In recent years, feed acidifiers have emerged as pollution-free, residue-free, and efficient feed additives. Together with probiotics, enzyme preparations, and flavoring agents, they have become green feed additives. They have successfully replaced some feed antibiotics, and their application in livestock production is becoming increasingly widespread.

Feed acidifiers are substances that can acidify feed. Initially, they were used as flavoring agents to improve feed palatability. Adding acidifiers to feed can lower the pH of the gastrointestinal tract, enhance the digestion of nutrients in the pig's gastrointestinal tract, inhibit the growth of harmful intestinal bacteria, promote animal growth, strengthen the animal's immune function, and reduce stress in pig production. They effectively replace antibiotics and are widely used feed additives.

1. Types of Feed Acidifiers

The acids used as feed acidifiers include inorganic acids and organic acids. Inorganic acids, primarily strong acids, currently mainly consist of phosphoric acid and hydrochloric acid. Inorganic acids have strong acidity and low addition costs. The efficacy of hydrochloric acid is influenced by dietary electrolyte balance. Phosphoric acid can act as an acidifier and also provide a source of phosphorus. A small amount of inorganic acid has an insignificant effect, while a high amount can affect feed palatability, damage oral mucosa, and corrode feed processing machinery. Organic acids are often used in pig production because they can improve feed palatability, participate directly in animal metabolism, improve feed quality, and enhance animal growth performance. Common organic acids include formic acid, acetic acid, propionic acid, butyric acid, lactic acid, citric acid, tartaric acid, malic acid, and fumaric acid. Although organic acidifiers are more expensive, they have good flavor and can enter the tricarboxylic acid cycle directly, which is beneficial for promoting the growth of young animals.

Both organic and inorganic acids have their specific advantages and disadvantages, and their mechanisms of action differ. Mixing them can produce complementary synergistic effects to enhance their effectiveness. To overcome the shortcomings or defects of a single acidifier, inorganic acids are now combined with organic acids or organic acids are combined with organic acids to form composite acidifiers. This improves the application effectiveness of acidifiers, rapidly lowers pH values, maintains good buffering capacity and biological performance, achieves optimal addition costs, and reduces the corrosiveness of acidifiers to equipment and irritation to skin and mucous tissues.

2. Mechanism of Action of Feed Acidifiers

2.1 Impact on the Intestinal Tract and Digestive Metabolism

Acidifiers can lower the pH of the digestive tract, activate pepsinogen in the stomach, stimulate the secretion of pancreatic enzymes in the duodenum, and enhance the activity of various digestive enzymes in the stomach. The digestive system of young animals is not fully developed, and the stomach acid secretion is insufficient, so acid must be supplemented from the feed to regulate the acidity of the digestive tract. A pH value of 2.03.5 in the stomach and 57 in the small intestine creates an acidic environment. Adding acidifiers to feed ensures that feed components are fully digested and absorbed, thereby improving growth performance and enhancing resistance.

Low pH can slow down gastric emptying, prolonging the residence time of feed in the stomach, and ensuring more complete protein degradation. The acidity can feedback to influence the emptying rate of the digestive tract, with a more pronounced effect in the stomach. Gastric emptying is mainly controlled by the pressure difference between the pylorus of the stomach and the beginning of the duodenum. When acidic chyme enters the small intestine, it chemically stimulates the duodenal wall, delaying gastric emptying, increasing the residence time of chyme in the stomach, and improving nutrient digestibility. Acids and fats can also promote the release of cholecystokinin and inhibit gastric movement, slowing gastric emptying. The inhibitory effect of the duodenum on gastric emptying disappears gradually as the acidic chyme is neutralized and food digestion products are absorbed.

Most organic acids are important intermediate products in energy conversion and can directly participate in metabolism. For example, the tricarboxylic acid cycle starts with the condensation of acetyl-CoA and oxaloacetate to form citric acid, and fumaric acid is also an intermediate product of the tricarboxylic acid cycle. Adding organic acids to the diet can improve the digestibility of dry matter, protein, and energy. Fumaric acid has a shorter energy supply pathway than glucose and can be used for emergency synthesis of ATP under stress, enhancing resistance.

2.2 Impact on Intestinal Microbes

A balanced and stable microbial environment is essential to prevent animal diseases. The optimal pH values for several pathogenic bacteria tend to be neutral or alkaline. For instance, E. coli prefers a pH of 6.0-8.0, streptococcus 6.0-7.5, staphylococcus 6.8-7.5, and Clostridium 6.0-7.5, while lactic acid bacteria prefer an acidic environment. Acidifiers can inhibit the growth of harmful microorganisms by lowering the pH of the gastrointestinal tract, reducing nutrient consumption, toxin production, and promoting the proliferation of beneficial bacteria. In addition to lowering the intestinal pH, some organic acids also have another function to kill gram-negative bacteria. After organic acids enter the animal's gastrointestinal tract, some dissociate to produce H+ to lower the pH, while the remaining parts exist in molecular form. Only the part existing in molecular form can enter the bacteria through the cell membrane. The intracellular pH of bacteria is neutral, so organic acid molecules dissociate into H+ and RCOO—. H+ reduces the intracellular pH of bacteria, and to maintain normal life, bacteria need to pump H+ out of the cell, consuming a lot of energy (ATP), thereby making bacteria lose vitality. RCOO— can inhibit the synthesis of DNA and proteins in bacterial nuclei, preventing bacteria from reproducing. Therefore, the direct bactericidal effect of acidifiers depends on the dissociation degree of the acid. The lower the dissociation degree of the acid, the stronger the bactericidal effect. Generally, inorganic acids (such as phosphoric acid) have a high dissociation degree, so their direct bactericidal effect is poor; organic acids have a low dissociation degree, and their bactericidal effect is relatively strong. Different organic acids also have different bactericidal effects, among which citric acid and lactic acid have a lower dissociation degree than other organic acids, resulting in a weaker bactericidal effect; 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 properties.

