THE EFFECT OF PH AND HEAT-TREATMENT ON THE KAPPA-CASEIN CONTENT AND THE XI-POTENTIAL OF THE PARTICLES IN RECONSTITUTED SKIM MILK

Reconstituted skim milk samples at a variety of concentrations and pH were heated at temperatures from 20 to 140 degrees C and the changes i n the soluble kappa-casein content and micellar zeta-potential were de termined. For the 10% TS milk samples, there was a low level of kappa- casein dissociation at 20 degrees C at all pH, and at pH below 6.7 reg ardless of the heating temperature. At pH 6.7 or above, the level of k appa-casein dissociation increased with increasing pH at each temperat ure, and with increasing temperature at each pH. For the 25 % TS milk samples, kappa-casein dissociation was pH dependent over the entire pH and temperature range, with increasing dissociation at higher pH and higher temperatures. Most of the kappa-casein dissociation occurred wi thin the first minutes of heat treatment, with little further change o n prolonged heat treatment. When compared to the unheated milk, heat t reatment of the 10% total solids milk at 80 degrees C for 15 min resul ted in only small changes to the micellar zeta-potential regardless of the pH of the milk at heating. In contrast, heat treatment at 100 deg rees C and pH 6.5 increased the micellar zeta-potential by about 1 mV. As the pH was increased, the zeta-potential decreased essentially lin early so that at pH 7.1, the zeta-potential was about 1.5 mV lower tha n for the sample at pH 6.5. A similar behaviour was observed on heatin g at 120 or 140 degrees C; however, the curves were progressively shif ted to lower zeta-potential as the temperature was increased. A simila r behaviour was observed for the 25 % TS milks over the 20 to 120 degr ees C temperature range. In general, the difference in zeta-potential between samples at pH 6.5 and 7.1, heated at 80-140 degrees C became m ore pronounced as the heating time was increased.