Performance of mycological media in enumerating desiccated food spoilage yeasts: an interlaboratory study

Dichloran 18% glycerol agar (DG18) was originally formulated to enumerate n onfastidious xerophilic moulds in foods containing rapidly growing Eurotium species. Some laboratories are now using DG18 as a general purpose medium for enumerating yeasts and moulds, although its performance in recovering y easts from dry foods has not been evaluated. An interlaboratory study compa red DG18 with dichloran rose bengal chloramphenicol agar (DRBC), plate coun t agar supplemented with chloramphenicol (PCAC), tryptone glucose yeast ext ract chloramphenicol agar (TGYC), acidified potato dextrose agar (APDA), an d orange serum agar (OSA) for their suitability to enumerate 14 species of lyophilized yeasts. The coefficient of variation for among-laboratories rep eatability within yeast was 1.39% and reproducibility of counts among labor atories was 7.1%. The order of performance of media for recovering yeasts w as TGYC > PCAC = OSA > APDA > DRBC > DG18. A second study was done to deter mine the combined effects of storage time and temperature on viability of y easts and suitability of media for recovery. Higher viability was retained at - 18 degreesC than at 5 degreesC or 25 degreesC for up to 42 weeks, alth ough the difference in mean counts of yeasts stored at - 18 degreesC and 25 degreesC was only 0.78 log(10) cfu/ml of rehydrated suspension. TGYC was e qual to PCAC and superior to the other four media in recovering yeasts stor ed at - 18 degreesC, 5 degreesC, or 25 degreesC for up to 42 weeks. Results from both the interlaboratory study and the storage study support the use of TGYC for enumerating desiccated yeasts. DG18 is not recommended as a gen eral purpose medium for recovering yeasts from a desiccated condition. (C) 2001, Elsevier Science B.V. All rights reserved.