274 APPLIED BIOLOGY 



microscope. Are yeast cells present? Where could they have 

 come from in 1 and in 2 ? Why does bottled cider or grape-juice 

 ferment soon after a bottle is opened? Why do we suspect the 

 presence of some antiseptic when grape-juice does not ferment in an 

 open bottle ? 



Experiment 12. "Salt-raising bread." (Optional.) Add enough 

 common salt to make some milk taste slightly salty, and set in a 

 warm place until foam appears on the surface, i.e., fermentation 

 begins. Yeast cells come from the air. The salt prevents the growth 

 of certain other microscopic organisms (bacteria) which would cause 

 souring of the milk. Such fermenting milk mixed with dough causes 

 it to "raise," but less than when yeast is used. Hence, bread made 

 in this way is heavier than yeast bread. 



252. Physiology of Yeast Plant. The foregoing ex- 

 periments have given information on which we may base 

 some general statements concerning the physiology of the 

 life-activities of the yeast plant. The plant is a one-celled 

 organism, without chlorophyll, and for its food dependent 

 upon materials built up by other plants. It is therefore a 

 saprophyte like the molds, mushrooms, Indian pipe, and many 

 other plants without chlorophyll. The studies which Pasteur 

 made with the solution which bears his name showed that 

 yeast plants must have nine elements in their food ; namely, 

 carbon, hydrogen, oxygen, nitrogen, calcium, sulphur, 

 phosphorus, potassium, and magnesium. These are neces- 

 sary for the formation of new yeast protoplasm, that is, 

 for growth ; and if fermentation is also to take place, sugar 

 (or starch convertible into sugar) must also be present. 

 Fermentation results in changing sugar into alcohol and 

 carbon dioxide. Within recent years it has been demon- 

 strated that this is due to an enzyme secreted by yeast plants. 

 Under great pressure this enzyme has been extracted and used 

 to cause fermentation without the presence of living yeast cells. 

 This is in principle similar to the diastase extracted from 

 plant cells and used in digesting starch to sugar in test- 

 tube experiments, or to pepsin from animal stomachs. 



