A microscopic view of good yeast dissolved in water should have the appearance as seen in [Fig. 2], shown above. When broken it should show a fine fracture, irregularly rounded. Should it be crumbly, deterioration has set in. In lukewarm water it should melt readily, and not be sloppy to the touch. The dissolved yeast placed in a glass tube should settle slowly and evenly with the water above it perfectly clear. During this test adulteration with plaster of paris is readily detected, as it would be the first to settle out, and by carefully decanting the fluid, examination of the sediment would disclose plaster of paris.
If the solution of yeast and water does not clear itself the yeast is spoiled, and is of no use for the fermenting of doughs. It is contaminated with wild yeast and harmful bacteria, and would be instrumental in starting putrefactive fermentation, ruining the flavor of baked goods. Should we desire to ascertain the amount of starch present in an adulterated yeast, the following method is applicable:
Weigh a small beaker and a small glass rod on a very accurate scale; or, better still, on an analytical balance, and assume the weight to be 17.5 g. In the beaker place 10 g. of the compressed yeast under examination; break it up fine with the glass rod, and place the beaker in a hot water bath for several hours, weighing occasionally until two consecutive weighings are exactly equal; for instance, 21.2 g. We deduct from this the weight of the beaker and glass rod, giving us the following figures: 21.2 g. - 17.5 = 3.7 g.
The quantity of moisture evaporated out of the yeast would therefore be 10 g. - 3.7 = 6.3 g. According to the findings of Hayduck, pressed yeast contains originally 73.5 per cent. and dry starch 36 per cent. of moisture.
We now proceed to make deductions to determine the quantity of starch contained as an adulterant in the mixture. We set the example: “What per cent. of starch is contained in a mixture of yeast and starch if 10 g. of the mixture gives off by evaporation 6.3 g. of moisture, yeast containing 75.3 per cent. and starch 36 per cent. of moisture?”
Solution.—One hundred g. of pure press yeast, heated to dryness, gives off 73.5 g.; therefore, 10 g. heated should give off 7.35 per cent. moisture.
In our test the loss is but 6.3 g., consequently a deficiency of 1.04 g. This in itself indicates starch adulteration.
Starch gives off 36 per cent. of moisture; therefore, 1 g. gives off 0.36 g., and 1 g. of yeast 0.735 g. of moisture. With each 1 g. of starch addition the moisture loss is found to be 0.735 - 0.36 = 0.375 g. deficient.
In the 10 g. mixture under examination there is contained as many times 1 g. of starch as 0.375 g. is contained in 1.05 g., which is equal to 2.8 g. In a 100 g. mixture the result would be 28 g., or 28 per cent., which is the per cent. of starch adulteration in our mixture examined.