Abstract
We report on the effect of ionic calcium on the gravity creaming of oil-in-water emulsions (volume-surface average diameter less than 0.5 μm) prepared at pH 6.8 with sodium caseinate as the sole emulsifier and n-tetradecane as the dispersed phase. The addition of CaCl 2 (5 or 8 mM) prior to homogenization was found to enhance greatly the stability of these emulsions (6 wt.% protein+10 vol.% oil or 4 wt.% protein+35 vol.% oil). Shear rheological measurements on pseudoplastic emulsions with no added calcium displayed increasing apparent viscosity with time. In contrast, identical emulsions with added Ca 2+ (5 or 8 mM) were low-viscosity Newtonian liquids. Optical microscopy confirmed that calcium-free emulsions containing unadsorbed caseinate were flocculated whereas those containing 5 or 8 mM Ca 2+ were not. The enhanced creaming and shear-thinning rheology of the emulsions without added calcium is attributed to depletion flocculation by unbound caseinate sub-micelles. It is proposed that addition of Ca 2+ results in association of sub-micelles into larger aggregates which, owing to their large size (similar to casein micelles in milk), are incapable of inducing depletion flocculation.
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