Many countries have banned the use of antibiotics in swine feed for prophylactic and growth promotion purposes. Organic acids are one of the candidates to replace antibiotics in pig diets. What is there to know about organic acids, and what are the effects of organic acids in the swine industry?
Organic acids are broadly dispersed in nature (animal, plant and microbial sources) and are produced by several fungi, yeasts and bacteria. Organic acids are categorised in the “weak” acid group that do not totally dissolve in water, and they comprise one or more carboxylic acid groups covalently linked in groups such as amides, esters and peptides.
The acid and base properties of organic compounds are very similar to the acid and base properties of inorganic compounds. Properties of acids include a pH less than 7, a sour taste, producing hydrogen ions when dissolved in water and being corrosive to human tissue and reactive with bases to form a salt and water. Common properties of bases include a pH more than 7, a “soapy” feel, a bitter taste and being corrosive to human tissue and reactive with acids to form a salt and water. Pigs have sensitivity for sour taste that is about tenfold than for sweet taste.
There are two types of organic acids. One has the carboxyl group (COOH group), for example acetic acid (CH3COOH) which is made by oxidising grain alcohol or by the fermentation of fruit sugar in cider. The second type has a phenol group (C6H5OH). Salicylic acid (OHC6H4COOH) is an example of an organic acid with both carboxyl and phenol groups.
Organic acids play a role in the regulation of basic cellular processes such as pH modification, signalling messengers and modulating transport across biological membranes, and they extensively modify the cellular, subcellular or extracellular compartments in which they are found due to their chemical properties. Therefore, organic acids can be involved in various biochemical and physiological processes in vivo. In addition, organic acids are involved in chemical modification of proteins, with high impact on the in vivo protein activity. The different roles of these compounds still remain to be explored.
Chemical synthesis or fermentation are among the most used methods for organic acid production. In recent years, new techniques have been developed for fast and efficient extraction of organic compounds from different plant materials. Citric, lactic, gluconic and itaconic acids are produced industrially by microbial processes, which is a promising approach to obtain building block chemicals based on renewable carbon sources. In addition, large quantities of acetic acid are produced by bioprocesses and chemical synthesis. Microwave-assisted extraction is another technique to isolate various compounds from plants or vegetal materials for both analytical and industrial purposes.
The positive effects of organic acids include:
Decreasing the emptying rate of the stomach;
Stimulating enzyme excretion and activity in the gut;
Supplying nutrients to gut tissue;
Improving mucosal integrity and function;
Enhancing pepsin and microbial phytase activity;
Inducing pancreatic excretion;
Increasing protein digestion;
Improving minerals utilisation;
Reducing competition between the microflora and the host; and
Improving pig health and productivity.
Organic acids can directly decrease the pH of the gut environment through the release of hydrogen ions, thus preventing or inhibiting the proliferation of acid-sensitive bacteria. The antimicrobial effect of organic acids is greater under acidic conditions and lesser at neutral pH. It is important to know that each organic acid has a microbial activity spectrum involved to a specific pH range, membrane structure and physiology in the cell of the microbiota species. In addition, organic acids are promising alternatives to antibiotics to promote nutrient digestibility by decreasing the pH of the upper region of the digestive tract.
Some of the most used organic acids in swine nutrition include citric, lactic and formic acids.
Citric acid is an odourless, colourless compound with an acidic, sour-tasting nature. It is used predominantly as a flavouring and preserving agent in soft drinks and sweets, as a disinfectant and to stabilise or preserve medicines. Citric acid releases minerals bound to the phytate molecules, which in turn increases the utilisation of calcium, phosphorus and zinc. Dietary supplementation of citric acid improves the digestibility of dry matter, nitrogen and energy in finishing pigs and lactating sows, and the digestibility of crude protein, dry matter, fat and energy in growing pigs. In addition, citric acid supplementation to the diet of growing and finishing pigs decreases Escherichia coli counts and increases Lactobacillus counts, which improves gut health. Furthermore, in growing pigs diets supplemented with citric acid reduce the emission of acetic gas and the ammonia concentration.
Lactic acid is an odourless, colourless liquid, corrosive to metals and tissue, which is produced during fermentation. Feeding post-weaning pigs with a diet supplemented with lactic acid improves average daily gain, average daily feed intake, feed efficiency and performance. In post-weaning pigs, supplementing the diet with lactic acid improves the digestibility of dry matter, increases bacilli and lactobacilli concentrations and reduces Salmonella and E. coli counts.
Formic acid is a colourless liquid with a pungent odour. It is corrosive to metals and tissue. Formic acid is the simplest carboxylic acid and is present in various sources such as bee venom and ant stings. It is also used as a preservative and antibacterial agent in livestock feed. Formic acid reduces bacterial nitrogen in the pig gut and improves apparent ileal digestibility of crude protein, essential amino acids, lipids, calcium and phosphorus. In addition, formic acid supplementation enhances the digestibility of dry matter in post-weaning pigs and improves gut health.
Organic acids reduce gastric pH, prevent the growth of pathogens, act as an energy source, increase apparent total tract digestibility, improve gut health and enhance growth performance and productivity. However, the effect of organic acids in practice is not always consistent due to the wide variety of available products and the various recommended effective dosages with the different combinations. Type composition, dosage, formula, feeding regimen, environment, nutrient composition of feed and the age and health status of animals all affect the efficacy of organic acids. Therefore, furthermore research is required to establish effective dosage and combination of organic acids to achieve the best possible results.