34. Standardisation of Media in the Acid Ranges with Special Reference to the Use of Citric Acid and Buffer Mixtures for Yeast and Mould Media

Sources of error in the adjustment of media to pH 3·5 have been discussed. Titration curves for whey and beer wort with citric, lactic and tartaric acids have been constructed. It is shown that citric acid is the most convenient acid to use for adjusting media for yeast and mould counts. The data obtainable from these curves cannot be relied upon to give a medium of pH 3·5 ± 0·1, owing to the variations in the composition of whey and wort, but the required accuracy may be obtained by adding to the medium a bufEer mixture of citric acid monohydrate (64 parts) and potassium citrate monohydrate (36 parts) in a concentration of 2 per cent. In this way reliable results can be obtained without standardising acids or determining pH. values. This method may be used for any pH between 3 and 6.