A simple technique for the detection of melibiose-fermenting yeasts

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Abstract
The ability to ferment melibiose is the most important differential characteristic separating Saccharomyces carlsbergensis and S. cerevisiae, the 2 most widely used industrial yeasts. Ramnose, a trisaccharide that yields melibiose and fructose on partial hydrolysis, is commonly used in this test. S. cerevisiae is capable of causing only a partial hydrolysis of the raffinose, but S. carlsbergensis ferments it completely. Currently used techniques require special apparatus, and some are of questionable accuracy. A method is described which requires a conventional fermentation tube with an insert and 4% raffinose-yeast extract-peptone-brom-thymolblue broth. The yeast to be tested is inoculated into the broth and when the fermentation is over and the amount of gas in the insert has diminished in volume, a vigorous melibiose-positive yeast is inoculated into the tube. If evolution of gas follows, the yeast under test was melibiose-negative; if no gas is produced the yeast was melibiose-positive. The melibiose-fermenting strain used in this test was S. carlsbergensis, NRRL No. 379.

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