The electroacoustics of milk suspensions

The zeta potential and size distribution of casein micelles and fat globules from bovine milk have been investigated with the relatively new technique of electroacoustics. This technique requires no dilution or change of environment of the casein micelles or fat globules. The zeta potential obtained at natural pH for casein micelles from a commercial skim milk suspension was —18 mV and the median diameter was 0.2 um A zeta potential of —19 mV was obtained for homogenized fat globules from a commercial milk suspension, a zeta potential of —22 mV was obtained for natural fat globules from a commercial cream suspension and a zeta potential of —36 mV was obtained for fat globules from a recombined milk suspension. The median sizes obtained for the fat globules from the various suspensions were reasonable but at present electroacoustics cannot give a true indication of the spread of the size distribution for the fat globules. Two processes used in the dairy industry were investigated with electroacoustics. The size and zeta potential of casein micelles were monitored during acidification and renneting. For both processes it was found that the trends in zeta potential obtained with electroacoustics during the two processes agreed extremely well with trends reported in the literature. However, the trends in size observed with electroacoustics were different fiom trends reported in the literature. The difi‘erences in the trends in size of the casein micelles obtained during the renneting and acidification processes have been explained in terms of structural changes occurring to the casein micelles during the two processes. Hence the technique of electroacoustics can not only measure zeta potential and apparent particle size but may be able to monitor structural changes in particles during various processes