Rumen fermentation, membrane

From the slope of the curve for addition of sodium carbonate to the rumen fermentation of starch, one can estimate tht the net rate of acid formation is equivalent to 36 millimoles of acetic acid per liter per hour. If a level of no more than 0.2% acid must be maintained in the rumen fermentor, then the exchange rate across the membrane must be at least one liter per hour or half of the total reactor volume each hour. This is well below the observed exchange rate of 20 liters per hour. 

A further mechanism which helps to maintain stability of the milieu of the rumen, and possibly of the large intestine, is that as the pH falls owing to the production of acidic products of fermentation the proportion of free fatty acid to anion increases, and as the free fatty acid penetrates the epithelial cells lining the rumen more rapidly than the anion, so the rate of absorption increases and the pH is prevented from becoming too acid. The fact that large quantities of lactic acid may be associated with a pH of the rumen that is more acid than normal, i.e., a pH below 5, suggests that lactic acid as such penetrates the epithelial membrane at a slower rate than the other fatty acids. ... 

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