Plant-based food hydrogels: Constitutive characteristics, formation, and modulation

Abstracts

Plant-based hydrogels have great potential for applications, motivated by a favorable sustainability profile compared with other sources. They can be engineered to exhibit tailor-made textural structures and functional properties via tuning interrelated factors. The interrelationships between hydrogel fabrication, constitutive nature, mechanical properties, and performance underlie the fundamental design rationale for applications in different disciplines. Herein, we comprehensively review the natural characteristics of network-building blocks (particularly legume proteins and starch) and gelation principles of plant-based hydrogels. Special highlights are focused on how these involved factors impact hydrogel formation and, in turn, modulate its network structures and performance. We also propose some trends in advanced hydrogel designs, which can inspire new applications and provide convenient anchoring points for enhanced functions in prospect.

Introduction

The term gel, by tentative definition, refers to a semi-solid intermediate that is engineered to retain liquid-like (flowing) and solid-like (a finite elastic modulus) rheological behaviors when gel–sol transition takes place [1]. Gels usually consist of three-dimensional polymeric networks of fibrous/chain or globular macromolecules, which are apt to encapsulate considerable proportions of dispersing mediums such as water (hydrogels), gas (aerogels), or oil (oleogels) [1,2]. In the food industry, gelling is an effective technique facilitating food hydrogel design whereby the textural and sensorial properties of end-products, as well as flavor release in the food matrix, can be amended [1,3]. As such, a number of products are presented in the form of gels such as jam, jelly, confectionery products, desserts, quick-set gels, and so on. Gelling agents, also called gellants or gellators, are the soul of food gels, most of which are natural biopolymers mainly deriving from dairy, fish, meat, and microorganisms. Structure nuances of those gellants that existed in disparate sources conceivably prompt an enormous effect on designing gel molecular structures with predictable functions [4,5]. Over the last decade, extensive progress has been made in the design of food gels using both animal-sourced protein (e.g. whey protein, casein, gelatin, and egg white proteins) and microorganisms-based polysaccharides (e.g. agar, carrageenan, gellan gums, and xanthan) as gellants.

Currently, there is an urgent demand for the utilization of low cost, sustainable, non-toxic, and abundant natural gellants in the development of food hydrogel. Plant-based biopolymers, including plant proteins, starch, and polysaccharides (gums), are considered as credible alternatives to animal-based counterparts as functional building blocks for various food systems. In fact, food hydrogels derived from non-starch polysaccharides (e.g. pectin, guar gum, locust bean gum, and so on) have long been the focus since they are not only chemically characterized by renewability and biodegradability but also presenting tunable structures and various versatilities. Hence, a number of review articles dealing with plant-based non-starch polysaccharides gellants on food hydrogel formation can be found in the literature [6,7]. In essence, most of those gellants resemble viscoelastic substances with a great ability to trap a high amount of moisture, possess specific structuring and processing mechanisms, and have been intensively investigated as carriers to achieve oral and topical controlled release [8]. Selectively using them in food gels affords products with improved bioavailability and nutritional qualities, as well as strengthening metastable systems stability [2]. Traditionally, non-starch polysaccharides are extracted from the plant cell wall, and the production is generally low, which is always associated with a higher price. It is not until recent years that plant protein and starch from cereal and legume grains have gained drastic importance in the global functional food ingredients market. The outstanding sustainability and low production costs have dramatically propelled them major contenders in the formation of food hydrogels [1,2,5].

Whereas the opportunity for plant protein and starch to replace other food ingredients in contriving food hydrogels is promising; their imperfect performance in tuning the properties of food hydrogels needs to be circumvented. In addition, the complexity of real food systems has also presented challenges for food scientists to use plant protein and starch in designing a proper food hydrogel. Numerous studies have shown that probing into the constitutive nature, synthesis routes, gelling principles, and rheological behaviors is a constructive means allowing to intelligently fabricate innovative food hydrogels with desired mechanical characteristics and functions. In recent years, some pilot studies to apply plant proteins and starch on food hydrogels have also been conducted which provide valuable insights to address these challenges on the design of plant-based food hydrogels with predictable properties. Herein, the focus of this review is placed on the construction of food hydrogels using plant-based gellants, particularly legume protein and grain starch. An in-depth evaluation of the synthesis routes to plant-based food hydrogels, gelling mechanisms, and interrelated factors are presented. We also highlight the vital role of performance-based designing concepts in driving forward the advancement of plant protein and starch-based food hydrogels.