“Brewers make wort, yeast make beer.” It’s a saying I’ve cited so many times over the years, I’d have stopped repeating it long ago if I didn’t like the sound of my own voice so much. But it’s still very, very true.
As brewers, our job is similar to that of an elementary school custodian: keep the place clean and make sure the environment is conducive for the little buggers to do what they need to do . . . which in this analogy is not learning and getting socialized but converting malt sugars into alcohol and CO2 gas. We can ensure post-boil surfaces are free of contaminants, provide a quality, nutrient-rich wort, and foster favorable conditions (dissolved oxygen, fermentation temperature control, etc.) but the biggest determining factor that tips the balance between gallons or liters of awesome homebrew or pint after pint of blah homebrew is the health and happiness of the yeast doing the work of fermentation.
There are many factors to manage in conducting a good, healthy fermentation, but the first and most fundamental factor is inoculating the wort with a population of healthy yeast cells at the right dosage or pitching rate. Pitching rate is basically the amount of yeast one uses to inoculate cooled wort (pitching, in brewers’ parlance), expressed as a ratio of the number of yeast cells to wort volume.
A good baseline pitching rate is 6 million cells of healthy yeast per milliliter of wort. This rate is recommended for ales of average strength, which is what many homebrewers make most of the time. The rule most commercial brewers use (most meaning lager) is 1 million cells/°Plato/ml wort, or about 12 million cells per milliliter. Ale brewers do often times pitch at lower rates.
Most homebrewers make 5-gallon (19-L) batches. There are 18,927 (give or take) mL in 5 gallons (19 L), which works out to a target pitching rate of 113.5 billion cells for 5 gallons (19 L) of standard-gravity (<1.060 SG) wort that will be inoculated and fermented at ale temperatures (~65 °F/18 °C or so). It’s important to note that this is a guideline rather than a rule — there is room for variation and adjusting this number up or down depending on the style you’re brewing and the temperature you use to ferment the beer (more on that later on in this article).
Another formula you’ll come across, if you’d care to double down on both the metric system and wort gravity in Plato, is x million cells per milliliter of wort per °Plato. For me, the nice features of working the numbers this way are that it’s easily scalable by wort gravity, regardless of beer style; and because of the linear nature of metric, it’s also easily scalable by volume x million per milliliter is also x billion per liter, which (for me at least) makes it easier to relate to both liquid yeast packaging and my batch size. (These reasons are also why many large-scale homebrewers and commercial brewers use metric/Plato for calculating recipes and pitching rates.)
What’s Going On In There?
Before we move on to other considerations and theory-into-practice, let’s take a step back . . . or rather a step way, way closer, and get a layman’s overview of what happens when yeast meets wort, to help us understand why pitching rate is so important to the finished product. A very simplistic perspective is that our brewers’ yeast produces three things while colonizing its new home in our carboys and buckets: it makes more yeast cells, catabolic waste and various metabolic byproducts through its use of wort resources like malt sugars and dissolved O2.
“More yeast” is the straightforward result of cell reproduction, and that catabolic waste is CO2 and alcohol (one fungus’s trash is another man’s treasure). The many various byproducts yeast cells create during fermentation, then, include things like esters, aldehydes and phenols; the production (or lack thereof) of these byproducts, and the level at which they pervade the batch, has a direct impact on the flavor and aroma of our finished beer.
What Does A Proper Pitching Rate Taste Like?
Because those flavor-contributing metabolic byproducts are created during the period of cell growth prior to what we lay homebrewers might think of as “fermentation” (kräusen, bubbling airlock, maybe some blowoff), the extent of the growth required of our yeast population determines how much of these compounds are present in the beer. In a nutshell, higher pitching rate = more yeast in = shorter growth phase = lower esters, etc., = “cleaner,” more neutral profile with less yeast character. Lower pitching rate = less yeast in = longer growth phase = higher levels of esters, etc. = fruitier, funkier, and generally more yeast character in the beer’s profile.
