Modification of plant and algal proteins through the Maillard reaction and complex coacervation: mechanisms, characteristics, and applications in encapsulating oxygen-sensitive oils

Zijia Zhang*^a, Bo Wang^b, Jie Chen^c and Benu Adhikari*^de
aSchool of Perfume and Aroma Technology, Shanghai Institute of Technology, Shanghai 201418, China. E-mail: zijia@sit.edu.cn
^bSchool of Behavioural and Health Science, Australian Catholic University, Sydney, NSW 2060, Australia
^cState Key Laboratory of Food Science and Resources, Jiangnan University, Wuxi, China
^dSchool of Science, RMIT University, Melbourne, VIC 3083, Australia. E-mail: benu.adhikari@rmit.edu.au
^eCentre for Advanced Materials and Industrial Chemistry (CAMIC), RMIT University, Melbourne, VIC 3001, Australia

First published on 15th March 2024

---

---

Sustainability spotlight The continuous growth of global population has exerted an immense strain on the global food system. The increasing demand for sustenance resulting from the need of feeding the 8 billion plus population will ultimately lead to a depletion of our finite natural resources. Moreover, the climate change-induced decrease in the availability of rain and irrigation water will likely impact agricultural output. Animal proteins are unlikely to meet the protein demand of human population in the future. Also, the production of animal proteins requires large input in terms of land, water, and energy resources. It also involves higher greenhouse gas emissions. Plant and algal proteins are more sustainable to produce. The society, as a whole, is paying increasing attention to the nutritional, technofunctional and processing aspects of these alternative proteins. In this context, this manuscript provides a concise review of the differences between animal, plant and algal proteins in terms of their protein structure, amino acid composition and digestibility. Most importantly it reviews the underpinning science and technologies used for modifications of plant and algal proteins through covalent conjugation and non-covalent complexation, aiming to enhance the technofunctional properties of these proteins. This review highlights the technological pathways for converting plant and algal proteins into protein ingredients. Thus, this work meets the Sustainable Development Goals (SDGs) (goal 12) established by the UN.

1. Introduction

Protein is one of the most essential macronutrients as it is essential for the physical and physiological functions of the human body.^1,2 It provides peptides and amino acids to support and maintain the physiological functions.^3,4 Proteins are also important ingredients in food formulations as they provide important technological functions such as emulsification, gelling, and texture modification. The contemporary food industry uses dairy and animal-derived proteins such as caseins and whey protein, and gelatin as emulsifiers and encapsulants to stabilise susceptible oils for long-term storage. When an oil-in-water (O/W) emulsion is prepared, protein molecules readily adsorb at the oil–water interface to reduce the interfacial tension due to their amphiphilic nature and flexible molecular structure.^5,6 The protein layer covering the oil droplets provides electrostatic forces and steric hindrance among the emulsion droplets and acts against aggregation and coalescence.⁷

On the other hand, the rising global population is putting significant pressure on the natural resources required to produce food.⁸ The production of animal proteins comes with sustainability constraints as it requires much higher input in terms of nutrients, water and land resources.^9,10 It has also been reported that approximately 14.5% of greenhouse gas emissions come from livestock production, thereby impeding environmental sustainability.^11,12 There is a growing ethical concern on animal welfare under current practices, which is also helping consumers choose alternative proteins.¹³ Due to these reasons, there is an increasing consumer demand for food products containing non-animal proteins.¹⁴ The fact that the population choosing vegan diet in the United States increased from 3 to 19.6 million in the last 10 years (2014–2023) underpins the future demands for non-animal proteins.¹⁵

Plant- and algae-based proteins are promising alternatives for this purpose. To date, various plant-based proteins derived from cereals, legumes, pulses, oilseeds, and nuts have been trialled to replace animal proteins as stabilising, foaming, thickening and gelling agents.^16,17 The contemporary food industry uses an alkaline extraction–isoelectric precipitation method to extract these (plant and algae-based) proteins, whereby the protein is solubilised in the alkali solution and precipitated around its isoelectric point, followed by separation and drying.¹⁷ The precipitation process causes molecule aggregation and folding, producing a protein with an increased size and compact structure. In addition, plant-based proteins are commonly composed of multiple protein fractions with a high molecular weight and globular structure (e.g., albumins, globulins, glutelins, and prolamins).¹⁸ Due to compositional and processing-related impacts, plant and algal proteins come with less than desirable functional properties (e.g. solubility and gelling interfacial behaviour).¹⁹ Compared with animal proteins, plant-based ones exhibit a slow adsorption rate at the oil/water interface.¹⁹ This ultimately compromises their application as ingredients and limits commercial application.

There is growing interest in using algae as a sustainable source of protein. The economic cost to grow algae is relatively low as they require less nutrients, water and space so long adequate sunlight is available.²⁰ Some of the commonly cultivated algal species are rich in protein (47–70% of dry biomass) and essential amino acids.²¹ However, similar to most plant proteins, algae protein has a relatively low solubility at acidic pH values, particularly near their isoelectric points compared with the animal-based ones. Schwenzfeier et al.²² reported that the acidic environment (pH of 3.0–5.0) reduced the solubility of protein extracted from green microalgae Tetraselmis sp. by more than 80%. In addition, the protein extracted from blue-green algae (e.g., Spirulina) is heat sensitive. It was observed that the spirulina protein underwent denaturation upon exposure to a thermal treatment at 67 °C.²³ This temperature-sensitivity further hinders the potential application of Spirulina and other algal proteins in food processing when thermal treatment is involved. Hence, it is important to modify plant and algae proteins in order to improve their techno-functional properties.

To overcome the above-mentioned hurdles, proteins can be reacted with polysaccharides to modify their native structure and improve their techno-functional properties.^24,25 The resulting protein–polysaccharide assemblies are known as “conjugate” if created through covalent conjugation and “complex” if created via non-covalent interactions. Compared with the native proteins, the appropriately modified ones (conjugates or complexes) exhibit significantly improved emulsifying and stabilising properties.²⁶ Among various modification methods, the protein–polysaccharide conjugation and complexation via the Maillard reaction and complex coacervation (electrostatic charge driven complexation) are effective and are widely employed.²⁷ In the conjugation process, covalent bonds are created between protein and polysaccharide molecules. Various studies reported that the solubility and emulsifying properties of protein were substantially improved upon protein–polysaccharide conjugation induced by the Maillard reaction.^28–30 On the other hand, protein and polysaccharide molecules can form complex coacervates driven by electrostatic forces. The resulting supramolecular assembly showed superior interfacial properties compared to the native protein, making them promising emulsifiers and encapsulating wall materials.³¹

Microencapsulation technology is commonly employed in the food industry to protect and extend the shelf life of unstable oils that are prone to oxidation and developing off-flavour during food processing and storage.³² The process of encapsulation involves the stabilisation of active compounds by entrapping them within a single or multiple substances.³³ During the encapsulation process, the active or sensitive food component is enveloped by a layer or layers of coating material. The encapsulated component is referred to as the core material, which can consist of small particles, liquids, or gases. The covalent conjugate or complex coacervate-based coating materials possess ability to form a network (shell)-like structure surrounding the active component.³² The physicochemical stability of the encapsulated core can thus be enhanced.

There are reviews on various aspects of Maillard reaction-induced conjugation and complex coacervation-induced complexation and the application of resulting conjugates and complex coacervates where animal (e.g., gelatin) and milk proteins are involved. There is no overview of knowledge (review) regarding the fundamental aspects of covalent conjugation and complex coacervation of non-animal proteins and characteristics and application of the resulting conjugates and complex coacervates. The application of plant- and algal protein- based conjugates and complex coacervates as emulsifiers and encapsulants for stabilising susceptible oils has not been adequately explored. Thus, this paper presents an overview of the mechanisms of modification of plant and algal proteins via their Maillard conjugations and complex coacervations with polysaccharides. The physicochemical, emulsifying and encapsulating properties of the resulting conjugates and complex coacervate will be compared with those of the unmodified proteins.

