This section is from the book "Elements Of The Theory And Practice Of Cookery", by Mary E. Williams. Also available from Amazon: Elements Of The Theory And Practice Of Cookery; A Textbook Of Domestic Science For Use In Schools.
T. M. Prudden.
We have learned that mould, yeast, and bacteria are microscopic plants (pp. 30, 128). Collectively they are called microorganisms (meaning little live things). How does yeast make bread light? How do bacteria make milk sour? Under what conditions does mould grow best?
Yeast causes fermentation. Some bacteria cause fermentation, but more cause that unpleasant kind of decomposition called putrefaction. Carbohydrate foods tend to ferment; protein foods tend to putrefy. In foods containing both carbohydrate and protein, whichever process starts first is likely to prevent or to check the other. The enzyms (p. 131) which exist in most foods also tend to decompose them.