We're always excited about a new recipe we want to try, our next brew day, or a new piece of equipment to brew with. Unfortunately, that same enthusiasm rarely carries over to fermentation, and that's where the magic happens. Great brewers will tell you: "Good beers are all about fermentation", "Fermentation, Fermentation, Fermentation", etc, but despite the importance, it seems to be the least romanticized part of brewing. So I decided to write a post about how I care for my yeast, and what happens after the flame goes off. FYI, a lot of this might be repetitive for some brewers, but I've had a number of requests for this topic.

The first thing every home brewer should be told is that the brew day isn't that important. You can commit the brewing equivalent of murder during the the brew day, and if the beer was fermented well, it'll most likely still be good. Let's define some terms first. The 'hot-side' is wort production; the part of the brew day where you're making wort. It starts when you mill your grains (or mix in your extract), and ends when you kill the flame on your kettle. The 'cold-side' is everything that happens after you turn off the kettle (fermentation, clearing, packaging, etc), and the handling of your yeast. Even though it's much more fun to add hops to a kettle, great beer is made during the cold-side.

Yeast make beer. It sounds silly, but wort itself doesn't taste good. The flavors and compounds that yeast generate are what make beer, beer. So yeast should be your primary focus. The first question every brewer should ask before starting a batch is: "Do I have the proper amount of healthy yeast for this beer?" Now in the case of a 1.035 Bitter, it could be as simple as a single vial or smack pack. For a 1.080 Imperial Stout, we're going to need a massive starter, multiple vials, or re-pitch yeast from a prior batch. I don't need to cover pitching rates and cell counts here, as there are a bunch of great calculators available. I use mrmalty.com, and get good results from his numbers. I find his viability percentages to be a bit too conservative, but it's otherwise spot on. The important thing is to stick with one calculator, try it, find out if it works for you, take notes, and adjust as needed.

Most home brewers oxygenate by some means that introduces atmospheric air into wort. Most home brewers (myself included) don't get enough oxygen saturated into their wort. It takes a lot of shaking, with a decent amount of head space to get 8ppm of O2. I finally broke down and bought an O2 regulator and stainless diffuser stone so I can aerate with pure O2. I noticed an immediate difference on high gravity beers, and batches where I re-pitched yeast. If you don't brew high-gravity beers or re-pitch yeast, you might not see the benefit. I, personally, will never go back.

People are unhappy when they are too cold or too hot; yeast are no different. Steady, controlled temperatures are very important to clean fermentation. A sizable chunk of yeasts' ester production occurs during the growth phase of fermentation. That happens to be the first 12-72 hours or so. For this reason, I am a proponent of pitching my yeast into wort that's a few degrees colder than my target fermentation temperature. This allows the yeast to slowly acclimatize to the environment, and ensures they won't take off like gangbusters. Lag time isn't something you should fear, assuming that you sanitize properly and pitch a proper amount of healthy yeast.

Now that the yeast have acclimatized, started to multiply, and warmed up the wort to my target fermentation temperature, my goal is to keep that temperature consistent. Firstly, ambient temperatures do not equate to fermentation temperatures. Buy some of those cheap little liquid crystal stick-on thermometers for like $1-2. They are surprisingly accurate, and will show you what the temperature of your wort is. There was BYO article a few years back that showed the temperature of fermenting wort in the center of the fermenter was no more than .5F off from the temperature sticker stuck to the outside (Do keep in mind we're talking about 5 gallon buckets and carboys.) For the skeptics, a sanitized probe thermometer will also work.

There are a number of methods to keep a fermenter cool. Swamp cooler setups, frozen water bottles, refrigeration, or just a cool basement all work. I live in Arizona, so it didn't take me long to opt for refrigeration and a temperature controller. I tape my temp probe to the side of the fermenter; it works well. Whatever method you choose, just make sure that your temps are reasonably steady. 1-2* of swing isn't too bad, but I'd work to keep it from swinging any more.

Fermentation will reach a noticeable peak, and then activity will start to slow down. Every stain is unique, but generally, allowing the fermenter to slowly warm up at this point is good idea. I ferment most of my ales around 62-66*, but at this stage, I'll let them free rise to 68-72. The yeast will take up and metabolize diacytl, as well as a host of other compounds that could lead to off flavors later on. Keeping the yeast warm and happy at this stage will also aid the yeast in entering their dormant stage. They'll floc better, and generally be healthier for re-pitching.

Re-pitching Yeast
Don't re-pitch yeast from a prior batch if you weren't completely clean and sanitary throughout the process, or if anything about the fermentation seemed off in any way. Since I typically only re-pitch for a couple generations, and don't store my yeast for more than a couple days, I don't bother to separate the trub from the yeast. Using a thoroughly sanitized ladle, I mix up the slurry at the bottom of the fermenter, scoop up a measured amount, and directly re-pitch to the next batch.

