Are the acidic metabolites of interest to you? Then perform a thin layer chromatography. It is the eye of the Organic Chemist. It will see them all and you will be properly informed. The acidic metabolites will depend on the nature of substrates metabolized. Look at glucose and molasses and follow normal glycolytic pathway. For olive oil and waste frying oil look at fatty acid metabolic pathway like Beta-oxidation. All kinds of pathway acidic intermediary or end products will contribute to pH decrease.

Thank you Maurice,

It is just to report in my paper from the literature the two possible type of metabolites and its direct effect on biosurfactant production, since the bacterial strain decrease surface tension on vegetable based oils rather than sugars.

Sugar metabolism is via a whole series of acidic metabolites, both anaerobic and aerobic pathways, such as the Embden-Meyerhof apthway and the citric acid cycle. Just about all intermediates are acidic. As for lipids, the hydrolysis products are fatty acids, which get further metabolized to acetyl-CoA feeding into the citric acid cycle. Essentially all oxidative metabolism is through acidic intermediates or end products. Any elementary biochemistry textbook would handle this question.