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I have recieved a tin of Coopers stout off my dad, so i brewed it half way through though i noticed it was out of date by 5 years so brewed it any way.
i used the yeast that came with it but nothing happened so i waited 2 days and pitched another yeast. The temp is 24-26 degrees the brew beside it is a coopers real ale and it is going off even bubbling thru airlock, i have gunk everywhere is this a problem when i opened the lid it was fizzing like soft drink, the real ale is at same temp what is going on. this is the first time it has happened and i've been brewing awhile.

shornsheep,

I reckon you'll be fine. Cans of concentrate won't go off. All that will happen is that the malt will darken over time, which when you're making a stout isn't going to be a problem!

The older yeast gets the fewer viable cells there are. So after probably seven years (a couple of years from packaging to use-by date then the five years since) it's unlikely there was any viable yeast in the pack. You did the right thing by pitching another yeast and not throwing out the beer.

As for the foaming, I find that darker beers tend to foam more than lighter-style beer. Try and get the temperature down towards 20 and you should find your problems solved. You'll also end up with a better-tasting beer.

Cheers,

Oliver