Improving the functional properties of lupin protein for food applications

Manufacturers are shifting to plant-based proteins as food ingredients as consumers are shifting to plant-based diets. Plant-based proteins offer a promising solution and provide environmental sustainability, such as lower production costs and require fewer resources like water and land. Therefore, food manufacturers are looking for ways to incorporate plant proteins into food applications. Lupin is a legume which has a high protein (~40%) and fibre content (~40%), but low contents of starch. These characteristics, taken in combination, make lupin highly nutritious and particularly significant for food applications. Therefore, its viability as a substitute for existing food products, or ingredients thereof, is the subject of interest in research in both academic and food manufacturing community and there have been attempts at commercialization of lupin in the food industry. It has been discovered through these endeavours, however, that lupin possesses functional properties that are inferior to those of existing food ingredients such as soy protein. This includes properties that are integral features of food processing, such as protein solubility. This limits the application potential of lupin in the food industry, at least considering the current state of food processing technology. Despite these setbacks, the goal of integrating the lupin as a nutritional food product into the food industry remains a strong incentive for the continuous research that has been conducted to improve its functional properties. The aim of this PhD thesis is to determine the effect of low-frequency ultrasound, high shear rotor stator mixing and high- pressure homogenization on the functional properties of lupin protein. This thesis also focuses on investigating the practical food applications of lupin protein using high moisture extrusion. In the first experimental chapter, lupin protein was isolated from flour and the isolated lupin protein was treated with low frequency ultrasound at a range of energy densities (457–2746 J/mL which correlates to 5-30 minutes sonication time) at pH 5 and 9. The functional properties such as particle size, zeta potential, solubility, viscosity, rheology and molecular weight of the sonicated lupin protein were analyzed. The effect of ultrasound reduced the particle size of lupin protein suspension at pH 5 and 9 which also resulted in an increase in solubility. Ultrasound had no impact on the zeta potential on the lupin protein for both pH 5 and 9. The viscosity of lupin protein sonicated at pH 5 increased, however viscosity decreased after sonication at pH 9. This trend was also observed in the increase in the storage modulus of lupin protein after sonicating at pH 5. However, when the lupin protein is sonicated at pH 9, the storage modulus decreases. In the second experimental chapter, commercial lupin protein isolate was treated with high shear rotor stator mixing (HSRSM). The lupin protein (at pH 5 and 9) was treated with different processing times (5-30 minutes) at 5000 rpm. The same function properties outlined in the first experimental chapter were also measured in this chapter. HSRSM reduced the particle size, and it also improved the solubility of lupin protein. HSRSM did not have an impact on the zeta potential and the molecular weight profile of lupin protein. HSRSM decreased the viscosity of lupin protein at pH 5 and 9 with increasing the treatment time. This finding also correlates to the storage modulus results where increasing mixing time, decreases the storage modulus. In the third experimental chapter, commercial lupin protein isolate was treated with high pressure homogenization (HPH). The chapter investigated the impact of different pressures (25- 200 MPa) and the number of homogenization cycles (1-10) on the functional properties of lupin protein at pH 5 and 9. HPH reduced the particle size and improved the solubility of lupin protein. HPH also increased the viscosity and storage modulus of lupin protein. However, high pressure levels (>100 MPa) and increased number of homogenization cycles (>5) caused adverse effects on the lupin protein. HPH at high pressure caused an increase in particle size which also decreased the solubility of lupin protein. In the fourth experimental chapter, high moisture extrusion (HME) was used to produced meat analogs. Given that the most common ingredient used in producing meat analogs is soy protein, this experimental chapter incorporated soy protein and lupin protein. Four different formulations with different soy protein isolate (SPI) with lupin protein isolate (LPI) ratios were used in the HME process. It was found that increasing the LPI ratio caused a decrease in the gelling properties of the extrudates. This was also correlated to the hardness of the extrudates after HME processing. The findings from this thesis suggest that all three technologies, ultrasound, HSRSM and HPH were able to improve the functional properties of lupin protein. All three techniques were able to reduce the particle size of lupin protein. Decreasing the particle size resulted in an increase solubility. However, for HPH, increased pressure (>100 MPa) and increased homogenization passes caused the particle size to increase which resulted in an increase in solubility. All three processing technologies didn’t have an impact on the zeta potential. Ultrasound and HPH had an impact on the molecular weight profile of lupin. By increasing the energy density of ultrasound and the pressure of HPH, the band intensity becomes darker, and some bands reappear. However, HSRSM doesn’t have an impact on the molecular weight profile of lupin protein. All three technologies have an impact on the viscosity and rheology properties of lupin protein. The work produced from this thesis will help to extend the understanding of lupin protein potential for incorporating lupin protein as a functional ingredient in food applications. This thesis investigated the use of a range of processing technologies such as ultrasound, HSRSM and HPH to improve the functional properties of lupin protein for food applications. Also, it investigated the process performance of lupin protein for producing plant-based meat using high moisture extrusion.