Molecular methods
Although it has proved difficult to develop immunological methods that will detect a sufficiently broad spectrum of yeast and mould species, molecular biology-based methods have been successfully commercialised and automated for food industry applications. A yeast and mould assay kit has been developed for the widely used BAX® System from Dupont Qualicon, which amplifies and detects a DNA sequence specific to microfungi. The kit is able to detect yeasts and moulds at levels of 50 CFU/g within hours after a 44-hour enrichment step, but can detect higher levels within a day.
A test for fermentative yeasts designed specifically for the beverage industry has been developed by vermicon AG. Based on gene probe technology, the VIT-Fermentative Yeasts test kit is claimed to detect low levels of viable yeast cells with a time saving of several days over conventional methods.
A yeast method is available from Pall and is based on the GeneDisc® technology. Designed for beverage industries, it provides results in 2 hours for direct analysis (e.g. yeast slurry testing) and colony identification. To determine the presence of down to one microorganism in the sample, the test is performed in as quick as 30 hours.
Identification
Few rapid methods are available that can be used to identify yeast and mould isolates from food products. However, several systems have been developed to identify fungi primarily from clinical samples and these may be of use for food applications. The best known and most widely used in food microbiology labs are Biolog automated and manual systems for yeasts and filamentous fungi and the API® 20C AUX and ID 32 C strips for identifying yeasts. Both use carbon source metabolic ‘fingerprints’ to obtain a preliminary identification within 24-48 hours.
In addition, the GeneDisc PCR solution for Yeast ID allows the identification of the 12 major beverage spoilage yeast species and genera, including Saccharomyces cerevisiae var. diastaticus and Brettanomyces bruxellensis.