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Media for preservative resistant yeasts: a collaborative study
1CSIRO Division of Food Science and Technology, NSW, Australia.
Abstract:
An international collaborative study was carried out to determine the most effective medium for selective isolation and enumeration of preservative resistant yeasts. Such a medium should prevent the growth of other yeasts such as Saccharomyces cerevisiae that are tolerant to lower levels of commonly used food preservatives, and sensitive yeasts such as Rhodotorula species. The study compared two non-selective media that are in common use for cultivation of yeasts from foods, Malt Extract agar (MEA) and Tryptone Glucose Yeast extract agar (TGY) with media made selective for preservative resistant yeasts by addition of 0.5% acetic acid to these two basal media (MEAA and TGYA). A fifth medium, Zygosaccharomyces bailii medium (ZBM) was also included in the study. These media were compared for their efficacy in selective isolation and enumeration of the preservative resistant yeasts Zygosaccharomyces bailii, Schizosaccharomyces pombe and Pichia membranaefaciens. MEA and TGY without acetic acid were used as control, non-selective media, and Rhodotorula glutinis was the preservative sensitive control culture. Seven laboratories in six countries took part in the study. Of the non-selective media, TGY generally gave the highest counts, and TGY amended with 0.5% acetic acid (TGYA) was the best medium for recovery of all three preservative-resistant yeasts. ZBM was found to be selective for Z. bailii, but counts of this yeast on ZBM were significantly lower than on TGYA. R. glutinis did not grow on any of the selective media.
Insights
The best medium for isolating preservative-resistant yeasts is Tryptone Glucose Yeast extract agar (TGYA) with added acetic acid. This selective medium effectively recovers yeasts like Zygosaccharomyces bailii, preventing growth of sensitive strains.
Area of Science:
- Food Microbiology
- Yeast Ecology
- Microbial Detection
Background:
- Accurate enumeration of preservative-resistant yeasts is crucial for food safety and quality control.
- Commonly used yeast media may not effectively inhibit non-target yeast species, impacting selective isolation.
- Developing a selective medium is essential to differentiate and quantify spoilage yeasts from other yeast populations.
Purpose of the Study:
- To identify the most effective selective medium for isolating and enumerating preservative-resistant yeasts.
- To compare the efficacy of various basal and modified media for yeast recovery.
- To assess the selectivity of media against common foodborne yeasts and sensitive strains.
Main Methods:
- An international collaborative study involving seven laboratories across six countries.
- Comparison of Malt Extract agar (MEA), Tryptone Glucose Yeast extract agar (TGY), and Zygosaccharomyces bailii medium (ZBM).
- Evaluation of media with and without 0.5% acetic acid (MEAA, TGYA) for selective isolation of Zygosaccharomyces bailii, Schizosaccharomyces pombe, and Pichia membranaefaciens.
Main Results:
- Tryptone Glucose Yeast extract agar amended with 0.5% acetic acid (TGYA) demonstrated the highest recovery rates for all three target preservative-resistant yeasts.
- Zygosaccharomyces bailii medium (ZBM) showed selectivity for Z. bailii but yielded significantly lower counts compared to TGYA.
- Non-selective media (MEA, TGY) generally provided higher total yeast counts, while all selective media successfully inhibited the growth of the sensitive yeast Rhodotorula glutinis.
Conclusions:
- Tryptone Glucose Yeast extract agar with 0.5% acetic acid (TGYA) is the optimal medium for the selective isolation and enumeration of key preservative-resistant yeasts.
- The addition of acetic acid to TGY provides superior selectivity and recovery compared to other tested media, including ZBM.
- This study provides a validated method for accurately assessing preservative-resistant yeast populations in food products.
