Chitosan-based Pickering emulsions for food applications

The encapsulation of nutraceuticals is one of the most important applications of chitosan-based Pickering emulsions in the food area. They have been reported to protect and enhance the bio-accessibility of the encapsulated active agents. In this context, Shah et al.

have successfully encapsulated curcumin in Pickering emulsions stabilized by chitosan/tripolyphosphate nanoparticles. The developed Pickering emulsions enhanced the stability of the encapsulated curcumin (Shah, Li, et al., 2016). Their findings indicate that chitosan/tripolyphosphate Pickering emulsions can be used as an effective oral delivery

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system for highly lipophilic bioactive agents. In another study, hesperidin was encapsulated in Pickering emulsions stabilized with chitosan and lecithin (a food-grade emulsifier) (Dammak & José do Amaral Sobral, 2018). The emulsions have shown high stability and were reported to have the potential to be utilized for the encapsulation of other active agents. In a recent study, β-carotene was encapsulated in Pickering emulsions stabilized by chitosan hydrochloride/carboxymethyl starch nanogel particles (X.-M. Li et al., 2020). The encapsulation of β-carotene in the developed Pickering emulsions protected β-carotene from photodegradation by UV (365 nm) and enhanced its thermal stability and bio-accessibility. Furthermore, in another recent study, it was found that the encapsulation of d-limonene in chitosan-based Pickering emulsions highly protected it against thermal decomposition (Alehosseini et al., 2021).

Rattanaburi et al. produced Pickering emulsions stabilized by chitosan/lactic acid bacteria/hydroxypropyl cellulose, which combines the health benefits of chitosan and the antimicrobial activity of bacteriocin produced by the lactic acid bacteria. Stabilization of the emulsion was aided by the hydroxypropyl cellulose, which was able to fill the gaps between chitosan/lactic acid bacteria network existing at the oil interface, and thus was considered as a co-emulsifying agent. These Pickering emulsions acted as producing and delivery vehicles for bacteriocin (Rattanaburi et al., 2019).

Pickering high internal phase emulsions (Pickering HIPEs) stabilized with gliadin/chitosan hybrid particles and encapsulating curcumin have been reported to be a promising substitute for margarine to reduce the use of partially hydrogenated oils that increase the risk of cardiovascular diseases (Zeng et al., 2017). In this study, the formed Pickering HIPEs have shown viscoelastic properties contributing to the formation of stiff solid-like materials (similar to margarine) owing to the percolating network structure of the emulsion droplets. The encapsulation of curcumin in the produced Pickering emulsions was found to protect it against UV-induced degradation compared to the curcumin present in bulk oil.

2.8.1.2 Controlling lipid digestibility

Controlling lipid digestibility is considered a unique feature for some colloidal food systems/products; as it renders them suitable for reducing obesity, boosting satiety and

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reducing caloric intake (Costa et al., 2020; Yao et al., 2013). Furthermore, it assists in improving the bioavailability of the lipophilic bioactive agents and their delivery (Tzoumaki et al., 2013). Chitosan conventional emulsions, when administered orally, have been reported to help in reducing body weight in rats, and thus, could be used as a low-calorie functional food to enhance satiety (Wani et al., 2016).

Some Pickering emulsions have been reported to be able to delay lipid absorption. This property is attributed to the irreversible adsorption of particles at the oil/water interface of the emulsion droplets, which subsequently restricts the access of lipase enzymes and bile salts to the dispersed phase, and hence delays its digestibility. The in vitro lipid digestion rate of Pickering emulsions stabilized by three different food-grade polysaccharide particles (chitosan, cellulose crystals and cellulose nanofibers) was compared in a recent study (Costa et al., 2020). It was found that Pickering emulsions stabilized with cellulose crystals and cellulose nanofibers were capable of inhibiting lipid digestion due to the strong adsorption of the particles at the oil/water interface that prevented the action of lipase enzymes on the dispersed phase. On the other hand, Pickering emulsions stabilized by chitosan particles delayed the lipid hydrolysis because of the interactions between chitosan chains at the near-neutral pH of the intestine and the intestinal digestive fluids at the emulsion droplet surface (Costa et al., 2020). Therefore, it could be concluded that Pickering emulsions with cellulose particles have the potential to be used as lipid digestion inhibitor systems, whereas chitosan-based Pickering emulsions can be used as oral delivery systems of nutraceuticals with delayed lipid hydrolysis

In another report, the digestion rate and bioaccessibility of curcumin-loaded Pickering emulsions stabilized by chitosan/tripolyphosphate particles were studied (Shah, Zhang, et al., 2016). The emulsions were prepared with either corn oil, as a long-chain triglyceride (LCT), or medium-chain triglyceride (MCT). Pickering emulsions have shown a slower rate of digestion in comparison with nanoemulsions, and subsequently, the amount of the released free fatty acids was lower from Pickering emulsions. Also, Pickering emulsions have shown lower bioaccessibility values of curcumin which is dependent on the slower digestion rate. The slow bioaccessibility is due to the deprotonation of chitosan at high pH value. At pH 7, the pancreatic lipase had little access to the lipids entrapped in the chitosan aggregates. These results show that chitosan Pickering emulsions reduce lipids digestibility.

