224 E.J. Waters and C.B. Colby
6C.7.2 Selecting an Appropriate Bentonite Type
Sodium (Na) bentonites swell more than calcium (Ca) bentonites, and thus pro-
duce a greater volume of lees for an equivalent dose, but are more efficient at
adsorbing proteins, and hence, require a lower dose than Ca bentonites to achieve
heat stability (Zoecklin 1988). However, there is little quantitative data that shows
what these relationships actually are. Analysis of a study of bentonites from more
than a decade ago by AWRI (Morant 1990) suggests that Ca bentonites, whilst
requiring (on average) a dose three times higher than Na bentonites to achieve
heat stability, will produce one quarter of the lees. Furthermore, Na/Ca bentonites
required a dose 50% greater than Na bentonites but generated one half the volume
of lees.
The data presented indicates that using Na/Ca or Ca bentonites could signifi-
cantly reduce values losses arising from bentonite fining. For example, estimates
from the above data suggest that a Na bentonite dose of 0.4 g/L produces 6
vol.% lees whereas a Ca bentonite, by reducing lees volume, could regain up to
5% of the original volume of the wine that would ordinarily be occluded in Na
bentonite lees.
However, if this is the case why are Na/Ca or Ca bentonites not more widely
used? There are a number of possible reasons. First, higher doses of Ca bentonite
require greater volume of slurry or more concentrated slurry. Calculations assuming
a 2% constraint on water addition to the wine suggest that for a Na bentonite dose
of 1 g/L, 5% w/v slurry would be required. At the higher doses of Na/Ca and Ca
bentonite necessary to achieve the same treatment effect, the slurry concentrations
are 7.5% and 15% w/v, respectively. It may not be possible or practical to achieve
these higher slurry concentrations and thus Na bentonites are favored. However, Ca
bentonites swell less and higher slurry concentrations may be possible when com-
pared with Na bentonites. Investigations of the rheology of bentonite slurries would
provide valuable data to answer this question. Second, Ca bentonites release Ca dur-
ing bentonite fining and may contribute to tartrate instability. Third, Ca bentonites
may contribute to heavy metal pick up (Boulton et al. 1996), and thus their use may
be undesirable at elevated bentonite dose rates. However, this was not observed by
Morant 1990) when she measured trace metal concentrations for arsenic and copper
in wine following fining. In fact, no difference in heavy metal content was observed
between the Na, Na/Ca and Ca bentonites.
6C.7.3 In-Line Dosing of Bentonite
The in-line dosing paradigm exploits the observation that protein adsorption by
bentonite occurs rapidly – within several minutes (Blade and Boulton 1988; Muh-
lack et al. 2006). Furthermore, if combined with centrifugation, the bentonite and
wine can be separated and the bentonite lees are simultaneously compacted to
reduce value losses. In-line dosing methods for bentonite fining are already used