2. Relationship between proteins' composition and structure and digestibility

The main difference among the animal, plant and algal proteins is presented in Table 1, focusing on the composition, essential amino acid content, structure and digestibility. Proteins from different sources come with unique structures, which can be attributed to distinct polypeptide sequences, the nature of protein fractions and the secondary and higher order structure.³⁴ The proteins derived from plant and algae sources exhibit a domination of globular conformation, with hydrophobic regions buried within the proteins' interior while hydrophilic regions are positioned on the exterior.¹⁸ The diversity and difference observed in the globular structures of proteins are determined by the nature of protein fractions and the configuration of secondary and tertiary structures.³⁴ The nature of the globular structure and nature of the protein fractions exert significant impact on the technofunctional properties of the proteins. In addition to the protein structure, the nutritional value of a dietary protein source is also an important factor to consider when assessing its application. Essential amino acids (EAAs) are a group of amino acids that cannot be synthesized by the human body and must be acquired through diet. Consequently, proteins rich in EAAs can be classified as ‘complete’ proteins, and the content of EAAs is used to evaluate protein quality.³⁵ The EAA content of plant and algal proteins is comparatively lower when compared to milk proteins. However, some plant and algal proteins such as soy, pea, lupin, flaxseed, sunflower, Spirulina and Chlorella vulgaris can meet the WHO/FAO/UNU requirements of essential amino acids.³⁶

---

The digestibility of proteins is also an importance factor in determining the nutritional value, as it governs the extent to which they can be digested and peptides and amino acids can be absorbed by the human body.³⁴ The Digestible Indispensable Amino Acid Score (DIAAS) is commonly used to evaluate the bioavailability of amino acids, and a higher DIAAS value indicates a better digestibility of protein. Milk proteins exhibit superior digestibility, characterised by relatively higher DIAAS values compared to plant and algal proteins (Table 1). Recent studies by Zeng et al.³⁷ showed that, based on the DIAAS values, algal proteins show better digestibility compared to wheat and bean proteins. Although there is a lack of published data on the DIAAS value of Spirulina protein due to limited adoption of this method, their relatively high digestibility is shown through the protein digestibility corrected amino acid score (PDCAAS). Notably, Spirulina protein exhibits a higher PDCAAS value compared to pulse proteins.³⁸

3. Maillard reaction driven conjugation and electrostatic charge driven complex coacervation between proteins and polysaccharides

The modification of proteins through these two methods is aimed to improve the techno-functional properties of proteins and expand their applications in the food industry.²⁵ The covalent bonding and electrostatic interaction between proteins and polysaccharides have been extensively investigated for improving the functional properties of proteins, as both methods enable modification of proteins' structure and function.⁵⁶

3.1 Maillard reaction between proteins and polysaccharides

3.2 Mechanism of complex coacervation

The non-covalent interaction between proteins and polysaccharides occurs through various mechanisms, as illustrated in Fig. 2. Briefly, the repulsive or attractive interactions between proteins and polysaccharides can be driven by multiple forces, including electrostatic and hydrophobic forces, hydrogen bonding, and steric exclusion.⁷²

The interaction between a protein and a polysaccharide in their mixture is affected by their characteristics (e.g., molecular structure, charge density and distribution) and environmental factors (pH, ionic strength, temperature, biopolymer concentration and ratio, and pressure).⁶⁵ At a relatively low polymer concentration, proteins and polysaccharides may co-solubilise in a single phase (Fig. 2). However, thermodynamic incompatibility between proteins and polysaccharides arises as the biopolymer concentration increases. It causes segregative phase separation into one phase enriched with proteins and the other with polysaccharides (Fig. 2).

In contrast, when proteins and polysaccharides carry opposite charges, their electrostatic attraction results in associative phase separation (complex coacervation). The aqueous phase is separated into a lower phase enriched with biopolymers and an upper phase enriched with water (Fig. 2). When the pH value of the system is adjusted to a value slightly below the protein isoelectric point (pI) but above the acidic dissociation constant (pK_a) of polysaccharides,⁷³ the “co-soluble one phase” and “incompatible two phases” in Fig. 2 can transform into soluble complexes. In this scenario, the initially negatively charged protein molecules become positively charged. It results in relatively weak electrostatic interactions between proteins and anionic polysaccharides and soluble protein–polysaccharide complexes are formed. With the progression of complex coacervation and subsequent pH reduction, more positively charged protein molecules are associated with negatively charged polysaccharides. It leads to electrical equivalence between biopolymers at a specific pH value. At this point, electrostatically neutral insoluble complexes are formed through strong electrostatic interactions and exhibit the lowest overall charge.⁷² The precipitation of insoluble complexes creates a dispersed complex coacervate phase and significantly affects turbidity.⁷⁴ This is the stage at which “insoluble complexes” are formed. If the pH is further reduced, the complexes can dissociate due to the extensive protonation of reactive groups along the polysaccharide backbone.⁷⁵

4. Maillard reaction between proteins and polysaccharides

Maillard reaction induced conjugation affects the structure of proteins and the resulting conjugates usually exhibit better techno-functional properties, making them highly effective emulsifiers and encapsulating materials.⁷⁶ The Maillard reaction between proteins and polysaccharides is influenced by various factors, including the nature of the reacting protein and polysaccharide, their mixing ratios, and reaction conditions such as heating methods, temperature, pH and reaction time.⁷⁷ The combined effect of these factors determines the final techno-functional properties of these conjugates as shown by examples listed in Table 2 and Fig. 3a.

---

4.1 Determination of protein–polysaccharide conjugates after the Maillard reaction

4.2 The effects of the Maillard reaction on the functional properties of proteins

The functional properties of proteins play an important role in the characteristics of food products in which they are part of. These properties can be modified through the Maillard reaction.⁵⁷ Solubility, emulsifying capacity, thermal stability, and antioxidative potential are the most important functional properties of proteins as well as their conjugates.^78,92

5. The effects of complex coacervation on the physicochemical properties of proteins

5.1 Optimisation of the complex coacervation process

As mentioned in Section 3.2, complex coacervation occurs between oppositely charged proteins and polysaccharides. The interaction is driven by electrostatic attraction to form soluble and insoluble complexes. The extent of complex coacervation is affected by several intrinsic factors, such as pH, mixing ratio, and ionic strength. These factors ultimately play an important role in the formation, structure, stability and rheological properties of the coacervates.^31,100 The complex coacervation process is optimised using the zeta-potential and turbidity data of the protein–polysaccharide mixture as a function of pH and the biopolymer mixing ratio (Table 3).

---

The pH of the solution is the most significant factor in the complex coacervation process, as it determines the degree of ionisation of functional groups (amino and carboxyl groups) present in biopolymers. Ultimately, their zeta-potential values of proteins, polysaccharides and their mixture as a function of pH can serve as indicators at what pH the formation of complex coacervates is at its peak. Complex coacervation occurs when the solution pH value decreases below the isoelectric point (pI) of proteins and above the acidic dissociation constant (pK_a) of polysaccharides. The optimal complex coacervation is achieved when the complex becomes electrically neutral. The pH value corresponding to this state is pH_opt.^105,106 During complex coacervation, the zeta-potential of the protein–polysaccharide mixture is monitored by carefully adjusting its pH values until pH_opt is detected.²⁵ Besides the zeta-potential of the mixture, its turbidity is another indicator to evaluate the extent of complex coacervation. The turbidity value of the mixture increases when insoluble complexes are formed during pH adjustment. Published literature on this theme reports that turbidity keeps on increasing as the pH value moves closer to the pH_opt and is highest at this pH.^103,110

The equilibrium of the electrostatic charge between proteins and polysaccharides also depends on their mixing ratio. Some examples of the optimum pH and biopolymer mixing ratio used to produce protein–polysaccharide complex coacervates are presented in Table 3.

Klemmer et al.¹⁰³ studied the effect of the protein-to-polysaccharide ratio on complex coacervation between pea protein isolate (PPI) and alginate (AL). The pH_opt for complex coacervation ranged from 2.1 to 3.3 with the increased PPI-to-AL ratio from 1:1 to 20:1. In Zhang et al.'s⁹⁹ work, a similar trend was observed when a Spirulina protein–maltodextrin mixture was complexed with carrageenan. An increase of the protein component in the system caused excess positively charged molecules. The excessive protein interacts with the relatively low polysaccharide content at higher mixing ratios and facilitates complex precipitation at relatively high pH values.¹⁰⁴ In other words, achieving the charge neutralisation becomes easier at higher pH values due to the relatively lower presence of negatively charged groups that need to be neutralised by the positively charged groups.¹¹¹

Apart from the pH and biopolymer ratio, the ionic strength of the solution also affects the complex coacervation process via charge screening effects.⁷⁴ The aqueous phase with high ionic strength usually hinders the formation of complex coacervates between proteins and polysaccharides, which ultimately reduces yield and turbidity. However, low ionic strength sometimes promotes the formation of complex coacervates. Zhang et al.¹⁰² investigated the effect of ionic strength on the complex coacervation between soy protein isolate and Flammulina velutipes polysaccharide. The authors reported that the complex coacervation process was accelerated in the presence of 10 and 50 mM NaCl. A further increase of NaCl concentration inhibited the interaction between polymers. The structure of the resulting complex coacervates was more robust and compact in the presence of NaCl.¹⁰²

5.2 Characteristics of complex coacervates

6. Application of protein–polysaccharide conjugates and complex coacervates to encapsulate oxygen-sensitive oils

Several methods or processes are used to microencapsulate unstable and oxygen-sensitive oils including spray and freeze drying, and fluidized bed coating using coacervates and conjugates as shell materials.¹¹⁹ The encapsulation efficiency of encapsulated oil can be significantly influenced by the properties of the wall material used.¹²⁰ This section briefly outlines the application of protein–polysaccharide conjugates, their complex coacervates, and conjugate-based coacervates as wall materials for encapsulating susceptible oils (Table 4).

