Early Saturday morning, starting at 4:45 am, I warmed up the still and began distilling 100 gallons of pear wine from Doug Fabbioli. The wine was aged in old Bowman casks, and I was curious if this would have any effect on the taste of the distillate, the amount of heads (perhaps more methanol in this run), or the quality/quantity of the hearts.
By and large, the take from the run was ample and of good quality. We got something like 22 proof gallons of very nice distillate, which is not bad. As usual, I turned off the still, opened the hatch to let the steam escape, and dumped the spent pear wine.
I wasn't ready for what I would see the next morning. Overnight, the entire still had turned a bright amethyst.
Perplexed, I posted a query on the American Distilling Institute forum, and got back this satisfactory explanation:
I think the purple stain is a little more complex than the suggestion of merely an oxide of Copper.
All stills which are made out of Copper quickly form a protective layer of Oxide which is just the dull layer which is normal if the copper has not been treated to keep it bright.
It seems form the picture that the coloration is strongest at the front of the still, around and above the entry port.
Pears have seeds or pits which contain amygdalin which under acidic conditions will be converted to cyanide, th "OCN" chemical group.
In the presence of moist conditions this will form cyanuric acid.
Cyanuric acid on leaving the still body as a vapor and meeting the copper oxide on the outside of the still will form copper cyanurate which is noted for its bright purple or amethyst hue.
That's my theory. anyhow!
Alan Wolstenholme, BSc Chemistry, Edinburgh, Scotland
Caledonian Solutions Limited
It seems the key factors here are the low pH of the wine and the presence of these compounds in pears specifically. We have distilled pears before without this purpling effect, so I'm guessing that the contact with the wooden casks increased the cyanuranic compounds in the wine.
Eventually, we'll need to remove the purple with polishing, but for now, I think it is rather pretty.