6C Proteins 221
depletion (across all protein fractions) observed in this study corresponded linearly
with the level of bentonite addition (percentage reduction in protein concentration
per g/L of bentonite added ranged from70% to 89%). These different conclusions
in the published literature might be attributed in part to the different methods used
to fractionate proteins and assess their levels.
Several authors have investigated the extent of adsorption of standard and model
proteins by bentonite. Gougeon et al. (2002, 2003) studied the absorption of two
homopolypeptide preparations with average MW around 20 kDa onto a synthetic
bentonite. Their data suggested that these polypeptides tended to unfold and take on
a more random coil structure upon absorption. Using a range of physical measures,
Gougeon et al. (2002, 2003) also hypothesized that the polypeptides were primarily
absorbed near the edges of the bentonite sheets rather than within the interlayer
spaces between the sheets. The adsorption of the standard protein, bovine serum
albumin (BSA) by bentonite in model wine solutions was studied by Blade and
Boulton (1988). Adsorption was shown to be independent of temperature, but varied
slightly with protein content, pH and ethanol content. In another study (Achaeran-
dio et al. 2001), bentonite adsorption was evaluated with three proteins (BSA,
ovalbumin, lysozyme) in a model wine solution. The effect of ethanol content and
protein molecular weight on the adsorption capacity of bentonite was also studied.
Adsorption capacity tended to increase with increasing ethanol concentrations with
regard to adsorption of BSA and lysozyme, however, no change was observed for
ovalbumin. Blade and Boulton (1988) showed that maximal absorption was reached
rapidly, and complete within 30 s of the addition. This is consistent with an earlier
study (Lee 1986), in which Gew ̈urztraminer wine fined with bentonite was rendered
stable one minute after bentonite addition and with later studies of in-line dosing
described below (Muhlack et al. 2006; Nordestgaard et al. 2006). In comparison,
bentonite fining in a winery setting typically takes one to two weeks, depending on
the tank size and rate of bentonite addition used.
Bentonite regeneration refers to the desorption of adsorbed wine protein from the
bentonite surface, and would permit bentonite to be reused. However, a commercial
process for bentonite regeneration does notcurrently exist, and thus bentonite is only
used once before being discarded. An early study (Armstrong and Chesters 1964)
investigated the effect of pH on the desorption of pepsin from bentonite. In this
study, 62.8% of the pepsin (pI 2.8), which had been adsorbed onto the bentonite at
pH 3.0, was desorbed by raising the pH to 5.2 using sodium hydroxide. In a more
recent study (Churchman 1999), the batch treatment of bentonite-protein complexes
with a range of bases at a variety of different concentrations, durations and agita-
tion methods was examined. The greatest degree of desorption was achieved with
sodium carbonate at a low solid: solution ratio and with agitation. Similar results
were obtained with sodium hydroxide at pH 12 and 13 (protein released per gram of
bentonite = 52 and 65 mg/g respectively). However, from the study it was concluded
that the use of alkaline pH and salts was ineffective for substantial removal of pro-
tein from bentonites. Instead, the study suggested protein degradation followed by
a treatment to displace the products of protein breakdown from the bentonite was
required. For example, a batchwise treatment that employed hydrogen peroxide,