---

6.1 Microencapsulation of susceptible oils using protein–polysaccharide conjugates

Proteins are widely used to stabilise susceptible oils by the contemporary food industry. However, proteins as encapsulating shell materials are susceptible to acidic pH near the proteins' isoelectric point, high temperature, and ionic strength. As in Section 4.5, protein–polysaccharide conjugates formed via the Maillard reaction are promising emulsifiers as they possess higher solubility, emulsifying properties and thermal stability. When a conjugate is used as an emulsifier, its protein component rapidly adsorbs at the oil–water interface to stabilise oil and the polysaccharide component provides strong steric repulsion and prevents the coalescence of oil droplets effectively.¹³⁵ Moreover, the antioxidant activity of the conjugate provides additional protection to the susceptible oils. The presence of a carbohydrate as a component of the wall matrix also lessens the drying-related stress to the protein and improves the powder properties.¹³⁶

The efficiency and effectiveness of various protein–polysaccharide conjugates to stabilise susceptible oils are a well-research theme.^137–139 Zhang et al.¹⁴⁰ produced a hydrolysed soy protein isolate–maltodextrin conjugate and used it to stabilise fish oil via emulsion evaporation technology. The authors observed that the microcapsules stabilised with a soy protein isolate–maltodextrin conjugate had lower surface oil and higher encapsulation efficiency than the microcapsules produced using their mixture. This improvement in the microencapsulation efficiency was attributed to the better emulsifying properties of the conjugate. The conjugates effectively decreased the O/W interfacial tension and help produce droplets with smaller size.¹⁴⁰ The conjugate stabilised microcapsule also exhibited improved oxidative stability compared to the one with the mixture as the wall material, which was attributed to the improved antioxidant acidity and emulsifying properties of the conjugate. A similar trend was observed by Wang et al.¹⁴¹ when hemp seed oil microcapsules were prepared with a soy protein isolate–maltodextrin conjugate. The encapsulation efficiency of the microcapsule was shown to increase with the increase of the glycation degree.

The microcapsules stabilised by protein–polysaccharide conjugates have shown improved ability to protect the encapsulated compounds from premature release in a harsh gastric environment and deliver them to the intestinal stage of digestion. Yang et al.¹³⁹ investigated the digestibility of a citral microcapsule in a simulated gastrointestinal environment with a soy protein-soy polysaccharide conjugate as the wall material. Compared with the microcapsule stabilised by a protein–polysaccharide mixture, the one stabilised by the conjugate exhibited increased encapsulation efficiency and enhanced resistance against gastric digestion.^142,143 Covalent bonding between proteins and polysaccharides partially masked the pepsin binding sites of proteins and slowed down their access.¹³⁹

6.2 Microencapsulation of susceptible oils using protein–polysaccharide complex coacervates

Protein–polysaccharide complex coacervates are effective encapsulants of susceptible oils as they have a stronger protective effect compared to individual proteins or polysaccharides.¹⁴⁴ In general, the microencapsulation process through complex coacervation commences with the formation of an oil-in-water (O/W) emulsion using a protein or protein–polysaccharide mixture as the emulsifier, depending on whether the in situ or the ex situ complex coacervation method is used (Section 4.2.4). Subsequently, the polysaccharide is added to the emulsion (in situ method), followed by pH adjustment to induce complex coacervation and facilitate the migration of coacervates on the surface of oil droplets.³¹

The microcapsules produced using complex coacervates as wall materials have desirable features including a compact structure, high oil loading and high encapsulation efficiency improved controlled release properties (Table 4).^122–125 For example, Timilsena et al.¹²¹ encapsulated chia seed oil using chia seed protein-chia seed gum complex coacervates as the wall material. Compared with the microcapsule produced using chia protein, the one produced using the coacervate as the wall material showed much higher encapsulation efficiency (93.9%) and smaller particle size. In Lan et al.'s¹²³ study, hemp seed oil was encapsulated in pea protein isolate-sugar beet pectin complex coacervates. The authors achieved higher oil loading as well as higher encapsulation efficiency. RiosMera et al.¹²⁶ crosslinked soy protein isolate–inulin complex coacervates with transglutaminase to stabilise fish oil. Crosslinking was implemented to make the complex coacervate compact and robust. Compared with the non-crosslinked microcapsules, the transglutaminase crosslinked ones showed improved oil retention even at 100 °C. It was because the crosslinking significantly enhanced the integrity of the complex coacervate wall material surrounding the oil.¹³¹

Compared with unmodified proteins as the wall material at the oil/water interface, those made with complex coacervates are usually more rigid and less porous. Thus, complex coacervates are more effective in protecting the encapsulated oil against environmental stresses.¹⁴⁵ Kaushik et al.¹²⁴ produced flaxseed oil microcapsules stabilised in flaxseed protein-flaxseed gum complex coacervates. The authors reported an improved oxidative stability of the encapsulated oil in terms of reduced peroxidative values during a 30 day storage. Similarly, mung bean protein isolate-apricot peel pectin complex coacervates were used to encapsulate rose essential oil.¹²⁸ These microcapsules exhibited remarkable stability during in vitro oral and gastric digestion. The steric hindrance provided by the polysaccharide component in the coacervates inhibited pepsin's access to the protein component of the wall matrix and helped preserve the integrity, especially in the gastric stage. The encapsulated rose essential oil was successfully delivered to the targeted (gastric) stage of digestion and achieved the desired targeted delivery.

6.3 Microencapsulation of susceptible oils by using conjugate-based coacervates

A small number of publications recently applied covalent conjugation followed by complex coacervation to encapsulate oxygen-sensitive and unstable oils. In this approach, a protein–polysaccharide (i.e., polysaccharide 1) conjugate is produced first. This conjugate is further coacervated with different polysaccharides (i.e., polysaccharide 2). Here, the protein is modified twice, first by conjugation with polysaccharide 1 and then again using polysaccharide (2). The Maillard reaction is used to achieve conjugation. In some cases, complex coacervation is carried out between a protein and a polysaccharide and then heat treated to induce conjugation. In Huang et al.'s¹³³ study, vitamin E was encapsulated in soy protein isolate–chitosan complex coacervates followed by thermal treatment of microcapsules over 50 °C for 12 hours to induce the Maillard reaction between the protein and polysaccharide. Compared to the microcapsule produced using complex coacervates as the wall material, the ones produced using conjugated complex coacervates showed significantly higher storage stability. Muhoza et al.¹³¹ prepared gelatin-low methoxyl pectin conjugates via the Maillard reaction and used these conjugates to stabilise cinnamaldehyde. Then, the authors adjusted the pH to induce complex coacervation between the protein and polysaccharide components in the conjugate. Compared with microcapsules produced using conjugates as the wall material, the ones stabilised by conjugate-based complex coacervates showed higher encapsulation efficiency and stability against oxidation. The study conducted by Ifeduba and Akoh¹²⁹ also showed a similar trend in which stearidonic acid soybean oil was encapsulated in gelatin-gum Arabic conjugate-based complex coacervates. The authors reported that the formation of a secondary layer on the droplet surface through polysaccharide adsorption significantly enhanced the steric repulsion and improved the oxidative stability of the microcapsule.

Zhang et al.⁹⁹ developed a (protein–carbohydrate (1) conjugate)-polysaccharide (2) complex coacervate as a novel wall material for microencapsulation purposes. The schematic diagram illustrating the preparation of the microcapsule is presented in Fig. 5. Briefly, Spirulina protein and maltodextrin were conjugated via a controlled Maillard reaction in the initial reaction stage. The resulting conjugate was used as an emulsifier to stabilise canola oil in an O/W emulsion. Carrageenan gum was then added to the aqueous phase and the pH of the mixture was adjusted to induce complex coacervation. Finally, microcapsule powders were obtained through spray-drying. The authors reported that the microcapsules produced using the (protein–maltodextrin conjugate)-carrageenan complex coacervate exhibited enhanced encapsulation efficiency and oxidative stability in comparison to the ones produced using the (protein–maltodextrin mixture)-carrageenan complex coacervate as the wall material. The improved encapsulation efficiency was due to the improved emulsifying properties and antioxidant activity of the conjugate. The conjugate also helped form a rigid wall material, inhibiting oxygen access to the encapsulated oil.⁹⁹ These conjugate-based coacervate delivery system showed notable resistance against digestion in the in vitro oral and gastric environment, evidenced by limited protein hydrolysis and oil release. It was primarily attributed to the covalent bonding between the protein and carbohydrates in the conjugate and the formation of the robust microcapsule walls during the complex coacervation process.¹³⁴ Ultimately, the stabilised canola oil was released and hydrolysed in the simulated intestinal digestion stage achieving more efficient targeted delivery of the oil.

