When to D-Rest

First, I apologize if this question has been covered, as I have seen that it seems to be a repeated topic. However, I could not find the answer I'm looking for.

The big question is, WHEN do I do a diacetyl rest?

The general consensus seems to be when fermentation is at 75%. But how do I know when it's at 75%? The logical assumption is, check the gravity, but in a previous thread, I read that you shouldn't check the gravity too often. Is the rate of fermentation even linear? I'm guessing it's probably a function of LN, so it would be extremely difficult to predict the rate of fermentation in order to calculate when it's at 75%, so as to limit the number of times I have to stick my wine thief into my beer.

So, is there a way to tell when it's at 75%? When the krausen starts to fall? After it's fallen? When the airlock starts to slow? I'd think that after the krausen has fallen, fermentation is a lot further than 75%.

I've also heard, "After N days." Well, if the fermentation is strong and rapid, it could be completely finished in N days. That's something that would vary too much for a simple rule of thumb like "after N days".

How do I know when to pull my lager out of my fridge and stick it somewhere warmer for a couple days? Thanks in advance.

I personally wait until the krausen falls.

Lagering is just a cold storage method, where the yeasts finish up fermentation that's already begun at a slightly warmer temperature. For example, you start your wort with some pilsner yeast and let it ferment for two weeks or so at 55*f in your fermenter. Once the cold fermentation has slowed down and peak krausen has ended, you chill the beer another 10 degrees to 45*f for "lagering" for another four weeks. This cold storage allows any residual fermentation to continue very slowly and settles a lot of crap out of your beer (proteins, yeast, etc.). The result is a very clean, smooth tasting beer.


A diacetyl rest may or may not be necessary for your beer. If you need to know whether your beer has any detectable diacetyl, take a sample with a sanitized tube/container/whatever and taste it. Does it have that buttery, butter scotchy flavor? Does it feel slick on your tongue? If so, your beer would benefit from being allowed to gradually warm up to 70*f for a week or so. When you warm yeast up, they'll go after that diacetyl as a food source and convert it into more pleasant esters.

So, ask yourself first, do I NEED to do a D rest? Did my beer ferment a little on the warm side for a lager yeast? Do I taste buttery flavors? Is it like an oil slick on my tongue? If those answers are no, there's no need to do it.

According to the things I've read and understand if you wait until the airlock just stops bubbling or is really slow, that is a good guesstimate on the 75% gauge.