Feed International - October/November 2017 - 25
FeedInternational ❙ 25
pKa, which is nothing but the negative logarithm of the
concentration of reaction participant bodies at equilibrium. It is a rather complicated index when it comes to
its definition and calculation. In biochemistry terms, and
this is where we need this index to help us understand organic acids, we can understand pKa as the pH at which 50
percent of the acid is dissociated. In even more practical
terms, the higher the pKa, the weaker our acid will be.
The pKa values of
several common acids
used in animal feeds
How organic acids benefit the animal
Organic acids are believed to affect pathogenic bacteria through two main ways. First, they can reduce the
pH of the surrounding environment, making it impossible
for undesirable bacteria to survive. This role is usually
performed by stomach hydrocloric acid that kills most
external bacteria consumed along with food or water, but
it does not affect naturally present bacteria that just consume nutrients. For organic acids to lower stomach pH,
aiding or substituting naturally secreted hydrochloric acid,
we need strong(er) acids that split (dissociate) under stomach conditions.
Second, organic acids in their intact (un-dissociated)
form are believed to penetrate the bacterial outer membrane entering its cytoplasm where they dissociate under
a neutral pH. This drops internal pH, and bacteria eventually die while trying to overcome the problem. Only weak
acids remain intact in the stomach, entering into the small
intestine where they can attack bacteria. Such action also
occurs in the stomach, but it is of limited significance
because feed does not stay long in the there. In contrast,
in the small intestine bacterial populations are substantial,
and this is where a general reduction in good and bad bacteria usually ends up improving feed conversion efficiency.
However, the action of organic acids can be curtailed by
the overpowering ability of the organism to maintain neutral pH in the intestines, causing organic acids to dissociate sooner or later. To this end, only covered or otherwise
protected organic acids can reach the large intestine.
Colorado State University
It is evident that hydrochloric acid is a most potent
acid as it dissociates completely and instantly in
testine, ignoring the importance of having a low pH barrier in the stomach, but this requires some explanation.
In the stomach of animals, there is water and hydrochloric acid, with the resulting pH being about 2, which is
very acidic - as needed to bar entrance to most pathogens. In young animals, with a still developing digestive
system, pH is often closer to 4, especially in animals that
What is the importance of pKa in the stomach?
We usually pick organic acids to act in the small inOctober/November 2017 ❙ www.WATTAgNet.com
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