134 R. Marchal and P. Jeandet
caused by cold stabilization is why some winemakers decide to fine with bentonite
during or following potassium bitartrate stabilization. In certain wines, free tartaric
acid can be complexed with proteins and polyphenolics, inhibiting potassium bitar-
trate crystal formation. Removal of a portion of these complexing compounds with
bentonite can enhance potassium bitartrate stability. Additionally, bentonite fining
of wines during cold stabilization allows potassium bitartrate crystals to compact
the bentonite lees.
However, because the use of bentonite alters the organoleptic characteristics
of wines (Lubbers et al. 1996; Main and Morris 1994; Miller et al. 1985), other
authors have searched for alternative treatments for the use of bentonites, particu-
larly mannoproteins (see Sect. 5.3).
5.1.2 Use of Gelatin in White Wine Fining
The primary reaction occurring with gelatin is a complex formation between poly-
phenols in the wine and the protein of gelatine to give the desired floccular precipi-
tate. The second reaction, less well understood but equally important, is the complex
formation between the natural wine proteins and the added protein, i.e. gelatin.
5.1.2.1 The Nature of Gelatin
Gelatin is prepared from derived sources of collagen. Gelatin is a protein, that is,
a polymer of amino acids joined together by peptide bonds. Hence, proteins can
be depicted as long molecules with many different side chains, which accounts for
their varying properties. Proline, that is, a very important amino acid of gelatin,
imparts a twist to the chain and affects the shape of the protein molecule and its
rigidity. The protein chain is amphoteric and can carry either a positive or a negative
charge depending on the pH of the medium. In wine and beverages at a pH of 2.9–
3.6, most of the amino groups are positively charged and most of the acidic groups
are uncharged. The molecule then behaves as a cation. It attracts and forms polar
associates with anions in solution. In addition, proteins form associations due to
hydrogen bonding using the negatively charged oxygen and nitrogen atoms in the
molecules.
The isoelectric point (pI) of a protein is the pH at which the protein will not
migrate in an electric field. At that pH, the molecule carries an equality of positive
and negative charges: the molecule is isoionic, in the absence of added ions other
than hydrogen and hydroxyl ions in solution. Gelatin, is rather unique in that it
can have an isoelectric point anywhere between pH 9 and pH 5, depending upon
the source and method of production. Gelatins used in enology are usually derived
from acid pre-treated pigskin (Type A) and have isoelectric points between 6 and 9.
Gelatins with a high gel strength (Bloom strength) have the higher pI and gelatins
with a low Bloom strength have a pI closer to 6. Some enological gelatins sometimes
derived from limed hide or limed ossein (Type B gelatins) and all of them have a pI
close to 5. The significance of pI is, of course, that the higher the pI, the greater the