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A Study of Oiling off of Cream in Coffee^*

New England Dairies, Inc., Boston
Massachusetts State College, Amherst

ABSTRACT

1. A suitable test for measuring the extent to which cream will oil off in coffee has been developed and is presented as a part of this paper. Creams yielding a reading of 3 or above with this test will show a noticeable oily separation when used in coffee.
   
2. A number of factors have been studied which affect the stability of the fat emulsion in cream and have a bearing on the problem of oiling off of cream in coffee. The more important factors are summarized as follows:
   1. When milk which has been partially frozen is separated, the resultant cream will produce an oily separation in coffee.
      
   2. As separating temperatures are increased above 90° F. the fat emulsion in the cream becomes progressively less stable when the cream is used in coffee. Mechanical agitation of the milk during preheating prior to separation proved undesirable. Separating milk into cream of more than 45 per cent fat also caused a decrease in the stability of the fat emulsion.
      
   3. The temperature of cream when standardized in fat content by the addition of whole or skimmed milk has no effect on the fat emulsion.
      
   4. If vats are filled rapidly with cream and heated slowly to 145° F. a minimal increase in the tendency to oil off results. When vats fill slowly equally desirable results are secured if the cream is held below 90° F. and not agitated. Agitation and the heating of cream to a temperature near that of pasteuriaztion are undesirable practices and cause the oily separation.
      
   5. Prolonged holding of cream at the pasteurization temperature increases the amount of destabilized fat in cream. The partial filling of pasteurization vats causes similar undesirable results. Stainless steel or glass-lined steel vats with slow propellor or paddle agitation, and a heating medium of 160° F., produced satisfactory results. However, an increase in the speed of agitation or in the temperature of the heating medium, in these same vats, increased the oiling off tendency. Coil vats, regardless of the temperature of the heating medium destabilized the fat emulsion. Heating cream to pasteurization temperatures higher than 145° F. increased the test for oil separation slightly. Whether cream is agitated slowly or not at all during the holding period apparently made no difference, but rapid agitation caused oiling off.
      
   6. Pumping cream from pasteurizer to cooler by proper size centrifugal pumps had no effect on the fat emulsion, while the use of oversize pumps destabilized the fat to some extent. Steam piston pumps do not affect the fat emulsion adversely; in fact, with cream which oiled off badly, the piston pumps partially re-emulsified the fat which had separated.
      
   7. Cooling cream in the pasteurizing vat was found to be very undesirable regardless of the type of agitation employed. The final temperature to which cream is cooled over a surface cooler does not affect the fat emulsion so long as freezing on the cooler does not occur. Freezing of cream to the cooler was found to be one of the most serious causes of oiling off of cream in coffee.
      
   8. Aging cream (without agitation) at low temperature caused no change in the stability of the fat emulsion. Shipment at low temperature also had no effect unless the cans were partially filled, thus allowing for agitation which proved harmful.
      
   9. Reducing the fat content of pasteurized cream by the addition of milk or skim milk had no effect on the tendency of the cream to oil off.
      
   10. Cream which had been improperly handled, so that droplets of oil would separate from it if used in coffee, was made entirely stable by homogenization of the cream at the pasteurization temperature. A pressure of 500 pounds was used. Attempts to re-emulsify the fat by the use of a colloid mill and manually operated emulsor were partially successful.
       
   11. Storage of cream for three days in a household refrigerator did not increase the tendency to oil off when the cream was used in coffee.
       
   
   

^* Contribution No. 280 of the Massachusetts Agricultural Experiment Station.