Rapid method for testing raffinose fermentation by yeasts.

The alcoholic fermentation of raffinose by yeasts has considerable taxonomic importance. Certain species do not ferment this trisaccharide at all; others ferment only the fructose part; whereas others ferment both parts (fructose and melibiose). All or some strains of a few species ferment melibiose incompletely. Most older methods for testing raffinose fermentation are based on quantitative determinations either of the CO2 formed or of the residual sugar; Henrici, according to Wickerham in 1943 used melibiose rather than raffinose. The first group of methods requires special equipment or time consuming chemical titrations, both incompatible with routine work; whereas the use of a broth prepared with pure melibiose requires in certain cases a prolonged time of adaptation (Wickerham, 1943). This latter difficulty is reduced when melibiose broth is prepared as proposed by Skinner and Bouthilet (1947); the method, however, requires the use of a raffinose and a melibiose tube, if full information is to be obtained regarding the ability of a given yeast to ferment raffinose. A practical method was described by Wickerham (1943), who used a single Durham tube with raffinose broth. After completion of the fermentation (if any) by the yeast under study, Wickerham inoculated the same tube with a strong melibiose fermenter (Saccharomyces carlsbergensis, NRRL Y-379). This yeast starts a fermentation if the first yeast has not fermented, or has only incompletely fermented, the melibiose part of the raffinose molecule. We have found that the same information may be obtained through use of a simple qualitative test for reducing sugar. This method is based on the facts that raffinose is a nonreducing trisaccharide, and melibiose and its constituent monosaccharides are reducing sugars. Therefore, nonreducing raffinose broth becomes reducing when