A study of the Gamma hypothesis: Predictive modelling of the growth and inhibition of Enterobacter sakazakii

Although the temperature growth profile of the opportunistic pathogen Enterobacter sakazakii is known, few other environmental factors affecting growth have been analysed. Using a model based on the Gamma hypothesis – that antimicrobial factors in mixtures exert independent effects – a range of weak acids (lactic, acetic, propionic, citric, sorbic and benzoic), pH, salt and temperature and some of their combinations were examined. The weak acids examined inhibited principally with the acid-form of the weak acid, however, benzoic, sorbic and propionic acids also displayed an inhibitory contribution from their respective anionic forms. In all cases pH could be considered an independent inhibiting factor. The minimum pH and maximum salt concentration for growth were calculated to be 3.89 and 9.1% respectively. In combination, there was no suggestion of any interactive effect between them. Studies performed on combinations of Na acetate/pH between 25 and 41 °C showed that temperature did not affect the relative inhibitory effects of the weak acid/pH mixtures.The results of this study support the Gamma hypothesis suggesting that there are no synergistic interactions between inhibitory factors and that growth can be predicted from a library of known effects. More importantly to the food industry, the results can be used to design good quality shelf-life challenge tests by reducing the number of studies required.