10. Still configuration

The recent growth of the craft spirits industry has spawned a wide range of still configurations, many of which focus on flexibility for different feeds. Such stills are referred to as hybrids. As mentioned above, ethanol is only part of the composition of the distilled spirit. A range of other compounds, especially a plethora of esters, short-chain fatty acids and methyl ketones are common secondary metabolites of whey fermentations. Their presence affects the final sensory performance of the spirit and therefore should be under control, either by fermentation management or by judicious distillation.

In principle, most "contaminating" secondary metabolites can be removed by employing four distillation approaches in sequence: stripping, rectification, hydroextractive distillation and another rectification step. A distiller may not want to remove all the additional flavor-active components. Using a simple pot still, the fermented whey will distil to yield a product of around 15–20% v/v ethanol, depending on the initial ethanol concentration. This ethanol concentration can be increased up to around 70% v/v with a second pot still. This approach will yield a spirit that will retain significant levels of flavor compounds and so will be most

"whey-like". If a "cleaner" spirit is required more complexity is required in the distillation set-up.

At the other extreme to the two-pot system is the four-stage system indicated above. Stripping is followed by rectification, a process that typically employs a column of plates to enhance the separation of ethanol from the stripped feed. This should yield an output of close to 96% v/v, close to the maximum concentration of ethanol possible at atmospheric conditions from an aqueous ethanol system (the "azeotropic limit"). But this ostensibly clean spirit still retains flavor from the initial stripping feed and needs further processing to clean up the final spirit. To do this, water is perversely added back to the rectifier column output. This has the effect of increasing the volatility of the secondary metabolites, so that they are more easily separated from the distilling ethanol. The output of this column is still relatively water-rich so an additional rectification stage is the final part of the distillation process to elevate the ethanol concentration toward the azeotropic limit of around 96% v/v.

As mentioned above, there is an option of applying a demethylizer as a final column stage. This is an essential operation for pectin-rich distillation feeds such as those from stone fruits and potatoes. The pectin content of whey is negligible so this is unnecessary. One point to note concerning the use of a demethylizer is that it is most effective at low water concentrations (in contrast to hydro-extractive distillation) and so it is best employed after the second rectification step.

For a plant that only distils whey fermentations, the four-column process has most to commend it, as it will yield spirit that is relatively clean or "neutral". From a craft perspective this is a relatively complex distilling operation (with associated fabrication costs), so novel still configurations are becoming increasingly common. From a customer perspective there are three points to keep in mind when seeking distillation equipment:


The two former points help to define the distillation stages and the columns that may or may not be required (columns add significant cost to still fabrication). The latter is an important consideration when considering heat source. Direct heating such as electrical elements can be problematic if heating causes precipitation (e.g. of proteins) as they can congeal on to the heating surfaces and can cause heat transfer and burn-on issues for the spirit. The latter in particular can give rise to burnt-on flavors that are difficult to remove from the spirit despite repeated distillations.
