6C Proteins 215
6C.3 Characterization of Wine Proteins
6C.3.1 Size
In the 1960s, electrophoresis enabled separation of different sized wine proteins,
with researchers at UC Davis (Bayly and Berg 1967; Moretti and Berg 1965),
finding four different protein bands, varying in concentration within and among
wines from different cultivars ofV. vinifera. These researchers were the first to
raise the possibility that certain protein fractions, rather than total protein, might
be responsible for protein instability. Somers and Ziemelis (1973) used gel filtration
chromatography to separate wine proteins from other constituents and, by using the
exclusion limit of their gel, concluded that the wine protein size was between 10 and
50kDa. Hsu and Heatherbell (1987a) discovered many different protein fractions
with a range of 11.2–65kDa. In a subsequent study (Hsu and Heatherbell 1987b),
it was suggested that low molecular weight proteins (20–30kDa) were important to
heat instability of wines, rather than those with high molecular weights. This was
later confirmed (Waters et al. 1992). In an electrospray mass spectrometry study of
the proteins in the juice of 19 cultivars ofVitis vinifera(Hayasaka et al. 2001),
a range of masses (13–33kDa) was observed. The proteins were identified as
mainly thaumatin-like proteins (range of 21,239–21,272 Da) and chitinases (range
of 25,330–25,631 Da). Small variations in the masses of these protein due to small
variation in their polypeptide sequence form the basis of a robust method of varietal
identification based on mass spectrometry (Hayasaka et al. 2001).
6C.3.2 Isoelectric Point
Research into the isoelectric point of wine proteins has often been concurrent with
studies of wine protein size. Proteins with low isoelectric points (pI) were found
to be significant contributors to total wine protein (Moretti and Berg 1965) and
to wine haze (Bayly and Berg 1967). Hsu and Heatherbell (1987a) confirmed this
observation and suggested that the majority of wine proteins had a pI of 4.1–5.8,
whilst Lee (1986) suggested the major protein fractions of wine had a pI of 4.8–5.7.
Dawes et al. (1994) fractionated wine proteins on the basis of their pI and found
that the five different fractions all produced haze after heat treatment. Haze particle
formation was found to differ between the fractions however, leading to a statement
that other wine components, such as phenolic compounds, need to be considered to
understand fully protein haze.
6C.3.3 Stability
The unusual aspect of wine protein instability is that the proteins responsible for
protein haze in the long term are, paradoxically, very stable themselves in the