130 R. Marchal and P. Jeandet
French town (Montmorillon) where it was first discovered. In the USA, bentonite
is principally mined in Wyoming – hence the term “Wyoming clay”. The type of
bentonite, the source, and its purity influence its properties (Marchal et al. 1995).
Bentonite is a complex hydrated aluminumsilicate with exchangeable cationic
components: (Al, Fe, Mg) Si 4 O 10 (OH) 2 (Na+,Ca++). The most commonly used
bentonite form in enology is the sodium bentonite. Sodium bentonite has enhanced
protein binding capabilities over calcium bentonite.
Bentonite exists as small plates which, when hydrated, separate to form a col-
loidal suspension with enormous surface area (300–900 m^2 /g). Its subsequent activ-
ity in solution is like that of a multiplated negatively charged structure able to
exchange its cations with positively charged components of the juice or the wine
(not only proteins). Bentonite absorption of uncharged molecules also occurs if they
are polar. Additionally, due to the fact that the platelet edges are positively charged,
some limited binding of negatively charged proteins may occur.
5.1.1.1 Mechanisms of Protein Removal
The mechanisms by which polypeptides and proteins adsorb on clay minerals have
been studied using homopolyamino acids (polylysine and polyglutamic acid) and a
synthetic montmorillonite (Gougeon et al. 2002, 2003). The use of complemen-
tary approaches (^13 C NMR, specific surface area by BET measurements, X-ray
diffraction, adsorption/desorption isotherms) have led to a detailed description of
the interactions between “positively” charged proteins and “negatively” charged
plate surfaces in a synthetic wine.^13 C NMR spectra showed that these polypep-
tides, which exhibit a mixture of helical and random coil conformations in the bulk,
tend to unfold and adopt a more extended random coil structure upon adsorption
on phyllosilicates. Values of the basal spacing measured by XRD on dehydrated
samples clearly indicated that, in the presence of adsorbed polypeptides, the silicate
layers do not collapse. Incorporation of polypeptide fragments within the interlayer
spaces was also revealed by the decrease in the specific surface areas. Alogether,
these results prove that polypeptides are adsorbed at the periphery of the montmo-
rillonite particles through specific interactions between the protein side chains and
the silicate sheets, whereas the polypeptide backbones do not enter the interlayer
spaces (Gougeon et al. 2002, 2003).
Bentonite may indirectly adsorb some phenolic compounds via binding with
proteins that have complexed with phenolics. However, the amount of phenols
removed is usually not high. Bentonite is also known to enhance membrane fil-
terability. Presumably, this is due to a reduction in the colloidal particle number in
suspension.
Despite the vast literature on protein instability, the actual protein levels at which
wines will remain protein-stable are unknown. Wine proteins are a mixture of prob-
ably more than 100 proteins derived from the grape, yeast (Dambrouck et al. 2003),
autolyzed yeast (Charpentier et al. 1986) and sometimesBotrytis cinereawhen
grape berries are not sound (Cilindre et al. 2007). Variety, vintage, maturity of the
fruit, pH, and processing methodology affect both the must and the wine protein
contents. Yeast proteins, however, have not been shown to play a role in white wine