Evaporation

[Hogarth]

Something that's been puzzling me ... when you boil the wort, what actually evaporates? I'd assumed it was just water vapour, but the stains on my wall have a brownish tinge, which suggests something else. Do the malt or the hops boil off too? (Not that I could taste either when I, um, had a lick.)

[Jim]

Some volatile components of the hops are driven off with the steam, maybe that's what it is.

[Hogarth]

Thanks, Jim. I thought it might be something like that.

Really this is a question about batch sparging. I've been reading DaaB's excellent introduction to it at 18000feet, but one thing puzzled me: why should you calculate the loss to evaporation and increase the quantity of wort accordingly? If it's only water that evaporates, shouldn't you simply let it do so, and replace it with more water? Otherwise the boil will also condense the wort.

Apologies if this is a clueless question --- and there'll probably be more of them. I'm just about to attempt my first all grain and it's All Very Confusing.

[Jim]

I've never batch sparged, but as far as boiling wort is concerned, the evaporation is all part of the process. Because of evaporation, (commercial) brewers have always collected more wort than they need and allowed some to evaporate to leave the correct brew length.

The wort is concentrated by this evaporation, of course, but the extra wort collected (from a fixed-size grist) dilutes the wort in the first place, so you end up with the correct concentration at the end (hope that makes sense - I know what I mean, anyway ).

[Hogarth]

That makes perfect sense! Thanks.

[Hogarth]

Thanks DaaB. Clueless first AG brew to follow shortly...

[steve_flack]

You don't just boil off water when you're boiling. As well as some volatile hop compounds - some of which aren't that tasty - you also boil off undesireables from your malt. For example, whilst boiling a wort made mostly from pilsner malt, a compound called dimethysulfide is generated. This has the aroma of rotting cabbage so is something best boiled away.

[BlightyBrewer]

Something that's been puzzling me ... when you boil the wort, what actually evaporates? I'd assumed it was just water vapour, but the stains on my wall have a brownish tinge, which suggests something else. Do the malt or the hops boil off too? (Not that I could taste either when I, um, had a lick.)

I can't believe you licked your wall!!

[andyp]

So.

During the boil is it best to keep the lid on (quicker boil, less electricity etc) or off (evaporate off those nasties)?

[Calum]

Definately off.

If you leave the lid on you end up with a really bitter beer (and not in a good way).

[BlightyBrewer]

Am I the only one here that finds this whole wall licking thing alarming?

Is it ok to have the lid on when the wort is coming to the boil - i.e. to help it get to the boil quicker? (Not that I am sure it will make a big difference? )

[Aleman]

Generally the first thing you notice when you have the lid on for the boil is that its boiled over

I actually have 90% lids for my kettle, so I'm boiling with the lids on, but more than enough area for volatiles to escape

[andyp]

I suppose if it boils over you can always lick the surface you're boiling on should you run out of wall space!

[Vossy1]

I suppose if it boils over you can always lick the surface you're boiling on should you run out of wall space!

[Hogarth]

I can't believe you licked your wall!!

All in the cause of science!

Can't say I noticed any volatile hop compounds, mind. The predominant flavours were paint and cheap bleach.