Pickering emulsions are garnering more attention in the food sector [1,2,3]^[1][2][3]. The stabilizing mechanism of a Pickering system is different from the mechanism commonly observed for traditional emulsions, formed by immiscible liquids like oil and water ^[4]. Most conventional emulsions use some surfactant or tensoactive chemical as stabilizer, reducing the surface tension between the polar and nonpolar phases. Pickering emulsions, on the other hand, are formed by solid stabilizing particles with polar and nonpolar domains that act as a physical barrier between the phases, preventing the coalescence of droplets. A chemical coats the interface between the droplet (dispersed phase) and the dispersing medium (continuous phase), with no solubilization in any of the phases ^[5], usually forming a single layer of solid particles. When the contact angle of one of these single particles towards the water phase lies between 15° and 90°, an oil-in-water Pickering emulsion forms, and if the contact angle ranges from 90° to 165°, it stabilizes the water-in-oil emulsion ^[6]. If the solid particle size with adequate contact angle ranges between 10 and 20 nm, the energy barrier prevents the separation between the droplet and the particle ^[6], creating an irreversible adsorption between these two surfaces. This adsorption increases the stability of the emulsion ^[7], allowing for the development of different products using solid particles with adequate particle size and wettability properties by the immiscible liquids.

Different types of particles can create Pickering emulsions with diverse combinations of immiscible liquids, such as metallic particles ^[5], synthetic polymers ^[7], or natural biomacromolecules [8,9]^[8][9] such as cellulose. The use of cellulose as a Pickering emulsion stabilizer has received more attention in recent years as a potential surfactant replacement following the trend of younger consumers looking to buy food products perceived as having a greater sense of naturalness (branded as “clean label”) ^[10].

2. Foodstuffs

One of the main applications of Pickering emulsions is the production of emulsion-based food products. There are many articles (more than 650 documents) that relate to biopolymers, including cellulose nanostructures, for this application. There are also patents regarding equipment and procedures that could be used to prepare Pickering emulsions for food applications [36,37]^[11][12]. In a recent search on the Google Patents website (6 September 2023), almost 280 patents were found with the terms “nanocellulose” and “Pickering”. Pickering emulsions can be an intermediate step for preparing powdered oil, a material similar to powdered milk. In one study, a dispersible powder of oil-in-water camellia oil Pickering emulsion prepared with CNC from microcrystalline cellulose, extracted with a high pressure homogenizer using hydroxypropylmethylcellulose (HPMC) and sodium carboxymethyl cellulose (Na-CMC) was prepared via spray-drying. The Pickering emulsion was successfully dried, encapsulating the oily phase, and then redispersed in water, reforming the camelia oil emulsion [38]^[13]. These results indicated that HPMC and Na-CMC improved the stability of the emulsion prepared with CNC from microcrystalline cellulose. In another study, CNC from peach palm agricultural waste emulsified avocado oil. The authors compared the efficiency of using nanocellulose (1% w/w) with the efficiency of sorbitan monostearate (3.5% w/w). The stability indices (percentage ratio of the volume of the emulsion layer and total volume) after 72 h of the systems prepared with sorbitan were between 2.99 and 4.37 and the stability indices of the Pickering emulsions were always above 97% in all tested conditions for pH (3, 7, 11) and temperature (2 °C or 25 °C) [39]^[14]. The authors of this study point out that the stability of an emulsion in a broad range of pH values and temperatures is imperative when producing a food product. For example, a sauce may contain acidic additives such as organic acids or citric juices and may be stored at room temperature (≈25 °C) or refrigerated (≈4 °C). In the case of this product, a cellulose-based Pickering emulsion may be more efficient than a system prepared with traditional surfactants. An exciting aspect of Pickering emulsions with respect to their potential food applications is that they could provide an approach to slow down the digestion of lipids while imbuing the desirable effects of those lipids into a food’s texture [40]^[15]. Pickering-stabilizing particles must resist gastrointestinal conditions (e.g., low pH in the stomach and the action of enzymes) to reduce lipid release to the intestine [41]^[16]. Cellulose is non-digestible by the enzymes in the upper part of the human gastrointestinal tract, an acidic environment with proteolytic enzymes. Therefore, nanocellulose materials appear to be promising structures in the context of stabilizing emulsions. In one study, a Pickering oil-in-water emulsion prepared with wood CNC reduced simulated corn oil small intestine digestibility from ≈60% to ≈20% [41]^[16]. This process maintains the same texture as regular emulsion-based products but can provide lower caloric intakes and the potential to reduce obesity.

