222 E.J. Waters and C.B. Colby
sodium carbonate and photo-oxidation with UV radiation was found to be effective
in removing residual protein from bentonite.
As discussed by Waters et al. (2005), in both these studies, protein was desorbed
from bentonite by increasing the pH, but only in a batch system. As adsorption
and desorption are equilibrium processes, it is hardly surprising that residual pro-
tein was retained on the bentonite surface. In contrast, in a continuous flow sys-
tem, fresh reactant could be continually fed to the adsorption system, while ion
exchange products could be continually transported away from the system, driving
the exchange process to completion. Continuous flow contactors, such as continuous
stirred tanks, packed bed columns and fluidised beds, are commonplace in many
chemical industries, as is continuous catalyst regeneration (Fogler 1992). There-
fore, it was postulated that effective regeneration of bentonite might be achieved by
treatment with a base such as sodium hydroxide in a simple continuous flow system
(Waters et al. 2005). However, the difficulty in regenerating bentonite by salts and
alkalis could also suggest that protein adsorption on bentonite may involve other
mechanisms than solely cation exchange; thus complete regeneration of bentonite
by such a process may not be feasible.
6C.6.1 The Sensory Impact of Bentonite Fining
One of the perceived disadvantages of bentonite fining is that it is thought to reduce
flavor and aroma of wines. This perception is not supported by strong evidence in
the normal operational range of bentoniteadditions by winemakers. Sensory evalu-
ation of wines treated with bentonite showed no significant differences between the
control and the fined samples (Leske et al. 1995). Similarly, using difference testing,
Pocock et al. (2003) reported that bentonite fining of a Chardonnay and Semillon
wine had no effect on wine aroma and palate. This contrasts with previous findings
(Miller et al. 1985) that demonstrated reduced concentration of aroma compounds
after bentonite addition to juice, mustor wine. More recently Pollnitz et al. (2003)
elegantly confirmed that aroma compounds can be absorbed by bentonite, as did
Cabaroglu et al. 2003, although the later study found no sensory effect of bentonite
fining ofV. viniferacv. Muscat Ottonel or Gewurztraminer wine. Rankine (1989)
stated that bentonite fining results in the loss of aroma and flavor and Mart ́ınez-
Rodriguez and Polo (2003) extended this conclusion to sparkling wines when ben-
tonite is added to the tirage solution. This has led to the widespread conclusion,
throughout both literature and industry, thatbentonite fining at typical addition rates
has a detrimental effect on wine aroma and flavor, despite the fact that the conclu-
sions of Miller et al. (1985) were influenced by many factors and “should not be
considered general” (Miller et al. 1985). Similarly, Leske et al. (1995) concluded
that “lack of significant differences observed in this trial suggests that the negative
effect of fining may not be evident to a consumer in a commercial situation” and
“the magnitude of any such [negative sensory] effect will presumably depend on the
individual wine, the bentonite chosen, and the rate of addition chosen”.