Interaction of dairy and plant proteins for improving the emulsifying and gelation properties in food matrices: a review

Abstract

A variety of variables influence food texture, two of which are gelation and emulsification. Protein interactions have an important role in influencing gelation and emulsifying properties. The utilization of plant proteins in the development of food systems is a prominent subject within the current protein transition paradigm. Plant proteins diminish gel strength compared to dairy proteins. Protein providers prefer to create their own networks rather than rely on tight ties. It may be feasible to resolve these challenges if the interactions between plant and dairy proteins are known at all sizes, from molecular to macroscopic. Therefore, the proteins and dairy proteins are the main emphasis of this review. The role of these proteins in interacting with food matrices is also discussed. Additionally, this data gives information on worldwide research trends. Finally, a glimpse into the future was discussed.

Introduction

The majority of the foods we eat contain protein in some form. They serve a number of technological purposes in addition to their nutritional value as a source of important amino acids (Price et al., 2022). They are essential in achieving the product’s macroscopic qualities, like appearance or texture. Proteins play a significant role in generating or stabilizing relevant structures such as aggregates, particles, droplets, bubbles, and combinations thereof, making them especially relevant when thinking about food matrices (Liu et al., 2022). The molecular structure, native assembly state, charge, and solubility of proteins are governed by the amino acid makeup and sequence of the proteins themselves (Hinderink et al., 2021). Furthermore, the techniques used to extract protein constituents (such as concentrates and isolates from laboratory scale to commercial ones) from raw materials and to construct the end-food products may significantly alter the structure, assembly state, and functioning of proteins.

Protein-based foods have a structure–function relationship that is intrinsically complex due to the wide range of relevant scales (from molecular to macroscopic features) and the large number of interconnected parameters that may be at play important role (Rout and Srivastav, 2023; Schmid et al., 2022). Animal proteins, especially those found in dairy and eggs, have been the focus of much of the relevant study (Kumar et al., 2023). Therefore, this aspect of the latter has been thoroughly defined, while further research into several associated systems is ongoing. However, the ongoing so-called protein transition presents a new challenge in this area of study by encouraging the intake of alternative food proteins, the majority of which come from plants (Andreani et al., 2023). Nutritional concerns (such as amino acid content and digestibility) aren’t the only ones raised by this kind of partial replacement; the functional characteristics for structuring food colloids are also likely to be significantly altered.

Protein bodies are thick, glassy micron-sized structures that may self-organize as storage proteins in plant seeds (Warsame et al., 2020). For instance, dairy proteins are essentially evolved to thrive in aqueous environments, and so lack the supramolecular organisation required to do so (Alrosan et al., 2022; Hinderink et al., 2019). The latter are further subdivided into caseins, whey proteins, lactoglobulin, lactoferrin, and lactalbumin. Micelles of casein, as opposed to the bodies of plant proteins, are highly hydrated colloidal structures (3–4 g water per g casein). For example, Yoghurt and cheese are made by destabilising the ionic structure of the milk protein curds using enzymes and/or an isoelectric process (Guinee, 2021).

This review article explores the most recent developments into the formulation of food colloidal systems (aggregates, gels, foams, emulsions) from protein blends derived from both animal and plant sources. To begin unravelling the mechanisms at play in systems further structured by emulsifying qualities or network formation (gels), it is required to take a look back at a few plants and dairy proteins. Due to the lack of available experimental data for several of these structures and systems using plant-dairy protein combinations, the authors additionally propose some potential outcomes based on findings achieved with other protein mixtures. Finally, we explore several outstanding scientific questions and practical hurdles in the field of food colloids formed of plant-dairy proteins, such as the potentially enormous effect of non-protein components in the plant protein constituents.

