6C Proteins 223
6C.7 Improving Bentonite Efficiency
As well as the perceived but questionable adverse sensory effects of bentonite fining,
principal criticisms of the traditional method for bentonite fining are as follows:
- Between 5 and 10% of the wine may be occluded in the lees (Tattersall et al. 2001).
During recovery by RDV filtration, this occluded wine may be downgraded in
quality (Waters et al. 2005).- Gravity settling of the bentonite normally requires several days to a week. Protein
adsorption during bentonite fining is rapid – the treatment effect is completed
within several minutes, as described above (Blade and Boulton 1988; Muh-
lack et al. 2006). Hence, the long processing time required for bentonite fining is
constrained by gravity settling of the lees and to maximize wine recovery during
racking. Furthermore, the long period of contact between bentonite and the wine
during gravity settling might be a contributing factor to perceptions of adverse
sensory outcomes.Consequently, there is interest is developing more efficient ways of using bentonite.
6C.7.1 Predictive Assays for Heat Stability
The amount of bentonite required to prevent protein haze formation differs from
wine to wine and is usually determined by a laboratory fining trial and assessed by
a heat stability assay or test. The heat stability test most commonly employed by
the Australian wine sector is to observe whether a wine sample produces excessive
(e.g.>2 NTU) turbidity following heating at 80◦C for 6 h. Similar accelerated
heating tests are used in other countries. Pocock and Waters (2006) have shown that
this test does not always predict haze formation accurately during long-term storage
trials of white wines. Furthermore, Pocock and Waters (2006) demonstrated that
most of the current predictive assays used by winemakers could be biased towards
over fining and it is also likely that technical errors performing both the heating and
the haze measurement steps add to theirinaccuracy. This could lead to fining with
more bentonite than is needed under most conditions encountered commercially by
bottled wines during transport and storage. Clearly, further trials with more wines
and with storage under a wider range ofconditions would be necessary to confirm
these results.
Therefore, an alternative test method, such as heating at 80◦C for 2 h, an ‘alkali’
modified Coomassie-dye assay (e.g. Boyes et al. 1997), or the more recently avail-
able reagent based test kits such as Proteotest or Prostab, could present an oppor-
tunity to decrease bentonite dose and reduce volume of wine occluded in bentonite
lees if confirmed to predict more accurately haze formation in wine. If these alter-
native test methods could replace the current heat test method of 80◦C for 6 h, this
would also reduce the amount of time and/or effort presently required by a winery’s
laboratory to conduct heat stability testing.