7. Concluding remarks and prospects

The modification of non-animal proteins, especially plant and algal ones, is gaining increasing research interest due to the need to improve their functional properties. Covalent bonding (i.e., conjugation) and non-covalent complexation that can occur between proteins and polysaccharides can be used to alter the structure of proteins and enhance their techno-functional properties. The resulting conjugates and complex coacervates can be preferably used as emulsifiers and encapsulating shell materials of unstable and oxygen-sensitive oils. This review presents the underpinning science and associated technologies based on the Maillard reaction and complex coacervation whereby they are used to modify and improve the physicochemical properties. The protein–polysaccharide conjugates formed through the Maillard reaction are able to improve the solubility, thermal stability, antioxidant activity and emulsifying properties of these proteins. The protein–polysaccharide complex coacervates are also quite effective as emulsifiers and encapsulating shell materials. Several recent studies reported that the covalent conjugation followed by complex coacervation or complex coacervation followed by covalent conjugation is a much more effective approach to create protein–polysaccharide based emulsifiers and encapsulating shell materials. These conjugate-based complex coacervates are much more effective in encapsulating unstable and oxygen-sensitive oils. However, there is still a paucity of information, particularly on how to control and optimise conjugation and complex coacervation processes. The wet route of carrying out Maillard reaction-based conjugation is attractive; however, it is important to precisely control the temperature and time of the reaction to prevent it from progressing to the advanced stage. These parameters depend on the nature of the protein and polysaccharide used. The question of whether conjugation should precede complex coacervation or vice versa, i.e., the optimal sequence, remains unresolved. The strength or tendency of the conjugated protein to electrostatically bind with a second polysaccharide remains unknown. The digestion or breakdown of conjugate-based complex coacervates under digestion conditions is still poorly understood. Most importantly, these approaches are not sufficiently applied to algal proteins.

Author contributions

Zijia Zhang has drafted this manuscript with input from and supervision of Benu Adhikari, Bo Wang, and Jie Chen. All authors provided critical feedback, wrote some parts, and contributed to the final version of this manuscript.

Conflicts of interest

The authors declare that there is no conflict of interests.

Acknowledgements

The authors declare that they received no funding for this review article.

