The expert homebrewers in the room can scroll down to the chart and explanation below. As background for the rest of us, it is said that "Brewers make wort, yeast make beer." and that "Happy yeast make good beer". In a quest to keep my yeast happy, I have read lots of posts, gained some sound advice via the NB Brewmaster's chat, and read the book Yeast. I've learned that, for the home brewer, the three most important things are sanitation, sanitation and sanitation. The forth is temperature. After that comes pitch rate and oxygen. Those last two are easily handled by a yeast starter and oxygenation. Temperature can be handled too, but it takes a bit more thinking and planning. Depending on where you are, and the time of year, it may involve either heating or cooling. Various heating and cooling methods are available.
I've learned that:
- fermentation produces heat, so wort temperature runs above room temp
- the yeast care about the temperature of the wort, not the room
- although the yeast's optimal temperature range may be wide, say 55 to 75 F (for a particular ale yeast), the yeast will be most happy if temps don't vary by more than 2 or 3 degrees during active fermentation.
I've seen a few posts offering estimates of how much hotter the wort is than the room. Some say 4 to 7 degrees (F). Others say it can be more, still others say it's only 1 or 2 degrees. It seemed to me that the temperature difference would reach some maximum at the peak of active fermentation and taper down to no difference once fermentation completed. Since some fermentations are very quick and active while others might progress at a more steady pace, it would seem that the size of the maximum peak is would probably vary according to how active the fermentation is.
To begin to find out, I bought a temperature controller and connected a window air conditioner in a small room holding my just brewed wort. The chart below shows the results.
I had pre-cooled the room to 65 then set the ac to max cooling to let the controller take over based on wort temp. The temperature controller was set to turn the AC off when the wort temp fell below 67.0 and on when it rose above 68.0. The brew was NB's Bourbon Barrel Porter Extract Kit
in a 6 gallon glass carboy
with no external wrapper or insulation.
Obviously, I should have cooled the wort further at the end of the brew before pitching the yeast. I'm still learning, but I think the large initial temperature difference is simply due to my failure to cool the wort enough. I was surprised that it then took nearly 12 hours to bring the wort down 10 degrees in the air conditioned room. At that point, the controller turned off the ac, and the room temp rose much faster than the wort temp. A few hours later, the wort reached 68 and the ac came back on, driving the room temp back down. About 20 hours in, I changed the ac unit from max cool to a minimum room temp of 63 so it would run any time the wort was above 68.0 but would not cool the room below 63. As the chart shows, after initial cooling, the wort runs about 4 degrees F above room temp at 24 hours after pitch to nearly room temp at less than 72 hours after pitch. The fermentation appeared to be most active at around 24 hours. It was active and steady with several inches of foam on the surface although it never did reach up into the blow-off tube. "Your results may vary."
For my next brew, I'll do a better job of initial cooling and report back. Comments, suggestions and feedback are strongly encouraged. I'm here to learn.