**1. Introduction**

Carotenoids are a family of terpenoid pigments rich in fruits and vegetables and are related to several potential health benefits because of their antioxidant and anti-inflammatory properties [1–5]. Lutein is one of the main carotenoids that can selectively accumulate in the eye, macula and retina in particular, and is known for eye protection effects, especially against photoinduced damage [6–8]. This is mainly because lutein is capable of quenching singlet oxygen and other reactive oxygen species and absorbing blue light [6]. Abundant epidemiological evidence has suggested that lutein intake is positively correlated with a lower risk of age-related macular degeneration and cataracts [9–11]. Nevertheless, lutein can only be obtained from diets, which is often insufficient due to the limited consumption of fruits and vegetables. Additionally, the biological activity of lutein is highly dependent on its gastrointestinal absorption, which may be hindered mainly as a consequence of its physicochemical properties [12,13].

One approach to overcome the challenges of insufficient lutein intake is to develop lutein-enriched staple foods, thus delivering lutein to humans regularly and continuously. However, merely incorporating lutein as an ingredient into foods and beverages hardly exerts the nutritional value of lutein due to its poor solubility and the fact that lutein is prone to oxidative degradation [13,14]. Encapsulation technology has shown to be an

**Citation:** Yao, Y.; Lin, J.J.; Chee, X.Y.J.; Liu, M.H.; Khan, S.A.; Kim, J.E. Encapsulation of Lutein via Microfluidic Technology: Evaluation of Stability and In Vitro Bioaccessibility. *Foods* **2021**, *10*, 2646. https://doi.org/10.3390/ foods10112646

Academic Editors: Hong Wu and Hui Zhang

Received: 28 September 2021 Accepted: 29 October 2021 Published: 1 November 2021

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attractive strategy to entrap the bioactive compounds within a carrier, which can improve their stability during food processing, storage and gastrointestinal absorption [15–18]. Microfluidics has become a trending topic in innovative food processing in recent years, with nutrients encapsulation being an emerging application of the microfluidic technique [19]. Microfluidics particularly focuses on accurate control over minute volumes of fluids within a system of microchannels [20,21]. This makes it possible to alter the way of working with dispersed food systems, and inherently manipulate structures at a micro-level [19].

Lutein has to be incorporated into mixed micelles for gastrointestinal absorption due to its hydrophobic property. Bioaccessibility describes the fraction of lutein solubilized in the mixed micelles and is usually determined in vitro via a simulated gastrointestinal digestion model [22]. Several lines of studies have indicated that fat-soluble carotenoids such as lutein, when dispersed in dietary oils, obtain greater bioaccessibility than when they are consumed alone [23–25]. This is because oils work as a delivery vehicle for these hydrophobic nutrients: The hydrolysis products of oils—free fatty acids and monoglycerides—together with phospholipids, bile salts and cholesterol, form the mixed micelles in the aqueous digestion fluid [26]; the formation of the mixed micelles facilitates the solubilization of hydrophobic lutein in mixed micelles and makes lutein become accessible during digestion [23,27]. Moreover, several studies have reported that the emulsion-based delivery system shows desired properties such as alleviating the degradation of bioactive compounds, improving the efficiency in micellization and promoting the digestive enzyme activity [28–30].

However, considering microfluidics is a relatively innovative technology, especially in "foods" area, limited studies have investigated its potential in nutrient encapsulation combined with an emulsion-based delivery system. Additionally, the evidence of applying this microfluidic technique to encapsulate carotenoids into foods is lacking. Therefore, this study aimed to encapsulate lutein into a staple food noodle using excipient emulsions via microfluidics-based continuous extrusion technique, and to assess the stability and bioaccessibility of lutein with different microfluidic assemblies and different types of oils.
