290 GENERAL MICROBIOLOGY 



Method. 1. Remove the (frozen) ice cream sample 

 from the container by means of the sterile butter trier. 



2. With the sterile knife discard the upper portion of the 

 sample and place in the sterile wide-mouthed bottle. 



Note. Pack the sample in ice if it cannot be examined at once. 



3. To examine, allow the ice cream to melt quickly by 

 placing it at about 37 C. and then treat as a milk 

 sample. 



4. Plate on litmus lactose agar, using the following dilu- 

 tions: 1 : 10,000, 1 : 1,000,000 and 1 : 100,000,000 and 

 incubate plates at 37 C. 



5. Add a large quantity (25 c.c. to 50 c.c.) to the melted 

 agar shake and incubate at 37 C. Examine in twenty- 

 four to forty-eight hours for acid and gas. Is B. coli 

 present? 



6. Count plates at the end of three days and estimate 

 the total number of bacteria present per cubic centimeter, 

 also the number of acid colonies and of any other predom- 

 inant type. 



7. Transfer predominant types to litmus milk tubes and 

 note action, also note rapidity with which each type pro- 

 duces changes in the litmus milk. What may these results 

 signify? 



8. Make a microscopic count, using the method in Exer- 

 cise 1, Dairy Microbiology. How do microscopic and plate 

 counts compare? 



9. Look up references for ascertaining bacteriological 

 standards for ice creams. What is the quality of the 

 ice cream you analyzed as compared with the maximum 

 bacterial limit? What do you think this limit should be? 



10. From what diverse sources do bacteria enter ice 

 cream? 



What is their significance in this product? 

 What relation may some of the common practices of 

 ice-cream makers have to the bacterial content of milk? 