Plant proteins

By 2030, the market for plant-based proteins is projected to grow from $29.4 billion to $162.1 billion (Kim et al., 2022). The events following COVID have prompted scientists to look for other protein suppliers. Plant-based proteins are popular among consumers since they are nutritious and kind to the planet (Bulah, 2020; Hertzler et al., 2020). Nutritional value and health are more important to consumers than sustainability and animal welfare (Knight and Light, 2021). The poor solubility and emulsification properties of plant proteins hinder their absorption into dietary matrices. Protein emulsification characteristics are invariably linked to their structural makeup. The protein’s instantaneous or potential amphiphilicity is one of its most critical characteristics for its application as an emulsifier. The form and location of the hydrophobic and hydrophilic regions of the structure, which characterise amphiphilicity, are determined by the individual amino acids’ positions and compositions throughout the polypeptide chain. By modifying the rate of structural reorganisation of the protein adsorbed from the interface, other factors like protein structure, flexibility, and isoelectric point can impact protein solubility and hydrophobicity, which are characteristics that directly affect protein emulsifiability. This in turn affects steric hindrance between droplets and changes protein emulsion stability. Due to their inflexible structures, the majority of plant proteins are less soluble and can only be used in small amounts as dietary additives (Rout & Srivastav, 2024; Hertzler et al., 2020). Thus, these proteins require a combination with other proteins to achieve optimal performance. The following plant proteins have major role for the for matrices.

Soy proteins

Soybeans (Glycine max L.) are often regarded as the most important oilseed crop due to their ability to produce vast quantities of cheap, useful, and nutrient-rich proteins. Therefore, meals made with soybean protein have one of the highest growth rates in the food industry (Messina et al., 2022). Formula for infants, milk-based drinks, and processed meats are all good examples (Albert et al., 2021). Foods made with soybeans are often subjected to both heat and cold processing. Researchers have been looking for non-thermal methods to kill harmful microorganisms like pathogenic bacteria and viruses without compromising food’s shelf life or flavour because of the detrimental effects of traditional heat treatments (Allai et al., 2023). Ultrasonication, high hydrostatic pressure, pulsed light, and cold plasma are only a few of the many techniques available for this purpose (Singh et al., 2022). Macromolecules like proteins and carbohydrates can undergo dynamic structural reformation under specific environments, which can alter their physicochemical and technical properties.

Pea proteins

Legumin (11S), convicilin (7S), and vicilin are only few of the globular proteins found in pea (Pisum sativum L.). As a hexamer, legumimin is held together by disulfide bonds (–S–S–) between its acidic and basic subunits (Helmick et al., 2023). Vicilin is made up of three 50 kDa subunits, but it is deficient in cysteine and contains only a little amount of methionine. Convicilin is a protein with subunits ranging in size from 4 to 10 kDa. The gelation of globular proteins is the most important feature of these proteins. Less disulfide linkages were generated in pea legumin gels because of the lower cysteine content compared to soybean glycinin (12 S fraction). The examples of different plant proteins along with their functional properties are enlisted in Table 1.

Table 1.

List of plant protein ingredients and their functional properties

Plant Protein	Molecular Weight (kDa)	Techno-Functional properties	Examples	References
Soy protein	300–600	Emulsification,gelation, viscosity	Soymilk and baby food, meat analogues, tofu	Rout & Srivastav (2024)
Pea protein	320–380	Water and fat binding, gelling, texture	Nuggets, sausages, meatballs	Gravel et al. (2023)
Mung bean protein	150	Emulsification, foaming properties, texture	Vegan burger patty, meat analogues	Hou et al. (2023)
Peanut protein	62	Oil absorption, foaming, gelling, binding	Meat emulsion, sausages, bakery products	Cui et al. (2023)
Wheat protein	50–116	Viscoelastic, texture, gelling, emulsifying	Gluten films, meat replacement, nanocarriers, plasticizer	Gasparre & Rosell (2023)
Quinoa protein	85	Foaming, whipping, water absorption, emulsification	Protein shakes, sausages, nuggets	Ren et al. (2023)
Grass pea protein	47	Gelling, texture, binding, emulsification	Meat balls, burger patty, sausages	Rout & Srivastav (2024)
Amaranth protein	250	Water and oil binding, texture, gelling	Dosa, cheela, meat analogues, patties	Rivero Meza et al. (2023)
Rice protein	10–66	Emulsifying and foaming properties, gelling, texture	Meatballs, sausages, meat analogues	Roy et al. (2023)
Chickpea protein	19–23	Water and oil binding, emulsifying, foaming, gelling	Nuggets, sausages, vegan burger	Wang et al.. (2023)

