Combined effect of water activity and pH on the inhibition of Escherichia coli by nisin

The Doehlert design and surface response methodology were used to study the influence of pH and water activity (aw) on Escherichia coli inhibition by nisin. Combining stress factors at levels where they are not inhibitory by themselves, a reduction of E. coli survival fraction can be achieved with lower nisin doses than in a single nisin treatment. For all the pH values assayed, a synergistic effect of aw and nisin concentration was detected, and the isoresponse lines showed the existence of an area of maximum inhibition. Factors that reduced viable cell counts by 4 to 5 log cycles were 1,000 to 1,400 IU of nisin per ml at pH 5.5 to 6.5 and a water activity of 0.97 and 0.98. The addition of different ionic and nonionic solutes to control aw suggested that the effect of aw in the inhibitory action of nisin on E. coli cells was not solute-specific. The use of the Doehlert experimental design was effective to determine the optimal combination of stress factors, as well as to point out the most important variables that affected E. coli inhibition.