The Effect of Particles on the Phase Separation of Waxy Corn Starch plus Galactomannan gums

This thesis was initiated with the aim of studying the effect of small particles on phase separation of polysaccharide mixtures with the expectation of stabilising the mixture for a longer period of time, completely inhibiting phase separation, or changing the phase-separated microstructure. The phase separation of a model system consisting 2 wt.% waxy corn starch + 0.25 wt.% guar gum or 0.3 wt.% locust bean gum was studied at 25 °C at pH 7. Phase separated networks occurred within 30 minutes after preparation where the upper phase was enriched with galactomannan gum whereas the lower phase contained starch. Macroscopic phase separation of mixed polysaccharides was studied at vary concentrations, i.e., 0.5 – 4 wt.% and 0.05 – 0.6 wt.% galactomannan gums. The phase diagram illustrating the binodal lines almost coincided with the axes representing the polysaccharide concentrations. Microscopic results showed that the morphology of both mixtures exhibited thermodynamic incompatibility via spinodal decomposition structure. The model systems were also studied in the presence of different kinds of particles. There are three main types of particles that were selected in this thesis; silica nanoparticles (at different hydrophobicities), oil-in-water stable microdroplets and whey protein microgel particles with an initial particle size of 20 nm, 280 nm and 149 nm, respectively. The key parameters investigated were particle concentration, size and effect of pH. This particle-stablised model W/W emulsion was shown to induce long-term stability. With silica particles, the results showed that the rate of phase separation was inversely proportional to particle concentration. The observation also showed that the phase separation of polysaccharide mixtures was significantly slowed down in the presence of 0.5 wt.% of the 80-SiOH and 65-SiOH particles and fully inhibited at 1 wt.% particle concentration. The microdroplets appeared to slow down the rate of phase separation but within one month of storage all mixtures showed the sign of phase separation. The phase separation of a model system, starch + locust bean gum, was significantly inhibited in the presence of 1 vol.% whey protein microgel particles at pH 4, whilst the microgel particles were not as effective in starch + guar gum systems