Friday, November 5, 2010

Five Gallons At A Time: Compensating for Yeast Starters

If you're a homebrewer who uses liquid yeast from Wyeast or White Labs, one of the steps you can take to ensure healthy fermentations is to make properly-sized yeast starters. In general, the size of your starter will be proportional to the gravity of the beer you're brewing. The Mr. Malty Pitching Rate Calculator will do the math for you.

A drawback of yeast starters is that they dilute your original gravity and hop bitterness. Thankfully, it's easy to compensate for a yeast starter in your recipe*. Let's say that you want to brew a 5 gallon batch with an original gravity of 1.060 and hop bitterness of 60 IBU. If you typically lose a half gallon of wort in your fermenter, your total fermentation volume should be 5.5 gallons. According to the pitching rate calculator (with the drop-down menu set at "Simple with O2 at Start"), you should make a 2-liter yeast starter. Because one gallon equals 3.785 liters, your yeast starter will be (2 liters)/(3.785 liters/gallon) = 0.5 gallons. To calculate the volume of your main batch that should end up in your fermenter, subtract the yeast starter volume from the total fermentation volume. The resulting value is 5.5 gallons - 0.5 gallons = 5 gallons. If you lose half a gallon between your kettle (measured hot) and fermenter (measured cold), your post-boil volume (measured hot) should be 5.5 gallons.

You can figure out the target gravity and bitterness of your main batch by rearranging the mixing formula:

Aa + Bb = Cc -> a = (Cc - Bb)/A

Capital letters represent volumes and lowercase letters represent either specific gravities or IBUs. A/a represents the main batch, B/b represents the yeast starter and C/c represents the combined "wort" in the fermenter (the formula treats the yeast starter as an unfermented liquid, which is necessary to determine the effective original gravity of the beer). Assuming your yeast starter will have no hop bitterness and a specific gravity of 1.040, the target gravity and hop bitterness of your main batch can be calculated like this:

OG = (5.5 gallons x 1.060 - 0.5 gallons x 1.040)/5 gallons = 1.062
Hop Bitterness = (5.5 gallons x 60 - 0.5 gallons x 0)/5 gallons = 66 IBU

When you plug the new numbers into your recipe software, your grain and hop bills should automatically be adjusted. To maintain a consistent color and flavor, I recommend tweaking your percentages of specialty malts and flavor/aroma hops so their weights remain the same. That way, your yeast starter simply replaces some of your base malt and the IBU correction is achieved in your bittering hop addition.

*Geeks: the most accurate way to do this is by calculating the total masses and extract masses of the two known quantities, and then using the relationship between specific gravity and degrees Plato to calculate the volume and gravity of the third quantity. However, for low-gravity liquids such as wort and yeast starters, the the simplified method described above will result in smaller margins of error than your measurement methods can detect. Where mass analysis becomes essential is for post-lauter additions of high-gravity liquids (e.g. honey) and solid adjuncts (e.g. corn sugar).


  1. Nathan Peck assistant brewer Sand Creek BreweryNovember 5, 2010 at 3:01 PM

    Well done Joe the Zynerdgy in you is strong. A cheater tip is to befriend a local probrewer they may be kind enough on occasion to provide you with pure pitchable yeast directly from the fermentor.

  2. Wow! Good information, and useful, but I'm still ok with just pitching in the smack-pac or dumping in rehydrated yeast...I brew for fun and too much math takes away the fun. FWIW-I don't follow recipes when I cook either.

    I look forward to more homebrewing posts - thanks!

  3. Hey Nathan, I don't think that's a cheater tip at all! I'd still make an effort to control the pitch rate, though. The Mr. Malty calculator provides good ballpark estimates in its "Repitching from Slurry" tab.

    David, have you noticed any differences in attenuation or fermentation speed between the beers you brew with liquid and dry yeasts? A typical 11.5-g packet of dry yeast provides about 230 billion cells, which is plenty for most ales under 1.060, but smack packs only provide about 100 billion cells. With smack packs, I'd rather make starters and not worry about dilution than pitch insufficient amounts of yeast.

  4. Being a pretty lazy brewer - I don't notice much. But, the cell count does explain why I get a better kick-off from a dry packet.

    FWIW - I usually don't brew a 5gal batch due to space considerations and my tendency to drink homebrews only in addition to the other gallons of product in the fridge. My standard batch size is 3gal tho I have brewed as little as 1/2 gallon at a time.

    I did geek out about homebrewing in the late 90's I built spreadsheets and all kinds of cheat-sheets but they got in the way of the fun aspect. When I got into all-grain brewing I went over the edge...I was getting pissed off at myself for not following 'the rules' and decided to just brew what I felt like, when I felt like it. It's led to some happy accidents like my Orval-style Belgian Pale that sat in a primary for 2.5yrs...

    Keep the info coming - it's a great addition to MBR.

  5. There's one trick i use when pitching a big or stepped-up starter:

    Allow the starter some extra time or crash cool prior to pitching in order to floc out the yeast, then decant the spent "wort". That way, you can utilize as much yeast as possible and not have to factor in extra volume the starter will be contributing. FWIW.

  6. Hey Garrett, I haven't had much luck decanting my starters. It's likely that I just didn't give them enough time to settle, though. On the flip side, most breweries I've worked in pitched their entire "starter" worts into their main batches. The tradeoff is essentially better yeast viability vs. less non-recipe wort. In my opinion, you can make much better beer with healthy yeast and mediocre fermentables than you can with great fermentables and mediocre yeast.


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