Diacetyl Rest and Lagering

You also have to keep in mind that active fermentation doesn't necessarily get rid of the excess diacetyl during fermentation. It is once fermentation has mostly ceased that the yeast begin to actively forage for alternative sugar sources - mostly diacetyl. So, in either case, it is important to let fermentation continue for at least a week or more after fermentation seems to have stopped. This means to not let the temperature drop to below the low 40's during that time. According to Chris White, crash cooling immediately after the diacetyl rest is not recommended. Rather, a slow drop of about 1.5 C per day (better 0.8 C every 12 hours) is recommended, until the low 40's is reached. Then leave it there for about another week before dropping it to the low 30's.

 

What I meant is that when the yeast have used up all the readily available sugars in the beer, they then look for alternative sources of food to build up their reserves, which includes diacetyl. Before that, they prefer the malt sugars to the diacetyl. Thus if the beer is crashed before the yeast have turned their attention to the diacetyl, the yeast become dormant and can't finish the job of reducing the diacetyl. Chris White and Jamil Zainasheff say, in their book Yeast , "Separating the yeast from the beer too soon or cooling the beer early can leave a considerable amount of diacetyl and diacetyl precursors in the beer. Even though you may not taste the diacetyl, the beer may still coNTain high levels of the diacetyl precursor acetolactate. Any oxygen pickup during transfers or packaging will most likely result in diacetyl, and once you remove the yeast, there is no simple way to get rid of diacetyl or its precursor. They also say: "while it is possible to do a diacetyl rest once the fermentation reaches terminal gravity, the proper time to do a diacetyl rest is two to five specific gravity points prior to reaching termInal gravity." Another key statement is: "What you do not want to do is allow the fermentation temperature to drop at the end of fermentation. This will greatly slow or stop diacetyl reduction. Many brewers make the mistake of lowering the beer temperature immediately upon reaching terminal gravity, because they assume fermentation is complete and the beer is ready."

 

Further Yeast states: "... very rapid reduction in temperature (less than 6 hours) at the end of fermentation can cause the yeast to excrete more ester compounds instead of retaining them. They recommend a slow rate of cooling the beer at the rate of 1 to 2 F per day (0.5 to 1 C). This is done to avoid sending the yeast into dormancy. Traditionally, once the beer reaches a temperature close to 40 F, the brewer transfers the beer into lagering containers. They say that some yeast strains require two week or more at 40 F to clear completely.

 

How long do you leave it after terminal gravity has been reached before you bring it down to below 40 F?

 

Wow, that's a very short time, and I'm wondering how you avoid diacetyl and esters,since it goes against what Chris White and Zainasheff have to say. Even my ales aren't kegged until at least 3 weeks have passed. My lagers are longer than that (about 4 weeks) before I drop it below 40 F. I've got one lager currently dropping slowly from a diacetyl rest (currently at about 51 F) and it's been 16 days since I added the yeast. Do you use a large and very healthy yeast starter as well as low oxygen methods? Which yeast do you use?

 

I broke down and purchased a three- as well as a five- liter Erlenmeyer flask for the starters. I used to do double or triple starters for lagers in the 2-liter flask, but it took too long, and there wasn't enough head space, so had to make 1-1/2 liter starters. The 2-liter flask seems almost useless for lager yeast starters. The 3-liter flask is much more practical for healthy yeast and has lots of head space and no need to reduce it to 2-1/2 liters. The 5-liter starter is better for yeast that has sat around for a few months.

 

I use them as back-up in case the yeast doesn't start, and keep several packages on hand That happened to me earlier this year. Other than that. I haven't used them as a primary yeast source as I understand that they don't compare well to the liquid yeasts.

 

Thanks, Jack. I might give it a try and compare it to the ones made with liquid yeast.

 

According to Chris White in Yeast, if you crash your beer to lagering temperatures shortly after a rest (or whenever), you can get diacetyl later. A d-rest alone won't ensure that you will not have diacetyl later.

 

Well I did write to Chris White (founder of White Labs and one of the authors of Yeast: The Practical Guide to Beer Fermentation"), about some of the questions raised, and he wrote back last month. I then asked for some additional clarifications and heard back from him yesterday.

Most of you will not like the answers that he provided, since it requires a LOT more time than most are willing to devote to fermentation/lagering, but here they are, along with my questions.

1. How quickly can you raise the temperature of the lager to the diacetyl temperature range?

Chris White: Most people do not have a way of heating a fermentor, so it happens slowly anyway. By starting the diacetyl rest around 1.020-1.025, the yeast are still creating heat which helps raise the temperature gradually.

1.b I guess I wasn’t too clear in my question. Let’s say the fermenter is in a freezer with a temperature controller and that the fermenting wort is held at 48 F. Then you want to do a diacetly rest when the SG reaches about 1.020-1.025. Is it OK to take the fermenter right out of the freezer and into a room with a temperature (room temperature) approaching that of the diacetly rest (68-74 F)? In other words, is it OK to let the temperature rise naturally 20-26 F over the course of a day or two, or is that rate of a rise too much of a shock to the yeast?

Chris White: YES,THAT IS WHAT MOST PEOPLE DO

2. At what rate is the temperature lowered immediately after the diacetyl rest?

Chris White: It is still best to go slowly. Traditionally it was lowered to 50-55, left there for 2 days, then lowered to below 40.

2.b You mentioned in your book (page 115) that the temperature should be lowered at about 1 to 2 F (0.5 to 1 C) per day. Is that the rate used from the diacetyl rest to 50-55, or from 50-55 down to 40? Or is the same rate used from diacetyl rest all the way down to 40 F?

Chris White: THAT IS ALWAYS A SAFE WAY TO CHANGE THE TEMPERATURE

3. Once you reach 40 oF or slightly above, approximately how long should you leave it at that temperature before ‘crashing’ it to near freezing?

Chris White: Everyone does it different- traditionally it was 90 days. Now 30 days is the longest norm.

3.b Is that 30 days AFTER the beer has reached 40 F? In other words, if the diacetyl rest was 72 F, and then reduced 1-2 F per day, it would take 16-32 days to reach 40 F. Are you then suggesting that it stay at 40 F for an additional 30 days minimum (i.e. a total of 46-62 days) before even bringing it down to lagering temperature?

Chris White: YES, AFTER REACHING 40F

4. What would be the best time to rack the beer from the yeast sediment?

Chris White: After the yeast settles, it does not do anything under the beer, it is the yeast still left in solution that does. So as soon after terminal gravity and yeast settles, rack it.

 

I specifically mentioned the expression "before even bringing it down to lagering temperature" (I should have said "cold conditioning"), which he did not address or correct, if that was his intention. His book also mentions leaving it just above 40 F before bringing it down to near freezing for lagering, suggesting that the yeast is still working above 40 F but not below it. I agree that he's not to clear on his meaning. Read pages 114-115.

 

He says: "...very little happens once you take the yeast below 40 F (4 C). If you want the yeast to be active and to carry on reduction of fermentation by-products, it happens much faster at higher temperatures. As far as yeast activity goes, crashing the temperature or lowering it slowly makes little flavor difference if you are dropping the beer below 40 F (4 C)."

Further along, he says: "After a few days (of slowly dropping the temperature) the beer has reached a temperature close to 40 F (4 C) and there are still some fermentable sugars remaining, about 1 to 2 P. "