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Acid concentration

FIGURE 14.2 Effects of acetic and gluconic acid on the destruction of Escherichia coli O157:H7 (cocktail of strains). The log reduction times (d values) for acetic (black bars) and gluconic (gray bars) acids at 0.002, 0.02, and 0.2 M concentrations in water at 25°C and pH 3.1. The error bars indicate the upper 95% confidence intervals. No statistically significant difference was detected for the gluconic acid D values (p > 0.05). (From Breidt, F., Hayes, J.S., and McFeeters, R.F., J. Food Prot, 67, 12-18, 2004.)

Acid concentration

FIGURE 14.2 Effects of acetic and gluconic acid on the destruction of Escherichia coli O157:H7 (cocktail of strains). The log reduction times (d values) for acetic (black bars) and gluconic (gray bars) acids at 0.002, 0.02, and 0.2 M concentrations in water at 25°C and pH 3.1. The error bars indicate the upper 95% confidence intervals. No statistically significant difference was detected for the gluconic acid D values (p > 0.05). (From Breidt, F., Hayes, J.S., and McFeeters, R.F., J. Food Prot, 67, 12-18, 2004.)

increasing temperature for a given pH and ionic strength. Gluconic acid may have wider application as a noninhibitory buffer for similar experiments with other organic acids.

In addition to the proposed mechanisms for the effects of weak acids on microorganisms mentioned above (acidification of the cytoplasm and intracellular accumulation of anion), the effectiveness of these compounds may be modulated by additional factors. Examples of these include: specific effects of the acid or acid anion on cellular enzymes or membranes, the internal buffering capacity of cells, proton pumping at the expense of cellular ATP, and facilitated transport of acid molecules, among others. To investigate the relative importance of these effects for the inhibition of yeasts with sorbic acid, Stratford and Anslow [71] compared the effects of acids with similar pK values (acetic acid, pK = 4.76; sorbic acid, pK = 4.74) and used structural analogs of sorbic acid that have similar lipophilic properties. Interestingly, a variety of structural analogs, including aldehydes and alcohols, had similar MIC values to sorbic acid (which was 3 mM) for the inhibition of a Saccharomyces cerevisiae strain, and a survey of yeast strains showed sorbate resistance correlated with ethanol tolerance [71]; they proposed that sorbic acid acted specifically on yeast membranes. Krebs et al. [69] examined glycolysis intermediates in yeast cells treated with benzoate and showed an increase in the intracellular concentrations of glucose 6-phosphate and fructose 6-phosphate, while frucotose 1,6-bisphosphate and triose phosphate concentrations were reduced. The specific inhibition of phosphofructokinase, however, could be attributed to a lack of ATP required for the function of this enzyme [69]. Alakomi et al. [72] showed that lactic acid had a specific membrane effect on Gram-negative bacteria. They found that lactic acid could sensitize E. coli O157:H7, pseudomonas, and salmonella to lytic agents such as detergents and lysozyme, presumably by disrupting the outer membrane. Lactic acid (5mM, pH 3.5) was found to have a greater ability to liberate lipopolysaccharides from the outer membrane of Salmonella serovar Typhimurium than a 1 mM EDTA solution under similar conditions [72]. The effect of sorbate on the germination of C. botulinum spores was investigated [110]. This study indicated that sorbate inhibited spore outgrowth by disrupting the cell membrane after the start of germination. In addition to membrane effects, organic acids may have a variety of other possibly minor effects on the inhibition of microorganisms. A review by Shelef [88] cites additional effects of lactate salts on the inhibition of microorganisms. These effects include lowering water activity, chelating iron, and the inhibition of lactate dehydrogenase.

14.5.2 Genetic Regulation of Acid Resistance

Induction of acid resistance genes in E. coli can be accomplished by growing cells statically to stationary phase in media containing an excess of glucose, resulting in a pH of about 5.5 [104]. The acid resistance systems in E. coli and other pathogenic bacteria are also subject to crosstalk, or regulation by

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