Studies on formation, oxidative stability and plausible applications of food-grade 'droplet-stabilised' oil-in-water emulsions : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Food Technology at Riddet Institute, Massey University, Palmerston North, New Zealand
This research was aimed at studying the structural characteristics, chemical stability and plausible functional applications of droplet-stabilised oil-in-water emulsions (DSEs). DSEs consist of oil-in-water droplets (the core) stabilised by submicron protein-stabilised oil droplets (the shell). The first objective was to increase our understanding of their structural properties and processing factors that contribute to DSE formation using food grade ingredients. To achieve this objective, milk protein concentrate (MPC) was chosen as the emulsifier. Four MPCs with different levels of calcium were used. The surface lipid (20 %) consisted of either a low (olive oil), medium (palmolein oil) or high (trimyristin) melting surface lipid. The core lipid (20 %) consisted of either a triglyceride (soybean oil) or pure fatty acid (linoleic acid). Protein-stabilised shell emulsions were processed either via the microfluidizer (170 MPa) or two-stage homogeniser (1st stage-20 MPa; 2nd stage-4 MPa). Results of the study showed that aggregated structure of protein emulsifier, shell droplet concentration, surface and core lipid types influenced the formation and structural properties of DSEs. The second objective focused on investigating the chemical stability of DSEs by evaluating their stability to oxidation and ability of its interfacial structure to protect polyunsaturated lipids incorporated within from oxidation. To achieve this objective, oxidative stability of high linoleic acid oil (safflower oil) stabilised by protein-coated low (olive oil), medium (palmolein oil) and high (trimyristin) melting lipid droplets was evaluated and compared with composition-matched conventional protein-stabilised safflower oil-in-water emulsion as well as a conventional protein-stabilised safflower oil-in-water emulsion (reference emulsions). Influence of physical state of high melting lipid droplets on oxidative stability of droplet-stabilised safflower oil emulsion was also evaluated. High linoleic acid (72.54% of total fatty acids) safflower oil (20%) was used because of its high susceptibility to oxidation. Olive oil (low acidity), palmolein oil and trimyristin were chosen because of their low susceptibility to oxidation. The study showed that safflower oil oxidation in DSEs was reduced by about 40-55% in
comparison to conventional emulsions. High melting surface lipid DSEs provided better protection for safflower oil than low and medium melting surface lipid DSEs. The third objective aimed at improving our understanding of the influence of antioxidant’s location in emulsions on antioxidant performance. The study was also focused on exploring a plausible functional application of DSEs by incorporating a hydrophobic antioxidant in shell droplets (at the interface) of DSEs rather than in the interior of the core unsaturated lipid. To achieve this objective,
butylated hydroxyanisole (BHA) a common commercially used synthetic hydrophobic antioxidant was chosen. BHA was incorporated either in shell droplets or core droplets of DSEs. The ability of BHA to counteract oxidation when incorporated in low (olive oil) and high melting (trimyristin) shell droplets of DSEs was evaluated and compared with BHA’s anti-oxidation performance when incorporated directly in core droplets (safflower oil) stabilised by low (olive oil)
and high melting (trimyristin) shell droplets without BHA. Results of the study indicate that ability of BHA-in-shell DSEs to counteract oxidation of core safflower oil better than BHA-in-core DSEs is influenced by BHA’s concentration and transfer mechanism to reaction sites. The fourth and final objective was aimed at investigating mobility of a hydrophobic antioxidant incorporated at the interface of DSEs to establish their location after emulsification. The study focused on determining if a hydrophobic antioxidant incorporated in shell droplets remained localised within or migrated overtime to core droplets. The study also investigated the use of two techniques (saturated transfer difference (STD)-nuclear magnetic resonance and confocal Raman microscopy) to determine partitioning of antioxidants in DSE. To achieve this objective, confocal Raman spectroscopy technique was employed to probe
antioxidant location without phase separation or destruction of DSE structure. Beta-carotene was chosen for the study for its excellent Raman scattering property. Beta-carotene was incorporated either in shell droplets (olive oil and trimyristin) or core droplets (safflower oil) of DSEs. Location and mobility of beta-carotene was evaluated after three days production. Beta-carotene migration from low (olive) and high melting shell droplets to core safflower oil was minimal. The present study provides processing conditions and structural characteristics required to form food-grade DSEs. The study confirms and establishes the potential of DSEs to effectively protect oxidation-sensitive lipophilic bioactives incorporated within from degradation and confirms the viability of concurrent incorporation of two different bioactives in DSEs emulsions by locating one bioactive in shell droplets and the second within the core.