LASER-LIGHT SCATTERING OF MODEL CASEIN SOLUTIONS - EFFECTS OF HIGH-TEMPERATURE

Static and dynamic light scattering studies of sodium caseinate soluti on in a Pipes-KCl buffer at pH 6.75 and a constant ionic strength in t he dilute concentration regime are presented. In the absence of calciu m ions, a bimodal distribution with comparable scattering intensity co ntributions has been consistently observed by using the CONTIN method to analyze the dynamic light scattering data. However, based on the we ight fraction of submicelles and that of the aggregates, the casein so lution remains overwhelmingly in the submicellar form with sizes in th e 20-nm range. The calcium-induced aggregation of these casein submice lles appears to be a time-dependent process. After reaching equilibriu m, two aggregate fractions of different average sizes (about 90 and 35 0 nm) have been detected in the casein solution. Casein submicelles ar e generally stable at high temperatures. But, a prolonged ultrahigh-te mperature treatment (10 min at 140 degrees C) can fully transform case in submicelles into very large aggregates. Calcium-induced aggregates exhibit reduced thermal stability. Modification of casein with iodoace tic acid to block the SH groups gives the protein a stronger resistanc e to calcium-induced aggregation, but significantly reduces its stabil ity at high temperatures when the calcium ions are present. (C) 1995 A cademic Press, Inc.