Chemical and Biomolecular Engineering - Theses
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ItemThe behaviour of almond proteins in purified, minimally processed and complex food systems( 2021)A growth in consumer preferences for plant-based diets has triggered the utilisation of plant ingredients in a variety of food and beverage applications, including substitutes to dairy products. The majority of commercial plant-based dairy alternative products, however, have reduced protein content compared to dairy products. Such protein plays an important role in dairy products, including yoghurt and cheese, where it imparts functionality, such as gelation and texture. These differences in protein content and properties in plant-based systems, compared to dairy products, may be responsible for the commonly encountered technological challenges, such as the replication of the texture and mouthfeel. With an attempt to fill these gaps, this thesis aimed to explore the potential of almond as an ingredient to develop novel high protein products. First, commercial almond-based yoghurt alternatives were studied to understand the role of almond proteins in these complex food systems and establish the difference between almond and dairy systems. Next, almond proteins were derived in the form of minimally processed extracts or purified isolates to develop almond milk and almond protein isolate. The effect of processing parameters, including temperature and pH, on the fundamental and functional properties of these protein extracts was then studied, with the objective of gaining deeper mechanistic insights into almond protein gelation. First, a range of objective instrumental techniques were used to assess whether commercial almond-based yoghurt alternatives behaved differently to soy and dairy yoghurts. All almond-based yoghurt alternatives contained added stabilisers and were lower in protein compared to dairy yoghurt (less than 2.7 wt percent in almond vs 5 wt percent in dairy). The interconnected protein network, which is known to structure dairy yoghurt, was absent in almond based yoghurts. Instead, these systems appeared to be flocculated and contained swollen starch granules, protein and fat particles/ aggregates. As a result, the almond yoghurts had lower colloidal and structural stability, in comparison to dairy yoghurt and also differed in their rheological and tribological properties. This study highlights the areas that require attention to further optimise almond yoghurts if product developers wish to mimic the properties of dairy yoghurt. Second, the effect of thermal treatment (45–95 degree Celsius for 30 min) on the structure of almond proteins extracted in minimally processed almond milk was assessed, as the unfolding and association of these proteins in response to heat, and its impact on colloidal stability and gelation of almond milk was not well understood. This temperature range was chosen based on the wide range of denaturation temperatures reported previously for almond proteins (45–115 degree Celsius). Above 55 degree Celsius, protein surface hydrophobicity and particle size increased, while alpha helical structure decreased, reducing the stability of skim or full fat almond milk. Fractal protein clusters were observed at 65–75 degree Celsius and weakly flocculated gels with a continuous protein network occurred at 85–95 degree Celsius, resulting in gels with high water holding capacity (approximately 70 percent) and a strength similar to dairy gels at similar protein concentrations of approximately 4 wt percent. The presence of almond fat increased the gel strength measured but led to a more heterogenous microstructure. The elasticity of almond gels could also be increased approximately 25 times with a threefold increase in protein concentration (i.e. from 3.6 percent in skim almond milk to 10.8 percent post concentration). This study provided a better understanding of the heat sensitivity of almond milk proteins and revealed temperatures that are critical to almond milk processing for a variety of applications, including heat treatment to induce denaturation prior to almond yoghurt formation or heat-induced gelation to form products like almond tofu/cheese. Third, the behaviour of purified almond protein isolate, containing predominantly amandin protein, was examined under conditions of neutral and acidic pH (pH 7 and 4). The isolate was highly soluble (70-80 percent) at either pH. An increase in acidity led to protein unfolding, an increase in random coil structure and the appearance of lower molecular weight proteins, potentially due to acidic and/or proteolytic hydrolysis. These structural changes at pH 4 increased the capacity for foam formation and foam stability, increased viscosity and led to concentration and age dependent thickening. Gels, similar in strength but with distinct microstructures and properties, were obtained following heating. At pH 7, a particulate type gel with an interconnected protein network was formed, while the gel produced at pH 4 had a dense continuous protein matrix. The gels differed in their susceptibility to chemical disruption, suggesting different underlying molecular interactions. This study illustrates the potential utility of almond proteins in foaming, thickening and gel formation and how almond preparations can be tuned by varying pH and temperature to obtain a range of tailored products with desired properties. Overall, these studies have increased our understanding of the response of almond proteins to process variables that are essential for both product and process development, performance and stability. Interconnected protein-based gel networks could be developed using either minimally processed or purified almond protein systems where the protein concentrations were equal to or greater than 4 wt percent. The formation of such gels and their pH tuneable properties may assist the formulation of novel almond based gelled vegan products, that are at par with dairy both in terms of protein content, functional performance and consumer acceptability.