3. Innovative Food Applications

Plant-based foods are getting more attention worldwide for many reasons, including their potential to reduce environmental impacts [50]^[17] and animal welfare concerns [51]^[18] and avoid future zoonotic diseases [52]^[19]. Fungi-based and cell-based food may also replace animal-based food with additional environmental benefits [53,54]^[20][21]. In one study, a foam prepared with a Pickering emulsion was a suitable substrate for the cultivation of human neuroblastoma cells [49]^[22], and biodegradable foams may be a scaffold for cell- or fungi-based meat analogs with reduced environmental footprints. A Pickering emulsion stabilized with CNC was used as 3D printing ink in [55]^[23] to prepare custom-made structures. The reseauthorchers of this studyresearch created oil-in-water Pickering emulsions with 80% oil that were stabilized with 0.5 wt% of CNC and suitable for 3D printing even with an ionic strength of 50 mM NaCl. The SEM of the printed materials is a scaffold of apparent high surface area [55]^[23]. This 3D-printed sponge may potentially be applied to grow cells and develop novel 3D food products. Pickering emulsions based on cellulose may be used for many food applications. Nevertheless, these applications must have environmental benefits (e.g., biodegradability or lower footprints) to improve sustainability. Researchers must work to solve the current issues related to food production, storage, and consumption while always keeping regulatory and environmental issues in mind.

4. The Sustainability of Pickering Emulsions

Governmental and financial institutions worldwide have recognized that designing and developing sustainable technological solutions for the bio-based and circular economy is of essential importance [56]^[24]. It is of fundamental importance to achieve a successful sustainable innovation to alleviate safety, environmental, and socioeconomic issues at the early steps of conceptualization and experimentation of the processes and products. Easier and cheaper modifications in techniques and material sources implemented during the early stages of product research, development, and innovation (RDI) may better reduce the environmental impacts of technological innovations [57]^[25]. However, at these stages, many unknowns are present in the technology development process, with uncertainties related to product properties and applications as well as processing equipment for mass production at an industrial scale being prevalent. The consideration of safety, environmental, and socioeconomic issues require the development and adoption of a framework that describes the steps needed to identify hotspots and optimize the sustainability performance of novel products during the early research and development stages. In alignment with this need, the Joint Research Centre (JRC) of the European Commission recently proposed a framework to assess the safety and sustainability of chemicals and materials at the early development stage [58]^[26]. The aim was to prepare the chemical sector to attend the forthcoming European regulation for eco-design (the proposed Ecodesign for Sustainable Products Regulation—ESPR). This new regulation would repeal the current Ecodesign Directive 2009/125/EC (focused on energy-related products), which encompasses many types of products except those related to the food, feed, and medicinal sectors. This novel directive intends to support RDI teams to develop long-lasting, safer, and sustainable products for consumers and workers [56]^[24].

5. Conclusions and Recommendations

The bio-based circular economy is an essential element for the sustainable development and continuity of humankind. This revisewarch presented nanocellulose as a viable nanoparticle for preparing Pickering emulsions. The reviewed articles are evidence that this is a feasible food technology. research on the sustainability and eco-design of products with Pickering emulsions is needed. There are LCA studies regarding emulsion components but not of emulsions themselves. More interaction is needed among health, material sciences, and food engineering researchers to address the complex issues regarding approving a food containing Pickering emulsions. Ultimately, legal professionals must be involved in the discussion of new food products to prevent public backlash and government misunderstanding, especially when these products have reached the technological maturity required for commercialization. Finally, consuming countries of this technology must approve a legal framework that allows food products with Pickering emulsions to be put on the market. Solving these shortcomings must happen concomitantly to allow for the successful release of these types of food products on the market. The partnership between private and governmental sectors will be of utmost importance to achieve these goals. If this system succeeds, it may provide a reference point for other innovative products, speeding up technology adoption and economic innovation and benefitting society and the environment.