References

1. M. Akhtar and R. Ding, Covalently cross-linked proteins & polysaccharides: formation, characterisation and potential applications, Curr. Opin. Colloid Interface Sci., 2017, 28, 31–36,  DOI:10.1016/j.cocis.2017.01.002.
2. L. Day, Proteins from land plants – Potential resources for human nutrition and food security, Trends Food Sci. Technol., 2013, 32, 25–42,  DOI:10.1016/j.tifs.2013.05.005.
3. M. Hadidi, S. Jafarzadeh, M. Forough, F. Garavand, S. Alizadeh, A. Salehabadi, A. M. Khaneghah and S. M. Jafari, Plant protein-based food packaging films; recent advances in fabrication, characterization, and applications, Trends Food Sci. Technol., 2022, 120, 154–173,  DOI:10.1016/j.tifs.2022.01.013.
4. B. H. Sarmadi and A. Ismail, Antioxidative peptides from food proteins: a review, Peptides, 2010, 31, 1949–1956,  DOI:10.1016/j.peptides.2010.06.020.
5. M. Evans, I. Ratcliffe and P. A. Williams, Emulsion stabilisation using polysaccharide–protein complexes, Curr. Opin. Colloid Interface Sci., 2013, 18, 272–282,  DOI:10.1016/j.cocis.2013.04.004.
6. Y. Wang, W. Liu, X. D. Chen and C. Selomulya, Micro-encapsulation and stabilization of DHA containing fish oil in protein-based emulsion through mono-disperse droplet spray dryer, J. Food Eng., 2016, 175, 74–84,  DOI:10.1016/j.jfoodeng.2015.12.007.
7. L. G. Gómez-Mascaraque and A. López-Rubio, Protein-based emulsion electrosprayed micro- and submicroparticles for the encapsulation and stabilization of thermosensitive hydrophobic bioactives, J. Colloid Interface Sci., 2016, 465, 259–270,  DOI:10.1016/j.jcis.2015.11.061.
8. J. Aschemann-Witzel, R. F. Gantriis, P. Fraga and F. J. A. Perez-Cueto, Plant-based food and protein trend from a business perspective: markets, consumers, and the challenges and opportunities in the future, Crit. Rev. Food Sci. Nutr., 2021, 61, 3119–3128,  DOI:10.1080/10408398.2020.1793730.
9. M. Henchion, M. Hayes, A. M. Mullen, M. Fenelon and B. Tiwari, Future Protein Supply and Demand: Strategies and Factors Influencing a Sustainable Equilibrium, Foods, 2017, 6, 53,  DOI:10.3390/foods6070053.
10. T. C. Pimentel, W. K. A. da Costa, C. E. Barão, M. Rosset and M. Magnani, Vegan probiotic products: a modern tendency or the newest challenge in functional foods, Food Res. Int., 2021, 140, 110033,  DOI:10.1016/j.foodres.2020.110033.
11. S. R. Hertzler, J. C. Lieblein-Boff, M. Weiler and C. Allgeier, Plant Proteins: Assessing Their Nutritional Quality and Effects on Health and Physical Function, Nutrients, 2020, 12, 3704,  DOI:10.3390/nu12123704.
12. K. Sakadevan and M.-L. Nguyen, Chapter four – livestock production and its impact on nutrient pollution and greenhouse gas emissions, in Advances in Agronomy [Internet], ed. Sparks D. L., Academic Press, 2017, pp. 147–184, available from, https://www.sciencedirect.com/science/article/pii/S0065211316301080 Search PubMed.
13. A. Vainio, M. Niva, P. Jallinoja and T. Latvala, From beef to beans: eating motives and the replacement of animal proteins with plant proteins among Finnish consumers, Appetite, 2016, 106, 92–100,  DOI:10.1016/j.appet.2016.03.002.
14. F. Boukid, C. M. Rosell, S. Rosene, S. Bover-Cid and M. Castellari, Non-animal proteins as cutting-edge ingredients to reformulate animal-free foodstuffs: present status and future perspectives, Crit. Rev. Food Sci. Nutr., 2022, 62, 6390–6420,  DOI:10.1080/10408398.2021.1901649.
15. L. Bourassa, Vegan and Plant-Based Diet Statistics for 2023 [Internet], PlantProteins.co., 2023 [cited 2023 May 13], available from, https://www.plantproteins.co/vegan-plant-based-diet-statistics/ Search PubMed.
16. M. Hadidi, S. Boostani and S. M. Jafari, Pea proteins as emerging biopolymers for the emulsification and encapsulation of food bioactives, Food Hydrocolloids, 2022, 126, 107474,  DOI:10.1016/j.foodhyd.2021.107474.
17. X. Yan, Z. Zeng, D. J. McClements, X. Gong, P. Yu, J. Xia and D. Gong, A review of the structure, function, and application of plant-based protein–phenolic conjugates and complexes, Compr. Rev. Food Sci. Food Saf., 2023, 22, 1312–1336,  DOI:10.1111/1541-4337.13112.
18. W. Kim, Y. Wang and C. Selomulya, Dairy and plant proteins as natural food emulsifiers, Trends Food Sci. Technol., 2020, 105, 261–272,  DOI:10.1016/j.tifs.2020.09.012.
19. L. M. Sagis and J. Yang, Protein-stabilized interfaces in multiphase food: comparing structure-function relations of plant-based and animal-based proteins, Curr. Opin. Food Sci., 2022, 43, 53–60,  DOI:10.1016/j.cofs.2021.11.003.
20. P. Geada, C. Moreira, M. Silva, R. Nunes, L. Madureira, C. M. R. Rocha, R. N. Pereira, A. A. Vicente and J. A. Teixeira, Algal proteins: Production strategies and nutritional and functional properties, Bioresour. Technol., 2021, 332, 125125,  DOI:10.1016/j.biortech.2021.125125.
21. C. Mellor, R. Embling, L. Neilson, T. Randall, C. Wakeham, M. D. Lee and L. L. Wilkinson, Consumer Knowledge and Acceptance of “Algae” as a Protein Alternative: A UK-Based Qualitative Study, Foods, 2022, 11, 1703,  DOI:10.3390/foods11121703.
22. A. Schwenzfeier, P. A. Wierenga and H. Gruppen, Isolation and characterization of soluble protein from the green microalgae Tetraselmis sp, Bioresour. Technol., 2011, 102, 9121–9127,  DOI:10.1016/j.biortech.2011.07.046.
23. I. S. Chronakis, Gelation of Edible Blue-Green Algae Protein Isolate (Spirulina platensis Strain Pacifica): Thermal Transitions, Rheological Properties, and Molecular Forces Involved, J. Agric. Food Chem., 2001, 49, 888–898,  DOI:10.1021/jf0005059.
24. Q. Du, L. Zhou, F. Lyu, J. Liu and Y. Ding, The complex of whey protein and pectin: Interactions, functional properties and applications in food colloidal systems – A review, Colloids Surf., B, 2022, 210, 112253,  DOI:10.1016/j.colsurfb.2021.112253.
25. B. Muhoza, B. Qi, J. D. Harindintwali, M. Y. Farag Koko, S. Zhang and Y. Li, Combined plant protein modification and complex coacervation as a sustainable strategy to produce coacervates encapsulating bioactives, Food Hydrocolloids, 2022, 124, 107239,  DOI:10.1016/j.foodhyd.2021.107239.
26. E. Dickinson, Interfacial structure and stability of food emulsions as affected by protein –polysaccharide interactions, Soft Matter, 2008, 4, 932–942,  10.1039/B718319D.
27. C. Ke and L. Li, Influence mechanism of polysaccharides induced Maillard reaction on plant proteins structure and functional properties: A review, Carbohydr. Polym., 2023, 302, 120430,  DOI:10.1016/j.carbpol.2022.120430.
28. W. Chen, R. Lv, W. Wang, X. Ma, A. I. Muhammad, M. Guo, X. Ye and D. Liu, Time effect on structural and functional properties of whey protein isolate-gum acacia conjugates prepared via Maillard reaction, J. Sci. Food Agric., 2019, 99, 4801–4807,  DOI:10.1002/jsfa.9735.
29. L.-H. Wang, X. Sun, G.-Q. Huang and J.-X. Xiao, Conjugation of soybean protein isolate with xylose/fructose through wet-heating Maillard reaction, Food Meas., 2018, 12, 2718–2724,  DOI:10.1007/s11694-018-9889-y.
30. L. Zhong, N. Ma, Y. Wu, L. Zhao, G. Ma, F. Pei and Q. Hu, Characterization and functional evaluation of oat protein isolate-Pleurotus ostreatus β-glucan conjugates formed via Maillard reaction, Food Hydrocolloids, 2019, 87, 459–469,  DOI:10.1016/j.foodhyd.2018.08.034.
31. N. Eghbal and R. Choudhary, Complex coacervation: Encapsulation and controlled release of active agents in food systems, LWT, 2018, 90, 254–264,  DOI:10.1016/j.lwt.2017.12.036.
32. A. M. Bakry, S. Abbas, B. Ali, H. Majeed, M. Y. Abouelwafa, A. Mousa and L. Liang, Microencapsulation of Oils: A Comprehensive Review of Benefits, Techniques, and Applications, Compr. Rev. Food Sci. Food Saf., 2016, 15, 143–182,  DOI:10.1111/1541-4337.12179.
33. A. K. Anal and H. Singh, Recent advances in microencapsulation of probiotics for industrial applications and targeted delivery, Trends Food Sci. Technol., 2007, 18, 240–251,  DOI:10.1016/j.tifs.2007.01.004.
34. L. Day, J. A. Cakebread and S. M. Loveday, Food proteins from animals and plants: differences in the nutritional and functional properties, Trends Food Sci. Technol., 2022, 119, 428–442,  DOI:10.1016/j.tifs.2021.12.020.
35. J.-A. Gilbert, N. T. Bendsen, A. Tremblay and A. Astrup, Effect of proteins from different sources on body composition, Nutr., Metab. Cardiovasc. Dis., 2011, 21, B16–B31,  DOI:10.1016/j.numecd.2010.12.008.
36. D. J. Millward, Identifying recommended dietary allowances for protein and amino acids: a critique of the 2007 WHO/FAO/UNU report, Br. J. Nutr., 2012, 108, S3–S21,  DOI:10.1017/S0007114512002450.
37. Y. Zeng, E. Chen, X. Zhang, D. Li, Q. Wang and Y. Sun, Nutritional Value and Physicochemical Characteristics of Alternative Protein for Meat and Dairy—A Review, Foods, 2022, 11, 3326,  DOI:10.3390/foods11213326.
38. Y. Wang, S. M. Tibbetts and P. J. McGinn, Microalgae as Sources of High-Quality Protein for Human Food and Protein Supplements, Foods, 2021, 10, 3002,  DOI:10.3390/foods10123002.
