. Bacteriology and mycology of foods. Food. 220 YEASTS AND MOLDS Microscopic Examination of Compressed Yeasts. Prepare a thm mixture of the yeast in distilled water. To this mixture or a small por- tion of it add a httle of a per cent solution of methylene blue. Mount a drop of this under a cover glass on a slide and enumerate the dead and living bacteria. The dead cells will stain but the living cells will not. Test for Adulteration with Beer Yeast. Practically all of the beer yeasts ferment rafiinose while the bread yeasts generally do not. One gm. of the yeast is mixed with about 10
. Bacteriology and mycology of foods. Food. 220 YEASTS AND MOLDS Microscopic Examination of Compressed Yeasts. Prepare a thm mixture of the yeast in distilled water. To this mixture or a small por- tion of it add a httle of a per cent solution of methylene blue. Mount a drop of this under a cover glass on a slide and enumerate the dead and living bacteria. The dead cells will stain but the living cells will not. Test for Adulteration with Beer Yeast. Practically all of the beer yeasts ferment rafiinose while the bread yeasts generally do not. One gm. of the yeast is mixed with about 10 of a 1 per cent solution of raflS-nose broth and placed into a fermentation tube. Incubate this tube for about twenty-four hours at 30° C. A blank tube without rafiinose should be prepared in order to test for gas produced from stored glycogen. If the difference between the two tubes is decided (about 50 per cent) adulteration with beer yeast is probable. A greater difference renders the test more conclusive. Efficiency of Bread Yeasts. This may be determined in different ways. A representative sample of the yeast should be obtained and divided into three parts. Store one part at 20°, 30°, and 40° C. At regular intervals test each part for dead ceUs, gas formation, and dough-rising power. Test once or twice for adulteration with beer yeast and with myco- derma. Dough-rising Power of Bread Yeast. Mix 100 gms. of flour, 2 gms. of yeast and 60 of distilled water by first making a thin batter of the yeast, water and a little of the flour. Then add the rest of the flour and knead for five minutes. About 3 gms. of flour should be kept with which to clean the glass rod. The dough should be formed into a cylinder and dropped into a warmed graduate cyl- inder which has been greased or powdered with flour. Press the dough down into the cylinder and place at 30° C. The volume is read every thirty minutes until the maximum is reached. Compute the maximum volume in terms of the
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