TYPES OF MICROORGANISMS IN ICE CREAM 289 



7. Plate the following dilutions of the condensed milk 

 in litmus lactose agar: 1 : 20, 1 : 2000 and 1 : 20,000. 

 Place plates at 25 C. 



8. Examine and count at the end of five days. 



9. Record, the numbers and types of organisms develop- 

 ing on the plates. Are any acid colonies present? Deter- 

 mine the morphology of the acid colonies. 



10. Transfer each type of colony to a tube of sterile 

 litmus milk and observe action from day to day. Are the 

 types which are present desirable? Is Bad. lactis acidi 

 present? Any organisms of the B. coli type? Are patho- 

 genic bacteria apt to be present? 



11. To what factors are due the keeping qualities of 

 each type of condensed milk? 



What care should be taken of opened cans of milk of 

 either type? Of the milk after it has been diluted accord- 

 ing to directions? 



In what other forms is concentrated milk sold? What 

 factors are responsible for the keeping quality of each of 

 these latter types? 



12. Give your results and conclusions in detail, 



REFERENCES 



MARSHALL: Microbiology (1911), 363-366. 



SAVAGE: The Bacteriological Examination of Food and Water (1914), 



pp. 111-113. 

 SADTLER: Industrial Organic Chemistry (1912), pp. 281, 288. 



EXERCISE 10. TO DETERMINE THE NUMBER AND 

 TYPES OF MICROORGANISMS IN ICE CREAM 



Apparatus. Litmus lactose agar shake; three tubes 

 sterile litmus lactose agar; three sterile Petri dishes; ster- 

 ile 1 c.c. pipettes; sterile dilution flasks; sterile wide- 

 mouthed glass-stoppered bottle; sterile butter trier; sterile 

 knife. 



Culture. From ice cream. 