Don't pitch onto an entire yeast cake; it's bad practice. You're almost always over-pitching, and you should be fermenting in a clean, sanitized fermenter. It's not that hard to measure out a specific volume of slurry; take the time to do it right. Also, yeast nutrients and oxygenation become increasing important with re-pitched yeast. Wyeast, Fermentis, White Labs, etc all grow their yeast in nutrient rich environments. They are healthy and ready to rock out the smack pack, satchel, or vial. Re-pitched yeast is not as healthy and ready to rock, so a healthy dose of O2 and some nutrients go a long way to a healthy fermentation.

Maturation and Packaging
I don't need to go into depth here, but there's a few things worth covering. First, I'm not a fan of the month-long primary. I just don't feel it does anything. It is a good idea to leave the beer on the yeast for a couple days after fermentation has ceased, but the yeast aren't contributing anything after that. The beer is maturing, which is a good thing, but it can mature off the yeast just as well as it can on the yeast.

At this point, your beer is fragile. It's vulnerable to sunlight, bacteria, and oxygen, so take care of it. Clean and sanitize your kegs or bottles well, and do everything possible to avoid oxygenating the beer at this stage. Those that keg might try flushing their kegs with Co2 prior to racking, especially for hoppy beers. Brewers that bottle shouldn't be as concerned, as the yeast will take up most, if not all, O2 in the bottle when they ferment the priming sugar. Also, don't keg or bottle in direct sunlight, as hoppy beers skunk very quickly in direct sunlight.

Wrap Up
As with every part of brewing, take detailed notes. Note how the fermentation is progressing, what's happening with the temperature, and anytime you change something. The more you work with a specific yeast strain, the more familiar with it you'll become. Every strain behaves a bit differently, and there's no substitute for experience with a strain. WLP007 for example will attenuate 2-5 gravity points after all signs of activity have stopped. Every yeast has it owns quirks, and detailed notes will help you better understand the yeasts you use most often.

I know most of my points in this post aren't earth-shattering, but the take-away should be to really sweat the small stuff when it comes to the cold-side of brewing. Minor and even major mistakes made during the hot-side portion of the brew day can be washed away with a solid, clean fermentation.


  1. Hey Scott,

    Thanks for this post. I do the exact same process as you describe here. The only step I have not taken is re pitching and I definitely want to try that next. What do you use to measure your yeast?

    1. Either use something that's easily sanitized, such as a pyrex measuring cup, or measure the fixed volume of your scoop ahead of time. I've got a small ladle that's 30mls when full. If I need 150ml of slurry, that's 5 scoops.

  2. I don't find the cold side boring :) However, I do agree many people don't give as much attention as they should to it. I have been using the 02 and stone for about a year now and love it. I will warn others, and your mileage may vary, after using 02 my lag in visible fermentation increased. Rather than seeing nice krausen in 6-12 hours it is closer to 18-24, but my beers have been better.

    How long do you use the O2? I have beening doing 45 seconds for 1.050 or so beers and and a minute or so for higher gravity beers.

    1. Very similar. My lag times also increased slightly, which doesn't bother me. I think it's due to the fact that the yeast are growing under aerobic conditions for longer. Once the O2 is gone, they start metabolizing via anaerobic pathways.

      My regulator is pretty cheap, so flow rate is total guessing game. I turn it on while my stone is still in star san, so I can see the flow. I slowly open the valve until O2 just starts to flow from the stone, then I dunk it in my wort, and count to 60. I'll hit higher gravity beers for 90 seconds. I couldn't tell you what flow rate I'm using, so how that transfers to your O2 stone is only a guess. I should really buy a pressure gauge for this thing.

    2. I do the exact same thing - have the O2 barely streaming out so that I can be consistent with how much O2 I'm adding batch to batch. I do 60 seconds for everything but 1.080 and above where I do 90 seconds. Not the most scientific, but it's repeatable - which is key.

      As per another comment I also noticed an increase in lag time with O2, but you can speed it up by pitching active starters (if I time it right my starters are no longer put in the fridge before brew day - if they are I add a quart or two of chilled wort to get them active a few hours before pitching). I don't know if it's that important, but it's simple so I typically do it.

      Great info in this post!

    3. Ya, I like to do that when I can. Doesn't work on my higher-gravity beers, since I can't bring myself to pitch a 1 gallon starter into 5 gallons of wort. =)

  3. One neat tip for the oxygen bottles. Weigh them in grams when they are new. Mine was 633g. Write that number with a sharpie on the label. They hold 1.4oz (39g) of O2, so you can occasionally weigh the bottle to see how much O2 is left.

  4. Scott, nice blog! I have been reading increasingly more of your posts and you have some great stuff. You've introduced me to some cool stuff, gelatin finings, wax tops, etc.. keep it up, I think I speak for a lot of ppl when I say, very enjoyable reading. Cheers.


  5. Nice post. I totally agree with the cold side being the most important. Spend more time with a good start, O2, and temp control....and the beer will get better.

    Have you ever used yeast nutrient? I only add for my high gravity beers...assume it is helping keep those yeast happy

    1. Ya, I use it every time I make a starter. It's super cheap at my LHBS, so i figure why not. I have no idea if it helps or not. I've heard people have had luck with Servo from White Labs, but at $2/batch, I haven't been able to justify the cost.

