Butting in here as Myles is not on-line
The process involves dilution of the low wines to below 30% - say about 27 or so - letting stand for 3+ days then siphon the middle third out and only use that - add to the remaining low wines and repaet the process - after I don't know how many passes, I guess the upper and lower portions need to be chucked out
Quote from "Whisky: Technology, Production and Marketing"
Alcoholic strength of the charge of combined foreshots, feints and low
wines should not exceed 30 per cent ABV; strengths in excess of this lead to
blank runs, when the demisting test fails to indicate potable spirit. In such
circumstances the demisting test protects the previously collected potable
charges from an influx of non-potable spirit, which, with its high concentrations
of higher fatty acid esters and long chain saturated carboxylic acids,
would impart a ‘feinty’ note to the spirit. The demisting test should always
be available, even if foreshots are collected on a timed basis.
Low wines and feints receivers and chargers act as separating vessels. The
last runnings of a spirit distillation contain the heavy oils or esters that are not
readily soluble in water. Such oils have an affinity for alcohol, especially at
high strength. At a strength of less than 30 per cent ABV these compounds
undergo a phase separation, where the esters float on top of the aqueous layer
while a small proportion are dissolved in the aqueous layer. If the concentration
of alcohol is allowed to exceed 30 per cent ABV, these floating surface oils
migrate into the higher alcoholic strength aqueous layer, being completely
dissolved. This effect eventually impacts not only on the demisting test, but
also on the whole spirit distillation – potable spirit cannot be collected as the
charge of low wines and feints contains a disproportionate concentration of
heavy oils, making it impossible to have a turbidity-free demisting test result.
With low wines and feints charges at less than 30 per cent ABV, it is still
possible to attract distillation problems. Presentation of the floating surface
layer of heavy oils or higher fatty acid esters as a charge to the still (by
completely emptying the contents of the charger into the still) will result in
an episode when the collection of potable spirit (as determined by the demisting
test) is unachievable. The whole spirit distillation system will have been
contaminated by these esters, and it can take several distillations before satisfactory spirit is again obtained.
To avoid such scenarios, when the low wines and feints appear to be
approaching higher strengths (or have even reached this situation) the charge
can be diluted with water, aiming for a combined strength of less than 30 per
cent ABV and thus stimulating hydroseparation. The surface phase must not
be allowed to enter the spirit still on charging.
Adherence to these principles will ensure a consistent product, both on nose
and analysis. The low wines and feints components reach a steady concentration
state, maintaining equilibrium during subsequent distillations.