Besides ensuring reproducibility and consistency from batch to batch, controlling the pitch rate means we can also fine-tune the sensory profile of our pint. A clean, crisp Pils, where high levels of esters are a stylistic defect, mandates a high pitching rate; likewise a Belgian Tripel, where the combination of high-gravity wort with highly expressive yeast strains could get out of control quickly with fusel alcohols and high concentrations of esters and phenols. But maybe you’re brewing a session bitter, or a weissbier with a hankering for a strong isoamyl acetate “banana” nose — a pitching rate on the lower end of the spectrum (without actually going out of the spectrum and underpitching) will help the yeast strain stand forward and put an authentic stamp on styles like these.
The Golden Mean
Too much of anything is too much, as the man said — but not enough of anything isn’t much good either. Underpitching a batch of homebrew, even for styles that benefit from some yeast character, can lead to higher-than-desirable levels of esters, fusel alcohols, or sulfur compounds, as well as more serious problems like excessive amounts of diacetyl and higher-than-planned final gravity. Underpitching is a very common flaw in homebrewed beers. In the September 2010 issue of Brew Your Own, Brooklyn Brewery Brewmaster Garrett Oliver, who has judged a great many homebrew competitions, said, “The most important factor to brewing any style, at home or professionally, is making sure you start from the very beginning with a healthy population of yeast. You should see the fermentation take off sooner rather than later. For example, for most ales, you ought to see a very active fermentation in under twelve hours. A beer from a struggling fermentation has a certain flavor. It’s one of the main things that tends to distinguish what a professional might say is a home-brew flavor.”
Overpitching, which might not seem like such a big deal on paper, can in practice significantly reduce ester production, cause overshooting of target final gravity and speed fermentation up to create its own set of problematic flavors from yeast autolysis, and can throw the beer out of style. So to repeat myself (one of my other favorite hobbies, besides homebrewing): a good baseline pitching rate for standard-gravity (less than 1.060) wort that will be inoculated and fermented at ale temperatures (~65 °F/18 °C) is 6 million cells of fresh yeast per milliliter.
High-Gravity and Lager Fermentations: What’s Different?
But what if you’re going to be brewing north of 1.060 SG? Or if you’re going to be fermenting south of 60 °F (16 °C)? Better bring more yeast cells.
In addition to the considerations of flavor and aroma impact, high-gravity brewing creates added environmental stress for the yeast; the greater density of the wort means increased osmotic pressure on the cell walls, and the higher concentrations of alcohol as fermentation progresses becomes increasingly toxic to the yeast. For this reason, successful fermentation of a strong beer calls for starting out with more yeast than a standard-gravity brew.
The cool fermentation temperature of lagers and many hybrid beer styles is another source of environmental stress, slowing cell metabolism to a crawl. Add to that the stylistic requirement to minimize the ester levels in the finished beer and we’re suddenly asking a lot of our yeast. For these reasons, cold-fermented beer styles also require a higher pitching rate than beers fermented at warmer temperatures.
A good guideline when brewing high-gravity beers or lagers is to pitch double or triple the baseline pitching rate of 6 million cells/mL for ales — 12 to 18 million cells per mL. And for a high-gravity lager, like a doppelbock operating under the weight of both high OG and low temperatures, a quadrupling of the pitching rate — 24 million cells/mL — wouldn’t be out of order. (For more on brewing high-gravity beers, visit BYO on the Web at https://byo.com/story1882.)
What’s Your Pitching Rate?
Given that today’s homebrew-sized liquid yeast packages (known as activator packs or smack packs, depending on the manufacturer), as well as sachets of dry yeast, from the major yeast labs contain roughly 100 billion cells when fresh, those of us brewing smaller-volume batches can brew even fairly high-gravity beers and some lagers without any extra preperation — the quantity of yeast right out of the package will be sufficient, or more than sufficient, for the needs of brew lengths like these.
