Emulsification properties of puka gum - an exudate of a native New Zealand tree (Meryta sinclairii) : a thesis presented in partial fulfilment of the requirements for the degree of Master of Food Technology in Food and Advanced Technology at Massey University, Palmerston North, New Zealand

Abstract

This study investigated the emulsification properties of puka gum (PG) by determining an optimum gum concentration by evaluating various PG amounts (1-10% w/w), added to stabilise 15% w/w oil-in water emulsions (O/W). The effect of pH (2-8.5) on the emulsification ability of the gum, as well as the thermostability (80 °C for 30 min) of the emulsions were also tested. The emulsification properties of PG were evaluated in terms of mean particle size (d₃₂) and the particle size distribution at the initial day and over a storage period, the zeta potential, the rheological and microstructural properties and the visual phase separation (at 20 °C for one-month storage) of the emulsions. The effect of shear on the rheological properties of a 4% w/w PG in water solution was also carried out. PG stabilized-oil-in-water emulsions exhibited monomodal size distributions with average d₃₂ below 2 µm at 4% w/w PG. Generally, PG emulsions are resistant to a wide range of pH changes and heat treatment (80 ºC for 30 min), especially at extreme acidic conditions (pH 2) and more alkaline conditions (pH 8.5). Acidification seems to improve the emulsification ability of PG by changing the d₃₂ from 1.9 µm at native pH (∼5) to 0.48 µm at pH 2. Meanwhile, there were no changes in d₃₂ of heated emulsions at pH 2 and 8.5. PG-stabilised droplets of unheated and heated emulsions were negatively charged with zeta potential ca. -30 – (-45) mV at pH >3.5. Emulsions around native pH (5 and 6.5) were observed to have a mixture of bridging and depletion flocculation, as confirmed by the microstructure images of the emulsion and after the addition of SDS. At pH 6.5, coalescence occurred in heated emulsions. The apparent viscosity of these emulsions was dependent on gum concentration generally showing shear-thinning behaviour above 100 s⁻¹ and a Newtonian plateau at low shear rate. The loss modulus (Gʺ) was higher than the storage modulus (Gʹ) at all concentrations indicating that the liquid-like behaviour dominated at all frequencies. The apparent viscosity of PG solutions was found to be shear-thinning and time dependant, with an irreversible loss of viscosity found after shearing at constant shear rate over a period of time. This could have major implications for the emulsion droplet sizes obtained depending on the shear history of the gum solution. Furthermore, PG-stabilized emulsions remained stable against phase separation for at least 30 days at all studied pHs and after heat treatment. Crude and purified PG exhibited no major differences in terms of their emulsification properties suggesting that a purification process may not be required for food formulations. Overall, PG can be considered as a promising natural emulsifier for emulsion-based foods and beverage products.