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In a different approach, some studies have shown that the incorporation of chitosan in Pickering emulsions stabilized by non-chitosan particles influences their lipid digestion pattern. For example, chitosan-coating of emulsions stabilized by octenyl succinic anhydride (OSA)-modified maize starch nanocrystals was found to decrease the lipolysis of the emulsions and enhance their storage stability under acidic conditions (Jo et al., 2019).

The lipolysis rate of the emulsions decreased with the increase of the thickness of the chitosan layer, which suggests that the incorporated chitosan in the emulsions has the potential to control lipid digestion.

2.8.1.3 Retardation of lipid oxidation in food formulations

Lipid oxidation is a problematic issue for food product manufacturers since it negatively affects the sensorial properties of food formulations, produces harmful substances, and thus decreases the shelf-life of the products.

Atarian et al. reported that the rate of lipid oxidation was remarkably reduced in sunflower oil-in-water Pickering emulsions stabilized by chitosan/stearic acid in comparison to sunflower oil-in-water emulsion stabilized with Tween 80. The enhanced lipid oxidative stability is attributed to the positively charged surface of the emulsion droplets owing to the presence of free amino groups of chitosan in the chitosan/stearic acid nanogels.

Subsequently, the positively charged transition metals ( pro-oxidants present in the aqueous phase ) were repelled away from the surface of the oil droplets in the emulsion, retarding the lipid oxidation (Atarian et al., 2019).

Food-grade Pickering emulsions stabilized by chitosan grafted with caffeic acid have been reported to have enhanced free radical scavenging properties (İlyasoğlu et al., 2019). The synthesis of caffeic acid grafted chitosan was promoted by ascorbic acid. The produced particulate emulsifier based on chitosan modification has improved the solubility of chitosan and was also reported to retard lipid oxidation as it provided excellent antioxidant properties.

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2.8.1.4 Enhancing the properties of edible films

Biopolymeric films have recently attracted increasing attention owing to their biodegradable, biocompatible and non-toxic properties, which make them an environmentally friendly alternative for plastic films/packaging. The edible films and coatings have been used to extend the shelf life of food products and improve their quality (Agulló et al., 2003). Interestingly, Pickering emulsions have been used in the formation of edible films that have excellent oxygen barrier properties. Shi et al. produced Pickering emulsions stabilized by zein/chitosan colloidal particles and subsequently incorporated the produced Pickering emulsions into a chitosan-based matrix under ultrasonication to form self-supported emulsion films as shown in Figure 2.7 (Shi et al., 2016). In another recent study, chitosan Pickering emulsions encapsulating hesperidin were incorporated in gelatin-based films (Dammak et al., 2019). The films were produced by incorporating the chitosan Pickering emulsions at different concentrations (5, 10, 20, 30 and 50 g oil/100 g gelatin).

The films have shown high flexibility but less resistance to tension. Furthermore, the water vapor barrier activity of the films was enhanced by increasing the Pickering emulsion incorporation levels. The films possessed good antioxidant activity owing to the encapsulated hesperidin.

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Figure 2.7. 3D CLSM images of chitosan films loaded with Pickering emulsions stabilized with zein/chitosan colloidal particles as a function of oil-to-chitosan ratios: A) 5%; B) 10%;

C) 20%; D) 30% and E) 50%. The green fluorescence from Nile Red indicates the spherical emulsion droplets. Reprinted from (Shi et al., 2016), Copyright (2016), with permission from Elsevier.

2.8.1.5 Incorporation in complex food systems

The incorporation of Pickering emulsions in complex food matrices is an innovative research area that has been explored just recently. Hosseini and Rajaei investigated the

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incorporation of fish oil-in-water Pickering emulsions stabilized with chitosan/stearic acid nanogels containing clove essential oil in mayonnaise (R. S. Hosseini & Rajaei, 2020). It was observed that the introduction of fish oil in an emulsified form resulted in enhancing the oxidative stability and the rheological properties of mayonnaise in comparison to non-emulsified (free) fish oil. The study also showed that the presence of clove essential oil in the nanogels significantly improved the elasticity and, subsequently, the textural properties of mayonnaise. This study presents a new fortification strategy of food systems with bioactive compounds based on the incorporation of Pickering emulsions.