39. F. Guyomarc’h, M.-H. Famelart, G. Henry, M. Gulzar, J. Leonil, P. Hamon, S. Bouhallab and T. Croguennec, Current ways to modify the structure of whey proteins for specific functionalities—a review, Dairy Sci. Technol., 2015, 95, 795–814,  DOI:10.1007/s13594-014-0190-5.
40. D. S. Kalman, Amino Acid Composition of an Organic Brown Rice Protein Concentrate and Isolate Compared to Soy and Whey Concentrates and Isolates, Foods, 2014, 3, 394–402,  DOI:10.3390/foods3030394.
41. B. Hewa Nadugala, C. N. Pagel, J. K. Raynes, C. S. Ranadheera and A. Logan, The effect of casein genetic variants, glycosylation and phosphorylation on bovine milk protein structure, technological properties, nutrition and product manufacture, Int. Dairy J., 2022, 133, 105440,  DOI:10.1016/j.idairyj.2022.105440.
42. G. Qi, K. Venkateshan, X. Mo, L. Zhang and X. S. Sun, Physicochemical Properties of Soy Protein: Effects of Subunit Composition, J. Agric. Food Chem., 2011, 59, 9958–9964,  DOI:10.1021/jf201077b.
43. A. C. Y. Lam, A. Can Karaca, R. T. Tyler and M. T. Nickerson, Pea protein isolates: structure, extraction, and functionality, Food Rev. Int., 2018, 34, 126–147,  DOI:10.1080/87559129.2016.1242135.
44. S. H. M. Gorissen, J. J. R. Crombag, J. M. G. Senden, W. A. H. Waterval, J. Bierau, L. B. Verdijk and L. J. C. van Loon, Protein content and amino acid composition of commercially available plant-based protein isolates, Amino Acids, 2018, 50, 1685–1695,  DOI:10.1007/s00726-018-2640-5.
45. S. Shrestha, H. L van't, V. S. Haritos and S. Dhital, Lupin proteins: Structure, isolation and application, Trends Food Sci. Technol., 2021, 116, 928–939,  DOI:10.1016/j.tifs.2021.08.035.
46. C. Burgos-Díaz, M. Opazo-Navarrete, T. Wandersleben, M. Soto-Añual, T. Barahona and M. Bustamante, Chemical and Nutritional Evaluation of Protein-Rich Ingredients Obtained through a Technological Process from Yellow Lupin Seeds (Lupinus luteus), Plant Foods Hum. Nutr., 2019, 74, 508–517,  DOI:10.1007/s11130-019-00768-0.
47. B. Wang, D. Li, L.-J. Wang, B. Adhikari and J. Shi, Ability of flaxseed and soybean protein concentrates to stabilize oil-in-water emulsions, J. Food Eng., 2010, 100, 417–426,  DOI:10.1016/j.jfoodeng.2010.04.026.
48. P. Kaushik, K. Dowling, S. McKnight, C. J. Barrow, B. Wang and B. Adhikari, Preparation, characterization and functional properties of flax seed protein isolate, Food Chem., 2016, 197, 212–220,  DOI:10.1016/j.foodchem.2015.09.106.
49. M. Sharma and C. S. Saini, Amino acid composition, nutritional profiling, mineral content and physicochemical properties of protein isolate from flaxseeds (Linum usitatissimum), Food Meas., 2022, 16, 829–839,  DOI:10.1007/s11694-021-01221-0.
50. E.-K. A. Taha, S. Al-Kahtani and R. Taha, Protein content and amino acids composition of bee-pollens from major floral sources in Al-Ahsa, eastern Saudi Arabia, Saudi J. Biol. Sci., 2019, 26, 232–237,  DOI:10.1016/j.sjbs.2017.06.003.
51. C. P. F. Marinangeli and J. D. House, Potential impact of the digestible indispensable amino acid score as a measure of protein quality on dietary regulations and health, Nutr. Rev., 2017, 75, 658–667,  DOI:10.1093/nutrit/nux025.
52. S. Barak, D. Mudgil and B. S. Khatkar, Biochemical and Functional Properties of Wheat Gliadins: A Review, Crit. Rev. Food Sci. Nutr., 2015, 55, 357–368,  DOI:10.1080/10408398.2012.654863.
53. A. L. Lupatini, L. M. Colla, C. Canan and E. Colla, Potential application of microalga Spirulina platensis as a protein source, J. Sci. Food Agric., 2017, 97, 724–732,  DOI:10.1002/jsfa.7987.
54. A.-V. Ursu, A. Marcati, T. Sayd, V. Sante-Lhoutellier, G. Djelveh and P. Michaud, Extraction, fractionation and functional properties of proteins from the microalgae Chlorella vulgaris, Bioresour. Technol., 2014, 157, 134–139,  DOI:10.1016/j.biortech.2014.01.071.
55. W. M. Qazi, S. Ballance, K. Kousoulaki, A. K. Uhlen, D. M. M. Kleinegris, K. Skjånes and A. Rieder, Protein Enrichment of Wheat Bread with Microalgae: Microchloropsis gaditana, Tetraselmis chui and Chlorella vulgaris, Foods, 2021, 10, 3078,  DOI:10.3390/foods10123078.
56. X. Kan, G. Chen, W. Zhou and X. Zeng, Application of protein-polysaccharide Maillard conjugates as emulsifiers: Source, preparation and functional properties, Food Res. Int., 2021, 150, 110740,  DOI:10.1016/j.foodres.2021.110740.
57. F. C. de Oliveira, R. Coimbra JS dos, E. B. de Oliveira, A. D. G. Zuñiga and E. E. G. Rojas, Food Protein-polysaccharide Conjugates Obtained via the Maillard Reaction: A Review, Crit. Rev. Food Sci. Nutr., 2016, 56, 1108–1125,  DOI:10.1080/10408398.2012.755669.
58. S. I. F. S. Martins, W. M. F. Jongen and M. A. J. S. van Boekel, A review of Maillard reaction in food and implications to kinetic modelling, Trends Food Sci. Technol., 2000, 11, 364–373,  DOI:10.1016/S0924-2244(01)00022-X.
59. J. E. Hodge, Chemistry of browning reactions in model systems, J. Agric. Food Chem., 1953, 46, 2599–2600 Search PubMed.
60. A. P. Echavarría, J. Pagán and A. Ibarz, Melanoidins Formed by Maillard Reaction in Food and Their Biological Activity, Food Eng. Rev., 2012, 4, 203–223,  DOI:10.1007/s12393-012-9057-9.
61. J. Liu, Q. Ru and Y. Ding, Glycation a promising method for food protein modification: Physicochemical properties and structure, a review, Food Res. Int., 2012, 49, 170–183,  DOI:10.1016/j.foodres.2012.07.034.
62. A. Shakoor, C. Zhang, J. Xie and X. Yang, Maillard reaction chemistry in formation of critical intermediates and flavour compounds and their antioxidant properties, Food Chem., 2022, 393, 133416,  DOI:10.1016/j.foodchem.2022.133416.
63. F. J. Tessier and I. Birlouez-Aragon, Health effects of dietary Maillard reaction products: the results of ICARE and other studies, Amino Acids, 2012, 42, 1119–1131,  DOI:10.1007/s00726-010-0776-z.
64. R. R. Naik, Y. Wang and C. Selomulya, Improvements of plant protein functionalities by Maillard conjugation and Maillard reaction products, Crit. Rev. Food Sci. Nutr., 2022, 62, 7036–7061,  DOI:10.1080/10408398.2021.1910139.
65. Q. Zhang, Y. Zhou, W. Yue, W. Qin, H. Dong and T. Vasanthan, Nanostructures of protein-polysaccharide complexes or conjugates for encapsulation of bioactive compounds, Trends Food Sci. Technol., 2021, 109, 169–196,  DOI:10.1016/j.tifs.2021.01.026.
66. Q. Liu, B. Kong, J. Han, C. Sun and P. Li, Structure and antioxidant activity of whey protein isolate conjugated with glucose via the Maillard reaction under dry-heating conditions, Food Struct., 2014, 1, 145–154,  DOI:10.1016/j.foostr.2013.11.004.
67. Y. Liu, G. Zhao, M. Zhao, J. Ren and B. Yang, Improvement of functional properties of peanut protein isolate by conjugation with dextran through Maillard reaction, Food Chem., 2012, 131, 901–906,  DOI:10.1016/j.foodchem.2011.09.074.
68. F. Xue, C. Li, X. Zhu, L. Wang and S. Pan, Comparative studies on the physicochemical properties of soy protein isolate-maltodextrin and soy protein isolate-gum acacia conjugate prepared through Maillard reaction, Food Res. Int., 2013, 51, 490–495,  DOI:10.1016/j.foodres.2013.01.012.
69. Q. Zhang, L. Li, Q. Lan, M. Li, D. Wu, H. Chen, Y. Liu, D. Lin, W. Qin and Z. Zhang, et al., Protein glycosylation: a promising way to modify the functional properties and extend the application in food system, Crit. Rev. Food Sci. Nutr., 2019, 59, 2506–2533,  DOI:10.1080/10408398.2018.1507995.
70. Z. Zhang, B. Wang and B. Adhikari, Maillard reaction between pea protein isolate and maltodextrin via wet-heating route for emulsion stabilisation, Future Foods, 2022, 6, 100193,  DOI:10.1016/j.fufo.2022.100193.
71. H.-H. Chen, H. Jiang, Y. Chen, Y.-P. Qu and Y.-S. Wang, A pH-controlled curcumin-loaded emulsion stabilized by pea protein isolate-maltodextrin-epigallocatechin-3-gallate: Physicochemical properties and in vitro release properties, Colloids Surf., A, 2022, 646, 129003,  DOI:10.1016/j.colsurfa.2022.129003.
72. E. A. Cortés-Morales, G. Mendez-Montealvo and G. Velazquez, Interactions of the molecular assembly of polysaccharide-protein systems as encapsulation materials. A review, Adv. Colloid Interface Sci., 2021, 295, 102398,  DOI:10.1016/j.cis.2021.102398.
73. J. Weiss, H. Salminen, P. Moll and C. Schmitt, Use of molecular interactions and mesoscopic scale transitions to modulate protein-polysaccharide structures, Adv. Colloid Interface Sci., 2019, 271, 101987,  DOI:10.1016/j.cis.2019.07.008.
74. S. N. Warnakulasuriya and M. T. Nickerson, Review on plant protein–polysaccharide complex coacervation, and the functionality and applicability of formed complexes, J. Sci. Food Agric., 2018, 98, 5559–5571,  DOI:10.1002/jsfa.9228.
75. C. Elmer, A. C. Karaca, N. H. Low and M. T. Nickerson, Complex coacervation in pea protein isolate–chitosan mixtures, Food Res. Int., 2011, 44, 1441–1446,  DOI:10.1016/j.foodres.2011.03.011.
76. Y.-Y. Lee, T.-K. Tang, E.-T. Phuah, N. B. M. Alitheen, C.-P. Tan and O.-M. Lai, New functionalities of Maillard reaction products as emulsifiers and encapsulating agents, and the processing parameters: a brief review, J. Sci. Food Agric., 2017, 97, 1379–1385,  DOI:10.1002/jsfa.8124.
