VFAs? The main ruminal acid is dCO2
VFAs in water environments are found as bases or acids, e.g., acetate and acetic acid (Arrhenius theory). The Henderson-Hasselbalch equation allow us to describe the dissociation and concentration of each Lewis pair, if we know the total amount, the dissociation constant for the pair (pKa) and the ruminal pH.
Below literature examples for different diets and under normal or SARA conditions.
VFAs in the ruminal liquor are mainly found as bases. The higher pKa of CO2 species means that dCO2 (acid) is the predominant ruminal form (Laporte-Uribe, 2024).
Table 1. Standard equilibrium constant for various molecules of significant importance in relation to the proper functioning and overall efficiency of the rumen system.
The decline in ruminal pH during fermentation reflects water dissociation product of the acid formation (Arrhenius theory). We can simulate this phenomenon by calculating the increase in acid concentrations at ruminal pH values found during SARA.
Please, observed which "acid" rose to levels that could have a strong physiological effect.
The average amount of dCO2 in the rumen is 60 mM at standard temperature and pressure and at pH 6.1 (Kohn and Dunlap, 1998). In contrast, the total amount of acetate/acetic acid is normally 50mM, from that around 7 mM are found as acetic acid, the rest is acetate, therefore the largest concentrations of “acid” in the rumen is ruminal dCO2.
In in vitro conditions VFAs are toxic for the rumen epithelium (Gabel et al., 1991), not doubt, however in in vivo ruminal dCO2 concentration of more than 80 mM for extended postprandial periods (CO2 holdup) might lead to CO2 poisoning (Laporte-Uribe, 2024).
Figure 1: Proportion of acid/base ratios for each Lewis pair of molecules that hold significant importance in the composition and functionality of the ruminal fluid at rumen pH thought to be the threshold for ruminal acidosis (pH 5.5).
Below is a detailed example drawn from the scientific literature that showcases average total concentrations of various molecules and their fluctuations at different ruminal pH levels, all according to their specific dissociation constant (pKa). This simulation was conducted utilizing the Henderson-Hasselbalch equation, which describe the relationship between pH and the ionization of weak acids and bases.
Figure 2. Model of concentrations of VFAs, ammonia/ammonium and bicarbonate/dissolved CO2 using concentrations described in the literature. The ruminal pH scale described the equilibrium between Lewis pairs as described by the Henderson-Hasselbalch equation.
The clear message from this model is that the primary acid found in the rumen is indeed dCO2, particularly when the pH level falls below the equilibrium threshold, ruminal pKa is 6.1 (Hille et al., 2017). VFAs are found mainly as bases, not acids.