Mung bean proteins

For over 3500 years, people in India and central Asia have been enjoying mung beans, often known as green gram. When compared to the protein content of kidney beans and soybeans, mung bean is on par. Glutamins (13.3%), prolamins (0.9%), globulins (62.0%), and albumins (16.3%) are all proteins that can be found in mung beans. There are 4% basic (7S) subunits, 89% vicilin (8S) subunits, and 8% legumin (11S) subunits in mung bean globulins (Gravel and Doyen, 2023; Pan et al., 2019). According to the findings of Shrestha et al. 2023, mung bean protein does not include cysteine, methionine, or disulfide linkages (Fig. 1). Due to the presence of a higher proportion of neutral amino acids, the gelation properties of this protein may vary from those of soybean protein.

Fig. 1.

SDS-PAGE profiles of soluble proteins from different pulse proteins isolates under non-reducing (NR) and reducing conditions (R) (Shrestha et al., 2023), (A colour version of this figure can be viewed online)

Dairy proteins

Proteins in food have the potential to operate as a natural emulsifier due to their amphiphilic nature, which allows them to adsorb at the interface, surrounding oil or air droplets, and stabilise the dispersions. Dairy proteins, including casein and whey proteins, are currently the most popular protein-emulsifiers on the market. Since casein has a flexible random coil shape and whey protein is a typical globular protein, these two dairy proteins have different emulsifying properties. For decades, researchers have examined oil-in-water emulsions made from casein, whey, or a combination of the two. Here, a comprehensive breakdown of these dairy proteins.

Casein

A phosphoprotein combination called casein makes up about 80% of the protein in cow milk. Whether it’s made through proteolytic coagulation or isoelectric precipitation at pH 4.6, casein is a highly adaptable and unstructured protein with a high surface hydrophobicity due to its open structure (Hill et al., 2022). The secondary structure of casein is the source of its highly conserved sequences, which promotes the spontaneous interaction of casein molecules without the need for protein folding (Horvath et al., 2022). Different hydrophobic and hydrophilic areas can be found at various locations throughout the protein chain that make up casein fractions with molecular weights between 19 and 25 kDa (Ma et al., 2023). Aggregates of casein are composed of four subunits: α_S1-, α_S2-, β- and κ-casein, which account for 38%, 10%, 36%, and 12%, respectively (Sadiq et al., 2021). Molten globule-like proteins, such as those seen in β- and κ-caseins have a compact structure with a high degree of hydration and side-chain flexibility (Carver et al., 2019). On the other hand, α_S1- and α_S2- caseins are naturally unfolded proteins with an extended coil-like conformation. Signal peptides are found at the N-terminus of all caseins, and a Pro-Gln rich region towards the C-terminus may serve as an amorphous calcium phosphate binding motif.

Whey

Whey protein is the liquid by-products of cheese and casein manufacturing, utilised extensively in the dairy industry, as a result of its exceptional functional and nutritional qualities. The tertiary structure of globular proteins induces a spherical shape with hydrophobic sites buried inside for protection and hydrophilic regions exposed for dipole–dipole interactions with solvents. Physical, chemical, and structural properties of globular proteins can be altered by denaturation and alterations in tertiary structure (Acharya and Chaudhuri, 2021). Small amounts of lactoferrin, immunoglobulins, serum albumin, glycomacropeptide, enzymes, and growth factors are also present in whey protein, although these other components make up the bulk of the protein’s molecular weight (Tsermoula et al., 2021). About 58% of whey protein is β-lactoglobulin, while the next most common component is α-lactalbumin (13%) (Vidotto and Tavares, 2022).

