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Journal of Dairy Science Vol. 11 No. 3 189-229
© 1928 by American Dairy Science Association ®
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Physical Factors Influencing the Formation and Fat Content of Gravit Cream*

H. C. Troy and Paul Francis Sharp

From the Chemical Laboratory of the Department of Dairy Industry, Cornell University, Ithaca, New York

ABSTRACT

  1. The rate of rise in milk plasma of individual fat globules varying in size from 1.8 to 41µ in diameter was determined. The experimental results were found to agree very well with the values calculated by Stokes' equation.
  2. The fat globules rise so slowly as independent individuals that it would require many times the normal creaming time of milk for them to reach the cream layer.
  3. The rate of rise in milk plasma of fat globule clusters varying in size from 10 to 800µ in diameter was determined. The experimental results were found to agree very well with the values calculated by Stokes' equation.
  4. The dense spherical clusters probably contain slightly less than 50 per cent of fat by volume.
  5. It is shown that clusters rise rapidly enough to account for the normal creaming time.
  6. Clusters of sufficient size to account for the normal creaming time of milk were observed in normal raw milk and carefully pasteurized milk.
  7. It was found that the clusters arranged themselves in the cream layer with volumes of plasma relatively free from fat between them. The packing of the fat globules into clusters, and then of the clusters in the cream, with volumes free from fat between them, readily explains the low fat content of gravity cream.
    The conclusion is drawn, that, for a given percentage of fat, the depth of the cream layer depends primarily on the clustering of the fat globules. Large irregular stable clusters form deep cream layers. Compact approximately spherical clusters, and especially weak clusters, form shallow layers and the fat content of the cream is high.
  8. The rigidity of the clusters gradually lessens as the creaming temperature increases permitting closer packing of the clusters at the higher temperatures. This explains why the deeper cream layers are obtained at the lower temperatures. If the creaming temperature is too high the clusters do not form at all or, if formed at a colder temperature, they tend to disintegrate on warming.
  9. The stability of the clusters in several instances was found to vary with the sample of milk and its treatment.
  10. Mechanical agitation was found to break up the clusters especially at near room temperature.
  11. Unclustered fat globules were found to pack themselves in the cream layer very closely. The fat content of such cream was very high and the cream required days to form.
  12. A microscopic creaming cell was described and it was pointed out that in order to compare definitely the creaming of a bulk sample of milk with clustering, it is necessary to carry out the creaming with the microscopic creaming cell under the same conditions as with the bulk sample.
  13. It was pointed out that creaming time may be divided into two parts; first, the time required for the clusters to form, and second, the time required for the clusters to rise after they have formed.
  14. The thickness of the adsorbed layer on the fat globules of raw milk and milk that had been heated to a temperature which destroyed its clustering and normal creaming power was shown to be the same in as far as this could be determined by the maximum packing of the fat globules by means of a centrifuge.
  15. The maximum packing of the fat globules indicated that the adsorbed layer was about 19 mµ in thickness.

FOOTNOTES

* Presented before the Summer Meeting of the American Dairy Science Association, at Michigan State College, East lansing, Michigan, June 24,1927.




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