The effect of immunoglobulins and somatic cells on the gravity separation of fat, bacteria, and spores in pasteurized whole milk1

Our objective was to determine the role that immunoglobulins and somatic cells (SC) play in the gravity separation of milk. The experiment comprised 9 treatments: (1) low-temperature pasteurized (LTP; 72°C for 17.31 s) whole milk; (2) LTP (72°C for 17.31 s) whole milk with added bacteria and spores; (3) recombined LTP (72°C for 17.31 s) whole milk with added bacteria and spores; (4) high-temperature pasteurized (HTP; 76°C for 7 min) whole milk with added bacteria and spores; (5) HTP (76°C for 7 min) whole milk with added bacteria and spores and added colostrum; (6) HTP (76°C for 7 min) centrifugally separated, gravity-separated (CS GS) skim milk with HTP (76°C for 7 min) low-SC cream with added bacteria and spores; (7) HTP (76°C for 7 min) CS GS skim milk with HTP (76°C for 7 min) high-SC cream with added bacteria and spores; (8) HTP (76°C for 7 min) CS GS skim milk with HTP (76°C for 7 min) low-SC cream with added bacteria and spores and added colostrum; and (9) HTP (76°C for 7 min) CS GS skim milk with HTP (76°C for 7 min) high-SC cream with added bacteria and spores and added colostrum. The milks in the 9 treatments were gravity separated at 4°C for 23 h in glass columns. Five fractions were collected by weight from each of the column treatments, starting from the bottom of the glass column: 0 to 5%, 5 to 90%, 90 to 96%, 96 to 98%, and 98 to 100%. The SC, fat, bacteria, and spores were measured in each of the fractions. The experiment was replicated 3 times in different weeks using a different batch of milk and different colostrum. Portions of the same batch of the frozen bacteria and spore solutions were used for all 3 replicates. The presence of both SC and immunoglobulins were necessary for normal gravity separation (i.e., rising to the top) of fat, bacteria, and spores in whole milk. The presence of immunoglobulins alone without SC was not sufficient to cause bacteria, fat, and spores to rise to the top. The interaction between SC and immunoglobulins was necessary to cause aggregates of fat, SC, bacteria, and spores to rise during gravity separation. The SC may provide the buoyancy required for the aggregates to rise to the top due to gas within the SC. More research is needed to understand the mechanism of the gravity-separation process.