A Buffer Capacity Model to Determine pH Changes in Acid and Acidified Vegetables Due to Microbial Activity

Introduction: Acid and acidified food safety depends on maintaining a pH below 4.6 to prevent spore germination. Acids and low pH also inhibit the survival of vegetative pathogens. In addition to lactic and acetic acids in vegetable brines, there are a variety of acidic, basic, and amphoteric compounds that affect buffering and brine pH. 

Purpose: Our objective was to develop a buffer capacity (BC) model that could predict pH changes that may affect safety in acid and acidified vegetables due to microbial activity.

Methods: The acid composition of fully, partially and non-fermented (recycled) commercial cucumber fermentation brines was determined by HPLC. The BC of these brines was determined by titration, and compared to the predictions of a preliminary BC model based on the measured acid concentrations. A non-linear optimization algorithm was then used to determine the pKa and concentration of a hypothetical monoprotic buffer that, combined with the known BC, matched the titration data

Results: Comparison of BC data from brines with control solutions containing only lactic acid, acetic acid and NaCl revealed additional buffering was present. The results show that a hypothetical buffer with a pKa of 3.0 and a concentration approximately 20 mM greater than lactic acid of each brine (R², 0.98) could predict the observed BC of the brines tested. The hypothetical buffer was presumably a composite of bacterial and plant cell components, including proteins, amino and other acids.  

Significance: The BC model can be used for predicting brine pH and pathogen survival in fermenting vegetable brines, as well as the pH changes due to growth of spore forming bacilli in acidified vegetable products. These predictions may be useful for the identification of critical hazards for acid and acidified vegetable products as may be required under the Food Safety Modernization Act.