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Influence of carboxymethylcellulose molecular weight on physicochemical properties and stability of whey protein-stabilized emulsions
Complexation of protein with polysaccharides at pH near the isoelectric point (pI) of protein has been widely used to improve the stability of oil-in-water emulsions. This is attributed to the fact that protein–polysaccharide complex formed a thick, dense and homogenous layer surrounding the droplets to prevent the flocculation. However, little is known about the effect of molecular properties of polysaccharides on emulsification properties of protein. The objective of this study is to assess the influence of carboxymethylcellulose (CMC) molecular weight on physicochemical properties and stability of whey protein-stabilized emulsions.
Emulsions containing 5 wt% oil, 0.5 wt% protein and 0 to 0.5 wt% CMC (molecular weights of 270 k, 750 k, and 2,500 kDa) were obtained by emulsification of oil with aqueous WPI–CMC solution at pH 7.0 through homogenization at 12,000 rpm for 1 min, followed by ultrasonic processing for 5 min. The emulsions were then slowly acidified to pH 5.2. Droplet size, zeta-potential, and rheological properties of emulsions were measured to characterize their physicochemical properties. Creaming index along with protein surface coverage were used to measure the stability of emulsions.
In the absence of CMC, WPI emulsions were prone to flocculation. WPI–CMC emulsion showed improved stability, but was highly dependent on the molecular weight and concentrations of CMC. Addition of less than 0.1% CMC enhanced the adsorption of protein at the interface and increased the repulsion between droplets, resulting in more stable emulsions compared to WPI emulsions. Further addition of CMC did not change the surface coverage of protein, but increased the viscosity of the continuous aqueous phase, further contributing to the stability of emulsions. Emulsions with low molecular weight CMC showed much faster creaming rate than those with high molecular weight CMC during 15 days of storage. This is likely due to combined effects of higher protein surface coverage on the droplets and increased viscosity of the aqueous phase. The flow behavior of emulsions changed from shear-thinning to Newtonian and back to shearthinning, however, the concentrations where these changes occurred were dependent on the molecular weight of CMC.
This study demonstrates that complexation of whey protein with high molecular weight CMC improves the surface properties of protein and enhances the repulsion between droplets, contributing to the stability of the emulsions. The outcomes could be applied to improve the stability of food emulsions having pH values near the pI of protein.
Keywords:
CMC, molecular weight, emulsion