When producing beers that must have a "clean" / "neutral" flavor like lagers, usually the fermenting temperature is low to prevent off-flavors to show. But then, is recomended to apply a diacetyl rest in order to reduce the buttery, butterscotch-like flavor to beer that the diacetyl provide. Here's the part I don't understand. You do a lager fermentation at lower temperatures to prevent off-flavors, but to do a diacetyl rest you raise the temperature from about 50° to 55° F to 65° to 68° F. What's the explanation for that? Isn't raising the temperature causes the yeast to produce undesired flavors to the beer? I don't get it!
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1 Answer
Most bad esters are made during growth phase and early fermentation, it's only at the end of primary fermentation that you do your diacetyl rest, after the risks of undesirable esters has past.
answered Dec 27, 2015 at 17:26
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But wouldn't that cause also yeast autolysis? Like if the 'food' for the yeast has ended, why diacetyl will reduce? Wouldn't the yeast start to eat its dead friends? Commented Dec 27, 2015 at 21:29
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True that's one reason to rack off primary yeast. Diacetyl rest is usually your last two days of primary. Commented Dec 27, 2015 at 21:34
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What should I understand by rack off? Like keeping it at the primary / not transfering to the second? But that wouldn't prevent the bad byproducts produced by the autophagy already made by the yeast right? Commented Dec 29, 2015 at 12:53
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I'm sorry some terms don't translate well. To "Rack" means to move beer or wort to a new container. To "Rack off" means to remove the beer or wort carefully leaving behind what is on the bottom in this case the yeast cake. And then there is "rack out" which removes your product from between two seperation layers, IE yeast and a starsan layer on top that siphoned in the fermenter. Commented Dec 29, 2015 at 13:06
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Autolysis really only happens when there is nothing else in the form of easy sugars and nuitrients for the yeast to consume. You do want to be on the primary yeast during diacetyl rest. Commented Dec 29, 2015 at 13:19