6^S: Reading-Course for Farmers' Wives. 



sauce, on our jelly or bread as in this glass box. If, however, the dust is 

 wiped off the floor, tables, chairs, etc., into a cloth where it is held until 

 washed out, it will never reach the sauce and other foods. If the cloth be 

 slightly damp or oiled, it will take and hold the dust much better. 



The pictures do not show a third kind of plant which, especially in 

 the country, is often present in house dust. This is yeast — also a single 

 cell, but reproducing by little buds which swell out from the parent cell 

 and may or may not break off later on. Those which float freely in the 

 air, both inside and outside of the house, are called " wild yeasts." So 

 far as shape, size and method of reproduction is concerned, they are lit- 

 tle different from the cultivated yeast plants which are used to raise 

 bread or to give the " sparkle " to sweet fermented liquids, as beer. 



As the invisible yeast plants can remain alive for a long time without 

 moisture, we may have them furnished to us in dried cakes as well as in 

 the fresh compressed form. 



Today, even with the cultivated yeasts, the housewife who mixes her 

 sponge in a dusty room, in dusty utensils, with old yeast, — or, with every- 

 thing clean and fresh, if she lets the sponge rise too long or keeps it too 

 hot, — is likely to have sour bread. The bacteria can grow well when and 

 where the yeast cannot, so that acid will be made out of the alcohol which 

 the yeast makes out of sugar. The yeast plants grow best at a medium 

 temperature, about 75° to 90° F., which is an average " summer heat." 

 Above 90° F. they cannot grow so well, but the bacteria grow better. 



The little yeast plant, although so small and simple in structure, is 

 endowed with many of the powers of the trees and vegetables or such 

 higher plants. It requires food, has a certain range of temperature in 

 which it grows best, will be injured or killed by too high or too low tem- 

 perature, or by too little moisture. If it be given the conditions which are 

 favorable, it will feed, grow rapidly, and reproduce itself by swelling out 

 one portion into a bud which may or may not break away from the 

 mother cell. The most favorable temperature for the rapid growth of 

 yeast plants, as already said, is from 75° F. to 90° F. Below this it will 

 not grow rapidly and therefore cannot do much work. At much above 

 90° it will be killed and dead plants cannot work any more than dead 

 animals. 



The work of the yeast plants is to change the sugar in the sponge 

 into two substances — alcohol, and a gas which is called carbon dioxide. 

 The millions of little bubbles cannot break through the sticky gluten of 

 the flour, so they raise the whole mass. When the bread is baked the 

 gas is dissipated, the gluten walls of these bubbles are hartlened and the 

 little holes remain, filled only with air. The alcohol, too, is driven out by 

 the heat. 



