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Abstract: The effect of plant proteins, specifically those from yellow field pea seeds on the in vitro digestibility of wheat starch was studied. Initially, the starch digestibility profile of pressure cooked pea seeds (dehulled and split), pea flour and isolated starches from pea and wheat was evaluated by determining the contents of RDS (rapidly digestible starch), SDS (slowly digestible starch) and RS (resistant starch). The RDS contents of cooked seed (80.3%) and flour (84.1%) samples were significantly lower than those of cooked starches (pea, 88.3%, and wheat, 88.6%), where the RDS content of cooked seeds was significantly lower than that of cooked flour. No significant difference was observed in the RDS contents of cooked starches from pea (88.3%) and wheat (88.6%). The SDS contents of pea seeds and flour generally were similar (12.2% and 13.2%, respectively), but significantly higher than that of isolated starches (pea, 6.4%, and wheat, 9.9%). The SDS content of isolated and cooked pea and wheat starches were significantly different. The RS content was significantly higher in split pea seeds (7.5%) than in pea starch isolate (5.3%), pea flour (2.7%) and wheat starch isolate (1.5%). Scanning electron microscopy (SEM) and differential scanning calorimetry (DSC) were used to characterize the morphological and thermal properties of the samples. The results indicated that endogenous protein and its interaction with starch during pressure cooking may be one of the factors that influence starch digestibility in pea seeds. In order to substantiate this preliminary finding, further research was performed using isolated plant proteins from different plant origins, and isolated wheat starch. The effect of protein isolates from wheat, corn, soybean, pea and rice grains in their native, heat-denatured and protease-hydrolysed forms on the in vitro amylase digestibility of wheat starch was investigated. Native proteins, except that from rice, did not cause significant reduction in the RDS content of cooked wheat starch when compared to controls. Heat-denatured proteins, denatured either by boiling (except for corn and pea) or pressure cooking, resulted in significant reduction in RDS content. Protease- hydrolysed proteins, produced with or without denaturation by boiling or pressure cooking prior to their addition to wheat starch, caused significant reduction in RDS (except for pressure-cooked plus hydrolysed pea protein, and boiling-denatured and boiling-denatured plus hydrolysed corn protein). Differential scanning calorimetry studies and confocal laser scanning microscopy of selected starch-protein mixtures suggested that protein denaturation and protease hydrolysis promoted starch-protein interaction, and thus mitigated RDS content. Further studies were conducted using digestion simulation models of the human gastro-intestinal system to understand the possible inhibitory effects of native and hydrolysed pea protein on the amylolysis of wheat starch in an extruded wheat snack matrix. A combination of an in vitro dynamic gastric model (DGM) and a static duodenal digestion model (SDM) was used in this study. The addition of native pea protein did not influence the release of soluble starch and glucose when compared to the control after complete gastric emptying. However, the addition of hydrolysed pea protein significantly reduced soluble starch at 0, 5, 20 and 40 min. Infrared (FTIR) scans of the extruded samples clearly indicated enhanced starch-protein interactions through hydrogen bonding, mainly in the blend with hydrolysed pea protein. The studies in general suggested the possibility of developing a protein-based strategy to formulate low-glycemic food products.

Field pea {Pisum sativum L.) is a well established crop around the world and over the last ten years, the production of this grain has been increasing in Australia. As with other grain legumes, field pea proteins contain high levels of lysine which may be important in balancing the deficiencies of this essential amino acid in cereal-based diets. In recent years, interest in plant proteins for feed and food led to the evaluation of field peas as an economical and nutritional source of proteins. Although some research work has been done on the production of field pea protein, starch and fiber as food ingredients in Europe and Canada, none of these attempts have resulted in a commercial scale applications of pea proteins in the food market. The primary limitation to the development of pea proteins as food ingredients is the lack of information on the technology and characterisation of the resulting products, in comparison with well-established soy proteins. The major purpose of this project has been focused on the isolation and characterisation of field pea proteins, both on a laboratory scale and on a pilot scale, as well as the functional properties evaluation of the resultant products for appropriate food applications. The feasibility of chemical modification in order to enhance the functional properties of the proteins is also included.