Mercaptans (was) Re: Help, please.
--- In Distillers@yahoogroups.com, "miciofelice2003" <miciofelice2003@...> wrote:
> Ciao a tutti.
> My distillate has a strange smell, look like rubber/plastic burned
> but, believe me, IT'S NOT an overheating of grape pomaces (schorching)
> because I used a lot of precautions, just because I know that red
> pomaces are more delicate than white pomaces.
> In my opinion is the owner of grapes (the wine maker) that added
> something to the grapes during the cultivation or during fermentation.
> So I'm planning to try with activated coal: I know that I'll loose
> some good perfumes, but for me is important to eliminate that bad
> smell (that become less if I let for a night the distillate into a
> cup to open air). I already bought a bag of powdered activatd coal.
> My question is to know the quantity to add to my distillate (about 5
> liters at 75%, as it flow out from the alhembic)), the time that the
> coal has to stay into the distillate, and if then I've to re-distil
> or if is enough to filter the grappa.
> Thank for helping: consider that for me is the first time I add
> something to my grappa, so ... be patient in explanations.
> Ciao a tutti
> micio felice
The 'burnt rubber/plastic' smell most often comes from excess furfural (furfurolo) produced by burning or scorching the pomace. But you have said that you did NOT do this. So, another reason needs to be found and there is one.
Aroma profile for Burnt Rubber A fault resulting from production of certain mercaptans. Chemicals contributing to aroma:
Mercaptans are very nasty things and can NOT be removed by carbon filtering alone. From several sources, I have put together a reason why they have formed, and a possible fix for your distillate. As always, you need to test the fix on a small amount before you do the whole batch. So to business...
Excess hydrogen Sulfide (H2S, rotten-egg gas) can often be found in Marc or Pomace due to breakdown of yeast proteins and/or Sulfur preservative used in storage. Longer pomace storage times can make the problem worse. When this pomace is distilled, the excess H2S cannot all be removed by Copper contact. So some H2S ends up distilling over into the spirit.
The H2S then reacts with ethanol (alcohol) and forms Ethyl Mercaptan, which is even more unpleasant (the burnt rubber/plastic smell). Airing out the distillate then makes another chemical reaction. Ethyl Mercaptan and Oxygen react to form Hydrogen DiSulfide, which is burnt rubber again and non-volatile. It cannot be removed by further distillation alone.
So now you know what you are dealing with and why it happened. Now to the possible fix...
According to Artisan Distiller [Prof Kris Berglund], the best approach for an improvement could be achieved using a product containing Silver Chloride . 'Ercolid' or 'Sulfidex' powders are available from good winemaking suppliers. They are non-dissolving powders. Use them at the rate of 100g per 100 litres of your smelly distillate. Leave it in for 3-10 days, stirring once or twice per day. Then filter the distillate to remove the powder and get a clean product.
In the future, you can avoid this problem by distilling the pomace with a cup (250 ml) of [Copper Sulfate 10% solution] poured in with the pomace per 100 litre still. This will convert any H2S to non-volatile Copper Sulfide, which will remain in the pot and not distil over. Of course you need to throw away those pot dregs. Don't keep them for further use.
I hope this will help you recover your Grappa as something worth keeping. Let us know the results.