## 2 Carotenoids: Structure and Functionality  
Carotenoids are hydrophobic compounds which essentially consist of a C\({}_{40}\) hydrocarbon backbone in the case of carotenes (_i.e._, they contain 40 carbon atoms in eight isoprene residues), often modified by various oxygen-containing functional groups to produce cyclic or acylicxanthophylls. So, all carotenoids possess a long conjugated chain of double bond and a near bilateral symmetry around the central double bond, as common chemical features [33].This chain may be terminated by cyclic groups (rings) and can be complemented with oxygen-containing functional groups [34].  
Carotenoids can be classified into different groups on the basis of the criteria used. Based on the basic chemical structure and the oxygen presence, carotenoids are classified into two types: carotenes or carotenoid hydrocarbons, composed of carbon and hydrogen only; and xanthopylls or oxygenated carotenoids, which are oxygenated and may contain epoxy, carbonyl, hydroxyl, methoxy or carboxylic acid functional groups [35]. Lycopene and \(\beta\)-carotene are examples of caroten carotenoids and lutein, canthaxanthin, zeaxanthin, violaxanthin, capsorubin and astaxanthin are xanthopyll carotenoids [36].  
The degree of conjugation and the isomerization state of the backbone polyene chromophore determine the absorption properties of each carotenoid. Due to the numerous conjugated double bonds and cyclic end groups, carotenoids present a variety of stereoisomers with different chemical and physical properties. The most important forms commonly found among carotenoids are geometric (_E-/Z_-). A double bond links the two residual parts of the molecule either in an _E_-configuration with both parts on opposite sides of the plane, or a _Z_-configuration with both parts on the same side of the plane. Geometrical isomers of this type are inter-convertible in solution. This stereoisomerism exerts a marked influence on the physical properties. The conjugation system described imparts carotenoids with excellent light absorbing properties in the blue-green (450-550 nm) range of the visible spectrum. Because of this reason, biochemical techniques such as UV-Vis spectrophotometry or Raman spectroscopy can be used to analyse carotenoids production by plants and microorganisms [37].  
When the criterion used to classify carotenoids is related to vitamin A, then the carotenoids can be categorized as follows: (a) vitamin A precursors that do not pigment such as \(\beta\)-carotene; (b) pigments with partial vitamin A activity such as cryptoxanthin, \(\beta\)-apo-8\({}^{\prime}\)-carotene acid ethyl ester; (c) non-vitamin A precursors that do not pigment or pigment poorly such as violaxanthin and neoxanthin; and (d) non-vitamin A precursors that pigment such as lutein, zeaxanthin and canthaxanthin [38].  
Some of the most important carotenoids in terms of biotechnological and biomedical uses explored so far are: Astaxanthin (3,3'-dihydroxy-\(\beta\),\(\beta^{\prime}\)-carotene-4,4'-dione) [36,39,40], \(\beta\)-Carotene (\(\beta\),\(\beta\)-carotene) [38,41,42,43,44], Canthaxanthin (\(\beta\),\(\beta\)-carotene-4,4'-dione) [45,46,47,48,49,50,51,52,53,54,55, 47, 48], \(\beta\)-Cryptoxinanthin (hydroxy-\(\beta\)-carotene) [38,49,51,52,53,54,55, 56, 57], Fucoxanthin [38,55], Lycopene (\(\psi\),\(\psi\)-carotene) [33,56,57], Lutein (\(\beta\),\(\varepsilon\)-carotene-3,3'-diol) [42,58,59,60,61], Zeaxanthin (\(\beta\),\(\beta\)-carotene-3,3'-diol) [38,62,63], and Violaxanthin (5,6.5',6'-diepoxy-5,5',6,6'-tetrahydro-\(\beta\)-carotene-3,3'-diol) [64,65,66,67,68].