116 GENERAL MICROBIOLOGY 



6. Is the cake made up mostly of starch grains or yeast 

 cells? What is the purpose of the starch in the yeast 

 cake? Do the starch grains remain intact or do they 

 disappear? Explain. What kinds of starch are used? 



6. Draw and measure the starch grains. A drawing of 

 the individual yeast cell may be made from this mount. 



7. Prepare a second hanging drop of yeast in water from 

 the fresh cake. 



8. Stain by adding a loopful of 0.0001% aqueous 

 methylen blue. Dead yeast cells are stained blue, while the 

 living cells remain unstained. 



9. Count the number of living and dead cells in each of 

 several fields. Estimate the per cent of living and dead 

 yeast cells. 



10. Repeat every seven days until all the yeast cells 

 are dead. 



11. How long does this take? What factors influence 

 the death rate? Do other microorganisms enter? If so, 

 what types? Why? From what source? Do they 

 influence the value of the yeast cake? How? 



12. Each time you record the percentage of living and 

 dead cells, note the macroscopical appearance of the cake. 

 Also note ,the presence of new microorganisms, consistency 

 of the cake, odor, and color. 



13. Record the results of this experiment in tabulated 

 form, and state, any conclusions that may be drawn or 

 practical application to be made. 



REFERENCES 



IAGO, WM. and IAGO, WM. C.: The Technology of Breadmaking. 



(1911), pp. 235-239. 



CONN: Yeasts, Molds and Bacteria, pp. 56-99. 

 SCHNEIDER: Bacteriological Methods in Food and Drugs Laboratories. 



(1915). Plate I, Figs. 2, 3 and 4. 



