Summary:

Plant proteins and phenolics are widely co-occurring in plant-based food resources and their interactions and associated changes are of great significance for both academia and food industry. Food proteins are macronutrients and also critical components to impart food quality and shelf-life based on their functional properties such as emulsifying, foaming and gelling. Phenolics possess a series of health-promoting features, particularly antioxidant, anti-microbial and anti-diabetic properties. Dietary proteins can form complexes with phenolics via non-covalent or covalent interactions. In earlier reports, it was believed that protein-phenolic interactions mostly deteriorated the properties of both ingredients, such as reduced protein digestibility, and masked bioavailability and antioxidant capacity of phenolics. However, recent studies indicates that protein-phenolic complexation may have minor or even positive effects on their merits, largely relying on the interacting compounds, surrounding conditions, and preparation procedures. Through appropriate design of protein-phenolic interactions, using their complexation to yield food products with improved functionality and nutritive quality can be feasible.

For phenolics, molecular weight (Mw), hydrophobicity, structural flexibility, and functional groups are the main factors affecting their binding of proteins. Reduced protein digestibility has been reported particularly for protein binding with large Mw polyphenols ascribed to their more binding sites, for example, polymerized proteins become resistant to proteolysis after tannin crosslinking. There are substantial studies on interactions of proteins with small Mw phenolics, however, most reporting binding parameters, not the effects on protein digestion. Phenolics can induce protein unfolding, followed by aggregation, crosslinking and then polymerization depending on the interaction type and strength. Such protein structure changes can make them less or more prone to proteolytic attack. The protein binding of small Mw phenolics via hydrophobic interaction can reduce the accessibility to proteases with preference for hydrophobic residues. If phenolic induces partial unfolding of food proteins without several polymerization, the exposed peptide bonds will be more accessible to digestive enzymes. Partial unfolding of protein structure also allows exposure of hidden hydrophobic groups that can contribute to enhanced emulsifying and foaming properties.

As for the phenolics, their binding to plant protein-based food matrix provides an opportunity to protect their stability and integrity during food processing and storage, and then sustainably release them in gut after protein matrix digestion, affording them a higher level of bioactivity compared to unprotected phenolic extracts. Suitable level of protein crosslinking enabled by phenolics also provide opportunity to improve protein matrix (including gels) mechanical strength. This is especially interesting to address the texture issues of plant protein-based food products and using phenolic compounds as natural food ingredients to replace synthetic food additives will make final products with cleaner label. The understanding and capacity to control protein-phenolic interaction, subsequently the level of protein unfolding, aggregation and crosslinking is the key to ensure successful food development. Nonetheless, most of our knowledge has been gained from dairy proteins, the structure-affinity relationship of phenolics for plant proteins are seldomly elucidated, highlighting the research necessity in this area towards heathy plant-based food development. In addition, most previous works studied interactions between protein isolates and purified phenolic compounds (e.g. catechin, EGCG). Nowadays, instead of using purified ingredients, investigations on plant fractions or extracts and their component interactions in actual food matrices are gaining interest. Plant protein fractions and phenolic extracts are good examples of this new area of research as both are highly demanded for plant-based foods and their interactions in food matrix directly determine the food texture, shelf-life, and nutritive quality.

This research aims to investigate component interactions between plant protein fractions and phenolic extracts from crop resources and agricultural by-products in Canada, and the associated changes in protein and phenolic structures in relation to food sensory, nutritive quality and potential health benefits. Phenolics from legume grain seed coats (lentils, faba bean, dark red kidney bean), cereal hulls/bran/aleurone (oat, barley, wheat, wild rice, sorghum) and winery by-products will be studied. Cereals are rich in ferulic acid, p-coumaric and sinapic acids. Oat is a good source of avenanthramides. Wild rice and beans are good sources of both phenolic acids and flavonoids such as apigenin and its C-glycosides, (+)-catechin, (-)-epicatechin. Anthocyanins are reported in dark kidney beans. While hydroxycinnamic acids predominate in the bound form in cereals, flavonoids in both cereal and legumes occur in the free extractable fractions. Food grade phenolic ingredients from winery waste streams provided by CrushDynamics will also be included in this study, which are rich in flavonoids (e.g. quercetin) and non-flavonoids produced by fermentation technique to reduce tannins. Protein fractions (albumins and globulins) from pulse and cereal crops (e.g. pea, lentil, faba bean, oat) will be mainly focused upon. These proteins have high nutritive value and good functional properties; yet unlike soy, are non-GMO and hypoallergenic. The brewing industry and flaxseed oil production play a significant role in utilizing Canadian agricultural produce. The by-product streams including barley spent grain and flaxseed meals are also good sources of proteins (globulins and prolamins), and thus will be included in this study. Investigation of phenolics and proteins from different sources allows a more systematic study of protein-phenolic interactions, which also give higher chance to identify suitable protein-phenolic complexations with desirable attributes for plant based-food applications.