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.