**2.1. Chemistry and structure of lutein**

Carotenoids are classified into two subgroups: carotenes, which are hydrophobic, consist of strictly hydrocarbons and xanthophylls, which are more hydrophilic, contain at least one oxygen atom in the polyene chain. The common characteristic of the carotenoid family is a C40H56 structure containing a long conjugated double-bound chain carrying the liner and cyclic alternatives. Lutein and zeaxanthin belong to the xanthophylls subgroup. They are characterized by the two hydroxyl groups attached to the end ionone rings in the nine conjugated carbon bounds polyene chain (**Figure 1**). The difference between lutein and its stereoisomer zeaxanthin is the position of the double bound in the terminal ring. In the human body, lutein and zeaxanthin could be transformed to each other via meso-zeaxanthin. Due to the presence of hydroxyl groups, lutein and zeaxanthin are more hydrophilic and polar in the serum and tissues, allowing them to react with oxygen produced in the liquid phase and scavenge reactive oxygen species (ROS) more efficiently. Due to the presence of chiral centers, lutein can exhibit eight stereoisomeric forms, of which (R,R,R) is mainly found in nature. On the other hand, zeaxanthin has three stereoisomeric forms, including (R,R), (S,S), and (R,S-meso).

orange pepper, kiwi fruit, orange, zucchini, and squash. Dark-green leafy vegetables are the major source of lutein, especially in kale and spinach, containing 15,800–39,550 μg/100 g and 7040–11,940 μg/100 g, respectively [1]. There are 44 and 26 mg of lutein per cup of cooked kale and spinach, respectively [8]. However, the dietary origin of lutein varies in different countries, depending on the preference for specific foods. In Canada, lutein mostly comes from spinach, broccoli, lettuce, corn, and oranges; while in Germany, spinach and green leafy salads contribute almost 50% of the total lutein supply [1]. Egg yolks, although does not contain lutein as high as kale and spinach, are treated as a great source of xanthophylls due to the high fat content in eggs, resulting in increased bioavailability. The concentrations of lutein and zeaxanthin are 292 ± 117 μg/yolk and 213 ± 85 μg/yolk (average weight of yolk is 17–19 g), respectively [9]. It has been demonstrated that consumption of 6 eggs/week increased the macular pigment optical density (MPOD) significantly, while the serum concentration of total cholesterol, triacylglycerols, high density lipoprotein cholesterol, and low density lipoprotein cholesterol stayed unaffected [10]. Because of the limitation in separating and quantifying lutein and zeaxanthin, most researches and databases frequently report the combined data of these two compounds in food. Thus, it may result in the inappropriate estimation of lutein content in several xanthophyll-rich foods (e.g. oranges and grapes). The microalgae, especially the genus Chlorella, are also an important natural source of lutein. Compared to the marigold flower, the conventional source of lutein in market, microalgae have faster growth rate and can be obtained throughout the year. Therefore, they can be used as a potential

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source for commercial lutein products.

**Figure 1.** Chemical structures of macular pigments.