Interaction of plant and dairy proteins

Most plant protein components insoluble in water, which is a crucial setback in the quest to completely substitute dairy proteins. Unlocking these applications may necessitate one of two approaches: (a) rethinking the production processes of plant protein ingredients to prevent extensive protein aggregation due to solvents and heat and the removal of fractions with high technological potential, such as albumins; or (b) utilising this low solubility to achieve food structuring via alternative routes, such as the formation of Pickering emulsions stabilised by insoluble protein particles. In the following section, the combined properties of dairy and plant proteins as emulsion stabilizer along with their nutritional benefits are discussed. The summary of dairy-plant protein mixture and their assembly in different media are given in Table 2.

Table 2.

List of dairy-plant protein mixture and their assembly in different media

Dairy protein	Plant protein	Assembly type	Ratio (DP:PP)	Inference	References
Lactoferrin	Soy protein	Associative	3:1 and 4:1	The heat-sensitive lobe in lactoferrin was more stable after the SPI/LF interaction	Zheng et al. (2020)
Β-casein	Pea protein	Co-solubility	7.5:2.5	In the presence of casein, the denaturation temperature of pea proteins rose, suggesting a chaperone-like action of casein micelles	Beghdadi et al. (2022)
Whey protein	Rapeseed protein	_	10:90, 20:80, 30:70, 40:60, 50:50, 60:40, 70:30, 80:20, 90:10	Depending on the protein concentration, RP altered the microstructural and/or denaturation properties of WP	Ainis et al. (2019)
Casein micelle	Pea protein	Co-dispersion	90:10, 80:20, 50:50	Mixed protein functional food products such as liquids, gels, and powders made with the CM:PP combination performed well	Krentz et al. (2022)
Whey Protein	Soy protein	Sol–gel	4:0, 3:1, 2:2, 1:3, 0:4	The elastic response and the transition to plastic behaviour of SPI-WPI hybrid gels have been enhanced over a single component	Xia et al. (2022)
Βovine milk whey proteins	Napin-rich rapeseed	Stable colloid	80:20, 60:40, 40:60, 20:80	Protein–protein assembly and, perhaps, phase separation are driven by charge anisotropy	Joehnke et al. (2019)
Whey protein	Soy protein	–	–	The packaging industry could benefit greatly from WPI-SPI based powder because it is simple to make, requires minimal space when stored, and contains no preservatives	Erdem et al.,(2021)
Whey protein	Mung bean	–	10:0, 9:1, 7:3, 5:5, 3:7, 1:9, 0:10	Wall material made from WPI-MG mixed starch is effective. In addition to improving the functional qualities of other foods, their addition provides adequate stability, decreased irritation, and good water solubility	Zhang et al. (2022a, b, c)
Whey protein	Soy protein	Biphase	9:1	Emulsion gel created from a mixture of SPI-WPI composite and Ca2 + is more stable and has highest potential value and smallest particle size	Zhang et al. (2022a, b, c)
Lactoferrin	Pea Protein	Associative phase	0.047:0.007	At pH 5.4, the coacervates were found to have a generally spherical size distribution with a max approximately 80 nm, as demonstrated by small angle X-ray scattering measurements	Adal et al., (2017)

DD dairy protein; PP plant protein; SPI WPI soy protein isolate-whey protein isolate; RP rapeseed, CM-PP casein micelle-pea protein; WPI-MG whey protein isolate-mung bean

Gelation properties

The gelation of plant-dairy protein mixtures has been the subject of a number of research, most of which have relied on plant protein isolates or the non-precipitated fraction following centrifugation. Gels formed from a combination of plant and milk proteins have been the subject of substantial coverage in recent reviews (Ouyang et al., 2022; Vogelsang-O’Dwyer et al., 2021). Proteins derived from plants have often come from soy or pea, but wheat, canola, sunflower, and hemp have also garnered interest. Both heat-induced and cold-set gels rely on the initial aggregates formed by the thiol/disulfide exchanges and/or hydrophobic effects of denaturing globular proteins at high temperatures (Rout and Srivastav, 2024). In most cases, these aggregates become larger with additional heating or become insoluble when acidified or salt is added. The gelling behaviours are presented in Fig. 2. Although the final microstructure and texture of such mixed gels vary on the type of materials used, the overall mechanism described above applies equally well to gels constructed of plant-dairy protein combinations.

