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September 1998 Volume 21, Issue 9

Competition Corner

by Bryan Gros

Rules of brewing:

1. pitch enough yeast
2. you never have enough yeast

There has been a lot of discussion this past month on the Homebrew Digest about proper yeast management and pitching. As Bob Jones and others have been saying for years, it is extremely difficult to get "enough" yeast to pitch. There are many advantages to pitching enough yeast. First, lag time is reduced, which minimizes the chance of wild yeast or bacteria growing in your beer. Second, your yeast will have a shorter reproductive phase and thus be more healthy during fermentation. Third, the healthy yeast will attenuate better, especially in bigger beers. Fourth, the healthy yeast will produce fewer off flavors (depending on temperature). And if you propagate your yeast from batch to batch, the benefits of healthy yeast will be even more apparent.

How do you get enough, healthy yeast? You make a starter. But remember, when you propagate yeast in your starter, you’re goal is making yeast, not beer. To encourage reproduction, aerate your starter as much as possible. Some people use a magnetic stirrer. If you don’t have that, shake your starter every chance you get. Adding oxygen encourages yeast growth by allowing the yeast cells to syntesize sterols, a vital component in their cell membranes. If cells cannot synthesize additional sterols, they can only divide once or twice before running out. This is why you won’t get enough yeast in your batch if you underpitch and/or underaerate your beer. And these weaker cells won’t be able to survive to complete the job—the acidic, high alcohol environment will kill them.

Next time you make your starter, taste it. Does it taste nasty? Unless it is sour or something, this is fine. All that yeast reproduction leads to higher alcohols, ethyl acetate, and other byproducts of growth. Remember, you’re making yeast, not beer, at this stage. So be sure to let your starter sit for a day or two and the yeast will settle out. Pour off the liquid before pitching onto your wort. It still isn’t clear to me if you want your yeast actively fermenting when you pitch. If you’ve built up a lot of yeast, you can easily add a bit of starter wort to your yeast in the morning before you brew. Aerate once when you pitch, and let it go. It should be actively fermenting when your wort is cool.

How much yeast do you need to pitch? The usual answer is one million cells per ml per degree Plato. So how do measure yeast? Well, here’s what Charley Burns figured out based on some data by Mort O’Sullivan, brew guru at a brewing school in Edinburgh, Scotland.

"Man did my brain sweat over this one. Scientific Notation and Metric systems tossed together with Degrees Plato is NOT my strong suit. But nevertheless, here's my attempt to use Mort's Magnificent Seven.

"Optimum pitching rate assumption: 1,000,000 cells per milliliter (ml) of 1 degree Plato of wort.

1,000 ml = 1 litre

5 gallons = 19 litres

therefore

19,000 ml = 1 each 5 gallon batch

In case you haven't figured it out yet, this is the long slow way to get around to final conclusion. Those of you good in this sort of math, can hit age down and go to the next post now.

1 batch of 1 degree plato wort therefore needs 19,000,000,000 (billion) cells.

Assumption 2: 1 degree plato = .004 points of specific gravity (please correct me if I'm wrong here - its memory fading quickly). So, 1 batch of 1.004 (SG) wort needs 19,000,000,000 (billion) cells of yeast.

1 batch of 1.052 Pale Ale wort needs 247,000,000,000 (billion) cells of yeast!

We're getting closer now.

The worst case (least dense) sample out of Mort's Magnificent Seven (See hbd#2802) provided 2.0e+9 cells per gram of yeast cake. This converts to a "normal" number of 2,000,000,000. I know, you guys that are mathematically inclinded think I'm nuts for keeping all the zeros around. Well, now that I'm getting used to it, it does seem like a lot of typing for nothing, but... ok, so now we divide cells needed by cells per gram to get total grams needed:

247/2 = 123.5 grams.

Mort was kind enough to provide the divisor in his follow up post to convert this semi liquid yeast cake in grams to fluid ounces: 29.5727.so, 123.5 / 29.5727 = 4.176 ounces (of yeast slurry for 5 gallon batch).

2 primary caveats:

Assumes 2,000,000,000 (2.0e+9) cells per gm of yeast slurry (low density)

Assumes 93 percent viability (very high).

Assuming a 50% viability, we'd need to pitch 8.34 oz of slurry.

Assuming a 25% viability, we'd need to pitch about a pint of slurry.

My last 5 gallon batch of brown ale (sg 1.053) I pitched with about a pint   1056 slurry (which had been in the fridge under distilled water for a couple of months). It took a good 18 hours to get action, even at 72F (ambient).

Prior to that I pitched a 2 quart starter from a smakpack of 2206 into a 1.075 Bocktoberfest (layer of yeast only about 1/8" thick at bottom of starter). It took off with a krausen within 2 hours - but took 25-26 days to ferment out. So, I think both quantity of yeast PLUS state (ie active or dormant) makes a huge difference in visible lag time and the total ferment time. More active yeast makes shorter lag time, but not necessarily a shorter ferment time. The brown finished at 1.010 in 4 days (quick total time, but long lag time).

So, bottom line is you need about 4 oz. (half a cup) of yeast slurry (or cement from Faultline) for five gallons of "normal" gravity beer. It is pretty hard to build up this much from a wyeast pack, so start early.

The other observation that becomes apparent from the digest discussion is that there is a lot of professional brewing literature out there (as you might imagine), but it is hard to follow. Not just because of the science either. Much of this work is aimed at producing light industrial lager at 16 Plato 24 hours a day 7 days a week. It is hard to figure out if the data can be applied to homebrew batches or how. A lot of this literature is what is taught at brewing schools like Seibels and probably UC Davis.

I’m certainly no yeast biochemist, so feel free to debate some of these things (preferably over a beer) and fire off letters to the editor (he needs mail).

******

Upcoming events:

5th annual Queen of Beer competition for women brewers (brewsters) in Camino CA. Entries due and Judging is Oct. 10.

Pacific Cup homebrew competition in Long Beach, CA. Entries due Oct 16 and judging is Oct. 24.

3rd annual Music City Brew Off in Nashville TN. Entries due Oct. 12 and judging is Oct. 24.

San Joaquin Beer Festival Homebrew Competition in Stockton CA. Entries due Oct 31 and judging is Oct. 14.

Congratulations: Lisa received a first place at the Stanislas Cty fair for her bock beer. Very impressive ribbon!



Updated: October 28, 1998.