Abstract
Seed coat mucilages are mainly polysaccharides covering the outer layer of the seeds to facilitate seed hydration and germination, thereby improving seedling emergence and reducing seedling mortality. Four types of polysaccharides are found in mucilages including xylan, pectin, glucomannan, and cellulose. Recently, mucilages from flaxseed, yellow mustard seed, chia seed, and so on, have been used extensively in the areas of food, pharmaceutical, and cosmetics contributing to stability, texture, and appearance. This review, for the first time, addresses the similarities and differences in physicochemical properties, molecular structure, and functional/bioactive properties of mucilages among different sources; highlights their structure and function relationships; and systematically summarizes the related genetic information, aiming with the intent to explore the potential functions thereby extending their future industrial applications.
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Key Points
Seed coat mucilages are mainly mucopolysaccharides covering the outer layer of the seeds and are secreted by seed coat epidermal cells in the process of seed coat differentiation. Biologically, seed coat mucilages play important roles in facilitating seed hydration and germination, improving seedling emergence, and reducing seedling mortality in an arid and sandy environment. Functionally, seed coat mucilages have been used in different food products as stabilizers, suspending agents, water retention agents, thickeners, texture modifiers, gelling agents, and film-forming agents. These mucilages have also been used in nonfood areas, for example, in the landscaping industry as binding agents, in cosmetics as stabilizers and in the pharmaceutical industry as drug delivery agents. From a health benefit perspective, their use has been associated with reducing the risks of diabetes, obesity, coronary heart diseases, mitigating colon and rectal cancer, decreasing cholesterol levels, and helping with intestinal functions.
In addition to the multiple functionalities, the biological properties of seed coat mucilages have garnered considerable interest. As a good source of water-soluble dietary fiber, seed coat mucilages have been widely reported for their health benefits, mainly with respect to satiety/satiation regulating, anti-diabetes, anti-colorectal cancer, and immunomodulatory effects.
To explore the potential functions and extend the industrial applications of seed coat mucilages in both food and nonfood areas, this review addresses specifically the extraction, physicochemical properties, molecular structural features, and functional and bioactive properties of different seed coat mucilages while reviewing the related genes acting on the synthesis, composition, and properties of mucilages. Flaxseed coat is very high in mucilage which is extracted and used in numerous commercial applications. Overall, four types of polysaccharides in the seed mucilages including xylan, pectin, cellulose, and glucomannan were summarized. The structure and functional/bioactive relationships of these seed coat mucilages were discussed. Some of these seed coat mucilages have similar physicochemical and structural properties to commercial hydrocolloids, therefore, are potential alternatives. However, it should be noted that although the seed coat mucilage has pronounced a potential in food, cosmetics, pharmacology, and other industries, large-scale industrialization is still challenging due to the difficulties in manufacturing. The commonly used solvents extraction methods are still time intensive, energy intensive, and labor intensive, leading to relatively high cost. Some solvents used such as acidic or alkaline solutions may also cause environmental concerns. Therefore, seeking a novel extraction method is still required. Furthermore, the detailed functional mechanism of the mucilages is still not clear due to their complex chemical composition and molecular structure. Exploring the relationship among the molecular structure, functionality, and bioactive properties of the seed coat mucilage is still required. Finally, the genes acting on the synthesis, composition, and properties of mucilages were also examined. By decoding the genetic information and conducting appropriate genetic modification, the control of molecular structure, yield, and chemical composition of seed mucilages is achievable.