Fig. 2.

Schematic diagram of gelling behaviours of plant proteins (Hinderink et al., 2021), (A colour version of this figure can be viewed online)

Emulsifying and foaming properties

Protein emulsifiers derived from legumes, such as the 11S fractions of soy and pea protein, glycinin, and legumin, have emulsifying and foaming properties. Soy proteins have been demonstrated to be prone to aggregation over time and to exhibit increasing droplet size with environmental changes such salt addition (Ding et al., 2023), whereas dairy proteins generally form stable emulsion (Rout et al., 2022). High surface activity via Pickering stabilisation in aggregates of legume proteins demonstrates their potential as protein emulsifiers (Tripathy et al., 2021a, b). Although heat treatment can produce an emulsion with small droplet size, cross-linking soy protein results in a denser nanoparticle layer at the interface and microgel production of pea protein. These proteins demonstrated a characteristic of Pickering emulsions by forming an extremely stable emulsion that remained dispersed without coalescing for over a month. With the goal of utilising alternative protein sources for the food sector, Cassani et al. (2023) effectively created lupin nanoparticles by heat treatment. Pea protein at the same concentration did not precipitate when salt was added; instead, the emulsion-gel phase transition at high salt concentration resulted in a weak gel-like emulsion structure and increased viscosity (Hu et al., 2023). The sodium caseinate-stabilized nanoparticle demonstrated the maximum encapsulation effectiveness (91.80%) and loading capacity (27%). This was found in a study by Liu et al. (2020) on curcumin encapsulation using sodium caseinate, whey, and soy proteins. When compared to whey protein (69.7%), soy protein showed a higher encapsulation efficiency (81.88%) and more uniform particle size (approximately 20 nm). Recent studies showed enhanced dispersibility of plant proteins, radical scavenging capabilities of plant proteins, enhanced interfacial stiffness, and increased repulsive interactions are the mechanisms for maintaining the physical and oxidative stability of emulsions stabilised with plant-dairy protein mixes (Fig. 3) (Feng et al., 2021; Hinderink et al., 2020). The stabilisation processes of protein blend-stabilised emulsions may be compromised by interfacial displacement (Hadidi et al., 2023).

Fig. 3.

Emulsion stability of plant proteins; Plant proteins (green triangles), whey proteins (orange spheres), caseins (orange loops), free radicals (red spheres) (Hinderink et al., 2021), (A colour version of this figure can be viewed online)

Due to their low solubility in water, cereal proteins (such as wheat, rice, maize, etc.) have not been thoroughly examined using the traditional emulsification approach. In instance, as compared to legume proteins, the emulsifying properties of cereal proteins including wheat, barley, and oat are comparatively poor. While β-casein underwent conformational rearrangement with an increase in α-helix structure upon heating, the emulsion made with deamidated wheat gliadin showed excellent emulsion stability against coalescence and thermal-induced aggregation (Anal et al., 2023). There was evidence that hydrolysed rice glutelin could generate a stable emulsion with tiny droplet size (Yan et al., 2022), however more study is needed because emulsions made with whey protein isolate (WPI) were more stable against environmental stresses (pH, temperature, and salt) (Kim et al., 2020).

