122 PLANTS WITHOUT CHLOROPHYLL 



minutes. This is known as pasteurization. Afterwards treat 

 exactly as you did the first jar of milk. 



Observations. — What is the odor of milk in each jar after 24 

 and 48 hours? What is the taste of the milk in each jar after 

 24 and 48 hours? 



Conclusion. — 1. What are found in milk that cause it to sour? 

 How do you know? 



2. What is the use of pasteurization ? 



Problem, 104 : How to care for milk bottles at home. 



Materials. — Recently used milk bottles. 



Method. — Place a recently used milk bottle in a warm place 

 for 24 hours. Note the odor. Rinse out a second milk bottle 

 with cold water, a third with boiling water. Set aside and note 

 odor after 24 hours, as before. 



Observations. — Describe the odor. Note any differences in 

 odor in the three bottles. 



Conclusion. — How should milk bottles be treated to prevent 

 rapid souring of milk ? 



Problem 105: To determine the bacterial content of milh. 



Materials. — Sterile Petri dishes containing agar culture media, 

 a sample of milk. 



Method. — Milk should be collected by pupils from some near-by 

 source as, for example, the lunchroom of the school. To 1 c.c. of 

 this milk add 19 c.c. of distilled water in a sterile pipette. Shake 

 well and then flood the surface of a sterile Petri dish with the mix- 

 ture. Pour off all excess fluid. Then cover quickly and place the 

 dish in a moderately warm place. 



Observations. — Notice that after 24 hours (or even less if the 

 temperature is warm) colonies of bacteria appear on the surface of 

 the culture media. Note the number of colonies of bacteria 

 present on the second, fourth, and sixth days after preparing the 

 experiment. 



Conclusion. — 1. What can you say of the number of bacteria in 

 this milk ? 



2. What do bacteria do to the milk ? (Smell the Petri dish.) 