For those of us brewing the traditional 5-gallon (19-L) batches, low- to moderate-gravity ales can also often be pitched directly from the package. But as gravity goes up and temperatures go down (or batch sizes increase), our target pitch rates will represent multiple packages’ worth of cells for a batch. Alternately, we could propagate one pack of yeast in a starter culture prior to brew day to build up our pitch rate (see the sidebar at the end of this story on making a yeast starter).
A starter culture is a great idea regardless of what you’re brewing, since it ensures maximum viability and health of the yeast population, and is an especially good practice if your yeast pack is out of date or was purchased through mail order and shipped in hot weather or other adverse conditions.
I’m happy to relate that it’s easier than ever to dial in your pitching rate for each batch (back in my day, uphill both ways through snow, etc. etc.) with the preponderance of calculators available online and onboard in many brewing software programs.
Disclaimer: It’s been said before, but it bears repeating: when most homebrewers talk about cell counts, it’s a ballpark estimate, and not an actual head count of our yeast. Pro brewers and lab types use hemacytometers and a microscope to get a more accurate count of their pitching population (see the December 2003 issue of Brew Your Own for more information), but for our purposes as homebrewers, the horseshoes-and-hand grenades approach to estimated cell counts will still yield great beer. (Using a hemocytometer is not a real head count either, by the way, but it is better than guessing!)
For homebrewing, you’re pretty safe with using an estimated target range from BYO’s pitching chart in the link I provided above, or by punching your numbers into Mr. Malty’s pitching rate calculator. (In case you didn’t know, Mr. Malty is actually BYO’s own “Style Profile” columnist Jamil Zainasheff, who is also the co-author of Yeast: The Practical Guide to Beer Fermentation (Brewers Publications, 2010), which is an excellent resource for questions about yeast.)
One of the most crucial keys to brewing beer at home (and commercially) is pitching the right amount of yeast cells to run a healthy, robust fermentation. Both underpitching and overpitching can create off flavors or adverse effects in your homebrew. The term “pitching rate” is the ratio of the number of yeast cells to wort volume.
Propagating a Starter Culture
– This content appears courtesy of Wyeast Labs.
Many brands of yeast for homebrewing are designed to directly inoculate 5 gallons (19 L) of standard ale wort (OG less than 1.060, fermentation temperature 65–72 °F/18-22 °C). However, high gravity worts (OG greater than 1.060) or cold fermentation temps (less than 65 °F/18 °C for lagers or hybrid ales) require a higher pitch rate than can be achieved with a single pack of yeast. Making a starter culture prior to brew day is an economical way to increase pitching rate and ensure consistent results in your brewing.
Determining Pitching Rate
First, determine the appropriate pitching rate for your beer. Once you have a target pitching rate, plug the numbers into a pitch rate calculator to determine the starter volume needed to achieve the target pitching rate.
Timing a Starter Culture for Brew Day
Because starter cultures are inoculated at high cell densities, growth is usually maximized within 24–36 hours. Preparing the starter one to two days prior to brew day is ideal. Starter cultures should be used immediately, or stored refrigerated for up to one week. Cell viability will decrease rapidly if the starter culture sits unused, especially if left at ambient temperatures for extended time.
Preparing a Starter Culture
The optimal media for cell growth and health is a malt-based wort of about 1.040 OG, fortified with yeast nutrients. Dried malt extract is ideal for starter culture wort, since it’s readily available, easy to measure, and the leftovers store well for use in future starters.
- Sanitized Erlenmeyer flask or jar, sized for the required volume of starter culture (optional)
- Sanitized cover for the flask or jar — aluminum foil, foam stopper or loose-fitting lid
- Oven mitts for handling hot liquid
- Magnetic stir plate and bar (optional)
- Foam control drops, such as Fermcap, (optional)
Basic Recipe (scale as needed)
- 3.5 oz./100 g plain dried malt extract (DME) (approx. ½ cup). Tip: measuring into a plastc cup makes transfer into the flask easier as you can crease the side of the cup.
