Agriculture Reference
In-Depth Information
Phenolics
Phenolics are a diverse group of compounds with a six-carbon aromatic ring and hy-
droxyl group. Colour in red varieties is a function of anthocyanin accumulation. In
most varieties anthocyanins are located in the skin and the immediately subtending
cell layers and its production depends on the availability of phenylalanine, which is
synthetised from sugars. The pH of the juice or wine will influence its red or blue
tint. Grape berry anthocyanin development is mainly driven by light and air tem-
perature (Kliewer and Antcliff
1970
; Buttrose et al.
1971
; Dokoozlian and Kliewer
1996
) but, in Shiraz, can also be negatively correlated to high crop load (Wolf et al.
2003
). Excess nitrogen can impair anthocyanin accumulation in the berry due to
slowed ripening as a result of competition with shoot growth (Keller et al.
1999
).
Astringency is a function of the berry tannin content. Seed tannins become fixed
to the seed coat as they mature and therefore tannin extractability declines with
ripening. These tannins also become less bitter with ripening due to polymerization.
Skin tannins can decline or increase with ripening (Bindon et al.
2013
). To date,
however, there has not been a clear link between berry tannin and the wine tannin
concentrations (Fournand et al.
2006
).
Aroma Development
A characteristic sequence of aromas develops as a berry of a particular variety rip-
ens (Suklje et al.
2012
). Environmental parameters, however, have a strong influ-
ence on the intensity of particular aromas so that some compounds are masked
by other more dominating ones. Aromas arise from volatile compounds, such as
terpenes, norisoprenoids, and thiols stored as sugar or amino acid conjugates in the
cell vacuoles of the skin and flesh (Lund and Bohlmann
2006
). Some key flavour
precursors have been identified in particular varieties. Isobutyl methoxypyrazine
is responsible for the green bell pepper attribute in Cabernet Sauvignon. The floral
and citrus characters of white varieties, however, can be attributed to a class of com-
pounds called terpenoids. Both light and temperature have been found to influence
the degradation and synthesis of aroma compounds in berries (Reynolds and Wardle
1989
; Marais et al.
2001
; Suklje et al.
2012
).
Yield
At the upper levels of normal production systems in hot climate irrigated vineyards,
grapevines may produce over 35 kg of fruit per vine, or around 40 t on a per hectare
basis. At the other extreme, fruit yields may be restricted to only a few kilograms
per vine were limited by water availability, soil fertility or deliberate regulation of
cropping level.
