216 E.J. Waters and C.B. Colby
short term and survive the vinification process. It is, therefore, not surprising that
wine proteins are resistant to low pH and enzymatic or non-enzymatic proteoly-
sis (Waters et al. 1992). The mechanism for the resistance has not yet been fully
elucidated, but resistance to low pH andproteolysis are characteristic of plant
pathogenesis-related proteins in general. Experimental evidence suggests that it
is also characteristic of the grape pathogenesis-related proteins involved, rather
than phenolic association or glycosylation (Waters et al. 1995b). Limited prote-
olytic processing of the wine proteins can, however, occur during white table wine
vinification (Waters et al. 1998) and during the Champagne winemaking process
(Manteau et al. 2003). These data indicate that there could be scope to exploit this
susceptibility to proteolysis under certain conditions and thus prevent wine protein
haze.
6C.3.4 Levels in White Wines
Protein levels in white wine have been reported by several authors and have been
shown to differ by variety. Lee (1986) reported a range of protein concentration
from 18 to 81 mg/L in 14 wines from different Australian regions and made from
different varieties. Some of these winesappeared to have been fined with bentonite
prior to analysis. Pocock et al. (1998) reported concentrations in unfined Australian
wines up to several hundred mg/L. Hsu and Heatherbell (1987b) found a range of
19–44 mg/L in four different unfined white wines from Oregon, while a very large
variation (20—260 mg/L) was noted by Bayly and Berg (1967).
6C.4 Effect of Growing and Harvesting Conditions
on Subsequent Wine Protein Levels
In grapevines, the synthesis of the PR proteins is regulated in a developmental and
tissue specific manner. InVitis viniferacv. Muscat Gordo Blanco berries, both the
expression of the VvTL1 gene, and the levels of the corresponding major thaumatin-
like protein increased dramaticallyafter the onset of berry softening (v ́eraison)
and continued throughout berry ripening (Tattersall et al. 1997). Similar develop-
mental patterns were also found in the expression of genes encoding chitinases,
some identical to those involved in wine protein haze (Derckel et al. 1996, 1998;
Robinson et al. 1997). An immunological study ofV. labruscanacv. Concord also
showed that thaumatin-like proteins and chitinases accumulate during berry ripening
(Salzman et al. 1998).
In all cultivars ofV. viniferastudied so far, thaumatin-like proteins and chiti-
nases are the major soluble components of grapes (Peng et al. 1997; Pocock et al.
1998, 2000; Tattersall et al. 1997). The predominance of these PR proteins was clear
at all stages of berry development following v ́eraison (Pocock et al. 2000). Impor-
tantly, as the concentration of these proteins in the berries continually increases