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Yeast Pitching Rate Results

Background

The ale yeast pitching rate generally recommended by commercial brewers is one billion cells, per liter of wort, per degree Plato. Assuming a 25% loss in viability prior to re-pitching results in the rule of thumb of 0.75 billion/L-°P. However, yeast products designed to inoculate at this level are not available on the homebrew scale. Directly pitching a vial or pack of commercial yeast without a starter results in about half of the recommended pitching rate. The effects of under-pitching are varied, but generally held to be undesirable. Wyeast Laboratories, for example, states that under-pitching can cause “excess levels of diacetyl, [an] increase in higher/fusel alcohol formation, [an] increase in ester formation, [an] increase in volatile sulfur compounds, high terminal gravities, stuck fermentations, [and an] increased risk of infection”.

Among home brewers, however, the effects of under-pitching are less universally agreed upon, with some brewers maintaining that under-pitching has no impact, or at least no negative impact, on the final product. Many others pitch multiple yeast products, or make starters prior to brewing, in the belief that it will result in better beer. To test these assertions, a controlled experiment needed to be conducted.

Experimental Setup

In order to provide a reasonable baseline for comparison, I brewed six gallons of an American Amber Ale, targeting a BU:GU ratio of about 0.5. The idea was to come up with a middle-of-the-road representation of a style that would be accessible to most craft beer drinkers. After chilling, the wort was split into two plastic bucket fermenters, with one pitched at 0.73 billion cells/L-°P (“control”), and the other at 0.29 B/L-°P (“under-pitched”), which is roughly the pitching rate that would result from using a month-old smack pack in a five gallon batch. Since the cell counts used to calculate the pitching rates are based on slurry volume rather a true count, the associated error is high – I would suggest ±10%. More complete information on the beer brewed and the yeast propagation procedures can be found in the experiment proposal and yeast ranching posts, respectively.

Seventeen sample sets of three bottles each were distributed to home brewers from Oregon to Florida. One (presumably) broke in transit and was not delivered, and there was one non-respondent. The 15 effective sets resulted in feedback from 37 individual tasters. Twenty-three received “Set 1″, which contained two controls and one under-pitched sample, with the remaining fourteen having “Set 2″, with two under-pitched beers and one control. All sets were labeled only as “A”, “B”, and “C”, resulting in a blind triangle test. Aside from being recruited via a topic on a brewing forum, no effort was made to establish the participants’ credentials with regard to brewing or judging beer. I’d like to think they can therefore be considered a representative sample of the (online) home brewing community.

The chief difficulty associated with collecting impressions from such a widely divergent group of respondents is converting them into unambiguous numerical data. My first thought was to have each taster complete a standard BJCP Scoresheet, but there were several obvious problems with that method. First, filling out the scoresheet can be a daunting, time-consuming task, especially considering that most respondents were not BJCP judges and would probably be completing it for the first time. Second, since by design it addresses only a single beer, a minimum of three sheets per “judge” would be required. Finally, since it lacks room for additional, comparative questions, at least one additional sheet, for a total of four, would be required. In order to minimize the amount of paper and the time commitment required, I laid out a simple feedback form, loosely based on the BJCP scoresheet, which was distributed to all participants.

In order to obtain objective, internally consistent data, two different methods were used. First, the volunteers were asked to perform two simple, quantifiable tasks: identify the control and under-pitched beers; and express a preference for one or the other. In addition to providing these definitive, binary answers, each respondent was also able to share more detailed impressions about appearance, aroma, flavor, and mouthfeel. The descriptive adjectives used were then compiled by means of a simple frequency count. In order to simplify the dataset as much as possible, some descriptors were combined – first, variations on the same root word (“malt” and “malty”, e.g.), then words with substantially similar meanings (“hot” and “solventy”, e.g.).

Personal Observations

IMG_2318IMG_2316My personal tasting notes for the two beers should be taken with a grain of salt, since they don’t represent a blind tasting, but the photographs are so dramatic that I thought they should be included. The photos show, left and right, the under-pitched and control beers. The first photo was taken two minutes after the beers were poured; the second after they had been drunk over the course of about 35 minutes.

The under-pitched beer is slightly lighter in color (10.5-11 SRM instead of 12), and has minimal head retention or lacing when compared to the control beer. The aroma is malty with a slight hot alcohol character, whereas the control has a perfumy, floral hop aroma, with the malt more subdued. The under-pitched beer has a vaguely spicy or vegetal off-flavor, particularly in the aftertaste, and a slightly solventy finish. The control also has a peppery or spicy taste (which I would attribute to the use of Munich malt), but a lingering citrus flavor predominates. The mouthfeel of the under-pitched beer is thin and astringent compared to relative fullness of the control, although it’s difficult to make a fair comparison due to the difference in head retention. The control may have a very slightly more compact, “stickier” yeast sediment.

