5 Use of Enological Additives for Colloid and Tartrate Salt Stabilization 135
cationic charge on the molecule at the wine pH. In other words, at pH 2.9–3.6, all
gelatins would be positively charged, but the charge density would be far higher for
high pI gelatins.
5.1.2.2 Polyphenol-Protein Reaction
Both tannins and anthocyanins in wines are molecules containing benzene rings
with adjacent hydroxyl groups which are proposed as a major source of hydrogen
bonds at the basis of complex formation between gelatin and tannins or antho-
cyanins in wines. Gelatin is held to be particularly suited to hydrogen bonding
because one third of the amino acids are glycine, where R = H, and hence steric
hindrance to hydrogen bonding would be far less than with proteins containing
less glycine. However, the tannin/gelatin complex is also very pH dependent and
disappears at approximately pH 8, which would be due to both molecules becoming
negatively charged and hence mutually repulsive. Hence, the role of polar bonding
between molecules of dissimilar charges must not be overlooked.
5.1.2.3 Protein-Protein Interactions
Beverage proteins essentially derived from the enzymes which are responsible for
the diversity of the biological processes occurring during grape maturation (plant
proteins) and during the conversion of the grape juice into a wine (protein secreted
by yeasts). In wine, growth and ripening enzymes of the sound grape and the fer-
mentation enzymes provide such proteins. With time, some of these proteins can
also associate to form insoluble precipitates and participate to “protein instability”.
For protein-protein interactions, it is necessary that the two proteins should be of
opposite charges at the beverage pH for polar associations to occur. These associa-
tions lead to a reduction of hydrophilic sites and hence precipitation. Also, further
hydrophobic bonding due to association of hydrophobic sites in aqueous media can
lead to an increase in effective molecular weight and precipitation.
Two other fining proteins must be mentioned, namely egg albumin and casein
(more precisely egg proteins and cow milk proteins). However, in both cases the
floc formation is due to the insolubility of the fining protein at pHs below their pIs,
hence the fining action is not the same as in the case of gelatin which is soluble at
all pHs, even at its isoelectric pH.
5.1.2.4 Gelatin Fining
In Europe, most of the available gelatins is Type A pigskin gelatin (ossein gelatin,
having a higher viscosity, is mainly employed in film forming applications). There
is a wealth of European data which show that low Bloom strength gelatin (Low
Molecular Weight) is optimum for fining (Calderon et al. 1968). This “dictate of the
art” is probably due to the facility of use of LMW gelatins when compared with
high molecular weight (HMW) gelatins that need to be heated before incorporation
in the wine. High Bloom Strength (HMW) gelatin used in fining have to be prepared