6C Proteins 219
to double the amount of bentonite required for prevention of protein haze when
compared to fruit harvested by hand and transported from the same vineyard before
destemming and crushing (Pocock and Waters 1998). This does not appear to be a
result of increased protein synthesis in response to wounding as comparisons among
hand harvested berries, mechanically harvested intact berries and the predominant
form of mechanically harvested fruit: a mixture of broken fruit and juice, indicated
little if any protein was produced as a result of stress caused by mechanical har-
vesting. Increases in protein content of juice from mechanically harvested fruit thus
appear to be due to extraction or release of protein from or associated with pulp and
skins rather than a physiological wounding response by the berry.
6C.5 Haze Formation in Wine
The mechanism of protein haze formation in wines is not fully understood. Slow
denaturation of wine proteins is thought to lead to protein aggregation, flocculation
into a hazy suspension and, finally, formation of visual precipitates. The importance
of non-proteinaceous factors in white wineprotein haze formation such as proan-
thocyanidins (Koch and Sajak 1959; Waters et al. 1995a; Yokotsuka et al. 1991)
have been suspected for some time. Other factors such as polysaccharides, alcohol
levels and pH have also been implicated (Mesquita et al. 2001; Siebert et al. 1996a).
It has been observed that grape protein added to model wine does not precipitate
or produce haze when heated, whereas visually obvious hazes occur when the same
protein is added to a commercial wine (Pocock 2006).
The current theory of the mechanism of haze formation formulated by Pocock
(2006) is as follows. Individual grape PR proteins probably exist as separate glob-
ular entities freely soluble in wine, tightly coiled and containing between six and
eight disulfide bridges. The first step in the process of haze formation is to uncoil
these proteins, or to denature them. This is accelerated by heating. The second step
involves aggregation of the denatured proteins as haze particles.
The size and amount of protein haze formed in a wine is strongly influenced by
other wine components. Pocock (2006) has demonstrated that one wine component,
the sulfate anion, previously referred to as factor X, is essential for haze formation.
If the sulfate anion is not present, heating does not result in sufficient denaturation
of the proteins to lead to their aggregation, thus a haze will not form.
Sulfate is one of the Hofmeister series of anions, a ranking of the ability of vari-
ous ions to precipitate proteins (Kunz et al. 2004). In simple terms, precipitation of
proteins by kosmotropic anions occurs due to ‘salting out’ – a competition between
the anion and the protein for water of solvation resulting in a loss of water from
the protein surface. This process is classically applied in ammonium sulfate pre-
cipitation as the first step in many protein purification schemes, although the levels
employed are several fold higher than those in wine. In the particular case of white
wine, this loss of water of solvation, even by a relatively low amount of sulfate
anion, by a protein in a solution containing a variety of cations and other anions and