Gypenosides as natural emulsifiers for oil-in-water nanoemulsions loaded with astaxanthin: Insights of formulation, stability and release properties
Introduction
Astaxanthin (AST) is a carotenoid that has beneficial effects on human health and wellness. AST exhibits stronger antioxidant activity than β-carotene and vitamin E, due to the presence of conjugated double bonds and hydroxyl groups in its molecular structure (Wang et al., 2017). The regular consumption of AST also provides other health benefits, such as boosting and modulating of the immune system, reducing the risk of cardiovascular diseases, certain cancers, oxidative stress, inflammation and cataracts (Higuera-Ciapara, Felix-Valenzuela, & Goycoolea, 2006). In addition, natural astaxanthin is a natural and safe alternative to synthetic colorants used in the food industry. However, AST cannot be produced in the human body and is obtained primarily through the consumption of AST-rich food, such as salmon (Sowmya & Sachindra, 2011).
Like other carotenoids, the absorption of AST by the human body from food is low due to its low water solubility. Furthermore, AST is unstable and sensitive to high temperature, pH changes, oxygen and light (Taksima, Limpawattana, & Klaypradit, 2015), which limits the utilization of AST as a nutraceutical ingredient in functional foods and beverages. Extensive efforts have been carried out to improve the stability and delivery properties of AST by entrapping this nutraceutical component into delivery systems that can be dispersed into aqueous-based food and beverage products (Li et al., 2015, Liu et al., 2016, Tamjidi et al., 2014a).
Among these colloidal delivery systems, emulsions and nanoemulsions are good candidates to deliver lipophilic bioactive compounds including AST. Nanoemulsion delivery systems provide improved physical stability and high optical clarity (Guttoff, Saberi, & McClements, 2015). Nanoemulsions can be prepared simply by homogenizing an oil phase containing oil-soluble functional compounds with an aqueous phase containing a water-soluble surfactant. Moreover, nanoemulsion-based delivery systems are effective in term of increasing the water-dispersibility and bioavailability for numerous non-polar bioactive compounds, such as carotenoids, and hydrophobic vitamins (Mayer et al., 2013, Ozturk et al., 2015).
Oil-in-water (O/W) nanoemulsions are thermodynamically unstable systems that contain small oil droplets (d < 200 nm) dispersed in aqueous phase. Selection of the correct emulsifier is crucial for producing a stable nanoemulsion. An emulsifier stabilizes an emulsion by protruding its polar group into the water phase and non-polar groups into the oil phase, then forms an interfacial layer to provide protection against droplet aggregation or coalescence (McClements, 2015). There are numerous types of food-grade emulsifier that are used to produce emulsion-based products in the food industry, such as polysaccharides (e.g., pectin and gum arabic), proteins (e.g., sodium caseinate and whey protein isolate), phospholipids (e.g., lecithin) and small molecule surfactants (e.g., Tweens) (Ozturk et al., 2014, Shariffa et al., 2016). Small molecule surfactants produce emulsions with relatively small droplet size, due to their relatively high adsorption kinetics (Mao et al., 2009). Small molecule synthetic emulsifiers, such as the Tween series, have been used in fabricating nanoemulsion-based delivery system for AST (Affandi et al., 2012, Kim et al., 2012, Tamjidi et al., 2014a). However, consumers are increasingly demanding the use of natural ingredients, including natural replacements for synthetic emulsifiers in emulsion-based foods and beverages.
In this manuscript we investigate the ability of gypenosides (GPs) to act as emulsifiers in the formulation of AST-enriched nanoemulsions. GPs are the major bioactive components isolated from the Gynostemma pentaphyllum, and are saponins with established beneficial effects, including lowering the serum lipid and cholesterol levels, antitumor, immune-modulatory functions, anti-inflammatory and anti-oxidative activities (Lu et al., 2010, Lüthje et al., 2015). To the best of our knowledge, there are no studies have reporting about the utilization of GPs as natural emulsifiers to formulate nanoemulsion-based delivery system for nutritional components, such as AST. These new nanoemulsion systems are expected to have dual health benefit, due to both the encapsulated AST and the emulsifiers themselves (GPs).
Thus, the aim of our present study was to fabricate a nanoemulsion-based delivery system for AST using GPs as natural emulsifiers via high-pressure homogenization methods. The properties and stability of GPs-stabilized nanoemulsions was also compared with those for a synthetic emulsifier (Tween 20) that is widely used in commercial food application. Moreover, the in vitro small intestinal digestion behavior and bioaccessibility of AST-enriched nanoemulsions were also investigated.
Section snippets
Materials
Gypenosides (SJGLD161207, saponins 98%) were purchased from Xi’an Season Biotechnology Co., Ltd (Shanxi, China). Zanthin® astaxanthin complex (purity 10%) was procured from Valensa International (Eustis, FL, USA). Standard astaxanthin (purity > 97%), pancreatin from porcine pancreas (P7545) and bile extract porcine (B8631) were purchased from Sigma-Aldrich (St. Louis, MO). The following chemicals were purchased from Wako Pure Chemical Industries (Osaka, Japan): refined soybean oil, Tween 20,
Properties of initial nanoemulsions
We examined the properties of AST-encapsulated nanoemulsions prepared by GPs and Tween 20, including droplet size, size distribution, ζ-potential and AST concentration (Fig. 1 and Table S1). The fresh nanoemulsions stabilized by GPs and Tween 20 exhibited monomodal droplet size distributions with relatively small and similar droplet sizes: d4,3 = 125 ± 2 and 145 ± 6 nm, respectively. These results indicated that both GPs and Tween 20 were capable of producing small droplet under the
Conclusions
The current work demonstrates the successfully encapsulation of AST within nanoemulsion delivery systems produced using high-pressure homogenization. GPs were used as natural emulsifiers and compared with Tween 20, a synthetic surfactant widely used in the food industry. The results showed that GPs were able to produce nanoemulsions with a small droplet size (d4,3 = 125 ± 2 nm), and had similar size distribution to those produced using Tween 20. The high-press homogenization conditions applied
Conflict of interest
The authors declare that there is no conflict of interest relating to the publication of this article.
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