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Mixes of organic acids are now commonly used as feed additives by swine and poultry nutritionists to promote better gut health and faster animal growth, particularly in young animals with immature digestive systems that may not produce sufficient hydrochloric acid to activate pepsinogen into pepsin

Various acid combinations are available on the market, but the specific actions of each acid are not always well understood. The five main acids found in these mixes are formic, lactic, propionic, fumaric, and citric acids. It's crucial to have a proper understanding of the role of each acid to tailor the mix to the specific needs of the animals.


The graph below is illustrated the differences between these 5 acids according to two dimensions that their coefficient of dissociation (pKa) and their molecular weight.

The lower the pKa the easier is for the acid to release its H+. Fumaric has the lowest pKa and is therefore considered the strongest acid. Depending on the pH of the stomach, each acid will dissociate more or less. If the pH is one unit above the pKa, the dominant form (>90%) is the dissociated form (in green in the graph below) therefore releasing their H+ in the process. Because of its higher pKa in comparison to Fumaric, Formic will stop releasing H+ as soon as the pH is below 2.75 (pKa-1) whereas Fumaric will keep on releasing its H+ until the pH reaches 2.03. 

You will notice that Fumaric and Citric have several pKa as they have two protons to release for Fumaric and three protons for Citric. These characteristics give these products a buffering property. They can release and capture protons depending on the evolution of stomach pH. When the pH increases, they will release up to 2-3 protons and when the pH decreases, they will capture some, preventing the pH from increasing or decreasing further. Thus, they contribute to maintaining the pH stable to avoid brutal variation that would affect the stomach ecosystem.

These acids are often recommended to improve gut health thanks to their antibacterial effect. But only the associated form can penetrate the bacteria membrane, mostly the gram negative bacteria. Once the acid meets inside the bacteria's higher pH, it releases

its protons destroying the bacteria by acidification. As formic acid has a higher pKa than fumaric, the proportion of associated form will always always be higher than for furmaric. Moreover, the smaller is the molecular size and the easier they can penetrate the membrane. Formic has a higher pKa but has a molecular size half the one of Fumaric. Therefore, when you are buying one kg of formic acid, you get twice more molecules per kg and twice more protons to be released. In summary, both formic and fumaric are considered strong organic acid but not for the same reason. Formic gets quicker to release a higher quantity of protons and penetrate bacteria while Fumaric release its protons on a wider range of pH making it a better buffering agent.

Formic acid primarily acts within the stomach of animals, where it exerts its pH-reducing and antimicrobial effects. Upon ingestion, formic acid quickly lowers the pH of the stomach contents, creating an acidic environment that inhibits the growth of pathogenic bacteria and promotes the activation of pepsinogen into pepsin, an enzyme critical for protein digestion. 

Because Formic releases all its protons in the stomach, its activity will be limited to the upper part of the Swine and Poultry intestinal tract while Lactic acid tends to exert its effects primarily in the lower regions of the GIT, including the small intestine (duodenum, jejunum, and ileum) and the cecum and colon. Lactic acid may also have some activity in the stomach, its impact is generally less pronounced compared to formic acid. 

Lactic acid contributes to modulating the composition and activity of the gut microbiota, particularly in the lower segments of the GIT where bacterial populations are more abundant. Lactic acid promotes the growth of beneficial bacteria, such as lactobacilli, while inhibiting the proliferation of harmful pathogens. 

In the small intestine, lactic acid helps to maintain an optimal pH environment for enzymatic digestion and nutrient absorption. By supporting gut health and microbial balance, lactic acid indirectly enhances nutrient utilization and absorption in the intestinal epithelium.

Citric acid, a natural acid found in citrus fruits, offers multifaceted benefits in animal feed, with a focus on antioxidant activity, mineral chelation, and flavor enhancement. As a potent antioxidant, citric acid scavenges free radicals and prevents oxidative damage to feed ingredients and nutrients, ensuring their stability and nutritional integrity. Furthermore, citric acid acts as a chelating agent, forming complexes with minerals such as calcium and magnesium, thereby improving their solubility and bioavailability for absorption in the gut. Additionally, citric acid enhances the palatability of feed, making it more appealing to animals and encouraging intake. 

The palatability enhancement effect of citric acid in animal feed is primarily attributed to its sour taste and aroma, which can stimulate the appetite and make the feed more appealing to animals. The sour taste of citric acid can stimulate the animal's appetite, triggering saliva production and digestive enzyme secretion. In some cases, feed ingredients or additives may have strong or unpleasant odors that deter animals from consuming the feed. Citric acid's acidic nature can help mask or neutralize these odors, making the feed more palatable by reducing sensory aversions. 

Citric acid's sour taste may also stimulate thirst in animals, encouraging them to drink more water. Adequate water intake is essential for proper digestion and nutrient absorption, so promoting increased water consumption can further support overall animal health and well-being.

As far as feed preservative, Propionic acid is often the preferred option due to several specific properties that make it effective in inhibiting the growth of mold and other harmful microorganisms in animal feed. Propionic acid is more lipophilic (soluble in fats) than other acid. This property allows propionic acid to penetrate microbial cell membranes more effectively, disrupting cellular functions and leading to cell death. Mold and yeast cells typically have lipid-rich cell membranes, making them more susceptible to the lipophilic nature of propionic acid. In conclusion, each of the five most common organic acids have a well defined and documented specialities,


Formic Acid: Antimicrobial Activity

Lactic Acid: Improved Nutrient Utilization

Citric Acid: Antioxidant Activity

Fumaric Acid: pH Regulation

Propionic Acid: Feed preservation


The nutritionist should discuss with their supplier of organic acids to adjust the mix depending on the situation of the farms he is formulating for.


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