Chapter 10 Weak organic acid resistance of spoilage yeasts

Abstract

Reduced reliance on the use of chemical preservatives is desirable in the light of recent evidence that raises concerns over whether the large-scale consumption of these compounds is completely safe. Resistance of spoilage yeasts to the organic acid preservatives currently approved for use in foods and beverages is often a major factor preventing a lowering of preservative levels. Recent work has unravelled the stress responses whereby Saccharomyces cerevisiae becomes resistant to such acids. Resistance to high acetic acid is achieved by loss of the plasma membrane channel, the Fps1p aquaglyceroporin, which facilitates diffusional entry of this acid into cells. Acetic acid stress activates the Hog1p MAP kinase, whereupon this active Hog1p causes phosphorylation, ubiquitination and delivery of Fps1p via endocytic pathway to the vacuole for degradation. Other carboxylic acid preservatives (propionic, sorbic or benzoic acids) are too large to enter the yeast cell through the Fps1p channel, but being more lipophilic than acetic acid, can enter cells at appreciable rates by passive diffusion across the lipid bilayer. Resistance to these involves the induction of an activity for catalysing active efflux of the preservative anion from the cell. In S. cerevisiae , this is the plasma membrane Pdr12p ABC-transporter, regulated by a transcription factor (War1p). The major role of War1p appears to be the induction of PDR12 gene in cells stressed by these moderately lipophilic acids. Still other mechanisms of weak acid resistance are present in the Zygosaccharomyces , most notably a capacity for oxidative degradation of sorbic and benzoic acids that is absent in S. cerevisiae .