Effect of the organic acid type on the heat resistance of bacterial spores

It has been recognised for several years that low pH reduces spore resistance. Mafart and Leguerinel (1998) proposed a model to describe the influence of pH on the heat resistance of bacterial spores. In addition to the conventional z value, the effect of pH on the thermal resistance of spores is characterised by a z(pH) value (Z(pH) is the distance of pH from a reference pH*, which leads to a ten fold reduction of D-value). However, according to the type of acid used, the level of pH influence changes (Blosher and Busta 1983). To study the acid effect on the Bacillus cereus spore heat resistance, the heat treatment medium used is a tryptone salt broth acidified at different pH with the addition of nine different organic acids (L glutamic acid, malonic acid, citric acid, malic acid, glucono-delta-lactone, lactic acid, succinic acid, adipic acid and acetic acid). A linear relationship between the calculated z(pH) value and its lower acid pKa was observed. This relationship showed that the acid form (dissociated or undissociated) modifies the thermal spore resistance in addition to the H+ ion. The influence of the dissociated acid form on the bacterial spore heat resistance is observed. At pH 7 higher D value observed for the highest of acid concentration shows the protective effect of the dissociated acid form on the heat resistance of spores. These studies should allow to construct an extended model to determine the combined effect of pH and organic acid on the heat resistance of bacterial spores.