EXERCISE 81 



MICROSCOPIC EXAMINATION OF MILK 



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Statement. Whole milk after having most of the butter fat removed as cream is called skim milk. 

 Skim milk is valuable as feed for calves, pigs, chickens, etc., as well as exceedingly nutritious for man. 

 It should be more generally used as a human food. Skim milk contains all the food materials found in 

 the whole milk except the small percentage of butter fat, averaging less than 4 per cent. The removal 



of butter fat changes the physical characteristics of the result- 

 ing products. 



Object. To determine the physical characteristics of skim 

 milk, whole milk, and cream. 



Materials. Three test tubes or shallow vessels ; samples of 

 skim milk, whole milk, and cream, respectively. 



Directions. Study the appearance of each sample and de- 

 scribe the differences. Place a slide showing skim milk under 

 the microscope and observe the size, shape, and number of the 

 fat globules. Also note the presence of any foreign materials, 

 as dirt or bacteria. In like, manner observe the slides showing 

 whole milk and cream. Compare the three and explain the 

 reasons for the differences noted. 



Examine under the microscope milk just drawn from a cow 

 recently fresh. Examine milk from a cow of the same breed 

 which has been in milk for several months. Compare the two 

 samples as to size, number, and color of the fat globules. Also 

 compare the fat content of milk samples from different breeds 

 of cattle. 



Make drawings on the opposite page of what the microscope 

 shows in the case of skim milk ; in the case of cream. 



Questions. Describe the fat globules. What is their average 

 size ? Does the length of time a cow has been in milk influence in any way the fat globules ? If bac- 

 teria are present in the sample, explain from what sources they might have come. Which is the heavier, 

 skim milk, whole milk, or cream? Which material contains the greater proportionate number 

 of fat globules, whole milk or cream? Explain the reason for the difference and how the con- 

 centration of globules occurs when milk is set, and how it is brought about when the centrifugal 

 separator is used. Did you observe any difference in the size of the fat globules in a given sample, 

 or were they all of uniform size ? Did you find that the globules were uniformly larger in some 

 samples than in others ? Are they larger in the milk of some breeds than in that from others ? Which 

 breed produces milk containing the largest globules, which the smallest? State what difference 

 there is between the size of the globules of skim milk and cream. As a rule, which globules are 

 left in the skim milk, the smaller or the larger ones, and why? What is the relation between 

 the size of fat globules and the rapidity and completeness of creaming either by the gravity or the 

 centrifugal method ? Name two methods by which cream may be removed from milk. 



References. Waters, H. J. Essentials of Agriculture, pp. 372-375. Ginn and Company. Eckles and 

 Warren. Dairy Farming, pp. 168-172. The Macmillan Company. Wing, H. H. Milk and its Products, 

 pp. 16-35. The Macmillan Company. 



Note. Slides of skimmed milk, whole milk, and cream should be prepared previous to the class period and adjusted 

 in the microscope. 



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Fig. 135. Butter-fat globules 



Reading from above downward, samples are shown 

 of cream, milk, and skimmed milk under the same 

 magnification. Proportionate numbers of fat glob- 

 ules are shown 



