216 EFFECT OF PASTEURIZATION 



The difference in the method of cooling between flash and 

 holding pasteurization is a further reason for the greater tendency 

 toward mealiness in the case of vat pasteurization. Slow cooling, 

 especially below the melting point of butterfat, invites crystalli- 

 zation or granulation o the butterfat. And crystallized butter- 

 fat means mealy butter. Slow cooling is often characteristic of 

 vat pasteurization. Rapid cooling, such as is usually accomplished 

 in flash pasteurization, is antagonistic to crystallization. There 

 is solidification but not crystallization of the fat. Hence flash- 

 pasteurized cream butter is seldom mealy and usually has a 

 more waxy body than much of the vat-pasteurized- cream butter. 



In the case of rich cream, cream testing over 35 per cent 

 fat, pasteurization tends to produce a salvy butter. This is 

 especially prone to happen with flash pasteurizers in which the 

 cream is violently agitated at a high speed, as is the case with 

 machines that are equipped with a rapidly revolving dasher. 

 In rich cream the milk solids which protect the fat globules 

 against mutilation are diminished, and in this very fluid and 

 expanded condition due to the high heat, the fat globules are 

 more. sensitive to the excessive friction that results from violent 

 agitation. Flash machines in which the cream flows gently 

 and in a thin layer between two heated surfaces, and vat pas- 

 teurizers, are less prone to mutilate the fat globules and there- 

 fore are less objectionable on this point. However, all pasteur- 

 izers tend to produce a salvy butter when operated with ex- 

 cessively rich cream. The salviness of butter made from rich 

 cream is further intensified during the churning process. With 

 cream testing from 28 to 33 per cent fat the danger of salviness 

 in butter is greatly minimized. 



Effect of Season of Year on Germ-Killing Efficiency of 

 Pasteurization. The resistence of micro- organisms to heat 

 varies with the species and types of germs present. The bac- 

 terial flora in milk and cream varies considerably with the sea- 

 son of the year. As a rule the predominating species in fall 

 and winter cream are more resistant to heat than those in 

 summer cream. This fact is brought out in the following 

 table 1 which shows the per cent reduction of germs due to pas- 

 teurization of summer cream and of winter cream : 



x Hunziker, Spitzer and Mills. The Pasteurization of Cream for Butter- 

 making, Purdue Bulletin, No. 203, 1917. 



