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Evaluation of ISO method 21527-1 compared with other yeast and mould detection systems

Yiping Chen and Ruth Fowler of the Institute of Life and Earth Sciences at Heriot-Watt University offer their insight.

Fungal contamination of food is a major problem, particularly in terms of food spoilage, but there is also the risk of spreading fungal pathogens to vulnerable people and contamination of agricultural food products by mycotoxin1-3. The Soleris system was initially developed in the 1990s to detect microbes in dairy products4,5 based on the growth of microbial contaminants in selective broths and the detection of pH changes induced by microbial metabolic by-products, causing colour changes in an agar indicator5. In this study, the automated Soleris 128 system was evaluated for its ability to detect yeast and moulds in various food matrices (one matrix of each food category defined by ISO). It detects carbon dioxide which is produced by fungal microbes growing in the system by measuring colour changes in an agar plug at the base of the vial. Only gases, such as carbon dioxide, can connect with the agar through a gas-permeable (and liquid-impermeable) membrane at the base of the vial6.

Materials and methods

The ability of the system was tested using four yeast species and two moulds in five different beverage and food samples plus one environmental sample. The challenge assays were: C. krusei in tap water (mimicking environmental sampling), S. cerevisiae in fruit juice, C. albicans in a tomato ketchup sauce, C. tropicalis in fruit yogurt, A. caseiellus mixed with deli ham and P. chrysogenum mixed with sliced smoked salmon. The food samples were purchased in a local supermarket and tap water collected from a local residence in Edinburgh city. All the solutions and media that were used in this study for plating were autoclaved at 121°C for 15mins. All the solutions and media were stored at 4°C.

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