Sometimes warm sometimes not, why?

[schmurf]

Just a thought, I've noticed this a couple of times and wonder if there's an explanation to it or if it's a sign of something bad. I know about yeast "at work" produces "heat" and warms up the fermenting wort. This is probably different for every yeast strains, some more able than others. But why would different batches of fermentation with same yeast act differently in this regard?
For example, some weeks ago I overbuilt a starter of wyeast 1318 and split it into 3 starters. I used one of them this Sunday and another one the Sunday before that. The last one made the temperature shoot up more than I wanted (peeked at 21C), the first one stayed within the expected range (max 19.5C). The ambient temperature is the same for both batches, 16-17C.
It wasn't the same beer I was brewing, would behaviour change with change of grist? Darker and lighter beer?

 

[Vale71]

Higher OG would certainly cause a higher temperature spike as more sugar essentially means more fuel available. Pitch rate and yeast vitality would also be important. When you say "the last one" do you mean chronologically or in the order you listed them? If by that you mean the one with the fresher yeast spiked higher than I'd say what you got is in line with what you would expect with fresher yeast, i.e. a faster and more vigorous fermentation.
Long story short, with the same fermenter geometry and the same ambient temperature the fermentation that will consume sugar (i.e. fuel) at a higher rate will also reach the highest peak temperature. This is true of all exothermic reactions.

 

[schmurf]

Thanks for the reply.

OG on brew #1 was 1.053, on brew #2 (this Sunday) it was 1.056. It was #2 that had the temperature rise.

 

[Still Brewn]

My guess...I'm no biologist..... starving yeast, waited a longer time to pitch.... different sugar make up of the wort and lastly actual room temperature was not actually the same for each fermentation.


Room temperature is the easiest for me to explain by my own example. The butler's pantry I ferment in during the winter at my house has different "zones" I can put my fermenting buckets. On the counter 69° on the floor 64° 4 inches above the floor 67°-68° . All of these temperatures are in the same 5x6 butler's pantry and vary with the outside temperature. If I put the fermenter on the floor the bottom is about 4-5 degrees colder than the top if I put the thermometer on the top of the fermenter it is (after fermentation completed) it 68-69 just moving across the 2 1/2 foot walk way can raise the temperature by 2 more degrees. All of these temperatures can vary with the temperature outside by a degree or two except the floor temperature which can get done to about 60°. Of course the temperature also varies in the summer to much warmer so I ferment in my basement then that, depending where I'm at, can vary from 40's in the winter to to upper 60's I. The summer. We have a wine cellar that keeps a pretty constant temperature of mid to upper 50's most of the year so most of the summer I keep something going in there. But even that varies from floor to counter to ceiling by about 5 degrees.

 

[schmurf]

Yes there's probably a lot of factors to take into account, but room temperature was same on both occasions without doubt. Fermented on the same place. Pitching temperature 20C on both and I know from previous experience the temperature get down to 18C before the fermentation starts (or visible signs of it).

 

[ESBrewer]

How high it goes depends on the rate and nature of exothermic reactions at any moment (mainly fermentation reactions of different types of sugars but there are more).

Oxygen probably makes difference. More oxygen means more yeast growth. More cells (fermenting units) probably means higher exothermic reaction rate and thus more heat. The same logic applies to pitch rate which may vary even if the starter volumes were same.

Also, there could be minor changes in wort composition. Certain sugars react easier and produce more or less heat.

Minor changes in ambient temp may have exponential effect on initial reaction rate causing a more prominent rise in temp. See the curves http://www.mikeblaber.org/oldwine/chm1046/notes/Kinetics/Temprate/Temprate.htm
So higher temp means more reactions, more heat and this causes more reactions and so on...a positive feedback unless temp is controlled or it runs out of reactants. Same thing with all other factors that might affect the reaction rate.

Then there are mutations and in some cases selection of yeast (multistrains etc)..external factors other than temp such as the fermenter shape and insulation, sunlight...

 

[ListerH]

Thanks for the reply.

OG on brew #1 was 1.053, on brew #2 (this Sunday) it was 1.056. It was #2 that had the temperature rise.

OG will only give an indication of the level of fermentable sugars in the beer... yeast activity will be influenced by the amount of food they have to chew on. If all other factors were the same (ambient temperature, pitch temp and rate etc), in general, I would expect a more vigorous fermentation with a lighter colored grain bill for this reason (of course the mashing process in terms time and temperature also play a part in the levels of fermentables).

 

[schmurf]

Thanks all, good replies. Yes I guess there is a lot of factors that come in to play and I shouldn't expect same behaviour from a yeast on every brew!