Wine Chemistry and Biochemistry

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8A Wine Aroma Precursors 255


threshold, and as it always occurs in wine with 4-vinylphenol, it is rarely perceived


positively (Chatonnet et al. 1993a,b). Furthermore, the levels of vinylphenols in


wine decrease dramatically with aging, particularly by addition of ethanol on the


vinyl double bond (Dugelay et al. 1995).


Therefore, volatile phenols play a minor role in the aroma of most wines, and


when their influence is significant in certain wines, they have mostly a negative


effect, which can definitely depreciate their aroma in limit cases (phenolic off-


flavors). Thus, the corresponding precursors in grape, phenolic acids, as well as


the above-mentioned unsaturated lipids, are hardly taken into account to capture an


essential characteristic of the varietal aroma, but to avoid their transformation into


off-flavors.


8A.4 Carotenoids


The carotenoids of mature grapes have all the corresponding bicyclic structures of


carotenoids associated with PSI and PSIIphotosynthetic systems, the multiprotein


complexes of plant chloroplast membranes (Britton 1982, 1995; Demmig-Adams


et al. 1996; Goodwin 1980; Moneger 1968). Their 40 carbon skeleton is biosynthe-


sized from eight isoprenic units: the two final 2,2,6-trimethylcyclohexylmethylene


moieties (each made up of two isoprene units) occur at different oxidation lev-


els and each is bound to both ends of a central linear conjugated system made


up of four isoprene units, mostly of alltransconfiguration (Fig. 8A.4). The most


common are -carotene and lutein, which represent nearly 85% of the total, and
the minor carotenoids are neoxanthin, violaxanthin, lutein-5,6-epoxyde, zeaxanthin,


neochrome, flavoxanthin, luteoxanthin (Bureau 1998; Marais et al. 1989; Mendes


Pinto et al. 2004; Razungles et al. 1987, 1996). The only carotenoid hydrocarbon,


i.e. carotene, is -carotene, as all others are oxidized carotenoids, i.e. xanthophylls.


While a mature Syrah berry has similar proportions of -carotene, luteine, violax-


anthin and neoxanthin (36%, 50%, 2% and 9% of the total) as do leaves (27%, 51%,


10%, 11%), the quantities in the berry are a hundred times less than those in the


leaves (Wirth et al. 2001). The levels of carotenoids in grape decrease from v ́eraison


to maturity, but depend also on climatic factors, agricultural practices, grape cul-


tivar and clone (Bureau 1998; Marais et al. 1989; Razungles et al. 1987, 1993).


Carotenoids are mostly found in the skin, at levels —two to three times higher


than in the pulp, while they are absent from the juice (Razungles et al. 1988). In


the same way, they are absent from wine,except in the case of fortified red wine,


like Port wine, which contains more xanthophylls than carotene (Guedes de Pinho


et al. 2001).


Carotenoids are regarded as part of the aroma potential of grape, as they are


the biogenetic precursors of C13-norisoprenoids, a chemical family with many


powerful odorants, which are mainly found as glycoconjugates in grape (Baumes


et al. 2002; Enzell 1985; Mathieu et al. 2005; Winterhalter 1993). Sunshine favors


the biosynthesis of carotenoids in the berry, from the first stage of fruit formation

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