77. Y. Liu, G. Zhao, M. Zhao, J. Ren and B. Yang, Improvement of functional properties of peanut protein isolate by conjugation with dextran through Maillard reaction, Food Chem., 2012, 131, 901–906,  DOI:10.1016/j.foodchem.2011.09.074.
78. F. Zha, Z. Yang, J. Rao and B. Chen, Gum Arabic-Mediated Synthesis of Glyco-pea Protein Hydrolysate via Maillard Reaction Improves Solubility, Flavor Profile, and Functionality of Plant Protein, J. Agric. Food Chem., 2019, 67, 10195–10206,  DOI:10.1021/acs.jafc.9b04099.
79. S. Tian, J. Chen and D. M. Small, Enhancement of Solubility and Emulsifying Properties of Soy Protein Isolates by Glucose Conjugation, J. Food Process. Preserv., 2011, 35, 80–95,  DOI:10.1111/j.1745-4549.2009.00456.x.
80. L. A. Arogundade, T.-H. Mu, M. Zhang and N. M. Khan, Impact of dextran conjugation on physicochemical and gelling properties of sweet potato protein through Maillard reaction, Int. J. Food Sci. Technol., 2021, 56, 1661–1670,  DOI:10.1111/ijfs.14787.
81. X. Ma, W. Chen, T. Yan, D. Wang, F. Hou, S. Miao and D. Liu, Comparison of citrus pectin and apple pectin in conjugation with soy protein isolate (SPI) under controlled dry-heating conditions, Food Chem., 2020, 309, 125501,  DOI:10.1016/j.foodchem.2019.125501.
82. Y. Yang, S. W. Cui, J. Gong, Q. Guo, Q. Wang and Y. Hua, A soy protein-polysaccharides Maillard reaction product enhanced the physical stability of oil-in-water emulsions containing citral, Food Hydrocolloids, 2015, 48, 155–164,  DOI:10.1016/j.foodhyd.2015.02.004.
83. X. Teng, M. Zhang, B. Adhikari and K. Liu, Garlic essential oil emulsions stabilized by microwave dry-heating induced protein-pectin conjugates and their application in controlling nitrite content in prepared vegetable dishes, Food Hydrocolloids, 2023, 136, 108277,  DOI:10.1016/j.foodhyd.2022.108277.
84. L. Han, S. Zhou, X. Zhang, K. Lu, B. Qi and Y. Li, Effect of carbohydrate type on the structural and functional properties of Maillard-reacted black bean protein, J. Food Sci., 2022, 87, 165–177,  DOI:10.1111/1750-3841.15992.
85. Q. Zhang, X. Long, J. Xie, B. Xue, X. Li, J. Gan, X. Bian and T. Sun, Effect of d-galactose on physicochemical and functional properties of soy protein isolate during Maillard reaction, Food Hydrocolloids, 2022, 133, 107914,  DOI:10.1016/j.foodhyd.2022.107914.
86. X. Yan, X. Gong, Z. Zeng, D. Wan, J. Xia, M. Ma, J. Zhao, P. Wang, S. Zhang and P. Yu, et al., Changes in structure, functional properties and volatile compounds of Cinnamomum camphora seed kernel protein by Maillard reaction, Food Biosci., 2023, 53, 102628,  DOI:10.1016/j.fbio.2023.102628.
87. Z. Zhang, G. Holden, B. Wang and B. Adhikari, Maillard reaction-based conjugation of Spirulina protein with maltodextrin using wet-heating route and characterisation of conjugates, Food Chem., 2023, 406, 134931,  DOI:10.1016/j.foodchem.2022.134931.
88. S. Pirestani, A. Nasirpour, J. Keramat, S. Desobry and J. Jasniewski, Effect of glycosylation with gum Arabic by Maillard reaction in a liquid system on the emulsifying properties of canola protein isolate, Carbohydr. Polym., 2017, 157, 1620–1627,  DOI:10.1016/j.carbpol.2016.11.044.
89. J. Teng, C. Zhang, Z. Xu, X. Li and J. Xiao, Preparation and characterization of the soybean protein isolate – chitosan oligosaccharide Maillard reaction products via wet-heating, J. Food Process. Preserv., 2022, 46, e16809,  DOI:10.1111/jfpp.16809.
90. Z.-Z. Xu, G.-Q. Huang, T.-C. Xu, L.-N. Liu and J.-X. Xiao, Comparative study on the Maillard reaction of chitosan oligosaccharide and glucose with soybean protein isolate, Int. J. Biol. Macromol., 2019, 131, 601–607,  DOI:10.1016/j.ijbiomac.2019.03.101.
91. W. Jiang, Y. Zhang, D. Julian McClements, F. Liu and X. Liu, Impact of pea protein-inulin conjugates prepared via the Maillard reaction using a combination of ultrasound and pH-shift treatments on physical and oxidative stability of algae oil emulsions, Food Res. Int., 2022, 156, 111161,  DOI:10.1016/j.foodres.2022.111161.
92. S. Zhao, Y. Huang, D. J. McClements, X. Liu, P. Wang and F. Liu, Improving pea protein functionality by combining high-pressure homogenization with an ultrasound-assisted Maillard reaction, Food Hydrocolloids, 2022, 126, 107441,  DOI:10.1016/j.foodhyd.2021.107441.
93. S. P. Chawla, R. Chander and A. Sharma, Antioxidant properties of Maillard reaction products obtained by gamma-irradiation of whey proteins, Food Chem., 2009, 116, 122–128,  DOI:10.1016/j.foodchem.2009.01.097.
94. L. B. Pham, B. Wang, B. Zisu and B. Adhikari, Covalent modification of flaxseed protein isolate by phenolic compounds and the structure and functional properties of the adducts, Food Chem., 2019, 293, 463–471,  DOI:10.1016/j.foodchem.2019.04.123.
95. C. Ke and L. Li, Influence mechanism of polysaccharides induced Maillard reaction on plant proteins structure and functional properties: a review, Carbohydr. Polym., 2023, 302, 120430,  DOI:10.1016/j.carbpol.2022.120430.
96. M. Nooshkam, M. Varidi and D. K. Verma, Functional and biological properties of Maillard conjugates and their potential application in medical and food: a review, Food Res. Int., 2020, 131, 109003,  DOI:10.1016/j.foodres.2020.109003.
97. Q. Zhang, L. Li, Q. Lan, M. Li, D. Wu, H. Chen, Y. Liu, D. Lin, W. Qin and Z. Zhang, et al., Protein glycosylation: a promising way to modify the functional properties and extend the application in food system, Crit. Rev. Food Sci. Nutr., 2019, 59, 2506–2533,  DOI:10.1080/10408398.2018.1507995.
98. F. Xue, C. Li, X. Zhu, L. Wang and S. Pan, Comparative studies on the physicochemical properties of soy protein isolate-maltodextrin and soy protein isolate-gum acacia conjugate prepared through Maillard reaction, Food Res. Int., 2013, 51, 490–495,  DOI:10.1016/j.foodres.2013.01.012.
99. Z. Zhang, B. Wang, G. Holden, J. Chen and B. Adhikari, Improving functional properties of Spirulina protein by covalent conjugation followed by complex coacervation processes, Future Foods, 2023, 7, 100239,  DOI:10.1016/j.fufo.2023.100239.
100. Y. P. Timilsena, T. O. Akanbi, N. Khalid, B. Adhikari and C. J. Barrow, Complex coacervation: principles, mechanisms and applications in microencapsulation, Int. J. Biol. Macromol., 2019, 121, 1276–1286,  DOI:10.1016/j.ijbiomac.2018.10.144.
101. G.-Q. Huang, Y.-T. Sun, J.-X. Xiao and J. Yang, Complex coacervation of soybean protein isolate and chitosan, Food Chem., 2012, 135, 534–539,  DOI:10.1016/j.foodchem.2012.04.140.
102. J. Zhang, H. Du, N. Ma, L. Zhong, G. Ma, F. Pei, H. Chen and Q. Hu, Effect of ionic strength and mixing ratio on complex coacervation of soy protein isolate/Flammulina velutipes polysaccharide, Food Sci. Hum. Wellness, 2023, 12, 183–191,  DOI:10.1016/j.fshw.2022.07.006.
103. K. J. Klemmer, L. Waldner, A. Stone, N. H. Low and M. T. Nickerson, Complex coacervation of pea protein isolate and alginate polysaccharides, Food Chem., 2012, 130, 710–715,  DOI:10.1016/j.foodchem.2011.07.114.
104. Y. Lan, J.-B. Ohm, B. Chen and J. Rao, Phase behavior and complex coacervation of concentrated pea protein isolate-beet pectin solution, Food Chem., 2020, 307, 125536,  DOI:10.1016/j.foodchem.2019.125536.
105. P. G. Chang, R. Gupta, Y. P. Timilsena and B. Adhikari, Optimisation of the complex coacervation between canola protein isolate and chitosan, J. Food Eng., 2016, 191, 58–66,  DOI:10.1016/j.jfoodeng.2016.07.008.
106. Y. P. Timilsena, B. Wang, R. Adhikari and B. Adhikari, Preparation and characterization of chia seed protein isolate–chia seed gum complex coacervates, Food Hydrocolloids, 2016, 52, 554–563,  DOI:10.1016/j.foodhyd.2015.07.033.
107. B. Naderi, J. Keramat, A. Nasirpour and M. Aminifar, Complex coacervation between oak protein isolate and gum Arabic: optimization & functional characterization, Int. J. Food Prop., 2020, 23, 1854–1873,  DOI:10.1080/10942912.2020.1825484.
108. F. Plati, C. Ritzoulis, E. Pavlidou and A. Paraskevopoulou, Complex coacervate formation between hemp protein isolate and gum Arabic: Formulation and characterization, Int. J. Biol. Macromol., 2021, 182, 144–153,  DOI:10.1016/j.ijbiomac.2021.04.003.
109. X. Liu, F. Xue and B. Adhikari, Hemp protein isolate-polysaccharide complex coacervates and their application as emulsifiers in oil-in-water emulsions, Food Hydrocolloids, 2023, 137, 108352,  DOI:10.1016/j.foodhyd.2022.108352.
110. Q. Ru, Y. Wang, J. Lee, Y. Ding and Q. Huang, Turbidity and rheological properties of bovine serum albumin/pectin coacervates: Effect of salt concentration and initial protein/polysaccharide ratio, Carbohydr. Polym., 2012, 88, 838–846,  DOI:10.1016/j.carbpol.2012.01.019.
111. M. S. M. Wee, S. Nurhazwani, K. W. J. Tan, K. K. T. Goh, I. M. Sims and L. Matia-Merino, Complex coacervation of an arabinogalactan-protein extracted from the Meryta sinclarii tree (puka gum) and whey protein isolate, Food Hydrocolloids, 2014, 42, 130–138,  DOI:10.1016/j.foodhyd.2014.03.005.
