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I see that's just 4°C less than the BP of water...so I get that part.
But why is 96°C the magic number? Why not 98°C or 94°C?
Or is it really the stiller's choice?
Stopping at 96C makes no sense. That's still about 5% ABV. Total waste.
Sorry, I should have been more clear - the 96°C is the vapor temperature at the top of the still.
Its the only reason I have a thermometer at the high point of the vapour path on a pot still. However, I don't stop at 96°C, I keep going to 98°C. Even then I turn off the boiler power, but leave the condenser running to take more on the residual heat in the boiler.
Even more so on a jacketed boiler where there is a hot fluid in the jacket. It might be a bit slower than continuing to run under power for longer, but to me it makes sense. Some folks strip even deeper into the end of the run than 98°C.
Can use a temp as a guide, but taste will tell you quite nicely when the alcohol is depleted.
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In my worthless opinion - there is no right temperature - mainly because there is cost/benefit component here.
What's your time worth? What's the energy and water cost involved? As you progress through the run, the marginal volume of alcohol recovered declines. You recover more usable alcohol in the first hour than in the last.
This doesn't even take into account the composition implications of going deep into tails.
Maybe you are reading 5-10% alcohol - but what alcohols? Don't forget about 1-propanol, isobutanol, 2-methylbutanol, isoamyl alcohol. You might want them, you might not, depends.
Do what works for you.
I thought I was stopping at 208F, until I boiled water using the same probe, and it barely got to 208... whoopsie! if you are going to trust a thermometer, it needs to be calibrated at the point you are trusting it at
Thanks for all the replies. You all have confirmed my suspicions and observations.
The big part of this, for me anyway, is the cost/benefit for trying to get out the last few drops of product. When I could actually see the higher density water in the product, at about 93°C, and couldn't taste any alcohol in the distillate, all that extra energy into the boiler seems like a waste for such rapidly diminishing returns.
(And yes, the thermos are calibrated, and atmospheric pressure is taken into account to determine boiling points.)
For stripping, I'm pretty much with @Myles, and 98 or 99 is my endpoint, depending a bit on whether I'd have to change in a new collection container when I got close to 98 or 99. If my only concern were alcohol(s), I might quit sooner, but my late stopping is as much for flavor as for ABV. No matter what the distillate tastes like at 93, simple physics tells me there's alcohol and flavor still coming across.
My spirit run goes late also, because that all goes back into the next strip run.
Zymurgy Bob, a simple potstiller
my book, Making Fine Spirits
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