Gamma study of pH, nitrite, and salt inhibition of Aeromonas hydrophila

The gamma hypothesis states that there are no interactions between antimicrobial environmental factors. The time to growth of Aeromonas hydrophila challenged with pH, NaNO2, and salt combinations at 30 degrees C was investigated. Data were examined using a model based on the gamma hypothesis (the gamma model), which takes into account variance-stabilizing transformations and which gives biologically relevant parameters. At high concentrations of NaNO2 and at pHs of > 6.0, the antimicrobial action of the nitrite ion has a strong influence (MIC = 2,033 mg liter(-1)), whereas at pHs of < 6, nitrous acid is dominant (MIC = 1.5 mg liter(-1)). This change is not due to a "synergy" between pH and the nitrite ion but is due to the shift in the equilibrium concentrations of nitrous acid and nitrite in solution caused by pH. In combination with salt, the parameters found for the action of Na nitrite were identical to those found when it was examined in isolation. Therefore, pH, NaNO2, and salt act independently on the growth of A. hydrophila. By expanding the gamma model with a cardinal temperature model, the results of fitting the model of Palumbo et al. (J. Food Prot. 54:429-435, 1994) to randomly produced environmental conditions could be reproduced, suggesting that temperature also has an independent effect.