Why does my frozen wort smell like black currants?

Question

I'm in the habit of freezing extra wort and using it for yeast starters. After I've racked the clean, cold wort from the kettle into the primary fermenter, I'm left with trub, hops, and wort in the bottom of the kettle. I filter this mess through cheese cloth, pour into 500ml PET bottles, and place in the chest freezer.

When the time comes to bulk up a smack-pack of yeast, I take a few bottles from the freezer, thaw them, boil and then cool the wort. I add some O2, and then the yeast. A few day's later, I've got enough yeast to pitch.

It's not really more convenient than using DME, nor does it save any significant cash, but I would feel bad throwing away any of that wort I just spent four or five hours producing.

So this evening I took some bottles from the freezer to build a starter for a lager yeast. Two of the four bottles had a very distinct black currant aroma to them when opened. If you know what Ribena or Creme de Cassis smells like, then you've imagined it perfectly. And I'm not talking about a subtle hint of black currant. It was very pronounced. The two affected bottles were dark wort, but I couldn't speak to their exact makeup. One was labeled 1.052 SG, the other 1.075.

Furthermore, the wort was not completely frozen. Most of the contents were solid ice, but on the very top there was maybe 5 ml of thick, dark syrup. The syrup tasted sweet, malty and bitter, and had the very distinct aroma of black currant.

I can imagine some scenario where syrup would form from the frozen wort, something similar to freeze distillation, but for the life of me, I can't think what sort of chemistry would make hopped malt smell like black currants. I'm curious if anyone has an idea how this could happen.

I went ahead and used the malt for my starter. I plan on cold-crashing and pitching just the slurry, so I don't expect the black currant flavour to affect the beer.

Answer 1

It could the the hops - some hops e.g. bramling cross have a clear blackcurrant aroma, or the use of a dark crystal, which can bring several different aromas, such as toasted biscuit, prunes, raisin and blackcurrant.

When sugar solutions are frozen to make frozen deserts, crystals can spontaneously form. This could have happened in the wort, pulling out also flavour and aroma compounds at the same time, and concentrating them into the sludge that you found.

Just in case you're interested, composition of blackcurrant aroma lists the 60+ compounds that make up blackcurrant aroma. If some of these compounds are still liquid at your freezer temperature, then fractional distillation could have occurred with these compounds, separating them from the rest of the frozen wort.