Controlling spoilage yeasts

Controlling spoilage yeasts in foods and beverages is the subject of ongoing research in the European Union. A collection of spoilage yeasts, isolated mostly from wines, olives, sugar syrups and yellow sauces, is being put together to obtain reference strains so that scientists can develop and apply rapid techniques for typing, identifying and testing the growth of these yeasts under a range of conditions.

Researchers found a specific strain of Z. bailli (ISA 1307) to be very resistant to sorbic acid. It was used for a number of studies together with S. cerevisiae. For example, the former grew at acetic acid levels of up to 3.5% and the latter at levels of up to 1% at pH 4. The presence of acetic or benzoic acids decreased the temperature range at which these microorganisms grow, indicating that their viability at high or low temperatures is strongly influenced by the presence of preservatives.

Z. bailli has an optimum temperature for growth at 32 C and has an associative temperature profile. In other words, death and growth coexist from 33 C to 39 C. The presence of ethanol, acetic acid or sorbic acid causes a shift of the lethal temperature to lower values.

However, this effect was less pronounced in Z. bailli than in S. cerevisiae. Both strains also can extrude benzoic acid during growth in the presence of almost any weak acid. The physiological relevance of this activity is being investigated. Resistant strains of Z. bailli and P. membranaefaciens can grow in sorbic acid concentrations of 650 ppm. The former can also grow on 70% glucose.

Further information. V. Loureiro, Instituto Superior de Agronomia, Department of Botony and Biology, P-1399 Lisbon Codex, Portugal; phone: [phone]; fax: [phone].