Investigating carotenoid loss after drying and storage of

35
1. Literature review
Enzymatic cleavage of carotenoids in plant is catalysed by a family of non-heme
carotenoid cleavage dioxygenases (CDDs) that can cleave regiospecifically the 9,10 or
11,12 double bonds of carotenoids leading to apocarotenoid products (Auldridge et al.
2006; Huang et al. 2009). Alternatively enzymatic cleavage leading to apocarotenoid
products can be operated unspecifically by co-oxidation (Bosser and Belin 1994). Co-
oxidation of carotenoids and another substrate, for instance fatty acids, such as linoleic
acid, using lipoxygenase and xanthine oxidase has been extensively studied by Bosser et
al. (1995) and Waché (2002; 2003; 2006). It has been reported that carotenoids are
oxidised by free radical species generated from another substrate by enzymatic reactions
(Waché et al. 2002; Zorn et al. 2003).
Co-oxidation of β-carotene by lipoxygenase from potato (Solanum tuberosum) has been
reported (Aziz et al. 1999). Presence of lipoxygenase has also been reported in cassava
(Isamah 2004), but it has not yet been identified in sweet potato. On the other hand,
sweet potato is reported as an excellent source of peroxidase activity, which is mostly
situated in the peel (Castillo-Leon et al. 2002). Peroxidases can also generate free radical
species (Yamazaki et al. 1985). Cleavage of β-carotene by peroxidase secreted by the
fungus Lepista irina was reported by Zorn et al. (2003). Cleavage products were the
same as with CDDs cleavage or co-oxidation by lipoxygenases or xanthin oxidases
(Zorn et al. 2003). However in studies on higher plants, catabolism of carotenoids by
peroxidase was reported to be dependent on the addition of a phenolic substrate (2,4
dichlorophenol) (Matilde and Martinoia 1982; Kennedy et al. 2002; Gandul-Rojas et al.
2004) and was enhanced by the further addition of hydrogen peroxide (Matilde and
Martinoia 1982). In order to determine the causes (enzymatic or non-enzymatic) of
carotenoid degradation one should take into account the presence of these substrates.
1.3.5 Products of degradation
"-carotene is degraded in a chain radical propagation reaction. The products of oxidation
of "-carotene are diverse and include: high molecular weight products, such as, various
apo-β-carotenal (8’; 10’; 12’; 14’; 15’), that further degrade; and short chain products,
such as, fragrant compounds called norisoprenoids (Siems et al. 2005)