EVALUATION OF THE INDIRECT CONDUCTANCE METHOD FOR THE DETECTION OF YEASTS IN LABORATORY MEDIA AND APPLE JUICE

The technique of indirect conductimetry was examined to asses growth of seven yeast species (Candida glabrata, C. parapsilosis, Debaryomyces hansenii, Pichia membranaefaciens, Rhodotorula glutinis, Saccharomyces cerevisiae, Zygosaccharomyces bailii) in terms of detection time, maximum rate of change in conductance and maximum change in conductance in laboratory media and apple juice. Conductance parameters were dependent on the species, population density, temperature, pH, aw and potassium sorbate concentration. Indirect conductimetry can be applied to detect yeasts at populations as low as 101 cfu ml-1 within 48 h at 30°C. A defined medium is not required to enhance conductivity. Measurements can be done directly in apple juice without interference of juice constituents with conductance signals. Compared to direct conductimetry, the indirect method is more sensitive, giving shorter detection times and higher rates of change in conductance. Conductance responses measured by the indirect method correlated well with viable yeast populations determined by conventional plate count techniques. Indirect conductimetry has potential as a rapid, convenient and reliable method to evaluate product quality and stability, and it may also have application in developing model systems to predict shelf-life of foods subject to yeast spoilage. © 1993 Academic Press Limited.