6C Proteins 225
by some wineries and anecdotal reports suggest that commercial applications are
growing.
Results obtained from pilot scale field studies of in line dosing by Colby and
colleagues (Muhlack et al. 2006; Nordestgaard et al. 2006) confirmed that a contact
time of only a few minutes was sufficient for adsorption of the haze-forming proteins
by bentonite. There was no detectable sensory impact on the wine of inline dosing
compared to batch addition.
These studies also observed a carryover of bentonite from the centrifuge during
in-line dosing of bentonite. This bentonite carryover could be reduced by decreas-
ing the operating flow rate to the centrifuge. Furthermore, Na-Ca bentonite was
separated more easily by centrifugation than Na bentonites, and thus produced less
carryover at equivalent flowrates.
This carryover problem, if centrifugation is used, is a disadvantage of the in-
line dosing method because it contributes to downstream wine losses from tanks
containing settled lees and/or constrains the throughput achievable. Some potential
solutions that could be used to overcome this problem include a more efficient cen-
trifuge, multiple clarification steps, an alternative and more efficient clarification
method, an alternative bentonite that is more easily clarified, and “counter-fining”
with another fining agent to improve bentonite separation.
6C.8 Summary
Grape PR proteins are the main proteins that persist through white winemaking and
remain in the wine. It is thought that their denaturation and subsequent aggregation
leads to the formation of hazes and precipitates. The formation of wine protein haze
is multifactorial and depends on the presence of components which are essential
for protein denaturation, such as the sulfate anion, and potentially others as yet
not formally identified. The main grape PR proteins in grape juice and wine are
thaumatin-like proteins and chitinases; they range in mass between approximately
21 kDa to 26 kDa, have acidic isoelectricpoints and are relatively stable to pro-
teolytic enzymes. The levels of these proteins in wines depend on grape variety,
fungal infection, water stress and skin contact during winemaking. Bentonite fining
is still the only commercially acceptable practical solution to avoid protein haze.
The efficiency of this process can be increased by using appropriate test proce-
dures to estimate the dose required, selecting the bentonite type best suited to the
application and using inline dosing and centrifugation to add and remove the ben-
tonite rather than the more time consuming process of batch addition and gravity
settling.
AcknowledgmentsWe thank Australia’s grape growers and winemakers for their support through
their investment body the Grape and Wine Research and Development Corporation, with matching
funds from the Australian government. We also thank our past and current colleagues and students
for their input into our understanding of this area.