Over the past decade, the food industry has shifted towards plant proteins, driven by growing demand for sustainable and health-conscious alternatives to animal proteins. However, this transition, while promising, presented several technical challenges. These include issues related to solubility, digestibility, and flavour of plant proteins, which limit their broader applications. Furthermore, plant proteins generally exhibit poorer techno-functional properties, such as emulsifying, foaming, and gelling abilities, compared to animal proteins. Recent research suggests that the strategic incorporation of polyphenols, natural compounds known for their antioxidant and therapeutic benefits, can address these limitations. By forming protein-polyphenol complexes, the functional attributes of plant proteins can be significantly enhanced, paving the way for their use in innovative food products such as functional beverages, protein bars, and nutrient-encapsulated foods. This paper aims to investigate the effectiveness of plant protein-polyphenol complexes in improving the functionality of plant proteins, to advance their potential application in plant-based functional foods.

Plant proteins have attracted considerable attention due to their sustainability and health benefits in the past decade, and the global market is expected to reach USD 20.5 billion by 2029 (1). Plant protein bodies are mainly contained within the storage cells of plants, requiring the destruction of the cell walls for extraction purposes (2), which can be classified into four types: albumin, globulin, prolamin, and glutelin (3). Plant protein extraction commonly involves pulses (soy, pea, lupin, fava bean, chickpea), cereals (wheat, rice, oats), nuts (almond, peanut), tuber (potato), and oil seeds (flaxseed). Generally, extraction methods for plant proteins include wet extraction and dry fractionation. Traditional wet extraction (alkaline solubilisation and isoelectric precipitation) can obtain proteins with high purity, but the protein structure will be denatured due to the intensive chemical extraction process (4, 5). In comparison, mild wet extraction preserves the native protein structure and original protein functionality by avoiding harsh extraction processes like isoelectric precipitation, though the purity of the extracted protein is low (4). Other than wet extraction, dry fractionation is solvent-free and can retain the native state of the plant proteins to a large extent (5), while the protein from dry fractionation is mixed with non-protein impurities (>10%) such as starch, fibre, etc (5). The overview of plant protein development is shown in Figure 1. The techno-functional properties of plant proteins (emulsifying, foaming, and gelling capacities) are crucial for their uses in food systems. Some studies have suggested that the functionality of extracted plant proteins can be preserved in their native state (5), while others have pointed out that plant proteins with moderate structural alteration exhibit the potential to achieve better functionality (6). For example, native plant proteins with compact structures can be treated with heating (7), ultrasound (8) , and high-pressure processing (9) to induce partial unfolding and become more flexible, improving protein adsorption at the oil/water interface. Plant proteins can change their secondary structures and improve their usability in food systems by enzymatic treatment (10) or by forming non-covalent or covalent bonds with polysaccharides (11, 12) or polyphenols (13). New focus of plant protein research is more on protein functionality rather than purity. Pea protein concentrates with limited fractionation achieved higher gelling ability and stronger gelling strength than pea protein isolates with high purity due to the difference in the extraction process and the different amounts of sugars and ash content (14). Plant albumins, the side stream from plant globulin extraction, show an excellent foaming capacity, which allows albumins to upgrade from a side stream into valuable foaming agents in food products (4, 15). Moreover, the cooking water of chickpeas (aquafaba) containing albumin, polysaccharides and saponins is a good egg replacement (16). The focus of current plant protein research is to use practical methods to modify protein structure and to improve their functionality. The goal is to develop underutilised plant proteins with improved functionality in food products.

Figure 1. Overview of plant protein resources, extraction, modification, and functionality.