Structuring and interfacial layer formation at oil/water interface

Interfacial film generated from a single protein source is often thinner and less dense than that formed from a mixture of proteins (Yang et al., 2022). Few contemporary literatures exist to reveal the interfacial properties of mixed protein emulsifiers, which would provide light on emulsion formation and stability (Gao et al., 2024; Grasberger et al., 2023; Yang & Cheng, 2024; ). This will examine the role of intermolecular contacts and the competitive adsorption behaviour between proteins in the creation and structuring of interfacial layers using a mixed protein system (Fig. 4). Adsorption behaviour must be understood in order to provide light on the layer creation and structural organisation of protein molecules at interfaces. While dairy proteins’ competitive adsorption behaviours are well-known, nothing is known about the adsorption behaviour of proteins derived from biologically diverse sources. Casein molecules, for instance, are gradually displaced by whey protein at the interface created by a two-dairy protein mixed system (Patel et al., 2019). Over time, the interfacial layer’s protein composition and structure in the mixed protein system are known to change. While the whey protein isolate (WPI) fraction concentration of whey-pea protein isolate (PPI) blends rose with time at the oil/water interface, the PPI fraction content of sodium caseinate (SC)-PPI blends increased with time. It was previously known that whey protein replaces casein, and this mechanism explains how this happens: on the droplet surface, SC can be displaced by PPI, and PPI can be displaced by WPI. This is likely due to intermolecular interactions in the mixed protein system and at the interface, which affect adsorption kinetics and the strength of the protein network (Yerramilli et al., 2017).

Fig. 4.

Structuring and network formation of mixed dairy and legume protein system in aqueous phase and at oil/water interface. a Network formation of mixed whey and legume protein in aqueous phase, b Network formation of mixed casein and legume protein in aqueous phase, c Structuring of mixed whey and legume protein at oil/water interface, d Structuring of mixed casein and legume protein at oil/water interface (Kim et al., 2020); copyright

© 2020 reprinted by permission of Elsevier publisher, (A colour version of this figure can be viewed online)

The adsorption behaviour of each protein at the contact can shed light on this phenomenon. The elasticity of the interfacial layer is a function of both the elasticity of the protein network and the elasticity of the proteins that have been adsorbed to the surface of the droplet (Zhang et al., 2023). The compact and massive structure of legume protein molecules causes sluggish adsorption due to disulphide and hydrophobic interactions (Lima et al., 2023). The smaller molecular weight whey protein would adsorb first and proceed through intermolecular alignment via disulfide bridges when both whey and legume proteins are present at the interface (Schröder et al., 2017). In addition, whey protein affects the permeability and reorganisation rate of legume proteins by entangling protein networks and blocking protein penetration at the interfacial layer. In contrast, casein molecules are more likely to get detached from newly produced oil droplets than whey protein molecules are, because casein does not form an interconnected network when adsorbed at the interface. The construction of the protein network in legumes can be influenced, for instance, by the movement of casein molecules between the interface and the bulk. When pea protein is added to casein, the protein does not form an intermolecular linked film, but rather, the surface concentration of the casein slightly increases. Adding dairy protein can improve emulsion qualities even when casein does not induce interfacial layer formation. This is because most plant-based proteins can stabilise emulsion through the Pickering mechanism. It has been shown, for instance, that adjusting the ratio of zein to sodium caseinate in a colloidal nanocomplex can improve the nanoparticles’ wettability and surface charge, hence enhancing their interfacial properties.

Heat-induced and cold-set gels

Even when the casein micelles alone are at a sufficient concentration and at an adequate pH (pH 6.0) to sustain the formation of a gel, the resulting networks are often weaker and less homogeneous, i.e., with larger pores. The yield stress and viscous moduli of heat-induced gels generated from a mixture of micellar casein and soy protein isolate in water at pH 7.0 were lower than those of a gel made from only soy proteins. Schmitt et al. (2019) found an antagonistic effect of the mixture on both the elastic moduli and the connectivity of the heat-induced gel by systematically adjusting the proportions of micellar casein and plant protein isolates (pea or soy). Similar to vicilin and casein micelles, heating pea legumin or PPI with casein micelles results in soft gels upon acidification due to the formation of very large and insoluble aggregates. The detachment of calcium from casein micelles by the legume proteins (or by residual phytates that are naturally present in legume seeds) and the chaperone and/or hindrance effects that caseins exert on the aggregation of heat-denatured globular proteins were responsible for the antagonistic effect of mixing legume proteins and caseins. Since native legume globulins lack reactive free thiols, electrophoresis analyses showed that casein micelles could inhibit the heat-induced denaturation and aggregation of the globular plant proteins. The networks formed from plant-dairy protein blends containing whey proteins, on the other hand, typically have a hard feel and uniform microstructures devoid of phase separation. For instance, when heated, gels containing a combination of milk and pea proteins were just as solid as gels containing pea protein alone and more so than those using skim milk powder alone. Acid-induced gels, the most common form of cold-set gels, gel at higher pH and produce stiffer networks when a combination of casein micelles, whey proteins, and soy proteins is preheated (Xia et al., 2022). Although the addition of soy or pea proteins favours an early commencement of gelation, unheated mixes or individually heated globular proteins and casein micelles produce coarse and soft gels with minimal water holding ability. Protein aggregation and the production of hard homogenous gels may be aided by the presence of heat-sensitive and thiol-containing -lg, since this protein is thought to facilitate thiol/disulfide exchanges with the other globular proteins and with casein upon heating. Aggregate size and interactions are sensitive to protein composition, temperature of heating, and ionic strength.

