Microorganisms in Relation to Man 569 



Domestic varieties of yeast are cultivated for specific purposes as 

 bread making and brewing. 



Yeast develops best in a dilute sugar or molasses solution. The 

 starch of flour is partly converted into sugar by diastase and so rendered 

 available for the yeast plants. 



Starch, sugar, and alcohol contain 'the same elements (carbon, 

 hydrogen, and oxygen) but in different proportions. 



REVIEW QUESTIONS 



1. Describe the experiment with " salt-rising " dough. 2. How 

 is bread made at home ? What materials are used, in what quantities, 

 and how mixed? 3. Why is bread kneaded? 4. What does the 

 baking do to yeast action? 5. What change does heat cause in the 

 surface of the loaf? 6. By what conditions can the rising of dough be 

 controlled? 7. What conditions hasten the bread-making process? 

 8. What qualities are gained by the slower process ? 9. What are the 

 sources of yeast for bread making? 



10. Describe the experiment with yeast and molasses, and state 

 what products were found. How are they identified? 11. Describe 

 the experiments with fruit juices, and state the results. Give the 

 reasons for the special treatment of each sample. 



12. What substances are produced in the rising sponge and dough? 

 What becomes of them? 13. Why does dough rise? 14. Describe 

 the yeast plant and its methods of reproduction. 15. Compare yeast 

 plants and molds as to size and methods of reproduction. 16. How 

 do yeast plants get into food? 17. Give instances of the number of 

 yeast plants in food products. 18. Under what conditions are yeast 

 spores produced ? 19. Distinguish between wild and cultivated yeast. 



20. Of what industrial importance are yeasts? 21. What yeasts 

 are used in brewing ? 22. How is wine made ? 23. What are Kumiss 

 and Matzoon? 24. What is the first action of yeast in dough? 

 25. Upon what does yeast live? 26. What are the products of alco- 

 holic fermentation? 27. What is diastase? What change in flour 

 is made by it? 28. What changes produce the fine flavors of bread 

 made by the slow process ? 29. If dough is allowed to stand too long 

 what change occurs ? 30. How and by what is lactic acid produced ? 

 31. How and by what is acetic acid produced? 32. What condition 

 is necessary to produce vinegar? 33. How can souring of bread be 

 prevented ? 



