Introduction: Role of Organic Acids in Monogastric Nutrition
Organic acids are well established as functional feed additives in monogastric nutrition, mainly for their antimicrobial effects and their contribution to reducing the use of antibiotics. Their characteristics encompass the control of the gastrointestinal tract pH, regulation of pathogenic microorganisms, maintenance of intestinal integrity, enhancement of growth performance and feed efficiency.
On a commercial basis, organic acids are provided primarily as free acids, mineral salts, or glyceride (esterified) form. From these, glyceride versions of organic acids offer unique physiological and functional benefits over salts with regard to pH kinetics, mechanism of activity, and antimicrobial effectiveness throughout the gastrointestinal tract (GIT).
Antimicrobial Properties of Organic Acids: Beyond pH Reduction
Organic acids have traditionally been recognized more for their role as effective preservatives than as dietary acidifiers. Their antimicrobial activity has often been attributed to their capacity to lower the pH of feed or the surrounding environment, thereby inhibiting or slowing the growth of susceptible microbial strains. However, their antimicrobial efficacy extends well beyond simple pH reduction.
A key feature underlying the effectiveness of organic acids is their ability to exist in either an undissociated or dissociated form, depending on the environmental pH. When organic acids are present in their undissociated form, they are able to freely diffuse across the semi-permeable cell membranes of microorganisms.
Importance of pKa and Molecular Structure in Antimicrobial Efficacy
The antimicrobial effectiveness of an organic acid is closely related to its pKa value, defined as the pH at which 50% of the acid is present in the dissociated form. Acids with higher pKa values retain a greater proportion of undissociated molecules at higher pH levels, enhancing their ability to penetrate microbial cells. In addition, antimicrobial efficacy generally increases with greater carbon chain length and a higher degree of unsaturation, characteristics that further improve the preservative and antimicrobial properties of organic acids.
Advantages of Glycerides over Free Fatty Acids and Salts
Glycerides are composed of a fatty acid linked to glycerol via an ester bond. The hydrogen atom (H) of the fatty acid reacts with the hydroxyl group (OH) of the glycerol molecule resulting in the formation of a water molecule (H2O) and creating a covalent bond between the oxygen atom of the fatty acid and the carbon atom of the glycerol molecule.
The strong covalent ester bond of glycerides confers several advantages over free fatty acids.
Structurally, glycerides are amphiphilic molecules, combining a lipophilic fatty acid chain with a hydrophilic glycerol backbone. This dual affinity enables spontaneous self-emulsification in aqueous environments, facilitating homogeneous distribution in feed and water matrices. As a result, glycerides remain biologically active across diverse environments, including feed, drinking water, the stomach, and the intestinal tract.
Due to their chemical structure, glycerides are pH-independent and do not undergo dissociation, allowing them to maintain functional stability across a wide range of gastrointestinal conditions. In contrast to free fatty acids, they are non-volatile, non-corrosive, and exhibit high thermal stability, making them well suited for feed processing applications. In addition, glycerides are characterized by a neutral taste and odor, which minimizes negative effects on feed palatability.
Intracellular Effects on Bacterial Metabolism
More importantly, multiple studies have demonstrated that glycerides (mainly monoglycerides) exert a substantially stronger antimicrobial effect than their corresponding free fatty acids. This enhanced activity is attributed to their ability to interact directly with microbial cell membranes in a pH-independent manner.
Monoglycerides enter bacterial cells primarily via membrane channel proteins known as aquaporins, which exhibit selectivity for glycerol due to its favorable energetic properties. This transport mechanism facilitates intracellular uptake of monoglycerides without requiring prior dissociation in the external environment. Once inside the bacterial cytoplasm, the ester bond between glycerol and the fatty acid is hydrolyzed, most likely through the action of bacterial lipases, releasing the active fatty acid in its dissociated form, consisting of a proton (H⁺) and an anion (A⁻).
Because bacteria lack a nuclear membrane, their nucleic acids are directly exposed within the cytoplasm. The anionic fraction of the dissociated fatty acid can therefore interact directly with nucleic acids, interfering with essential cellular processes such as transcription, translation, and replication. Since bacterial pathogenicity is largely dependent on the expression of virulence factors encoded in DNA, disruption of these processes negatively affects pathogenic potential, including the synthesis of virulence determinants and the expression of resistance mechanisms.
Simultaneously, the release of protons from the fatty acid lowers intracellular pH. This acidification leads to protein denaturation and impairment of enzymatic activity, as many bacterial enzymes exhibit optimal activity within a narrow pH range. A reduction in intracellular pH alters the isoelectric point required for enzymatic catalysis, thereby suppressing metabolic pathways and resulting in bacteriostatic effects.
To mitigate intracellular acid stress, bacteria activate membrane-bound proton extrusion systems, such as ATP-dependent proton pumps. The operation of these systems requires substantial energy expenditure, diverting ATP away from growth and maintenance processes. This additional energetic burden further compromises bacterial viability and proliferation.
Membrane Disruption and Antiviral Effects
Additionally, owing to their amphiphilic structure, glycerides readily form micellar structures, which facilitates their interaction with microbial lipid membranes. This property enables their incorporation into the phospholipid bilayer of microorganisms, leading to alterations in membrane integrity and permeability. Through these interactions, glycerides can destabilize or disrupt bacterial cell membranes and the lipid envelopes of viruses, resulting in leakage of intracellular components and loss of structural integrity. Gram positive bacteria and lipid-enveloped viruses are particularly susceptible to this mechanism, as their ability to adhere to host tissues and invade host cells depends on the preservation of an intact membrane. Disruption of membrane structure therefore compromises pathogen viability, preventing infection and replication.
Conclusion: Glycerides versus Salts of Organic Acids
In summary, glycerides of organic acids differ fundamentally from salts’ form in their stability, dissociation behavior, and antimicrobial efficacy along the gastrointestinal tract. Because the fatty acid is esterified to glycerol, glycerides do not dissociate, remain pH-independent, and do not require encapsulation to maintain functionality. This allows them to retain antimicrobial activity under both acidic and neutral pH conditions, including in the small intestine, where pathogen control is critical. In contrast, organic acids’ salts are rapidly dissociated compounds that depend on encapsulation to delay release; once this protection is lost, particularly at physiological pH, they dissociate almost immediately and lose antimicrobial effectiveness. Consequently, while salts may contribute to acidification in specific segments of the digestive tract, glycerides of organic acids provide a more consistent, pH-independent, and targeted antimicrobial effect throughout the gastrointestinal tract, making them better suited for supporting intestinal health in monogastric, especially in poultry production.
| Glycerides of Organic Acids | Ca/Na Salts of Acids | |
| Active Component | Glycerides of organic acids with short & medium chains | Calcium or sodium salts of organic acids |
| Need of encapsulation | no | yes, otherwise hydrolysis by water |
| % of dissociation-pH 3 | 0% | 0%/ Encapsulation is dissolved until in small instestin (pH about 6) |
| Efficiency at pH 3 Antibacterial effect | yes | no/ Encapsulation is dissolved until in small intestine (pH about 6) |
| Time of dissociation of organic acid after hydrolysis at pH 7 | never | <1 second ,immediately after hydrolysis |
| % of dissociation-pH 7 | 0% | 98%-99% |
| Efficiency in high pH 7 Antibacterial effect | yes | no |
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