- ½ tsp. yeast nutrient
- 1 qt. (1L) H2O
- Mix DME, nutrient and water. Shake or stir to dissolve.
- Boil starter wort 20 minutes to sterilize. Use a saucepan on a kitchen stove; if using a laboratory-grade glass flask, you may be able to boil directly in the flask — double check with your supplier or the manufacturer first. Use foam control drops (optional) to prevent foam from boiling up in the flask.
- Cool to 70 °F (21 °C). A cold-water bath will help speed things along.
- Transfer to sanitized flask or jar. Carefully pour the cooled starter wort into the sanitized flask or jar.
- Add yeast pack. Cover loosely with sanitized aluminum foil, a foam stopper, or the jar’s lid and swirl gently to mix.
- Incubate 24–36 hours at 70 °F (21 °C). Agitate the starter culture periodically, or use a stir plate for constant, steady agitation and aeration.
- Pour the starter culture into the cooled, aerated wort in your fermenter; the entire volume of starter may be added to the main batch, or you may prefer to decant some of the spent wort first: chill the starter during brew day to encourage cells to settle, then decant the top layer into the sink, pouring just the yeasty bottom layer into the fermenter.
Propagating a Starter Culture
Any yeast strain, including lagers, should be incubated at 70 °F (21 °C) to ensure rapid growth. It’s not uncommon for a starter culture to display less visible fermentation activity (kräusen, etc.) than a full 5-gallon (19 L) batch; due to the high pitching rate of a starter culture, fermentation can more or less happen while we’re not looking. Just as with a full-sized batch of homebrew, the best indicator of activity is a gravity reading; also look for CO2 bubbles coming out of solution (especially if using a stir plate), “yeasty” rather than “worty” aromas, and turbidity — starter cultures will usually look milky or cloudy while fermenting, and clear with a layer of whitish-tan sediment when finished.
Stirring and O2
Agitation of the culture aids in removing inhibitive CO2 from suspension as well as adding small amounts of oxygen. Small additions of oxygen periodically throughout the growth of a starter will replenish sterols and improve cell yield. Stirring or shaking the starter periodically, or using a stir plate, will improve cell growth in a starter culture. The use of stir plates has been shown to increase cell growth 25–50% over a non-stirred starter.
To increase cell count even further, brewers making strong lagers or very high-gravity beers, or those brewing 10-gallon (38-L) (or larger) batches can opt to make a two-stage starter culture: Allow an extra 24–36 hours before brew day and follow the basic procedure; when the initial culture is fermented out, chill and decant the spent wort, then replenish the flask or jar with another volume of fresh starter wort and repeat the incubation process.
Sound sanitation practices are critical when propagating a starter culture. It is important to understand that creating a starter can increase the risk of infection by undesirable organisms. A small level of contamination in a starter culture can multiply to unacceptable levels in the main batch, creating undesirable effects in the finished beer.
Target Pitching Rates
STYLE: GRAVITY PITCHING: FERMENTATION PITCH TEMPERATURE: TEMPERATURE RATE: (Million Cells/ml.)
Ale: <1.060 (15 °P) : >65 °F (18 °C): >65 °F (18 °C): 6.00
Ale: 1.061-1.076 (15–19 °P): >65 °F (18 °C): >65 °F (18 °C): 12.00
Ale: >1.076 (19 °P): >65 °F (18 °C): >65 °F (18 °C): >18.00
Lager: <1.060 (15 °P): >60 °F (16 °C): >60 °F (16 °C): 12.00
Lager: 1.061-1.076 (15–19 °P): >60 °F (16 °C): >60 °F (16 °C): 18.00
Lager: >1.076 (19 °P): >60 °F (16 °C): >60 °F (16 °C): >24.00