Strategies for developing lentil protein stabilized canola oil in water nanoemulsions
The overall goal of this research is to utilize the lentil protein isolate (LPI), prepared with isoelectric precipitation by POS Bio-Sciences (Saskatoon, SK, Canada), in the development of canola oil-in-water nanoemulsions. The effect of LPI concentration and the effect of high-pressure treatment of LPI on the formation, stability and rheological behaviour of canola oil-in-water nanoemulsions was investigated. According to a previous study of coarse emulsions, LPI showed the best emulsifying properties at pH 3; therefore, all nanoemulsions were prepared at this pH. In the first study, nanoemulsions were prepared by adding 5 wt% canola oil to the LPI solutions at various concentrations (0.5 – 5 wt% LPI) in a citric acid buffer at pH 3 and homogenized at 20,000 psi pressure. The storage stability of the nanoemulsions was recorded for 28 days. All nanoemulsions showed bimodal droplet size distribution, where the smaller peak was attributed to the oil droplets while the larger peak was attributed to unadsorbed protein aggregates in the continuous phase which grew in the size with increasing LPI concentration. The protein aggregation was also confirmed with confocal microscopy. Concentration of 0.5 wt% LPI was not sufficient for long term stabilization of oil droplets therefore the nanoemulsion separated out over the 28 days. The best stability of the nanoemulsions was observed with 1, 1.5 and 2 wt% LPI, confirmed by a photocentrifuge, which evaluates oil droplet movement and hence emulsion stability under accelerated gravitational force. Nanoemulsions stabilized with 3 and 5 wt% LPI transformed to a thick gel, most likely due to a network formation between the oil droplets and free proteins in the continuous phase. The viscosity and the gel strength of nanoemulsions increased with increasing protein concentration because of increased aggregation of free proteins in the continuous phase of the nanoemulsions. In the second study, the effect of high-pressure homogenization of LPI on the formation and stability of the nanoemulsions were investigated. The most stable and flowable nanoemulsions at 1, 1.5 and 2 wt% LPI concentrations were chosen based on the previous results. Prior to nanoemulsion formation, LPI solutions (1 -2 wt% LPI) were homogenized at 5,000 and 15,000 psi pressure for six cycles. Nanoemulsions were then prepared by adding 5 wt% canola oil to 95 wt% pre-treated LPI solutions at pH 3 and homogenized at 20,000 psi pressure. High-pressure homogenization of LPI significantly improved long term stability of the nanoemulsions by decreasing the large protein particles into small ones, which was confirmed by particle size distribution, light microscopy and photocentrifuge dispersion analysis. Small particles improved migration of proteins to the oil-water interface and facilitated formation of oil droplets and resulted in a decrease in the average oil droplet size from ~ 250 nm to less than 200 nm. No significant difference was observed between 5,000 and 15,000 psi pressure indicating that 5,000 psi homogenization of LPI solution was sufficient to brake large protein particles into small ones. High-pressure homgenization of LPI solutions also decreased protein aggregation in the continuous phase of the nanoemulsions which was confirmed with the confocal microscopic imaging and this might be due to the lower surface hydrophobicity created by high-pressure homogenization of LPI. Results from the interfacial rheology indicated that weaker interfacial film was formed by the high-pressure homogenized LPI solutions compared to un-homogenized proteins. Storage stability of the nanoemulsions prepared with high-pressure homogenized LPI solutions was significantly improved compare to the nanoemulsions prepared without high-pressure treated LPI due to a smaller droplet size and less protein aggregation in the continuous phase. Lipid digestibility showed an increase for nanoemulsions prepared with high-pressure homogenized LPI solutions (1 wt%) due to a smaller droplet size and weaker interfacial film, however no significant difference was observed for 1.5 and 2 wt% LPI homogenized solutions. This might be due to a higher LPI concentration covering the oil droplet surface and preventing digestive enzymes to access the oil. Overall, high-pressure homogenization improved emulsification properties of LPI and shelf life and lipid digestibility of the prepared nanoemulsions thereby increasing the nutritional value of the product. Lentil protein-stabilized nanoemulsions containing low oil volume fractions have many applications in the development of beverage type products due to their increased stability, flowability and longer shelf life.
Nanoemulsions, lentil protein isolate, high-pressure modification
Master of Science (M.Sc.)
Food and Bioproduct Sciences