Results

As a means of gauging fermentation performance, a refractometer reading was taken every twenty-four hours after pitching. The difference in the time required to start and finish fermentation is striking.

The control beer not only exhibited faster fermentation to begin with, but reached terminal gravity approximately twice as fast as the under-pitched fermenter. This provides a clear rationale for the use of higher pitching rates in commercial breweries, where fermenter time is extremely valuable. It could also point to a potential advantage for the higher pitching rate in allowing the yeast to out-compete any contaminating microbes.

The gravity of the two beers over time, as indicated by (uncorrected) refractometer readings.

Of the 30 tasters who attempted to differentiate the beers, thirteen were able to do so, with nine tasters correctly identifying the beers. This seems to support a hypothesis that there is a difference between the beers – if the three samples were truly indistinguishable, ten respondents could be expected to differentiate the beers, and five identify them.

Since the results follow a binomial distribution (a given sample is either identified or not, with no middle ground), the probability that these results are purely due to chance can be assessed. Given a random distribution, 13 of 30 respondents (or more) would be expected to differentiate the samples about 16.6% of the time. Based on these results, one can conclude, with 83% confidence, that the two beers do in fact taste different.

But what, specifically, are the differences between the beers? In addition to the more subjective descriptors I’ll get to in a bit, we can make a reasonable inference from the 13 tasters who correctly differentiated the samples. If the beers tasted different, but those differences revealed nothing about their identities, we would expect half of the 13 to guess correctly; in fact, the probability that nine or more would guess correctly is only 13.3%. This leads me to believe that at least some of the flavors generally associated with under-pitching – increased esters, fusel alcohols, diacetyl, acetaldehyde, etc. – are in fact present.

Twenty-four of the participants expressed a preference for one beer over the others, with the control being preferred sixteen to eight. When weighted appropriately for the number of samples, the control beer was preferred by 54.9% of tasters. Interestingly enough, this would seem to suggest that while the beers almost certainly are different, there is no consensus about which is better. Simply put, nearly half of people prefer under-pitched beers. An argument could be made, though, that only the opinions of the participants who actually differentiated the samples should be considered. And the data would seem to bear that out. Five of the differentiating tasters preferred the under-pitched beer; however, fully eight of the thirteen tasted Set 2, and when weighted accordingly they overwhelmingly prefer the control, 65.8% to 34.2%.

It is also worth noting that the first half of the respondents (those who tasted the beers after four weeks in the bottle or less) also preferred the control about two to one. So it’s entirely possible that additional conditioning time can reduce the off-flavors that result from under-pitching, but another controlled experiment, with the date of the tasting as a variable, would be needed to satisfactorily answer that question.

descriptors

Finally, we come to the subjective tasting results. I think the above image largely speaks for itself, so I won’t elaborate too much further. The ten most common descriptors for the control beer are:

    1: Malty
    2: Hoppy
    3: Bitter
    4: Fruity
    4: Sweet
    6: Estery
    6: Smooth
    8: Thin
    9: Solventy
    9: Clean
    9: Dry
    9: Cidery

For the under-pitched beer, they are:

    1: Bitter
    2: Malty
    3: Hoppy
    4: Astringent
    5: Fruity
    5: Estery
    5: Solventy
    8: Clean
    9: Smooth
    9: Thin

Based on the relative frequencies of some words, I think one can reasonably conclude that the under-pitched beer was perceived to be more bitter, more astringent, more solventy, less sweet, and – bizarrely – cleaner than the beer using the standard rate. Obviously, the increased perception of negative characteristics makes a persuasive case for the use of higher pitching rates.

Summary

  • No difference in attenuation was observed, but fermentation in the control finished twice as quickly.
  • Under-pitching negatively impacts head retention and lacing.
  • There is a 43% chance that an “average” home brewer will be able to distinguish between under-pitched and standard-pitched ales.
  • There is a 30% chance that he will be able to identify which beer is which.
  • Overall, home brewers exhibit no strong preference for either beer.
  • Among tasters who can differentiate the two beers, the standard pitching rate is preferred nearly two to one.
  • The control beer was described as malty, hoppy, bitter, fruity, and sweet.
  • The under-pitched beer was described as being more bitter, more astringent, more solventy, less sweet, and cleaner than the control.

Summary of the Summary

Using a starter makes better beer.
 
 
 
Download the full dataset:
pitchrate_experiment.ods | pitchrate_experiment.xls

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