The shear-induced destabilization of oil-in-water emulsions using caseinate as emulsifier

Calcium-induced flocculation of caseinate-stabilized emulsions in Couette s hear flow was studied. Emulsions, consisting of 20% v/v soybean oil in 20 m M imidazole buffer (pH 7), emulsified with 1% w/w sodium caseinate, were ho mogenized with a valve homogenizer or a microfluidizer. The concentrated em ulsions were diluted in imidazole containing 8-14 mM CaCl2, to a final oil content of 1%, and sheared at 670-740 s(-1) in a Couette shear device. The average particle size, measured with integrated light scattering, increased with shearing time, typically showing a sigmoidal shape. Handling of the s amples after shear treatment appeared very critical. Shear-induced flocs co ntinued to grow rapidly after taking away the shear stress, unless they wer e directly diluted in buffer of sufficiently low calcium content. On the ot her hand, flocs were disrupted in the mixing unit of the Malvern Mastersize r, even at minimal mixing speed. Rate and extent of shear-induced flocculat ion were dependent on preparation and storage of the emulsion; during stora ge the emulsion became more stable against shear-induced flocculation. As n o proteolysis or exchange of protein between the oil-water interface and th e serum could be detected electrophoretically, presumably the increased sta bility was due to rearrangement of protein molecules at the interface. Emul sions prepared with valve homogenizer and microfluidizer, with similar part icle sizes as measured with the Malvern Mastersizer, had different suscepti bilities to shear-induced flocculation, indicating that the type of homogen izer affected the functional properties of proteins on the oil-water interf ace. (C) 1998 Elsevier Science B.V. All Fights reserved.