Caseinate yields stable oil-in-water emulsions.

Surface active proteins have been used as emulsifiers not only to form physically stable emulsions, but also to increase the oxidative stability of oil-in-water emulsions. However, we do not completely understand the differences in the ability of various proteins to produce emulsions that have good physical and oxidative stabilities.

It was the goal of scientists at the University of Massachusetts to investigate the physical and oxidative stability of corn oil-in-water emulsions stabilized with caseinate (CAS), whey protein isolate (WPI) and soy protein isolate (SPI) at pH 3. A coarse emulsion-containing protein (0.2% to 1.5% CAS, WPI or SPI); an acetate-imidazole buffer at pH 3; and 5% corn oil were processed and homogenized.

The researchers found that CAS is able to produce oil-in-water emulsions with both physical and oxidative stabilities. In their work, the investigators measured particle size in order to monitor physical stability. They used lipid hydroperoxide and headspace hexanal to follow the progress of lipid oxidation.

Physically stable, monomodel emulsions containing 5% corn oil-in-water at pH 3 were made using 1.5% CAS, 0.5% WPI, and 1.0% or 1.5% SPI, respectively. At pH 3, the oxidative stability of CAS-stabilized corn oil-in-water emulsions was greater than that of emulsions made with WPI and SPI. For an emulsion made with 0.5% CAS, lipid hydroperoxide concentrations were basically unchanged after 185 hours of incubation.

The scientists did not see lipid hydroperoxides in the WPI- and SPI-stabilized emulsions after 24 hours of oxidation. Hydroperoxide concentrations increased to 113.2 mmol per kg of oil and 205.5 mmol per kg of oil, respectively, after 185 hours of storage. Hexanal formation in the emulsions that were stabilized with 0.5% protein exhibited trends similar to CAS-stabilized emulsions in which there was no detectable headspace hexanal for up to 48 hours of storage. Hexanal was detected in the WPI- and SPI-stabilized emulsions after 24 hours of storage.

Further information. D. Julian McClements, Department of Food Science, University of Massachusetts-Amherst, Chenoweth Laboratory, Box 31410, Amherst, MA 01003; phone: [phone]; email: [e-mail].