As with the gelation of milk or plant proteins separately, only recently has research begun to emerge on the gelation mechanisms and gel properties of mixtures of milk and plant proteins. Given the variety of plant protein ingredients available, it is essential to perform systematic investigations to provide criteria for making informed decisions. Wet fractionation procedures involving organic solvents and/or acidification can have significant effects on plant protein composition, structure, and concentration, as was discussed above. For example, it has been shown that when whey and soy proteins are cooked together, phase separation occurs because the plant ingredient aggregates uncontrollably (Grace et al., 2021). Additionally several proteins, polysaccharides, and fibres included in flours or concentrates are often removed during isolate purification and their potential impact on the textural qualities of mixed gels has thus far been understudied. There has been some new research linking the fractionation of plant proteins to the gel characteristics of both plants and dairy (Mefleh et al., 2022). It was demonstrated that opportunities exist for the development of plant-dairy protein-based gels with desirable gelation and textural features. Using a less refined pea protein concentrate or an albumin fraction of pea proteins can also increase gel strength. Using high temperature or pH values far from neutrality during fractionation of pea proteins appears to diminish protein solubility and to cause uncontrolled pre-aggregation, which hinders the gelling capability (Drusch et al., 2021; Klost and Drusch, 2019).

Enzymatically-induced gels

Despite the potentially vast range of substrates and enzymes, enzymatically-induced mixed gels have been relatively understudied. Gel hardness was increased and syneresis was decreased when chymosin (or rennet), primarily utilised as a coagulant of the caseins in cheesemaking. Gels made by applying chymosin and transglutaminase together to a mixture of unheated and non-acidified pea and soy proteins, then adding the resulting mixture to casein micelles or skim milk, were softer than those made by using only the individual protein components alone. The formation of the gels can be done without the use of any harmful chemicals, making these processes eco-friendly and more efficient. In this way, protein allergenicity can be reduced, and the water-holding capacity and sensory characteristics (appearance, colour, and flavour) of protein-based food products can be improved (Anzani et al., 2020).

Food-grade enzymes of interest for fermenting raw materials of animal and plant origin can be provided by microorganisms as well. In particular, proteolytic enzymes like cell-envelope proteinases can have a major impact on the final texture of treated gels. The resulting peptides can subsequently be used to modify the gel’s overall consistency. Changes in physicochemical circumstances during fermentation likely to additionally enhance cold gelation of proteins, as microorganisms like lactic acid bacteria can acidify the medium. Fermentation serves as a toolbox that can give a wide variety of proteolysis reactions and other important reactions by using a wide variety of enzymes. To prevent uncontrolled formation of globular protein and/or protein-quinones aggregates during pH cycle, several enzymes can reduce the amount of phytates or polyphenols during plant protein isolation or product processing. The nutritional value of these products can be improved by the addition of other bacterial enzymes (such as endo- or amino-peptidases and amino-transferase, which are involved in vitamin production) (Annapure et al., 2023). Plants have the ability to produce molecules like peptides and free amino acids, and they can also mitigate the beany, green, or chalky markers associated with plant protein ingredients (caused by enzymes like alcohol dehydrogenase) that have hampered their widespread use in processed foods. This method has been found to be effective in reducing sensory abnormalities in mixed systems including both dairy and legume (pea or lupin) proteins.

