COMMERCIAL YEAST 



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plants. In its simplest form a yeast plant is a single cell, ovoid 

 in shape and usually containing one or more vacuoles. The cells 

 reproduce by a process called budding. Under certain conditions 

 spores are found. 



An enzyme causes fermentation. It has been proved that if 

 yeast cells are ground up until they are destroyed, the juice 

 filtered from them is able 

 to cause fermentation. 

 Similar experiments have 

 been made with bacteria, 

 showing that enzymes 

 formed within the cells 

 cause fermentation. 

 These enzymes are called 

 zymases. 



Commercial yeast. 

 Cultivated yeast is now 

 supplied in compressed 

 or dried yeast cakes. In 

 both cases the yeast 

 plants are mixed with 

 starch and other sub- 

 stances and pressed into a cake. The compressed yeast cake must 

 be used fresh, as the yeast plants begin to die rapidly after two or 

 three days. The dried yeast cake contains a much smaller number 

 of yeast plants, but is probably more reliable if the yeast cannot 

 be obtained fresh. 



Life history of yeast. Follow the arrows and work out 

 what happens after the germination of the spores. How 

 many spores are produced in the sac or ascus shown at the 

 bottom of the diagram? 



Home Experiment. To determine the conditions favorable for the 

 growth of yeast in bread. Make a small amount of dough by mix- 

 ing flour, sugar, salt, and water in proportions to make a thick paste. 

 Knead with a little yeast which has previously been mixed with 

 water. Now place one lot of dough in the ice box, one at the tem- 

 perature of the room, and one in a warm place (over 95° F.). Later 

 bake 'each lot and use in the laboratory. 



Which of the three lots has risen the most? Which, after baking, 

 has the best appearance? The best taste? What makes the holes 

 in the bread ? 



What caused the dough to rise ? What are the best conditions for 

 this to take place ? Will the mixture rise if no yeast is added ? Why ? 



