Separating lactic acid and pH effects on the Listeria innocua inactivation

In food technology, organic acids (e.g., lactic acid) are popular preservatives. Predictive models describing their antimicrobial effect are often restricted to growth of the target organism. Next to this, the effects of the influencing factors pH and the undissociated form of the acid are insufficiently (individually) quantified. The purpose of this study was to separate the individual effects of the influencing factors pH and undissociated lactic acid LaH on the Listeria innocua inactivation. The inactivation process is investigated at controlled conditions of pH and undissociated lactic acid (i) situated on a trajectory, or (ii) forming a rectangular shape in the (pH,[LaH])-plane. The resulting inactivation curves consisted of a (sometimes negligible) shoulder, followed by a descent phase. In a few cases, a tailing phase was observed. Depending on the conditions, the descent phase contained one or two loglinear parts, or had a convex or concave shape. Four types of inactivation models were applied to the experimental data using the freeware tool GInaFiT: (i) the classical loglinear model, (ii) the loglinear model with shoulder and/or tail, (iii) Weibull-type models, and (iv) a biphasic inactivation model. Models of type (ii) and (iii) appeared most suitable. Future application of statistical tests must enable a further model discrimination step. Afterwards, a secondary model for the variation of the primary model parameters, for example, inactivation rate, with pH and [LaH] could be developed.