Polyphenols include diverse phenolic structures derived from plant sources and are broadly divided into flavonoids (including anthocyanins, flavanols, flavanones, flavonols, and isoflavones) and non-flavonoids, such as phenolic acids, tannins, lignans, xanthones, and stilbenes (17). These compounds are abundant in a variety of plant-based foods and beverages, including fruits (citrus fruits and berries), nuts, vegetables (broccoli and onions), cocoa, pulses, cereals, tea, coffee, and wine. They play a vital role in human health by exerting a range of therapeutic effects including maintenance of cardiovascular health (18), reduced risks of stroke and myocardial infarction (19), and improvements in lipid profiles (20), blood pressure, and systemic inflammation (21). Dietary polyphenols are generally recognised for their antioxidant properties, supporting the reduction of oxidative damage to biological molecules like lipids, proteins, enzymes, carbohydrates, and DNA (22, 23). They perform by scavenging radicals or activating the body’s inherent cellular antioxidant system, thereby boosting defenses against oxidative stress or by impacting gene transcription related to antioxidant defense and various cellular processes via nutri-epigenomic mechanisms (17, 23).

Apart from their therapeutic effects, polyphenol-rich foods including soy products, cocoa, and green tea-based beverages provide a preservative effect by scavenging reactive species, inhibiting lipoxygenase, and acting as reducing agents for metmyoglobin on food. Polyphenols also exert antimicrobial effects by interacting with components of bacterial cell walls, inhibiting microbial enzymes, or interfering with protein regulation thereby controlling biofilm formation and bacterial growth (24, 25). This effect can be incorporated into food products via the addition of polyphenols as conjugates or as ternary mixtures (26, 27).

Compared to the traditional solvent extraction or maceration, advanced extraction technologies (Fig. 2) including supercritical fluid extraction, ultrasonic-assisted extraction, microwave-assisted extraction, high-hydrostatic pressure extraction (HHPE), and glycerol and citric acid-based deep eutectic solvents (DES), efficiently extract polyphenols from food wastes like hazelnuts, coffee, grapes, spinach, tomato peels, onion waste and orange residues, providing high yields and antioxidant activity (28, 29, 30). Additional methods include Hybrid Molecular Imprinted Membranes (HMIMs) synthesised with quercetin molecular imprinted polymer nanoparticles, adsorption, and membrane technologies using Amberlite resins or zeolites, followed by ultrafiltration, which is effective for recovering polyphenols from wastewater, such as blanch water from marzipan production, or lemon waste (31). Nonthermal extraction techniques like cold plasma, pulsed electric field, and pressurised liquid extraction offer promising alternatives to conventional methods, reducing the need for heat and solvents (32).

All the above-mentioned methods utilise non-toxic solvents, maintain moderate temperatures, achieve fast extraction times, and require low energy input to produce bioactive compounds from food wastes for use in the cosmetic, pharmaceutical, and food industries, paving the way for more potentially sustainable alternatives for extraction.

Figure 2. Polyphenol isolation techniques that are segregated by conventional and novel methods.

The incorporation of polyphenols into food matrices typically improves the functional properties and quality of food products. The interactions between plant protein and polyphenols can offer improved functional properties of plant protein by modifying the protein structure (Fig. 3). With one or more hydroxyl groups in the aromatic systems, polyphenols interact with proteins to alter the structure and antioxidant capacity of the proteins (33). Covalent bonds formed between protein and polyphenol alter protein structure to a larger extent compared to non-covalent interaction, leading to a greater impact on their functional properties (e.g. emulsification, foaming properties). Methods to form covalent interactions, including alkaline reactions, free radical grafting, or enzymatic reactions, initially promote polyphenols to form semi-quinone radicals, which then rearrange to quinones, conducive to cross-link with proteins (C-N or C-S bond) (26). On the other hand, non-covalent interactions between protein and polyphenol, such as hydrogen bonding and hydrophobic interaction, are generally weaker and are potentially reversible (34). However, non-covalent interactions are considered important for the extension of the shelf life of food products by altering protein structure, being the latter more unfolded and less aggregated, providing antioxidant protection to protein and lipids, and positively affecting the sensory properties. For example, the incorporation of Tasmanian pepper leaf or berry successfully prevented lipid oxidation of pea protein-stabilised emulsions by improving the antioxidant activity of pea protein. Phenolic acids and flavonols in Tasmanian pepper leaf with lower molecular weights showed stronger non-covalent interactions with pea protein, leading to superior antioxidant activity compared to anthocyanins in Tasmanian pepper berry (35).

