Sunflower-seed oil body emulsions: Rheology and stability assessment of a natural emulsion

The viscoelastic characteristics of a purified oil body cream (67.6±0.7% lipid, 5.4±0.7% protein and 25.2±0.1% moisture) recovered from sunflower seeds (Helianthus annuus L.) have been determined. Moreover, the effects of pH (2–7) and CaCl2 concentration (0–150 mM) on rheology and physical stability of oil body emulsions have been studied. Oscillatory measurements showed that the purified oil body cream exhibited weak gel-like behaviour. Diluted oil body emulsions (⩽20 wt% oil) showed extensive creaming (creaming index=56–59%) at pH 5–6 resulting in significant (P<0.05) increases in perceived droplet size (D(3,2)=11–13 μm compared with 0.3 μm at pH 7) and viscosity (0.025–0.035 Pa s at shear rate 10 s−1 compared with 0.008 Pa s at pH 7). Microscopic examination revealed that the emulsion droplets aggregated at pH 5–6 but did not coalesce. The influence of CaCl2 was investigated at pH above (pH 7) and below (pH 3) the isoelectric point (IEP) of the intrinsic oleosin proteins (pH 5⩽IEP⩽pH 6) associated with the surface of the oil bodies. At pH 7, oil bodies were stable up to a CaCl2 concentration of 1.5 mM; at 5–150 mM CaCl2 creaming occurred (66–70%), and significant (P<0.05) increases in perceived droplet size (D(3,2)=10–13 μm) and viscosity (0.015 Pa s at shear rate 10 s−1) were observed. At pH 3 there was no significant (P>0.05) influence of CaCl2 on emulsion stability or rheology. These findings demonstrate that rheology and stability of oil body emulsions are, like processed emulsions, affected by pH and by ionic concentration when the pH is above the IEP of the surface components of the oil body. The novelty in this work therefore lies in the source of the emulsion, a natural, pre-formed oil-in-water emulsion derived from seed tissue.