An investigation of the Gamma hypothesis: A predictive modelling study of the effect of combined inhibitors (salt, pH and weak acids) on the growth of Aeromonas hydrophila

The Gamma hypothesis, that multiple inhibitory factors combine independently, is the underlying hypothesis for the quantification of the Hurdle concept used in food manufacture. The literature, however, is confused as to whether interactive effects exist and under which circumstances they occur, if at all. Using the method of time to detection (TTD), the inhibitory effect of pH, salt and specific weak acids (acetic, propionic, sorbic and benzoic) and combinations of these with respect to the growth of Aeromonas hydrophila (ATCC 7966) were analysed. A model based on the relative rate to detection described all combinations analysed as having independent effects on the TTD. No synergistic interactions were found between pH and salt, between pH and individual weak acids or between combinations of weak acids and pH for any of the systems under study.This study supports the validity of the Gamma concept — that individual environmental effects act independently and should, in turn, facilitate attempts to model the growth of other microorganisms under a variety of conditions.