Modeling the inhibitory effects of organic acids on bacteria

The inhibitory effect of acids on microbial growth has long been used to pr eserve foods from spoilage. While much of the effect can be accounted for b y pH, it is well known that different organic acids vary considerably in th eir inhibitory effects. Because organic acids are not members of a homologo us series, but vary in the numbers of carboxy groups, hydroxy groups and ca rbon-carbon double bonds in the molecule, it has typically not been possibl e to predict the magnitude, or in some cases even the direction, of the cha nge in inhibitory effect upon substituting one acid for another or to predi ct the net result in food systems containing more than one acid. The objective of this investigation was to attempt to construct a mathemati cal model that would enable such prediction as a function of the physical a nd chemical properties of organic acids. Principal Components Analysis (PCA ) was applied to 11 properties for each of 17 acids commonly found in food systems; this resulted in four significant principal components (PCs), pres umably representing fundamental properties of the acids and indicating each acid's location along each of these four scales. These properties correspo nd to polar groups, the number of double bonds, molecular size, and solubil ity in non-polar solvents. Minimum inhibitory concentrations (MICs) for each of eight acids for six te st microorganisms were determined at pH 5.25. The MICs for each organism we re modeled as a function of the four PCs using partial least squares (PLS) regression. This produced models with high correlations for five of the bac teria (R-2 = 0.856, 0.941, 0.968, 0.968 and 0.970) and one with a slightly lower value (R-2 = 0.785). Acid susceptible organisms (Bacillus cereus, Bac illus subtilis, and Alicyclobacillus) exhibited a similar response pattern. There appeared to be two separate response patterns for acid resistant org anisms; one was exhibited by the two lactobacilli studied and the other by E. coli. Predicting the inhibitory effects of the organic acids as a functi on of their chemical and physical properties is clearly possible. (C) 1999 Elsevier Science B.V. All rights reserved.