The effect of temperature on food emulsifiers at fluid-fluid interfaces

Heat-induced interfacial aggregation of a whey protein isolate (WPI) with a high content of beta -lactoglobulin ( > 92%), previously adsorbed at the oil-water interface, was studied by means of interfacial dynamic characteristics performed in an automatic drop tensiometer. Protein concentration in aqueous bulk phase ranging between 1 x 10(-1) and 1 x 10(-5) % wt/wt was studied as a variable. The experiments were carried out at temperatures ranging from 20-80 degreesC with different thermal regimes. During the heating period, competition exists between the effect of temperature on the film fluidity and the increase in mechanical properties associated with the interfacial gelation process. Interfacial crystallisation of food polar lipids (monopalmitin, monoolein, and monolaurin) previously adsorbed at the oil-water interface, was studied by interfacial dynamic characteristics (interfacial tension and surface dilational properties). The temperature, ranging between 40 and 2 degreesC, and the lipid concentration in aqueous oil phase, ranging between 1 x 10(-2) and 1 x 10(-4) % wt/wt, were studied as variables. Significant changes in interfacial dynamic characteristics associated with interfacial lipid crystallisation were observed as a function of lipid concentration in the bulk phase. Interfacial crystallisation of food polar lipids (monopalmitin, monoolein, and monolaurin) at the air-water interface, was studied by pi -A isotherms performed in a Langmuir trough coupled with Brewster angle microscopy (BAM). A condensation in monoglyceride monolayers towards lower molecular area was observed as the temperature decreased. This effect was attributed to lipid crystallisation at lower temperatures. BAM images corroborated the effect of temperature on the monolayer structure, as a function of the monoglyceride type. (C) 2001 Elsevier Science B.V. All rights reserved.