Thanks for your interest in the article and your question.
As you already know, pH values vary in each part of the intestinal tract. The stomach pH of an adult pig is usually around 2.5 to 4.0, because of the secretion of hydrochloric acid. In the piglet after weaning, the pH of the stomach can sometimes be as high as 4.5 to 7.0. We want to avoid a too high pH as a low stomach pH is required to ensure good protein digestion and to prevent bacterial growth. The pH of the small intestine is usually between 4.5 and 6.5, as it increases after the addition of bile salts and pancreatic enzymes. In the large intestine, we can expect pH values from 6.0 to 8.0. This higher pH favors the growth of the bacterial population and undigested feed substrate reaching this point will be fermented by the bacteria into lactic, acetic, propionic and butyric acids.
Now, to answer your question regarding the effect of acids in the gut: when looking at published research, we can see that gastric pH reduction has been observed when using acids with relatively low pKa, such as citric, formic, lactic and orthophosphoric acids. Based on reports available, we can say that pH reduction in stomach is most likely to be observed in piglets just after weaning, and rarely after 2 weeks post weaning.
An interesting review of the literature by Tung and Pettigrew (2006) has shown that acidifiers have a clear ability to reduce the pH of the diet. However, they did not significantly influence pH in the digestive tract as explained below:
- Out of 59 trials, acids reduced the pH of the diet in 58 cases (average reduction 21%, highly significant)
- Out of 22 trials, acids reduced the pH of the stomach in 11 cases, resulted in increased pH in 9 trials and had no clear effect in 2 trials (small average reduction)
- Out of 12 trials, acids reduced the pH of the small intestine in 6 cases, and resulted in increased pH in the other 6 trials (small average increase, not significant)
- Out of 11 trials, acids reduced the pH of the cecum in 4 cases, and resulted in increased pH in the other 7 trials (no difference when looking at averages)
- Finally, in the 11 trials studying the pH effect of acids at the colon level, pH was reduced in 8 cases, and was increased pH in the other 3 trials (no difference when looking at averages)
However, it has to be noted that in all the published data, diet characteristics (pH, buffer capacity) are not always reported which makes interpretation quite difficult.
So to summarize, pH is not going to be much affected in the gut, especially after the stomach. The antibacterial effect of the acids is dependent on their pKa as you know, which means that acids with a high pKa value (for instance butyric) have more chances to be effective against the pathogens.
As an additional comment, you have to remember that homeostasis (the tendency toward a relatively stable equilibrium between interdependent elements, especially as maintained by physiological processes) is there to ensure that pH does not go outside the values required for a good digestion process and optimum activity of the intestinal enzymes. For instance, secretin (a polypeptide hormone produced by S cells in the duodenum, especially on contact with acid, will stimulate secretion of pancreatic juice which is alkaline due to the high concentration of bicarbonate ions. So there is little way to change dramatically the pH of the intestines with the addition of acids. This is not what we want anyway.
I hope that helps.