Encapsulation of bitter peptides in water-in-oil high internal phase emulsions reduces their bitterness and improves gastrointestinal stability
Introduction
Several food-grade peptides are claimed to exhibit biological activities that may be beneficial to human health, including the ability to reduce microbial growth, lipid oxidation, and blood pressure (Saadi, Saari, Anwar, Hamid, & Ghazali, 2015). However, many of the biologically active peptides isolated from foods have an undesirable taste due to the presence of bitter peptides (BP), which reduces their acceptability to consumers (Fu, Chen, Bak, & Lametsch, 2018). These peptides are typically characterized by a high proportion of hydrophobic amino acids, as this feature increases their tendency to interact with the bitterness receptors on the human tongue. Moreover, peptides are susceptible to hydrolysis by pepsin in the stomach, thereby reducing their desirable biological activities (Giroux et al., 2019, Mohan et al., 2015). There is therefore interest in developing strategies to reduce the bitterness and gastric instability of peptides, so they can be incorporated into functional foods designed to promote human health. Researchers have developed various methods of reducing the bitterness of foods containing bitter peptides, including removing, masking, and chemically modifying them (Fu et al., 2018). However, many of these methods lead to a loss in the biological activity of the peptides and are therefore unsuitable for functional food applications.
Encapsulation is an alternative strategy for reducing the bitterness of peptides, as well as for protecting them from gastric conditions. Ideally, an encapsulation system should retain the peptides in the mouth and stomach but then release them in the small intestine where they can be adsorbed. Several technologies have been explored for their potential to encapsulate peptides, including spray drying, liposomal formulations, and coacervation, but each of these has its own limitations. The high temperatures used during spray drying can promote denaturation and chemical degradation of the peptides (Mohan et al., 2015). Liposomes are relatively expensive to produce and have limited stability in foods and the gastrointestinal tract (Mohan et al., 2016, Mohan et al., 2015). Coacervates are relatively inexpensive and simple to prepare but they are also highly unstable under food and gastrointestinal conditions (Tkaczewska, 2020). Advanced emulsion technologies have great potential for encapsulating and protecting bioactive molecules (Ying et al., 2021). Several studies have shown that peptides can be encapsulated within the internal water phase of water-in-oil-in-water (W/O/W) emulsions (Giroux et al., 2016, Giroux et al., 2019). However, these double emulsions are not widely used in the food industry because they are relatively expensive to prepare, and they tend to breakdown when exposed to food conditions. In the current study, we examined the potential of using water-in-oil high internal phase emulsions (W/O HIPEs) for encapsulating bitter peptides.
Studies have shown that oil phases can provide a physical barrier that restricts the ability of flavor molecules to interact with the taste buds on the tongue, thereby reducing their perceived intensity (Chabanet et al., 2013, Lynch et al., 1993). Consequently, it should be possible to reduce the bitterness of peptides by encapsulating them within the water droplets in W/O emulsions, as they will then be shielded from the taste buds by the oil phase surrounding them. Moreover, trapping the peptides in the internal water phase may also be useful for protecting them from hydrolysis by pepsin in the stomach. However, one drawback of using conventional W/O emulsions is that they contain a large amount of oil, which contributes to their cost and calorie content.
W/O HIPEs contain a high concentration (>74%) of water droplets, which greatly reduces their cost and oil content (Cameron and Sherrington, 1996, Okuro et al., 2019). Moreover, it increases the space within the delivery system where hydrophilic bioactive substances can be encapsulated. Finally, HIPEs usually have semi-solid characteristics because the oil droplets are so closely packed together that they cannot easily move past each other when an external stress is applied (Liu et al., 2021b, Zeng et al., 2017), which may be advantageous for some applications. Previously, we showed that encapsulating capsaicin within W/O HIPEs improved its bioaccessibility and reduced its tendency to cause irritation of the gastric mucosa after ingestion (Wu et al., 2022). In this study, we examined the ability of this kind of emulsion to encapsulate bitter peptides, as well as to reduce their bitterness and gastrointestinal instability. In addition, we examined the impact of incorporating the peptides on the physical and oxidative stability of the HIPEs. The results of this study may therefore be useful for formulating effective delivery systems for bitter peptides with beneficial biological activities.
Section snippets
Materials
The bitter peptides (origined from gulten, molecular weight <1000 Da) were donated by Jiangzhong Pharmaceutical Co., Ltd. (Nanchang, China). PGPR was supplied by Dahe Food Technology Co., Ltd. (Zhengzhou, China). Camellia oil was supplied by Luhai Grease Co., Ltd. (Jian, China). Palm oil was purchased from Langfeng Chemical Co., Ltd (Guangzhou, China). Copper sulfate pentahydrate (C805353) was purchased from Macklin Biochemical Co., Ltd (Shanghai, China). Methyl orange (M101227), FITC (46950),
Microstructure of BP-HIPEs
W/O HIPEs with different bitter peptide concentrations (0%-30%) were fabricated and then their microstructures were evaluated using CLSM and Cryo-SEM. For CLSM, the oil phase was stained red with Nile Red and the water phase was stained green with FITC. The confocal microscopy images (Fig. 1A) show that the emulsions consisted of tightly-packed water droplets (green) surrounded by oil (red), confirming that W/O-type HIPEs had been formed. Lee et al. (2019) also observed similar structures in
Conclusion
In this study, we showed that W/O HIPEs could be successfully used to encapsulate bitter peptides, which reduced their bitterness and improved their gastric stability. These effects were attributed to the fact that the peptides were located inside the water droplets in the W/O emulsions and so were isolated from the taste buds and gastric fluids by an oil barrier. We also showed that incorporation of the peptides into the HIPEs altered their rheological properties and stability. It was
CRediT authorship contribution statement
Yi Gao: Writing – original draft, Methodology, Investigation, Data curation. Xiaolin Wu: Data curation, Formal analysis. David Julian McClements: Writing – review & editing. Ce Cheng: Methodology. Youfa Xie: Investigation. Ruihong Liang: Writing – review & editing. Junping Liu: Investigation. Liqiang Zou: Conceptualization, Supervision, Funding acquisition. Wei Liu: Supervision, Funding acquisition.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
The authors are grateful for the financial support on this study from National Natural Science Foundation of China (32160563, 31972071); Key Research and Development Program of Jiangxi Province (20202BBF63017); “Shuangqian Project” of Scientific and Technological Innovation of High-end Talents-Natural Science, Jiangxi Province (S2019GDKX2836); “Shuangqian Project” of Scientific and Technological Innovation of High-end Talents-Youth, Jiangxi Province (S2019GDKX2835), Innovative and
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