Acidifiers can combine with certain mineral elements to form easily absorbable complexes. For instance, citric acid and fumaric acid can form high-bioavailability complexes with minerals like Ca, Zn, Fe, P, and Mg, promoting the absorption and retention of these elements in the body. Acidifiers not only form easily absorbable complexes with minerals but also prevent the formation of insoluble salts in an alkaline environment, enhancing mineral absorption. Additionally, an acidic environment is conducive to the absorption of vitamins (e.g., VA, VD). Some organic acids like fumaric acid have antioxidant properties, and citric acid acts as a synergist for antioxidants.

Impact on Stress Reduction and Immune Function

Stress factors such as early weaning, grouping, regrouping, transportation, and vaccination can reduce the resistance of livestock and poultry, leading to decreased quality of livestock products and reduced production and economic benefits. Adding fumaric acid to feed enhances piglets' resistance to stress from vaccination and strengthens immune responsiveness. Fumaric acid itself has a sedative effect, inhibits the central nervous system, reduces organism activity, and can effectively alleviate heat stress reactions. Fumaric acid has a shorter energy production pathway than glucose and can be used for emergency ATP synthesis during stress, serving as a stress reliever. Adding organic acids to feed can promote the secretion of gastric juice and digestive enzymes, overcoming the stress caused by early weaning in piglets. In the high temperatures of summer, pigs often experience heat stress, increasing water intake and reducing feed intake, leading to insufficient milk production. Adding a certain amount of organic acids to the diet can increase feed intake, improve milk quality, and enhance the physique of piglets in farrowing rooms.

Application of Acidifiers in Pig Production

3.1 Application of Acidifiers in Piglet Diets

Studies have shown that the addition of acidifiers to weaning piglet diets has the most significant effects and is therefore more commonly applied. Piglets have weak stomach acid secretion abilities, with a stomach pH of 4.2, while the pH of piglet feed is between 5.8 to 6.5. A high pH can decrease pepsin activity in the stomach, leading to the proliferation of harmful bacteria like E. coli and Salmonella, resulting in indigestion, diarrhea, and slow growth. Adding organic acids to pig feed can improve pig production performance. Zhang Xinru et al. (1995) fed weaning piglets with 1% citric acid in a corn-soybean meal diet, resulting in a 7.52% increase in daily weight gain and a 5.05% improvement in feed conversion rate, with a 41.88% reduction in diarrhea incidence. Adding 1.5%–2.0% fumaric acid to feed can increase piglet daily weight gain by 9%, feed intake by 5.2%, and feed conversion rate by 4.4% (Kirchgessner, 1982). Bolduan (1988) reported that after adding propionic acid, feed intake and daily weight gain in weaned piglets both increased. Wan Weerden et al. (1987) added 1.5% calcium formate to the diet, resulting in a 12% increase in weekly weight gain and a 4% improvement in feed conversion rate. Adding 1% calcium formate and 0.5% propionic acid improved daily weight gain and feed conversion rate by 15% and 8%, respectively. Huang Guoqing et al. (1999) reported that adding a certain amount of citric acid, phosphoric acid, and composite acidifiers to the diet for 28 days increased daily weight gain by 7.46%, 4.48%, and 13.93%, respectively, and improved feed return by 2.91%, 1.16%, and 5.23%, respectively. Research shows that acidifiers can improve piglet production performance.

3.2 Summer Application of Organic Acids in Suckling Sows

In the high temperatures of summer, sows experience heat stress, reducing feed intake and milk production, resulting in weakened piglet physique in farrowing rooms. Adding an appropriate amount of organic acids to the diet of lactating sows can improve feed palatability, increase sow feed intake, improve milk quality, and enhance piglet physique in farrowing rooms.

3.3 Use as Feed Antimicrobial Agents

Propionic acid and calcium propionate are excellent feed antimicrobials widely used for feed preservation. Sorbic acid is also a good feed antimicrobial. Adding fumaric acid improves the stability of vitamins A and C in premixes. Currently, there are many products abroad specifically designed for feed preservation and mold prevention, such as a foreign company's KEMEI, which mainly contains propionic acid, fumaric acid, sorbic acid, and tannic acid.

In conclusion, acidifiers can improve gastrointestinal microbial flora, reduce gastrointestinal pH values, increase the digestive and absorptive efficiency of nutrients in pig herds, participate in metabolic processes, effectively inhibit intestinal pathogenic microorganisms, promote pig herd health, reduce disease incidence, and serve as natural alternatives to antibiotics. This is significant for protecting the environment and food safety. There are many types of acidifiers available on the market, each with different chemical characteristics and functions. We need to determine their use based on actual production purposes to achieve the most reasonable economic benefits through rational and scientific use.