Emulsifying properties of basil seed gum: Effect of pH and ionic strength

Abstract

The emulsifying properties of Basil Seed Gum (BSG) were evaluated as a function of pH (1–12), ionic strength (0.01–0.24 M NaCl) and heat treatment (30 min at 80 °C, at extreme pHs). BSG-stabilised emulsions showed a significant surface charge (ξ > −30 mV) at low NaCl concentrations (≤0.04 M) and pH ≥ 4. The droplet size of o/w BSG-emulsions was sensitive to changes in pH and ionic strength as confirmed by the formation of larger droplet sizes and decrease in zeta potential values when reducing pH and increasing salt content. Additionally, heating at extreme pH and ionic strength led to a reduction in the emulsifying ability of BSG. In general, emulsions at alkaline conditions (≥pH 7.0) had small initial droplet sizes and were stable against coalescence. Confocal laser scanning microscopy images confirmed the inclusions of hydrophobic segments of BSG inside the oil droplets and the occurrence of coalescence over the storage time at acidic pH. The addition of salt drastically changed the particle size distribution of all the emulsions, making them also prone to coalescence. Although the addition of acid, increase in salt and heating resulted in flocculation and coalescence, all the emulsions remained stable against phase separation during one month storage probably due to the strong unchanged gel-like behaviour of the emulsions shown in the rheological properties. The mechanism by which BSG imparts emulsion stability is likely to be via a combination of electrostatic and steric stabilization, dominated by the latter; steric stability seems to be seriously compromised when altering slightly the charges on the gum – probably due to its strong hydrophobic character – affecting: (i) self-aggregation and ultimately adsorption behaviour, (ii) droplet surface coverage and (iii) droplet–droplet aggregation mediated by attractive hydrophobic forces.