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Pig Acidifiers

Organic acids in pig feed and drinking water

The potential of acidifiers for feed preservation and swine nutrition has been known for decades and is documented by many scientific publications.

Organic acids are compounds that contain a carboxyl group (-COOH). This carboxyl group dissociates in solution releasing a proton, thus giving the compound acidic properties (Voet and Voet, 1995). Organic acids typically are weak acids, thus they are only partly dissociated in solution.

Although there are numerous organic acids, many of which are found naturally occurring in the body (fatty acids, amino acids, nucleic acids), only a few of these are used in piglet, sow and hog feeds. Short carbon chain lengths of up to seven generally characterize the organic acids commonly used, as these are the acids most associated with antimicrobial activity (see Table 1).  Since these organic acids are naturally occurring in the body, they are readily metabolized by the animal (Voet and Voet, 1995).

Figure 1. Properties of Organic Acids
Figure 1. Properties of Organic Acids
Table 1. Commonly used organic acids in pig feed (adapted from Hajati, 2018)
AcidChemical nameFormulapKa
FormicFormic acidHCOOH3.75
AceticAcetic acidCH3COOH4.76
Propionic2-Propionic acidCH3CH2COOH4.88
ButyricButanoic acidCH3CH2CH2COOH4.82
Lactic2-Hydroxypropanoic acidCH3CH(OH)COOH3.83
Sorbic2,4-Hexandienoic acidCH3:CH:CHCH:CHCOOH4.76
Fumaric2-Butenedioic acidCOOHCH:CHCOOH3.02
MalicHydroxybutanedioic acidCOOHCH2CH(OH)COOH3.40
Tartaric2,3-Dihydroxy-Butanedioic acidCOOHCH(OH)CH(OH)COOH2.93
Citric2-Hydroxy-1,2,3-Propanetricarboxylic acidCOOHCH2C(OH)(COOH)CH2COOH3.13
BenzoicBenzenecarboxylic acidC6H5COOH4.19

Modes of action

Reduction of Intestinal pH

Organic acids and their salts exert growth-inhibiting effects on gastric and intestinal microbes through localized pH reduction. Proliferation of many pathogens like Clostridium perfringens, Escherichia coli, and Salmonella spp. is significantly diminished below pH 5, while acid tolerant microbes, such as Lactobacillus spp., are unharmed.

Inhibition of Bacterial Growth

In addition to inhibitory effects due to low pH, organic acids have a direct bactericidal action. While in the undissociated form, these acids can diffuse through the cellular membrane of bacteria where, due to cytosolic pH being maintained near 7, dissociation of the acid occurs inside the bacterial cell causing pH reduction and inhibition of metabolic pathways. These disruptions ultimately lead to bacterial cell death (Russell and Diez-Gonzales, 1998, Van Immerseel et al. 2006).

Antimicrobial activity is completely dependent on the acid used. For example, formic and proprionic acids their broad spectrum activity against both bacteria and fungi, whereas lactic acid is mostly effective against bacteria and sorbic acid is better known for being effective against mold (Hajati, 2018).

The ability of an acid to inhibit microbes is dependent on its pKa value, which is the pH at which 50% of the acid is dissociated. Most acids with antimicrobial activity and used as feed additives have pKa values between 3 and 5 (Hajati, 2018).

Energy substrate for Enterocytese

Some organic acids can be directly metabolized by the enterocytes lining the gastrointestinal tract and may enhance growth and development of the gut, leading to enhanced nutrient digestion and utilization and, ultimately, optimized performance (Miller and Slade, 2006).

Factors Influencing Efficacy of Organic Acids

  • pKa-Value

  • Chemical form (acid, salt, coated or not)

  • Molecular Weight

  • Micro-organism-related minimum inhibitory concentration (MIC) value of the acid

  • Target micro-organism

  • Animal species

  • Buffering capacity of feed

What is pKa?

The efficacy of an acid in inhibiting microbes is dependent on its pKa value, which is the pH at which 50% of the acid is in its undissociated form and 50% is in its dissociated form.

pKa value explanation

Choosing the Right Acid

Because of their acidic nature, organic acids are corrosive and many of them tend to have a strong, and often unpleasant, smell. Due to this unfavorable feature, acids in their pure form are less popular and difficult to handle in the feed mill.

Furthermore, the shorter the chain length of the acid the more volatile the product, thus there is continuous loss of acid from processed feed causing the storage life of the product to be limited.

As an alternative, organic acid products may come in a variety of forms such as salts, glycerides, and microencapsulated products. These variations create a product that is less corrosive, offensive, and volatile, thus making handling and processing much more manageable. Palatability may also be improved when the acid salt is used rather than the free acid.

The short carbon chain length combined with the acidic conditions in the upper gastrointestinal tract mean that organic acids are quite rapidly absorbed across the gastrointestinal wall. This means that the effects of supplementation with the free acid will be limited to the upper part of the digestive tract, which may be most appropriate for some applications and purposes.

Acids can be incorporated into the diet with a special inorganic carrier that acts as a sequential release medium (SRM) and that will ensure their gradual delivery all along the gastrointestinal tract. It is therefore of utmost importance to understand the mode and site of action of the acid when choosing which form of which acid to utilize.


Acidifiers are powerful tools to maintain animal health and assist with food safety goals. Consistent beneficial effects on productivity in livestock and poultry have been reported in numerous scientific studies with results showing decreased microbial counts and improved animal growth performance, reduced diarrhea, morbidity, and mortality rates. Overall, numerous livestock producers consider acidifiers as an ideal solution to enhance performance and, therefore, profitability.


Hajati, H. 2019. Application of organic acids in poultry nutrition. International Journal of Avian & Wildlife Biology 3(4): 324-329.

Miller, H.M. and Slade, R.D. (2006) Organic acids, pig health and performance. The Pig Journal, 57, 140-149.

Russell JB, Diez Gonzalez F. 1998. The effects of fermentation acids on bacterial growth. Adv Microbial Physiol. 39:205–234.

Van Immerseel F, Russell JB, Flythe MD, Gantois I, Timbermont L, Pasmans F, Haesebrouck F, Ducatelle R. 2006. The use of organic acids to combat Salmonella in poultry: a mechanistic explanation of the efficacy. Avian Pathol. 35:182–188.

Voet, D. and Voet, J.G. (1995) Biochemistry. John Wiley and Sons Inc. New Jersey, Second edition.