Two-Day Yeast and Mold Enumeration Using the ISO-GRID® Membrane Filtration System in Conjunction with YM-11 Agar

Evaluation of the Hydrophobic Grid Membrane Filter for the Enumeration of Moulds and Yeasts in Naturally-Contaminated Foods

Over 240 food samples from six food groups (tree nuts, grains and grain products, dried fruits, fresh produce, fruit juice, and dairy products) were tested for levels of fungal contamination using the NEO-GRID hydrophobic grid membrane filter (HGMF) and the FDA official (BAM) method. Results showed that HGMF performed very well for all tested commodities giving yeast and mould (YM) counts similar to those of the BAM (reference) method. Statistical analysis of the data (t-test) revealed no significant differences between the two methods for all foods tested. Regression analysis showed that there was a good fit linear relationship between the two methods for most of the commodities examined. Some difficulties were encountered during counting of the colonies on HGMF since the size of the grid is very small and the number of possible colonies per plate can reach 1600.

Detection of Kluyveromyces marxianus and other spoilage yeasts in yoghurt using a PCR-culture technique

A combined PCR-culture technique was developed for the detection of viable yeasts in yoghurt samples. Yoghurt samples were inoculated with either viable or heat-inactivated Kluyveromyces marxianus cells, and analyzed before and after incubation for 24 h at 25 degrees C under agitation. DNA was extracted from the samples and amplified using yeast-specific primers targeted at the gene coding for the 18S rRNA. A 251-bp fragment was amplified by the Polymerase Chain Reaction from the yoghurt samples containing initial yeasts counts of 10 cfu g(-1) or higher, whereas no PCR product was generated from control uninoculated yoghurt samples. Comparison of PCR results obtained before and after the incubation step was used to assess yeast viability. Viability was also confirmed by plating on Sabouraud-Dextrose-Chloramphenicol Agar. Moreover, comparison of the results obtained using PCR-culture and plate count methods for the analysis of commercial yoghurt samples, demonstrated that the PCR-culture technique developed in this work can be very useful for the rapid detection of viable spoilage yeasts in dairy industries.

Enumeration of yeasts in dairy products: a comparison of immunological and genetic techniques

Enzyme-linked immunosorbent assay (ELISA) and PCR techniques have been developed for the detection of spoilage yeast species in dairy products. Polyclonal antibodies against live yeast cells (AY) were raised in rabbits by inoculation of a mixture of 10 yeast species frequently associated with dairy products spoilage. AY antibodies were used for the development of two ELISA formats (indirect and double-antibody sandwich ELISA) for the detection of yeast species in milk and yogurt. A PCR assay was also developed for yeast detection in dairy products, using primers designed to amplify a conserved 250-base pair fragment of the 18S rRNA of the yeast species. The results obtained in this work show that ELISA techniques using polyclonal antibodies against viable yeast cells are of limited value for the detection and enumeration of spoilage yeast species in dairy products. On the contrary, PCR amplification of a conserved region of the 18S rRNA of the yeast species allows the homogeneous detection of all the yeast species tested and, combined with an overnight enrichment of samples, could be used for the detection of low levels of viable spoilage yeast species in dairy products.

Comparison of methods for enumeration of yeasts and molds in shredded low-moisture, part-skim mozzarella cheese

Two studies were conducted to compare established and new methods for enumerating yeasts and molds in shredded low-moisture, part-skim mozzarella cheese stored under refrigeration and temperature-abuse conditions. Yeast and mold counts covered a range of 6 log10 units. In study 1, the potato dextrose agar plus chlortetracycline (PDA) pour plate, dichloran rose bengal chloramphenicol (DRBC) spread plate, Petrifilm, and Iso-Grid hydrophobic grid-membrane filtration methods were used to analyze samples after < or = 1 day of unopened storage at 7 degrees C and after opening, resealing, and 2 days of storage at 25 degrees C. The results of all methods were highly correlated (r2 > or = 0.96). In study 2, the PDA, DRBC, and Iso-Grid methods were compared with the Simplate 2-day method in an analysis of 42 samples stored for various times at 8, 11, 15, and/or 22 degrees C. The results of all methods except the Simplate method were again highly correlated (r2 > or = 0.94), although yeasts and molds were not always detected by all methods. Compared with the PDA, DRBC, and Iso-Grid methods, the Simplate method most often (10 of 42 samples, 23.8%) failed to detect yeasts and molds when at least one other method did, and the results were less highly correlated with those of other methods (r2 = 0.88 to 0.90). Our results suggest that the PDA, DRBC, Petrifilm, and Iso-Grid methods are equivalent for enumerating yeasts and molds in shredded low-moisture, part-skim mozzarella cheese samples.