Application of both the proteins in various food model systems

Demand for plant-based proteins has increased as people become more conscious of their influence on the planet, the treatment of animals, and the quality of their own health. However, poor consumer acceptance of vegan goods is slowing the shift towards greener diets. The addition of novel plant-based ingredients to conventional dairy products opens up a whole new realm of possibility. Mixed protein emulsifiers have many potential applications in the food industry, including microencapsulation, dairy powder, and infant formula. Encapsulation with mixed protein emulsifiers has been shown to improve the oxidative stability of the core material through processing methods and protein sources. Microencapsulation with a mixed emulsifier (sodium caseinate and pea protein), for instance, can lower the perceived oxidative off-flavour by increasing the homogenization pressure and the number of passes (Mehany et al., 2023). It was also emphasised that it is important to maintain sensory qualities by maintaining small droplet sizes. As previously mentioned by Li et al. (2019), sodium caseinate’s increased molecular flexibility improves oxidative retention in comparison to other dairy protein sources. The use of mixed-protein emulsifiers is advantageous because to their increased heat stability. Recent years have seen the development of semi-industrial plant- and dairy-based infant formula (Le Roux et al., 2020). Even though legume protein partially aggregated and showed a little greater particle size, the study revealed the possibility of employing mixed protein emulsifier in infant formula. Despite satisfying the physicochemical criteria for dispersibility, solubility, and viscosity, mixed whey and legume protein infant formula powders still need to be improved in these areas and in the free fat content to satisfy the strict standards for shelf life and quality. Plant protein can play a larger role in human society if its use in blended goods is increased and its use in dairy products is widened. The use of enzymes, raw material selection based on protein quality, cutting-edge processing, and technology interventions are all areas of research that have the potential to increase nutritional characteristics through processing. It’s also important to make sure that people enjoy and are willing to consume double-protein dairy.

Global research trends

Research trends often entice researchers for advances in a certain subject. Hence, it is essential to quantify the statistical data. The pattern of research activity for the keywords “dairy and plant proteins” associated with scoups is shown in Fig. 5. The data for this trend was collected on August 20, 2023. As a consequence of this, it was discovered that there are a total of 2997 research items available. It can be observed from Fig. 5a that the total number of publications has been growing throughout the duration of the years. The patterns that have emerged over the last 15 years are detailed below. The years 2021 and 2022 had the highest number of publication totals ever recorded. Figure 5b displays the different categories of publications, and among them, research articles account for 80.9% of the total number of published works (2425). Figure 5c illustrated the contributions made by each country; here, we displayed the top five nations in terms of their contributions. To this date, the United States of America has made the most contribution of any other nation, followed by China, Canada, France, and Italy. According to the Scopus database, the majority of the research in this topic has been contributed by Wageningen University and Research, which is located in the Netherlands.

Fig. 5.

Global research trends a number of publications with respect to year, b types of publications, and c number of publications with respect to countries; data taken on August 20, 2023, (A colour version of this figure can be viewed online)

Future trends and conclusions

Plant–dairy protein combinations have been gaining interest in food research lately, notably for systems based on pea or soy proteins combined with dairy proteins to generate gels and emulsions. To conclude, despite some intrinsic restrictions for using combinations of dairy and plant proteins (and especially the fact that they are out of reach for e.g., the vegan market), such mixtures nevertheless seem to offer a significant promise in the food science sector. In particular, they may be important in minimising several concerns currently experienced when employing plant proteins just to build and stabilise colloidal systems, such as physical destabilisation, off-flavours, oxidative stability, and perhaps lower nutritional quality. Future work to comprehensively define the composition of the selected plant protein constituents (in particular, addressing non-protein components), and their interactions with dairy proteins, will be essential next steps from scientific and application perspectives.

Funding

This work was supported by the National Science and Technology Council (NSTC) Taiwan under grant numbers NSTC 111-2221-E-155-003-MY2 and NSTC 112-2811-E-155-002-MY2.

Declarations

Conflict of interest

The authors declare no conflict of interest.