568 Introduction to the Study of Science 



acid of one kind or another. The acidity of the bread may be 

 due to lactic or to acetic acid or to a combination of the two. 

 You may identify the lactic acid by means of the acid in sour 

 milk, for milk is made sour by bacteria which produce lactic 

 acid. Acetic acid is that which is found in vinegar. It is 

 produced by yeast and a species of bacteria called acetic bacteria. 

 Yeasts cause fermentation, in which are evolved carbon dioxid 

 and alcohol. Then the acetic bacteria, which can act only 

 with great difficulty upon sugar, begin to work rapidly on the 

 alcohol, which they oxidize to form acetic acid. 



It is comparatively easy to prevent the dough from becoming 

 acid when one knows the causes which produce it. A poor 

 grade of flour usually contains more kinds and more bacteria 

 than a high grade of flour. With a strictly high-grade flour 

 and a good, perfectly fresh yeast, and all materials mixed and 

 kneaded in clean utensils with clean hands, the primary con- 

 ditions for a sweet bread are provided. The dough should be 

 kept at a temperature of not lower than 75 F. in warm weather 

 and between 85 and 90 F. in cold weather. This will or- 

 dinarily insure complete fermentation within six to eight hours. 

 Detrimental bacterial action does not pass beyond control or 

 affect the flavor of bread which is prepared within this period 

 of time. 



SUMMARY 



Leavened bread is made, (1) by the " salt rising " method, (2) by 

 the use of dough from a former baking, (3) by prepared yeast. 



In each case yeast plants grow and multiply in the dough, produc- 

 ing carbon dioxid and alcohol. 



If the sponge is kept warm, yeast develops more rapidly than the 

 bacteria which make the bread sour. 



The universal distribution of yeasts is indicated by their appear- 

 ance in " salt rising " bread and in fruit juices. 



Yeast plants reproduce rapidly by budding, and in unfavorable 

 conditions produce spores. In many food products enormous numbers 

 may develop. 



Most varieties of yeast are useful and harmless. 



