220 YEASTS AND MOLDS 
Microscopic Examination of Compressed Yeasts. Prepare a thin 
mixture of the yeast in distilled water. To this mixture or a small por- 
tion of it add a little of a 0.1 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 raffinose while the bread yeasts generally do not. One 
gm. of the yeast is mixed with about 10 c.c. of a 1 per cent solution of 
raffinose broth and placed into a fermentation tube. Incubate this 
tube for about twenty-four hours at 80° C. A blank tube without 
raffinose 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°, 80°, and 40° C. At regular intervals test 
each part for dead cells, gas formation, and 
aa 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 c.c. 
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. 
Fre. 54.—Culture Flask The dough should be formed into a cylinder 
for Fermentation Tests. 
Fitted with Alwood 
(1908) Ventilation 
Tube. 
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 80°C. The volume is 
read every thirty minutes until the maximum is reached. Compute 
the maximum volume in terms of the original. Plot a curve which 
will show the activity of the yeast. 
Determination of the Fermenting Power of Yeasts, Meissl Method. 
Prepare a saccharose phosphate mixture as follows: 400 gms. of sac- 
charose, 25 gms. ammonium phosphate, 25 gms. potassium phosphate. 
Mix this thoroughly and weigh 4.5 gms. into an Erlenmeyer flask which 
