Making a Yeast Starter
More than one successful brewer has said, “we don’t make beer. We make yeast food… yeast make beer”. The ultimate success of our brew will depend on how well we’ve anticipated and met the demands of our microbial friends, and correctly predicted the amount of alcohol and the nature of the flavors they produce as they churn through sugars and turn our barley tea into beer.
One of the most impactful steps you can take while brewing at home will be to make sure there are adequate healthy yeast cells to properly ferment your wort. Your beer will benefit from a yeast starter when you are pitching liquid yeast, such as White Labs or Wyeast products, and when your Original Gravity is above 1.060 (6.5°Plato). A well made starter ensures that your yeast pitch is healthy and has the best chance possible to make the amazing brew your looking for. In this article, we’ll outline a process for making a healthy yeast starter, but first we’ll focus on three major factors that will affect your fermentation that can be controlled with the production of a yeast starter 1) yeast health, and 2) pitch rate, as it should be adjusted based on your starting gravity.
Before we begin let’s start with a little background about yeast metabolism. Metabolism breaks down to catabolism and anabolism. Catabolism is the process of breaking down energy-rich nutrients (glucose and other fermentable sugars where we’re concerned), and anabolism is the process of biosynthesizing new cellular components (essential for yeast reproduction). In our wort, the yeast catabolizes sugars by breaking the chemical bonds in fermentable sugars which release energy. The yeast stores this energy in a compound called ATP (adenosine triphosphate), and uses it later in its anabolic reactions. At its heart, the process of fermentation is this process of capturing and storing biochemical energy.
Yeast health is largely dependent on the availability of nutrients it needs to conduct its metabolic processes. Of primary concern when making a yeast starter will be the presence of oxygen and zinc.
Oxygen plays a crucial role in the anabolic processes. While yeast can respire, they prefer fermentation in the presence of abundant simple sugars (which is lucky for us, because this fermentation is what makes us beer). While oxygen does not play a role in fermentation, it does facilitate anabolic processes that produce sterols and fatty acids crucial to the health of the cell membrane. Without healthy membranes, yeast become more susceptible to alcohol and will stop reproducing. Properly oxygenating our starter (and our wort) ensures the maximum of healthy cell growth in the smallest amount of time.
Yeast require many additional nutrients, most of which are readily available in wort with the exception of zinc. Zinc facilitates faster fermentation, better flocculation, and healthier yeast due to its affect on enzyme production within the cell. There are many available yeast nutrients available. Dosing your starter and your wort with a zinc-containing nutrient will ultimately help your yeast as it ferments the sugars in your starter and in the wort.
Pitching an appropriate number of yeast cells into your wort will affect the quality and efficiency of fermentation. Underpitching can result in off flavors due to excessive levels of esters, diacetyl, fusel alcohols and volatile sulfur compounds. Underpitching can also cause stuck fermentations and/or high finishing gravities, and presents a greater chance of infection. While underpitching is more likely to affect your beer quality, overpitching also presents risks, including: very low ester production, very fast fermentations, think body and mouthfeel, and autolysis.
The basic rule of thumb for ales (double these numbers for lagers, whose cold fermenting temperatures inhibit the metabolic processes) is to pitch 1million cells per milliliter per degree plato. Or, to make that easier to read:
1 million cells/ml/°Plato
Wyeast and White Labs both attest that the 100 billion cell packs are sufficient for worts up to 1.048 (12°Plato). This is a pitch rate of 5.5 to 6 million cells/ml. Both manufacturers indicate that this pitch rate is sufficient because the viability and vitality of their cells, coming straight from the lab, are superior to the cells that you’d harvest from a previous fermentation (such as a starter). However, both manufacturers indicate that worts above 1.060 (14.5°Plato) should be pitched from a starter.
You should not make a yeast starter for dry yeast, which is bundled with it’s own nutrients. It is possible to deplete these nutrients in trying to make a starter and stunt the yeast. However, you should absolutely follow the manufacturer’s recommendation to rehydrate the yeast in sterile water before pitching.
The basic idea in making a starter is to make one liter of low gravity (1.030-1.040) wort without hops, then pitch one pack of White Labs or Wyeast liquid yeast. This should produce a 50% increase in cell count, and get you in the zone you want for your 5 gallon batches. A basic process follows:
Make 1 liter of 1.030 to 1.040 wort using DME. Boil for 10 minutes to sterilize the starter
Cool the starter wort to between 65° and 75°F.
Pitch one pack of liquid yeast
Aerate constantly for 12-18 hours
Pitch yeast starter into fresh wort.
Those are the basics, but there’s some variation in technique available to you. You’ll want to follow the same rigid cleanliness and sanitary procedures that you apply to your fermenters to all equipment you use to make a starter. You are, in essence, making a mini batch of beer to feed to your full size batch of beer. Following is a more in depth description of the process above.
Yeast will become stressed under fermentation conditions in worts above 1.020. This is mostly because of the alcohol content. Your choice between 1.030 and 1.040 is between greater yeast vitality at the low end, and greater yeast cell counts at the higher end.
Yeast will go through greater reproduction at higher temperatures up to 98°. However, at these elevated temperatures, yeast viability and stability will be negatively impacted. Temperatures that are too low will result in less than adequate cell growth.
The efficacy of your starter will depend greatly on your ability to adequately oxygenate your starter wort. The most common and most effective way of doing so on a homebrew scale is to use a magnetic stir plate and a glass jar. Erlenmeyer flasks are popular choices, but any sanitized glass jar will work. Do not use an airlock, as you want to induce oxygenation in the wort. Instead, use a sanitized piece of aluminum foil lightly crimped over the opening of your jar or flask. The magnetic stirrer will create a vortex that pulls oxygen into solution while encouraging metabolic activity due to the solution being in constant motion. If you do not have a magnetic stir plate, you can approximate this by shaking the starter as often as possible.
There is some debate over pitching the starter while it is at high krausen (the peak of fermentation; 12-18 hours after pitching yeast into the starter) or letting the starter ferment completely, settling the yeast to the bottom, and only pitching the yeast slurry (4-6 days after pitching yeast into the starter). Both have advantages and disadvantages. Pitching at high krausen ensures that your yeast is at the peak of it’s health, but it introduces the flavor (or lack of) of the low gravity starter wort into your batch of beer. Settling the yeast at the end of fermentation gets you away from pitching the starter wort, but produces yeast that is not quite as viable as the high krausen option. This is a choice between rigid control of the flavor of your beer or not, and it is up to you.
Regardless of the methodology you use, making a yeast starter will increase your yeast count and improve your fermentation. This is especially valuable in high gravity beers, or in any recipe where your final gravity is constantly higher than you’d like it to be. This is a relatively easy to do process. As for any of your homebrewing needs, we are always happy to help you with any questions you might have about making a starter for your next batch.