Book of American Baking / A Practical Guide Covering Various Branches of the Baking Industry, Including Cakes, Buns, and Pastry, Bread Making, Pie Baking, Etc.

by Various

We have now seen that yeast is capable of producing alcohol and carbon dioxide. This is called alcoholic fermentation.

Wine, beer, brandy and other spirituous liquors are produced by alcoholic fermentation, and the same is attributed to the raising of bread doughs.

The yeast cell in its search for nutriment consumes and changes the sugar, to facilitate growth, finally reducing it into simpler bodies of alcohol and carbon dioxide.

The chemical changes of the sugar are due to the ever-changing composition of the albuminous plasma of the yeast cell. When the plasma has lost the power to renew itself, it dies and putrefaction sets in.

Worts of sugar and diffusible albuminous solutions are ideal foods for yeast, as they readily permeate the fine, porous coverings of the yeast cells to nourish the plasma, which at the same time, by its own action, creates the requisite warmth by the dissolution of the sugars with alcohol—carbon dioxide.

The following description will illustrate how this is accomplished:

Make a drumhead, by stretching and fastening a piece of bullock’s bladder or either vegetable or animal parchment paper over a cylinder of glass. Place this in a vessel containing pure water, and pour into the cylinder a strong solution of common salt. The salt brine and the pure water are only separated from each other by the thin membrane of the bladder or the parchment. After a little while it will be noticed that the salt solution will have diffused out through the membrane until the liquid, both outside and inside the floating cylinder, has the same strength. This is called osmose, or dialysis.

In choosing its nutriment yeast is very selective. Of the carbohydrates, glucose, maltose and those of C₆H₁₂O₆ group are capable of direct fermentation, and are quickly and vigorously changed by yeast. In direct opposition, we find that cane sugar, beet sugar, as well as the starch of flour, are not fermentable until chemically changed. This change is brought about by yeast itself.

The plasma of the yeast contains an albuminous substance called Invertin. As explained above, the Invertin, by dialysis, is diffused out through the cell covering and changes cane sugar and sugars of the same class, as well as part of the flour starch, into fermentable sugar, known as invert sugars.