Weak acids: dissociation in complex buffering systems and partitioning into oils

Abstract

A method is developed to predict pH changes in complex buffering systems resulting from the addition of weak acids. Such systems are often encountered in foods and microbiological growth media which contain proteins and proteinaceous digests. The ‘buffering capacity’ of the system is characterised by a strong acid titration curve, and the known dissociation constants of weak acids are used to predict pH changes resulting from the addition of either one or a mixture of weak acids to the system. Predictions by the method show close agreement with experimental data. The familiar Henderson–Hasselbalch equation describing the dissociation of weak acids is modified to account for partitioning of undissociated acid into an oil phase. The equation may be used in conjunction with the pH prediction method to calculate both the pH and the concentration of undissociated weak acid in the aqueous phase of an emulsion containing complex buffering species. The equations should find application wherever weak acids are used for microbiological control in complex systems such as foods and pharmaceutical preparations, for predicting pH changes resulting from microbial growth during the production of fermented food commodities (eg cheese and yoghurt) or during food spoilage, and in biotechnological processes ranging from industrial bioreactors to agricultural silage production. © 2000 Society of Chemical Industry