Given that plant proteins possess complex globular structures or a network formed by prolamin/glutelin, the processing strategies for plant protein-polyphenol interactions are critically distinct from those for animal proteins. For example, grafting methods and enzymatic catalysis are common techniques to promote animal protein-polyphenol interactions. In contrast, physical processing technologies such as high-pressure processing, microwave treatment, and high-intensity ultrasound treatment with high energy intensity have demonstrated effectiveness in inducing plant protein-polyphenol interactions while also modifying secondary/tertiary structures of proteins (26). In particular, high-intensity ultrasound treatment is of interest due to its ability to generate hydroxyl radicals, leading to the formation of covalent bonds between plant proteins and polyphenols (36).

When polyphenols are combined with proteins, the emulsifying and foaming abilities of plant proteins are generally reinforced with increasing polyphenol concentration (37). This phenomenon is due to the modification of the unordered structure of proteins by polyphenols that results in better adsorption at the oil/water or air/water interface, creating smaller oil droplets and/or air bubbles, leading to the improved stability of the emulsion and foam-based systems (26). However, adding excessive polyphenols can cause protein aggregation, leading to negative effects in emulsions and foams (38). On the other hand, the impact of polyphenols on the gelling ability of plant proteins remains to be fully understood. We know that polyphenols can modulate gel strength (39), promising for future studies.

Figure 3. The formation, processing, structure, and functional food application of plant protein-polyphenol complexes.

Plant proteins could be an integral part in developing functional foods as natural emulsifiers, gelling, and film-forming agents. Plant protein has proven to be a viable solution to encapsulate lipophilic and hydrophilic bioactive ingredients for sustained and targeted release and protection (40). Enhancing the properties of plant proteins by incorporating polyphenols should be investigated further.

In the emulsion systems, the modification of plant protein with polyphenol resulted in smaller oil droplet and improved uniformity (26). The complexation of anthocyanins with soy protein in emulsions could improve the storage stability of emulsion by reducing creaming by 20% over 7 weeks of storage (41). The antioxidant capacity of anthocyanins has shown a reduction in the rate and extent of lipid oxidation, implying that its addition could increase the stability of a sensitive cargo. Plant protein-polyphenol complexes have also been used to create hydrogels and emulsion gels. The presence of polyphenols is seen to increase the physical interactions between protein molecules and protein-stabilised droplets, thus increasing the mechanical strength of the gel (42, 43). Shear-thinning properties were reported in a pea protein isolate-tannic acid-hyaluronic acid complex in high internal phase emulsion, rendering them suitable for creating custom designs in 3D-printing. Customisable plant protein-polyphenol gels allow manufacturers to create attractive functional food with highly detailed appearances that could be tailored for any demographic. The shelf life and bioaccessibility of protein-polyphenol functional food are highly competitive. The stability is enhanced by the dense three-dimensional networks in gel-based functional food, and the inherent antioxidant activity of polyphenols culminated in the increased bioaccessibility of encapsulated bioactive ingredients.

The benefits of polyphenol modification are not limited to enhancing the physical properties of plant protein products, as polyphenols may also act as active ingredients. Polyphenol supplementation has been shown to improve athlete recovery from muscle damage by minimising cell damage and post-exercise muscle inflammation, as well as supporting the growth of gut microbiota (44, 45). The multi-dimensional benefit that plant protein-polyphenol complexes introduce is promising in delivering more benefits while slimming the ingredients list.

The combination of plant protein and polyphenols holds enormous potential as functional food ingredients with improved emulsifying, foaming, and gelling ability of plant proteins. The fortified antioxidant activity of plant protein-polyphenol complexes has been shown to prevent lipid oxidation, slow down the deterioration of food products, and provide proven therapeutic effects. Improvements in sensorial and nutritional profiles are critical to stimulate their applications in plant-based foods, including as functional beverages, food gels, and 3D printing materials.