Acidifiers are compounds classified as organic or inorganic acids. Organic acids include formic, fumaric, lactic, benzoic, propionic, and citric acids. Inorganic acids include hydrochloric, sulfuric, and phosphoric acids. Salts of acids also have been used as acidifiers, including calcium-formate, potassium-diformate, sodium-diformate, and sodium-fumarate. Blends of acidifiers are often commercially available because organic and inorganic acids may have a synergistic effect. In addition, some commercial acidifiers contain protected acids that are coated with fatty acids or other molecules, mainly to allow the release of the acid in a targeted location in the gut with the goal to improve effectiveness (Upadhaya et al., 2014).
The mode of action of dietary acidifiers has not been fully understood, but several mechanisms have been proposed. Acidifiers are believed to enhance growth performance via pH reduction in the digestive tract, which improves nutrient digestibility and promotes growth of beneficial bacterial while inhibiting pathogenic bacteria (Jacela et al., 2009a).
Acidifiers have been commonly targeted for weanling pigs. Organic acids have been shown to improve growth performance of weanling pigs more consistently than inorganic acids (Kil et al., 2011; Suiryanrayna and Ramana, 2015; Liu et al., 2018). However, inorganic acids have been often considered as an alternative to organic acids because of lower cost. Acidifiers may also benefit grow-finish pigs (Tung and Pettigrew, 2006), particularly under transition or stressful conditions. In sows, use of acidifiers in the diet improves nutrient digestibility and reduces urinary pH, which aids in controlling the incidence of urinary tract infections (Kluge et al., 2010).
The magnitude and consistency of the responses to acidifiers are variable depending on the nature of acids, inclusion rate, combination of acids, and diet composition (Jacela et al., 2009a). For most acidifiers, the inclusion of excessive levels in the diet affects palatability and decreases feed intake. Also, some acidifiers are corrosive and pose handling and equipment issues during feed manufacturing. Generally, inorganic acids are the most corrosive and salts of acids are the least corrosive acid forms.