Agar in media with a low pH is hydrolyzed when autoclaved. Therefore, the sterilized molten agar is cooled to approximately 45°C and a predetermind volume of acid is added. The medium and acid are quickly but gently mixed to avoid air bubbles and immediately poured into petri plates. The addition of approximately 0.7% (v/v) 1 N hydrochloric acid to YM agar or glucose-peptone-yeast extract agar usually gives the desired pH of 3.7 to 3.8 many yeasts can be recovered at pH 3.7, but some species, such as those of the genus Schizosaccharomyces, are inhibited by high acid media and moderately acidic media with a pH in the range 4.5 to 5.0 give a higher recovery (Yarrow, 1998).