5 Use of Enological Additives for Colloid and Tartrate Salt Stabilization 131
protein clouding. Many wine proteins are not free but bounded to a minor quantity
of grape phenolics (flavonoids).
White wines contain relatively large amounts of insoluble proteins that slowly
precipitate from the solution (the initial protein precipitation begins during press-
ing). Most white wines are to deficient in phenolics, causing a quick and complete
protein precipitation. Protein haze may be due to the fraction of residual wine pro-
teins that have been rendered prone to precipitation by their interaction with reactive
phenolics (from the grape, and also rarely from the cork). Bentonite removes differ-
ent amounts of grape protein fractions (Moine-Ledoux and Dubourdieu 1999).
5.1.1.2 Electric Charges of Wine Proteins
At a certain pH, the positive and negative charges of each protein fraction are
equal and the protein is less soluble. This pH value is known as the isoelec-
tric point, or isoionic point, of the protein. The lower the difference between
the juice or wine pH and the isoelectric point of the protein fraction, the lower
the net charge on that protein fraction and the lower the solubility of that frac-
tion. If the juice or wine pH values are substantially different from that of the
protein isoelectric point, the protein charge is high and thus the higher will be
the ability of that protein to bind electrostatically to fining agents. Therefore, the
isoelectric properties of proteins influence not only their natural tendency to pre-
cipitate but also their affinity to be removed with various agents. The relation-
ships between wine pH and isoelectric points can be illustrated in the following
example. The Champagne base wine pH is 3.2. Then the protein fractions having
apI>3.2 will be all positively charged and those with a pI<3.2 will be nega-
tively charged. The positively charged proteins will react with a fining agent of
mainly the opposite charge (–) such as bentonite. In this case, there would remain
in wine the protein fractions which, due to their negative charges, are not easily
removed by the use of bentonite. Those protein fractions with isoelectric points
closest to pH 3.2 have a limited charge and are not able to bind electrostatically to
bentonite.
Protein clouding in white wines seems to be a greater problem when the wine pH
is close to the isoelectric point of the various protein fractions. This is due to the fact
that bentonite will remove, preferentially, the most positively charged proteins. The
electrostatic charge of various protein fractions explains the observable phenomena
of not being able to stabilize certain wines with the use of bentonite alone, or only
with excessive amounts that can strip the wine character. But the pI of proteins only
partially explains wine haze formation. It is also important to note that other factors,
as yet not clearly identified, can intervene.
5.1.1.3 Methods for Bentonite Preparation
The method of preparation significantly affects the ability of bentonite to remove
wine or juice proteins. Bentonite is made up of small platelets that are separated by
a layer of water molecules. During hydration, the charged platelets repel each other
and pop apart. As this occurs, swelling begins. Water molecules partially neutralize