112. J. Carpentier, E. Conforto, C. Chaigneau, J.-E. Vendeville and T. Maugard, Complex coacervation of pea protein isolate and tragacanth gum: comparative study with commercial polysaccharides, Innovative Food Sci. Emerging Technol., 2021, 69, 102641,  DOI:10.1016/j.ifset.2021.102641.
113. F. N. A. Aryee and M. T. Nickerson, Effect of pH, biopolymer mixing ratio and salts on the formation and stability of electrostatic complexes formed within mixtures of lentil protein isolate and anionic polysaccharides (κ-carrageenan and gellan gum), Int. J. Food Sci. Technol., 2014, 49, 65–71,  DOI:10.1111/ijfs.12275.
114. R. S. H. Lam and M. T. Nickerson, The properties of whey protein–carrageenan mixtures during the formation of electrostatic coupled biopolymer and emulsion gels, Food Res. Int., 2014, 66, 140–149,  DOI:10.1016/j.foodres.2014.08.006.
115. A. K. Stone, L. Cheung, C. Chang and M. T. Nickerson, Formation and functionality of soluble and insoluble electrostatic complexes within mixtures of canola protein isolate and (κ-, ι- and λ-type) carrageenan, Food Res. Int., 2013, 54, 195–202,  DOI:10.1016/j.foodres.2013.06.009.
116. Y. Yuan, Z.-L. Wan, X.-Q. Yang and S.-W. Yin, Associative interactions between chitosan and soy protein fractions: effects of pH, mixing ratio, heat treatment and ionic strength, Food Res. Int., 2014, 55, 207–214,  DOI:10.1016/j.foodres.2013.11.016.
117. C. Schmitt and S. L. Turgeon, Protein/polysaccharide complexes and coacervates in food systems, Adv. Colloid Interface Sci., 2011, 167, 63–70,  DOI:10.1016/j.cis.2010.10.001.
118. S. A. Vargas, R. J. Delgado-Macuil, H. Ruiz-Espinosa, M. Rojas-López and G. G. Amador-Espejo, High-intensity ultrasound pretreatment influence on whey protein isolate and its use on complex coacervation with kappa carrageenan: Evaluation of selected functional properties, Ultrason. Sonochem., 2021, 70, 105340,  DOI:10.1016/j.ultsonch.2020.105340.
119. N. V. N. Jyothi, P. M. Prasanna, S. N. Sakarkar, K. S. Prabha, P. S. Ramaiah and G. Y. Srawan, Microencapsulation techniques, factors influencing encapsulation efficiency, J. Microencapsulation, 2010, 27, 187–197,  DOI:10.3109/02652040903131301.
120. R. V. Tonon, R. B. Pedro, C. R. F. Grosso and M. D. Hubinger, Microencapsulation of Flaxseed Oil by Spray Drying: Effect of Oil Load and Type of Wall Material, Drying Technol., 2012, 30, 1491–1501,  DOI:10.1080/07373937.2012.696227.
121. Y. P. Timilsena, R. Adhikari, C. J. Barrow and B. Adhikari, Microencapsulation of chia seed oil using chia seed protein isolate⿿chia seed gum complex coacervates, Int. J. Biol. Macromol., 2016, 91, 347–357,  DOI:10.1016/j.ijbiomac.2016.05.058.
122. M. G. Bordón, A. J. Paredes, N. M. Camacho, M. C. Penci, A. González, S. D. Palma, P. D. Ribotta and M. L. Martinez, Formulation, spray-drying and physicochemical characterization of functional powders loaded with chia seed oil and prepared by complex coacervation, Powder Technol., 2021, 391, 479–493,  DOI:10.1016/j.powtec.2021.06.035.
123. Y. Lan, J.-B. Ohm, B. Chen and J. Rao, Microencapsulation of hemp seed oil by pea protein isolate−sugar beet pectin complex coacervation: Influence of coacervation pH and wall/core ratio, Food Hydrocolloids, 2021, 113, 106423,  DOI:10.1016/j.foodhyd.2020.106423.
124. P. Kaushik, K. Dowling, S. McKnight, B. ColinJ and B. Adhikari, Microencapsulation of flaxseed oil in flaxseed protein and flaxseed gum complex coacervates, Food Res. Int., 2016, 86, 1–8,  DOI:10.1016/j.foodres.2016.05.015.
125. D. Dong, Z. Qi, Y. Hua, Y. Chen, X. Kong and C. Zhang, Microencapsulation of flaxseed oil by soya proteins–gum arabic complex coacervation, Int. J. Food Sci. Technol., 2015, 50, 1785–1791,  DOI:10.1111/ijfs.12812.
126. J. D. Rios-Mera, E. Saldaña, Y. Ramírez, E. A. Auquiñivín, I. D. Alvim and C. J. Contreras-Castillo, Encapsulation optimization and pH- and temperature-stability of the complex coacervation between soy protein isolate and inulin entrapping fish oil, LWT, 2019, 116, 108555,  DOI:10.1016/j.lwt.2019.108555.
127. X. Jun-xia, Y. Hai-yan and Y. Jian, Microencapsulation of sweet orange oil by complex coacervation with soybean protein isolate/gum Arabic, Food Chem., 2011, 125, 1267–1272,  DOI:10.1016/j.foodchem.2010.10.063.
128. L. Qiu, M. Zhang, B. Adhikari and L. Chang, Microencapsulation of rose essential oil in mung bean protein isolate-apricot peel pectin complex coacervates and characterization of microcapsules, Food Hydrocolloids, 2022, 124, 107366,  DOI:10.1016/j.foodhyd.2021.107366.
129. E. A. Ifeduba and C. C. Akoh, Microencapsulation of stearidonic acid soybean oil in complex coacervates modified for enhanced stability, Food Hydrocolloids, 2015, 51, 136–145,  DOI:10.1016/j.foodhyd.2015.05.008.
130. E. A. Ifeduba and C. C. Akoh, Microencapsulation of stearidonic acid soybean oil in Maillard reaction-modified complex coacervates, Food Chem., 2016, 199, 524–532,  DOI:10.1016/j.foodchem.2015.12.011.
131. B. Muhoza, J. D. Harindintwali, B. Qi, S. Zhang and Y. Li, Conjugation Induced by Wet-Heating of Gelatin and Low Methoxyl Pectin Improves the Properties and Stability of Microcapsules Prepared by Complex Coacervation, Food Biophys., 2023, 18, 95–106,  DOI:10.1007/s11483-022-09754-7.
132. Y.-L. Du, G.-Q. Huang, H.-O. Wang and J.-X. Xiao, Effect of high coacervation temperature on the physicochemical properties of resultant microcapsules through induction of Maillard reaction between soybean protein isolate and chitosan, J. Food Eng., 2018, 234, 91–97,  DOI:10.1016/j.jfoodeng.2018.04.020.
133. G.-Q. Huang, H.-O. Wang, F.-W. Wang, Y.-L. Du and J.-X. Xiao, Maillard reaction in protein – polysaccharide coacervated microcapsules and its effects on microcapsule properties, Int. J. Biol. Macromol., 2020, 155, 1194–1201,  DOI:10.1016/j.ijbiomac.2019.11.087.
134. Z. Zhang, B. Wang, G. Holden, J. Chen and B. Adhikari, Investigating the effectiveness of coacervates produced from conjugated and unconjugated Spirulina protein in delivering unstable oil to the intestinal phase of digestion, Food Biosci., 2023, 56, 103198,  DOI:10.1016/j.fbio.2023.103198.
135. C. Encina, C. Vergara, B. Giménez, F. Oyarzún-Ampuero and P. Robert, Conventional spray-drying and future trends for the microencapsulation of fish oil, Trends Food Sci. Technol., 2016, 56, 46–60,  DOI:10.1016/j.tifs.2016.07.014.
136. D. A. Botrel, R. V. de Barros Fernandes, S. V. Borges and M. I. Yoshida, Influence of wall matrix systems on the properties of spray-dried microparticles containing fish oil, Food Res. Int., 2014, 62, 344–352,  DOI:10.1016/j.foodres.2014.02.003.
137. N. Aminikhah, L. Mirmoghtadaie, S. Shojaee-Aliabadi, F. Khoobbakht and S. M. Hosseini, Investigation of structural and physicochemical properties of microcapsules obtained from protein-polysaccharide conjugate via the Maillard reaction containing Satureja khuzestanica essential oil, Int. J. Biol. Macromol., 2023, 252, 126468,  DOI:10.1016/j.ijbiomac.2023.126468.
138. K. Li, M. W. Woo, H. Patel and C. Selomulya, Enhancing the stability of protein-polysaccharides emulsions via Maillard reaction for better oil encapsulation in spray-dried powders by pH adjustment, Food Hydrocolloids, 2017, 69, 121–131,  DOI:10.1016/j.foodhyd.2017.01.031.
139. Y. Yang, F. Niu, S. W. Cui, J. Gong and Q. Wang, Spray-drying microencapsulation of citral with soy protein-soy polysaccharide Maillard reaction products: stability and release characteristics, Food Hydrocolloids, 2022, 132, 107842,  DOI:10.1016/j.foodhyd.2022.107842.
140. Y. Zhang, C. Tan, S. Abbas, K. Eric, S. Xia and X. Zhang, Modified SPI improves the emulsion properties and oxidative stability of fish oil microcapsules, Food Hydrocolloids, 2015, 51, 108–117,  DOI:10.1016/j.foodhyd.2015.05.001.
141. T. Wang, K. Chen, X. Zhang, Y. Yu, D. Yu, L. Jiang and L. Wang, Effect of ultrasound on the preparation of soy protein isolate-maltodextrin embedded hemp seed oil microcapsules and the establishment of oxidation kinetics models, Ultrason. Sonochem., 2021, 77, 105700,  DOI:10.1016/j.ultsonch.2021.105700.
142. C. N. Copado, B. W. K. Diehl, V. Y. Ixtaina and M. C. Tomás, Application of Maillard reaction products on chia seed oil microcapsules with different core/wall ratios, LWT, 2017, 86, 408–417,  DOI:10.1016/j.lwt.2017.08.010.
143. L. B. Pham, B. Wang, B. Zisu, T. Truong and B. Adhikari, In-vitro digestion of flaxseed oil encapsulated in phenolic compound adducted flaxseed protein isolate-flaxseed gum complex coacervates, Food Hydrocolloids, 2021, 112, 106325,  DOI:10.1016/j.foodhyd.2020.106325.
144. M. Nooshkam and M. Varidi, Maillard conjugate-based delivery systems for the encapsulation, protection, and controlled release of nutraceuticals and food bioactive ingredients: A review, Food Hydrocolloids, 2020, 100, 105389,  DOI:10.1016/j.foodhyd.2019.105389.
145. X. Liu, F. Xue and B. Adhikari, Encapsulation of essential oils using hemp protein isolate–gum Arabic complex coacervates and evaluation of the capsules, Sustainable Food Technol., 2023, 1, 426–436,  10.1039/D3FB00004D.

This journal is © The Royal Society of Chemistry 2024