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Organic Acids for Swine

Organic acids in swine feed and drinking water

Organic acids are organic compounds that contain a carboxyl group (-COOH). The carboxyl group dissociates in solution releasing a proton, thus giving the compound acidic properties (Voet and Voet, 1995). Organic acids 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 swine 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 livestock and poultry feeds (adapted from Hajati, 2018)
AcidChemical nameFormulapKa
FormicFormic acidHCOOH3.75
AceticAcetic acidCH3COOH4.76
Propionic2-Propanoic acidCH3CH2COOH4.88
ButyricButanoic acidCH3CH2CH2COOH4.82
Lactic2-Hydroxypropanoic acidCH3CH(OH)COOH3.83
Sorbic2,4-Hexandienoic acidCH3CH:CHCH:CHCOOH4.76
Fumaric2-Butenedioic acidCOOHCH:CHCOOH3.02
MalicHydroxybutanedioic acidOOHCH2CH(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 pathogens such as Clostridium perfringens, Escherichia coli, and Salmonella spp. are significantly diminished below pH 5, while acid tolerant microbes, such as Lactobacillus spp. are unharmed.  

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 pH 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 display 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 possess antimicrobial activity have pKa values between 3 and 5 (Hajati, 2018). 

Energy Substrate for Enterocytes 

Some organic acids, most notably butyrate, 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 Acid  

  • pKa value 
  • Chemical form (acid, salt, coated or not) 
  • Molecular weight 
  • Micro-organism-related minimum inhibitory concentration (MIC) value of the acid  
  • 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, many organic acids in their pure liquid form are corrosive and may tend to have a strong, and often unpleasant smell.  Due to this unfavorable feature, acids in their pure form may be difficult to handle in the feed mills. 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 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. Due to their different properties, pure acids are beneficial for acidifying feed, while the other forms (salts, glycerides, etc.) tend to be used for their antimicrobial effect in the gastrointestinal tract. 

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 may be limited to the upper part of the digestive tract, which could be most appropriate for some applications and purposes. Acid incorporated into the diet as salts, glycerides, and encapsulated free acids each have different dissociation rates compared to their free acid form. With these alternatives, the site of action may be moved from the upper gastrointestinal tract to further down the small intestine and hind gut. 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 young pigs have been reported in numerous scientific studies with results showing decreased microbial counts and improved growth performance, reduced diarrhea, morbidity, and mortality rates. Overall, the swine industry has found 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.