Rapid detection of contaminants in pitching yeast using automated turbidometry

An automated turbidometric technique was tested for rapid detection of contaminants in pitching yeast. The turbidometer detects changes in optical density caused by microbial growth in a liquid medium. Pitching yeast samples were contaminated with three different cell concentrations of Lactobacillus casei subsp. casei, Pediococcus damnosus, Enterobacter agglomerans, Pectinatus cerevisiiphilus, and Saccharomyces cerevisiae (ex diastaticus). Detection times for the contaminants were calculated from the growth curves obtained with the turbidometer. Enrichment of contaminated yeast samples in broth and plating on selective agars were used as reference methods. The results obtained with the turbidometer were in most cases available one day before the appearance of visual turbidity. The time saved with the turbidometer compared with selective agars was from two to four days depending on the contaminating organism. The detection time was dependent more on the specific growth rate of the test organism than on its initial concentration. The detection limit of the turbidometer was 106 cfu/ml and the initial contamination level detected was 100 cfu/ml of yeast slurry.