Influence of protein-mineral interactions on physicochemical properties of model nutritional beverage emulsions
Nutritional oil-in-water emulsion products are usually fortified with minerals, which can interact with other ingredients and cause physiochemical instability reducing product shelf-life. The objective of this study was to investigate the impact of mineral-ingredient interactions on properties and stability of protein stabilized emulsion in order to find potential approaches to prevent the emulsion instability.
Commercial nutritional beverages had mean particle diameters between 0.5–11 μm and pH values around 7. The monovalent and divalent minerals present in the products at the highest average concentrations were potassium and calcium, respectively. Both calcium and potassium ions induced droplet aggregation in a model emulsion, which had a mean particle size of 0.6–0.7 μm, pH value at 7, 7% oil, and was stabilized by whey protein isolate (WPI). The aggregation led to an increase in emulsion particle size, apparent viscosity, shear-thinning behavior, and creaming instability with increasing mineral concentration. The critical concentration of the aggregation was 3–7 mM for calcium and 200 mM for potassium. The emulsion particle surface charge was decreased by minerals, probably because of electrostatic screening and binding of the mineral ions to negatively charged adsorbed proteins.
Calcium binding affinity and enthalpy of EDTA was greater than those of citrate and WPI, respectively. EDTA and citrate bound calcium ions in 1:1 ratio, whereas WPI bound to calcium ions in about 1:3 to 1:4 ratio. WPI that was heated at 70–90°C had an increase in calcium binding affinity and enthalpy. Citrate consisted of two types of binding sites.
EDTA, citrate, and non-adsorbed WPI prevented or reduced calcium-induced droplet aggregation with increasing their concentration. EDTA was more effective than citrate and WPI, respectively. The chelating agents lowered free-calcium concentration and magnitude of particle surface charge. The emulsions had less free-calcium ions to induce the instability. Heat treatment of non-adsorbed WPI at 70 or 90°C did not improve the performance of non-adsorbed WPI. EDTA, citrate, and non-adsorbed WPI could not prevent droplet aggregation induced by thermal treatment, when emulsions containing calcium were heated at 80–120°C for 15 min. It was suggested that hydrophobic attraction upon heating induced droplet aggregation.