3 Conclusion
In the past, fumarate or citric acid was often used to acidify the diet. The organic acid was used as an animal feed additive, and its mechanism of action was different. Mixed use may have complementary effects, thereby increasing the use effect. In recent years, feed production has also begun to pay attention to the use of a variety of acids (organic and inorganic acids) in order to overcome the disadvantages of expensive single acidifiers and unsatisfactory acidification effects, and to concentrate their respective advantages. However, it is also necessary to consider its buffering capacity. Therefore, it is necessary to formulate a novel composite acidifying agent by forming a low pH buffer pair according to the characteristics of the acid and its salt. However, the complex acidifying agent absorbs too fast in the stomach and cannot achieve the effect of acidifying the stomach and intestinal contents. Therefore, the problem of slow release should be considered. At present, protective acidifiers have been developed abroad. By lipid coating, the other active ingredients in the feed and the feed mill equipment can be protected from damage and corrosion, and the acidifier can be slowly released, and the acidification can be extended to the small intestine, thereby enhancing the acidification effect, thereby reducing the acidifier in the diet. The ratio in the feed also reduces feed costs. In addition, the research on the compatibility of acidifiers with antibiotics, high copper, high zinc, enzyme preparations, and Chinese herbal medicines is still limited, and has great development prospects. This research should be increased. With the widespread use of organic acids, the adaptability of bacteria to acids will be a huge challenge for future work. The content of future research should also include: 1 determining the acidity and pH value of common feed ingredients; 2 establishing the influence model of acidity and pH value of the diet; 3 surveying the acidity and pH value of common diets; 4 determining The requirement of the acidity of the diet for piglets with different patterns and physiological conditions; 5 further reveals the relationship between the body base, the acidity of the diet and the acidifier through animal production performance, digestive physiology, blood physiology and urine physiology. Interaction; 6 to strengthen the mechanism of microbial tolerance to acid. In addition, while using the acidification of the diet, other methods can be combined to ensure that the digestive tract maintains a certain acidity and lowers the pH of the digestive tract. For example, to control the protein level and mineral content of the diet, add acidic skim milk and whey, add probiotics (lactic acid bacteria preparation), replace some bean cake powder with yeast, and ensure a certain proportion of crude fiber in the diet (generally 5%) ), limiting the use of protein feeds with a large degree of acid binding (Luo Modeling et al., 1996). The effect of the acidifier is remarkable, non-toxic, non-polluting, and non-resistance, which makes the acidifier a hot research topic after the antibiotics are banned. However, further research is needed to understand and clarify its mechanism of action, to find out the most suitable methods for the existing problems, to develop new products, to rationally use acidifiers in livestock production, and to target the physiological characteristics of animals and acidifiers. The physicochemical properties of the acid, the optimization of the compound acid ratio and the improvement of the production process, the research on the compatibility of acidifiers with antibiotics, probiotics, enzyme preparations, copper, zinc and other additives to continuously improve the use of acidifiers.
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