ReviewRecent trends in oil structuring using hydrocolloids
Graphical abstract
Introduction
Oils and fats are important to our diet due to being an important energy source and solvent for key nutrients like vitamins and bioactive compounds. In foods, fats provide desirable functionality, texture, and palatability (Pehlivanoğlu et al., 2018). Fats had long been considered a bad influence on health, but, studies have shown that, principally, trans and some saturated fats are to blame (Liu et al., 2017; Zhu, Bo, & Liu, 2019). However, saturated fats have a technological role in foods because they are responsible for texture properties (Co & Marangoni, 2018). Solid-like lipids are preferred when processing some industrial products due to their specific features like better oxidative stability and solid lipid functionality. However, widely used methods, such as hydrogenisation, interesterification, and fractionation produce trans fatty acids which have severe adverse effects on coronary heart disease, blood lipoprotein profiles, diabetes, and cancer (Pehlivanoğlu et al., 2018). Academy researchers and research development experts have moved toward investigating ways for cheap and healthy production of solid fats with vegetable oil. Still, to replace saturated and trans fats in foods, maintaining their physicochemical and sensory properties is still a challenge (Iqbal, Xu, Huang, & Chen, 2019).
In the last decade, considerable progress in oil structuring technology has been made. Semisolid colloidal systems like oleogels incorporate a high percent of liquid oil within a structured network (composed by structurants) to give solid-like properties (Patel, Cludts, Sintang, Lesaffer, & Dewettinck, 2014). Moreover, highly concentrated gelled emulsions such as high internal phase emulsions (HIPEs) have gained popularity in this scientific field. However, an oleogelation limitation is the available food-approved structurants suitable for gelling liquid oils; of which hydrocolloids appear to be the most promising candidate. Generally, the gelling concentrations of hydrocolloids are remarkably low because of their large molecular sizes, making them efficient structuring agents. Still, most food-approved polymers are fundamentally hydrophilic and cannot be dispersed easily in oil to achieve the necessary structure and network formation required for gelation (Patel, 2018; Pernetti, van Malssen, Flöter, & Bot, 2007).
Hydrocolloid-based oleogelation is possible using indirect methods, an innovative solution to structure oil (Martins, Vicente, Cunha, & Cerqueira, 2018). This group includes: i) The emulsion-template approach where the emulsion formation is followed by evaporation of water, giving tightly packed droplets. ii) A solvent exchange procedure replacing the internal aqueous phase with an intermediate solvent followed by liquid oil. iii) The physical sorption of oil in porous structures such as aerogels or cryogels (Nephomnyshy, Rosen-Kligvasser, & Davidovich-Pinhas, 2020). In addition, HIPEs are templates to create oil continuous gels using low-temperature-triggered gelation of closely packed water droplets (Patel and Dewettinck, 2015).
Several types of hydrocolloids can produce novel structures because of their supramolecular interactions. Proteins are amphiphilic molecules that can adsorb strongly at an oil-water interface, thus are highly effective emulsifiers. Still, the functional properties of proteins can be lost in acidic conditions, at high temperature, high ionic strength, and in organic solvents, limiting their industrial uses (Akhtar & Ding, 2017). Polysaccharides are high-molecule-weight, hydrophilic, and biodegradable polymers regularly used as thickeners to affect the viscosity of aqueous phases to stabilise emulsions. However, many have poorer oil-water interfacial activity than proteins because of the lack of hydrophobic segments (Iqbal et al., 2019). The availability of many food-grade proteins and polysaccharides, and the versatility of the protein-polysaccharide interaction mechanisms, are fundamental to design suitable hydrogels that will later become oleogels (Anal, Shrestha, & Sadiq, 2019). These interactions have countless applications to improve the physical stability of the emulsion systems, and therefore in oleogels stabilisation.
Among indirect methods that require emulsification (such as the emulsion-template approach or HIPE gelation), a new category of food emulsifiers have increased interest from the oleogelation research field. Colloidal solid particles can form strong mechanical interfacial barriers via Pickering emulsions, anchoring at the interface as they can have an affinity for both oil and water (Berton-Carabin & Schroën, 2019). Interfacial layers containing Pickering particles give emulsions high physical stability, essential to develop stable gelled emulsions and oleogels (Lee, Tan, Ravanfar, & Abbaspourrad, 2019; Ma, Zou, McClements, & Liu, 2020).
In this review, the most recent insights of oleogelation using protein and polysaccharides are argued regarding the principal reasons that can influence the interactions and consequently the oleogelation. The promising hydrocolloid-based indirect oil structuration strategies, the development and incorporation of HIPE gels, and using Pickering emulsions with micro- and nanoparticles are discussed. Furthermore, functional colloid formation, current applications, and potential applications of structuring hydrocolloids are discussed.
Section snippets
Polysaccharides and proteins as structurants
Oleogelation is achieved by combining a structuring agent (structurant or oleogelator) with liquid oil to confer solid-fat functionality. Oil structuring agents can be classified into two groups based on their molecular weight: low and high molecular weight oil gelators (Co & Marangoni, 2018). High molecular weight oil gelators are mainly proteins and polysaccharides, which can form three-dimensional polymeric networks through physical interactions. Such oleogels have viscoelastic properties
Indirect methods to structuring liquid oils with hydrocolloids
The most common indirect methods include i) the emulsion-template approach, ii) a solvent exchange procedure, iii) physical sorption of oil into porous structures such as an aerogel or cryogel, and iv) HIPE gelation.
Conclusions
Under current demands for healthy, natural, and clean-label foods, plus a transition to more eco-friendly and sustainable ingredients, oil structuring using hydrocolloids has emerged as an outstanding strategy to substitute saturated and trans fats. Creation of oleogel systems using proteins and polysaccharides profit from hydrocolloid's proven structuring ability in food. However, hydrocolloids used as oil structuring agents raise a challenge due to their hydrophilic nature. Here, we have
Author statement
Pere Morell: Conceptualization, Writing – original draft, Isabel Hernando: Conceptualization, Writing- Reviewing and Editing, Funding acquisition. Amparo Quiles: Writing- Reviewing and Editing, Funding acquisition, Santiago Bascuas: Writing – original draft
Declaration of competing interest
The authors confirm there are not conflicts of interest.
Acknowledgements
The authors would like to thank Universitat Politècnica de València by FPI-UPV 2017 grant and the project RTI-2018-099738-B-C22 from the ‘Ministerio de Ciencia, Innovación y Universidades’. They would also like to thank Phillip John Bentley for assistance in correcting the English manuscript.
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Santiago Bascuas and Pere Morell contributed equally to this work as first authors.