Comparison of selective mycological agar media for the isolation and enumeration of xerophilic moulds and osmotolerant yeasts in granulated white sugar

The objective of this study was to evaluate the efficiency of selective mycological agar media for the isolation and enumeration of xerophilic moulds and osmotolerant yeasts present in white sugar. The sugar was heat treated to minimize interference of background contents of moulds and yeasts and inoculated with four xerophilic moulds belonging to the genera Aspergillus, Erotium and Wallemia and four osmotolerant yeasts belonging to the genera Zygosaccharomyces, Candida and Pichia. The recovery was evaluated using membrane filtration and four different agar media based on Wort agar and Sartorius Wort agar nutrient filters supplied with 50% glucose (Wort50; Wort50S), malt extract agar supplied with 40% sucrose (MEA40), Dichloran 18% glycerol agar (DG-18) and Dichloran 18% glycerol agar supplied with 20% sucrose and 5% glucose (DG-18S). The suggestion to use De Man, Rogosa, Sharpe agar (MRS) to enumerate osmotolerant yeasts was also addressed in this study as recommended in ICUMSA Proceedings 22nd session [11].

The recovery on MEA40 and DG-18S was the highest for the xerophilic moulds as well as for the osmotolerant yeasts. Recoveries on Wort50 and Wort50S were lower for both xerophilic moulds and osmotolerant yeasts while recoveries on DG-18 were lower for xerophilic moulds but equal to MEA 40 and DG-18S for osmotolerant yeasts. The recovery of osmotolerant yeasts on MRS was among the lowest of all tested media. The statistical evaluation of the efficiency of the media showed no significant differences between MEA40 and DG-18S, whereas colony counts for Wort50, Wort50S, DG-18 and MRS were significantly lower compared to DG-18S. The advantage of DG-18S is the low water activity (aw) of the media due to the addition of sucrose and glucose, which allows rich growth of the most common xerophilic and osmotolerant microorganisms present in white sugar as well as the presence of dichloran, which inhibits the spreading of mucoraceous species such as Mucor and Rhizopus and restrict the colony size of other fast-growing genera.

In conclusion, DG-18S can be recommended to be used as the favored agar media for microbiological examination of white sugar quality with regard to xerophilic and osmotolerant micro- organisms. Furthermore, this conclusion is most likely valid when examining sugar solutions since the yeasts used in this study regularly may be found in liquid sugar products. The poor performance of the MRS-medium, in essence due to the high water activity (aw = 0.993), makes it inadequate to use for regular detection of osmotolerant yeasts in white sugar as well as sugar solutions.

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