Shellac: A promising natural polymer in the food industry
Graphical abstract
Introduction
Natural polymers, which include proteins, starches, and other polysaccharides, are used not only as ingredients or additives to endow food with specific structural and functional characteristics, but also as processing materials for food applications (Djagny, Wang, & Xu, 2001; Jobling, 2004; Wijaya, Patel, Setiowati, & Van der Meeren, 2017). The intrinsic characteristics of these natural polymers favoring their application, especially as processing materials, include their good biocompatibility, biodegradability, and non-toxicity (Rhim & Ng, 2007). In addition, global health and environmental awareness has caused a tremendous shift in consumer preferences, from synthetic to natural polymers. Hence, the subsequent creation of more natural products in the food industry. Interestingly, natural polymers show comparable or superior performance to synthetic polymers in specific applications. For example, konjac glucomannan, agarose, and sodium alginate have important applications in the processing of non-plastic packaging films (Wu et al., 2019; Yuan, Hong, et al., 2017; Yuan, Mu, et al., 2017), thermoreversible hydrogels (Yuan et al., 2018), and pH-responsive microspheres (Yuan et al., 2019), respectively. Shellac possesses a combination of these characteristics, including good film formability, pH-responsiveness, and amphiphilicity, making it a promising processing material in the food industry.
Shellac is a natural polymer refined from a resinous substance excreted by an insect, Laccifer lacca, which is parasitic on certain trees, especially in India, Burma, Thailand, and southern China (Wang, Ishida, Ohtani, Tsuge, & Nakayama, 2004). In these regions, shellac has been recognized for about 4000 years (Gibson, 1942) and was originally used as a natural dye for architecture, silk, and leather dyeing. Subsequently, the applications of shellac gradually extended to include the pharmaceutical field, because of its good enteric properties (Azouka, Huggett, & Harrison, 1993). Additionally, shellac is generally recognized as safe (GRAS status) by the Food and Drug Administration (FDA), allowing its use as a food additive and raw material in food coatings and film formulations (Farag & Leopold, 2011).
Structurally, shellac is a low-molecular-weight resin mainly composed of oxyacid polyesters. The oxyacids are divided into aleuritic acids and cyclic terpene acids linked by ester bonds, which, respectively, constitute the hydrophobic and hydrophilic components of shellac (Luo et al., 2016). Therefore, shellac possesses the additional advantage of exhibiting excellent amphiphilicity, compared to the biodegradability and biocompatibility of other natural polymers (Patel, Drost, Seijen ten Hoorn, & Velikov, 2013). Furthermore, shellac is characterized by carboxyl groups in its molecular structure, which are exempted from the cyclic terpene acid esterification reaction, thereby endowing shellac with weak acidity (about pKa 6). Consequently, shellac dissolves in alkaline solutions rather than in acidic solutions, prompting its pH-responsiveness (Buch, Penning, Wächtersbach, Maskos, & Langguth, 2009). Owing to these exceptional characteristics of shellac (amphiphilicity and pH-responsiveness), its application in the development of novel foods is on the rise. These novel food applications, which have attracted increasing attention over the last few decades, include food delivery systems, food foaming agents, oil-gelling agents, and food emulsifiers.
Although a substantial number of studies in recent years have captured the use of shellac in the food industry, to the best of our knowledge, there is a paucity of relevant reviews on the application of shellac in the food industry. Therefore, we retrieved related articles from Web of Science for a comprehensive review of the applications of shellac in the food industry. First, this review will introduce the sources, extraction methods, chemical composition, identification, and excellent performance of shellac. Then, the applications of shellac in the food industry will be comprehensively introduced. Finally, the health considerations and future perspectives of shellac, regarding its food-related applications, are discussed. Through this comprehensive review, we aim to provide more theoretical references and practical directions to inspire the further developments and applications of shellac-based food systems.
Section snippets
Sources, extraction, and processing
Generally, there are as many as 100–150 Laccifer lacca (the insect that produces shellac, as previously mentioned) larvae per inch of host-tree twig. To survive, these larvae move onto specific host trees for 2–3 days each, inserting their proboscis into the phloem tissue to reach the sap. Consequently, shellac is secreted to form cells around their bodies, aiding their adherence to the host-tree branches. After that, male insects are moved out from their cells, while females still live in
Chemical composition and identification
Shellac is an amphiphilic biomacromolecule with a distinctive molecular structure consisting of aleuritic acid and cyclic terpene acids, as mentioned earlier (Fig. 1). Depending on the differences between the R and R’ groups, the cyclic terpene acid moiety in shellac can be aleuritic acid, shellolic acid, jalaric acid, laccijalaric acid, laksholic acid, laccishellolic acid, or laccilaksholic acid. The aleuritic acid and cyclic terpene acids are linked by ester bonds and act as the hydrophobic
Thermal properties
Shellac is a hard amorphous resin that contains small amounts of wax, yellow coloring matter, and odiferous matter. The color of shellac ranges from extremely light blonde to extremely dark brown, with many varieties of brown, yellow, orange, and red in between. The color is influenced by the sap of the tree from which the lac is harvested from and the time of harvest. Shellac is heavier than water with softening and melting temperatures of 65–70 °C and 75–80 °C, respectively (Azouka et al.,
Applications of shellac in the food industry
Owing to its distinctive structure and properties, shellac is widely used in the food industry. Fig. 3 depicts the various possible applications of shellac, including as a raw material for fabricating food waxes, food coatings, and biodegradable films. It is also applied as a food foaming agent, oil-gelling agent, and food emulsifier. Furthermore, shellac is employed in the preparation of food delivery systems, such as microcapsules, coated carriers, nanofiber films, nanoparticles, and
Health considerations
Shellac has been used in agriculture, medicine, and various other fields in the past few decades, increasing their chances of coming into contact with humans. Therefore, it is important to take cognizance of its health and safety features. Fortunately, shellac has been recognized as safe (GRAS status) by the US FDA since 1939 (Hagenmaier & Shaw, 1991), which has been supported by subsequent research.
Shellac used in cosmetics as a coating material is often topically applied, coming into contact
Conclusion and future perspectives
The numerous advantageous properties and potential applications of shellac have gained much recognition, especially in the food industry. Its origin as a natural resin, extracted from a wide range of sources distributed in the forests of several Southeast Asian countries, confirms its renewability. Currently, the relevant extraction technology is mature and, as such, the possibility of large-scale production exists. As introduced earlier, shellac has distinctive molecular structure, which is
Acknowledgments
This work was supported by the National Key R&D Program of China (No. 2016YFD0400203).
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The authors contributed equally to this work.