  6. Hey there, Scott. I also live in Chandler and broke down and got a chest freezer and Johnson Controls temp controller. We're cheap and keep our AC at 81, which is no bueno for fermentation. I'm still trying to dial it in based on others specs on Homebrewtalk. Do you use that controller, and if so, how do you have it configured?



    1. I feel you, we can't afford a $250 electric bill either. I use one of those aquarium controllers from ebay, the STC1000 model, so I'm not much help with the Johnson Controls unit. Although, my friend Greg has one, and it works well for him.

      Is yours working at all, or just not working the way you want it to?

  7. Hey there. It's working, I just don't think I have it dialed in right because the temp ranges are too large. I have it set for 68 but I see temps going as low as 61 then it creeps back up to 68. These specs might be Greek to you but maybe not to your buddy.

    SP 68
    DIF 1
    ASD 12
    OFS 0
    SF 1

    I think I still need to do some more searching around on Homebrewtalk.

    1. I'm guessing you have your temp probe reasonably exposed to the air in the freezer? My Kegerator does the same thing; it's fine.

      Basically, the controller tells the freezer to kick on, and the freezer runs until it hits your Set Point (68). The 'problem' is that once the freezer kicks off, the air inside the freezer continues to drop a few more degrees. I wouldn't worry too much about that, as it probably climbs back to 65-66* fairly quickly.

      I'd say the important thing to note is when you are actually fermenting beer, what's the average temperature of your fermenter. I personally tape my temp probe to the outside of my fermenter bucket to minimize the temperature swings. I use a .7C DIF (1.5F), and I find my average temp to be about 1-1.3F above my set point. So 62 (16.7C) on my controller really means about 63-63.5F for my fermenter.

      1*F obviously isn't a big deal, but it's nice to know what you're working with so you can adjust from there.

  8. Hey Scott, it's been a while since you've looked at this thread, but hopping you may be able to impart a little knowledge.

    I noticed in a few of the recipes you were using US-05, and wanted to know more about your experience using dry yeast. What are the benefits/disadvantages of using dry yeast? Also, what inspired you to move from dry yeast to liquid yeast?

    And as always, thanks for the thoughtful posts. I'm always looking forward to them.

    1. I actually really like dry yeast. I didn't stop using it because it's 'dried', more because I like WLP090 better as a strain. If WLP090 came in a dry packet, I would use that in a heartbeat.

      The advantages are, if properly re-hydrated, you get ~200-220 billion viable cells in a $4 packet. The important thing there is 'properly rehydrated'. You really need to rehydrate the yeast, and rehydrate it in 90-95*F water.

      I switched because I found US05 took a couple days longer to ferment out than WLP001, and it also takes almost a week longer to really clear up in the keg. WLP090 lets me turn out beer much faster with a similar flavor profile, so that's how I justify the extra work on liquid yeast.

  9. Alright, strain preference stems from efficiency, particularly speed to ferment/clear.

    So when using US-05, instead of 7 days in the primary, am I looking more at 10-14? Did gelatin do a fine job cleaning up the yeast upon kegging, or were you still finding it hazier than preferred once cold conditioned? It sounds like you were, but I'm just trying to verify and prepare for my first dry yeast fermentation experience in a long time.

    1. WLP090 ferments out in 3-6 days for me. Usually around 4-5. I give it an extra day or two before I rack to keg or dry hop. US05 was a solid 7 day ferment, sometimes approaching 10 days. I'd then give it a day or two before racking, so it was typically 9-12 days.

      Same deal clearing. Even after gelatin, US05 took about 2-3 weeks of cold conditioning before it dropped brilliantly clear. I get that in roughly a week from WLP090. It does eventually drop absolutely crystal clear.

      If you're patient, none of this is a big deal. Unfortunately, I'm not patient. I loved the beers US05 makes though, so from a final presentation standpoint, it's great. It just takes a little longer to get there.

      Think of it this way: I loved US05. Used it almost exclusively for a couple years. Then WLP090 comes out, they claim in ferments faster. I gave it a try, and found it had a similar flavor profile, similar attenuation, fermented faster AND cleared faster. It was a pretty easy decision from there what I wanted to use as my primary yeast.

    2. Question for you Scott. I know you use low gravity 5-gallon batches as 'starters' fairly often (just pitching the vial and then harvesting from that batch for the next). Just wondering what you would recommend as the maximum OG of the 'starter' batch when doing this. 1.040? Seems like you wouldn't want to go past that. Any info. is appreciated. Thanks.

    3. Ya, right around 1.040 I make a small starter. Or dried yeast, or repitch.

  10. Hi Scott, just wondering if you have any experience with the Wyeast version of the Chico strain - Wyeast 1056? If so do you find it more similar to WLP090 or US05 in terms of fermentation/clearing time?

    Great blog btw!

    1. I've only used once or twice. In my experience it's identical to WLP001. So slightly less attenuative than US05, but slightly more flocculate.

  11. is the aquarium controller reliable? I'd like to do some experiments with another brewing system (I have a Pixsys temperature controller for the one I have at the moment) but I don't want to spend too much :)


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