Weak-acid preservatives: modelling microbial inhibition and response.
Weak-acid preservatives are widely used to prevent microbial spoilage of acidic foods and beverages. Characteristically, weak-acid preservatives do not kill micro-organisms but inhibit growth, causing very extended lag phases. Preservatives are more effective at low pH values where solutions contain increased concentrations of undissociated acids. Inhibition by weak-acids involves rapid diffusion of undissociated molecules through the plasma membrane; dissociation of these molecules within cells liberates protons, thus acidifying the cytoplasm and preventing growth. By modelling preservative action in yeast, using a thermodynamic and kinetic approach, it was possible to demonstrate that: (i) inhibition depends more on the degree to which individual preservatives are concentrated within cells, rather than on undissociated acid concentration per se; (ii) it is entirely feasible for microbes to pump protons out of the cell during extended lag phase and raise internal pH (pHi), despite further influx of preservatives; (iii) the duration of the lag phase can be predicted from the model, using a Gaussian fit of proton-pumping H(+)-ATPase activity against pHi; (iv) theoretical ATP consumption for proton pumping can be directly correlated with the reduction in cell yield observed in glucose-limited cultures.
Microbiology Section, Unilever Research, Sharnbrook, Bedford, UK. RONNIE.LAMBERT@UNILEVER.COM
R J
RJ
Lambert
M
M
Stratford
D000144:Acids, Acyclic / Q000494:pharmacology*; D055598:Chemical Phenomena; D002627:Chemistry, Physical; D005520:Food Preservatives; D006863:Hydrogen-Ion Concentration; D017136:Ion Transport; D008954:Models, Biological; D012441:Saccharomyces cerevisiae / Q000187:drug effects* / Q000254:growth & development
D000144:Acids, Acyclic; D005520:Food Preservatives