does fermentation itself generate heat

Most ales ferment at room temp or 68-72F, if you are making a lager then temp plays more of a role as it needs to be fermented cold and aged colder using a lager yeast. for your particular kit your temp may be off but you need to know the range for your yeast.

Yes fermentation does generate heat,

Kasper Denmark "sozman" skrev i en meddelelse news:3f91d8de$0$28119$ snipped-for-privacy@news.optusnet.com.au...

Kasper,

Thanks - do you know if this is this significant enough to raise the wort temperature by a couple of degrees in an insulated cabinet?

Yep I have had a temp raise of 3-4 degrees C in an open room..

Cheers Kasper Denmark

Warning: Geek alert.

Yeah, I believe it should give off heat.

Make the incredibly simplifying assumption (which I hope at least balances):

Glucose (C6H12O6) + Happy Yeast ---> 2 CO2 + 2 CH3CH2OH

Heat of formation of glucose: -1273 kJ/mol (A) Heat of formation of CO2 (gas): -393 kJ/mol (B) Heat of formation of ethanol (l): -277 kJ/mol (C)

Thus:

Total enthalpy = 2*B + 2*C - A Total enthalply change = -67 kJ/mol which mean it gives off heat

Of course, this is incredibly simplistic as wort has different sugars and the actual reaction is very complex. But since the net effect is busting up some carbon bonds, it should be exothermic.

I leave it to the motivated student to use delta G and delta S to prove it's spontaneous (or not)....;^)

But how much would it raise the temp? Hmmm....let's make the incredibly simple assumption that all heat generated stays in the beer (ie, perfectly insulated container) and use the specific heat capacity of water (ignore ethanol's cp). Also assume that all s.g. above 1 is due entirely to fermentable sugars, and that we have 100% attenuation. Use a

5 gallon batch * 0.060 g/ml (glucose contribution) * 3785 mLs/gallon *

6.3 mol * 67 kJ/mol * 1000 J/kJ * (1/4.18 J/g/K) * (1/18925 g) (mass of

Ethanol's cp is about 50% higher than water if I'm not mistaken, which would mean the actual temperature increase would be lower (though probably not by much). It'd also be lower due to unfermentables and less than 100% attenuation.

Anybody wanna check my math or correct any misc> I started my first ever homebrew yesterday. I put the brewer in the fridge

Thanks Christian and Kasper.

So for practical purposes does this mean I should pitch my yeast when the brew is well below the recommended top temperature for that yeast so that the natural temperature rise doesn't "cook" it?

BTW my temperature has stabilised to 22°C for over 24 hours now so at least my heater calibration is ok, bubbling is very slow now.

It's not so much that it will hurt your yeast as the higher temps. will hurt your beer. You'll produce higher alcohols and phenolics that will make the beer harsh and give you killer hangovers. ------------>Denny

There are six on the right. 2 from the CO2 (2 molecules) and 4 from the ethanol (2 * 2 carbons in the molecule).

Of course, I'm a chem e, too, so I leave balancing to the chemists. I've got better things to do....;^)

Chris

David M. Taylor wrote:

(snip)

Huh huh. Chemistry is cool. I used to think about this stuff myself back in the day when I first started brewing. This ain't nuts. It's real science. Cool stuff, ain't it?

Actually, I believe there is supposed to be an oxygen molecule on the input end of the chemical reaction. And, not to be rude, but your equation above does not balance properly. Take, for instance, the Carbon atoms... if there are 6 on the left, then there better be 6 on the right. But anyway... I believe that based on the amount of oxygen in the fermenting brew, differing amounts of CO2 and alcohol will be produced, i.e., in the beginning stages of fermentation, more CO2 is produced and less alcohol. Then when the brew is more anaerobic, more alcohol and less CO2. Something like that anyway. I've got it all in my old notes somewhere... fun stuff.

-- Dave Bachelor of Chemical Engineering by degree

Oops. You got me on that one. But you are basically right: Fermentation produces heat.

-- Dave "Just a drink, a little drink, and I'll be feeling GOOooOOooOOooD!" -- Genesis, 1973-ish