Using Yeast For Brewing

Once you've chosen the right yeast for your brew, you aren't finished. You will need to make sure you pitch the right amount of yeast into well-oxygenated wort at the right temperatures, etc., in order to get the best beer. The conditions under which your yeast do their job will have a major impact on the outcome. Understanding these variables and the yeast life cycle is critical to getting optimal performance from your yeast.

  1. The Lag Phase—This stage takes place during the first few hours after inoculation of the wort (i.e., pitching the yeast). During this stage, oxygen is critical. The yeast uses it to create lipids and nonsaturated fatty acids which are required for healthy growth, which in turn are required for the health of the cell walls of the yeast. Lack of adequate oxygen supply at this stage will cause poor fermentation and poor long-term health of the yeast.

  2. During the next stage, yeast begin to grow at an acelerating rate.

  3. The Log Phase—This stage is characterized by extremely rapid yeast growth. All of the processes the yeast went through during the lag phase start to work here and allow the yeast to rapidly reproduce. This reproduction is asexual and occurs by budding—the adult cell forms another cell which is an exact copy of itself. The primary flavor and aroma compounds from the yeast are produced as by-products of this cell reproduction process. Nitrogen, amino acids, nutrients, and sugar are consumed here.

    The quantity of yeast initially pitched will affect the production of aromatic and flavor compounds and will also impact the overall health of the yeast. Proper pitching rates (discussed later) are required for the best balance.

  4. During the next stage, yeast growth begins to slow down.

  5. The Stationary Phase—This is the stage where all yeast growth comes to a halt. As the nitrogen and sugars are all used up, the yeast begin to prepare for a period where there will be a lack of nutrients. When the sugars are all consumed, the yeast will begin to flocculate out (i.e., settle to the bottom of the fermenter). As this is happening, the yeast will reduce the levels of diacetyl it produced during earlier stages.

Factors That Impair Yeast Health

There are many ways you can impair the health of your yeast. Over-pitching and under-pitching are two ways. Low oxygen rates in the wort the yeast is pitched into, poor temperature control, yeast autolysis, and other adverse environmental conditions are equally bad, if not worse.

According to Yeast Supplemental Material - From the American Brewer's Guild, the following are the major stresses that impair the vitality of the yeast (the following is a summary of the original list):

  • Fermentation of strong beers or barley wine.
  • Yeast washing (particularly acid washing).
  • Poor selection/harvesting. See below for details on harvesting.
  • Fermentation of fruit/spice beers.
  • Fermentation under CO2 saturation conditions.
  • Poor aeration of wort.
  • Poor sanitation leading to contamination.
  • Low pitching rates

Note that excessive pitching rates will also impair your yeast by not allowing adequate yeast growth (new yeast grows to replace old yeast, keeping healthy younger yeast in the culture; excessive pitching keeps more of the older yeast involved, and the viability is reduced).

Per (Wyeast's Recommended Pitch Rates page):

A low pitch rate can lead to:

  • Excess levels of diacetyl
  • Increase in higher/fusel alcohol formation
  • Increase in ester formation
  • Increase in volatile sulfur compounds
  • High terminal gravities
  • Stuck fermentations
  • Increased risk of infection

High pitch rates can lead to:

  • Very low ester production
  • Very fast fermentations
  • Thin or lacking body/mouthfeel
  • Autolysis (Yeasty flavors due to lysing of cells

Another quote from the above Wyeast page, regarding re-using yeast:

The following are guidelines that should be followed when re-using yeast:

  • Never re-use yeast from a high gravity fermentation (greater than 1.065 original gravity).
  • Never re-use yeast from an irregular fermentation (long lag-time, long fermentation, poor clearing, high terminal gravity, etc.)
  • Never re-use yeast from a fermentation with off flavors and aromas (excess diacetyl, phenols, sulfur compounds, etc.)
  • Never re-use yeast if you are not confident in your sanitation and brewing practices.
  • Do not store yeast for long periods before re-use (longer than 2 weeks typically).

So, How Much IS The Right Amount To Pitch?

Hell, if you look around, you'll find that everyone has the "Right Amount" for batch size 'x'. More often than not, someone will spout off about using a gallon of starter for a 10 gallon batch of beer. This is over-pitching in the extreme. Here's what they're really telling you: We want you to over-pitch by a huge quantity to minimize lag times and thus make up for improper cleaning and sanitizing and/or to get a very fast fermentation. Forget about re-using your yeast afterwards...they want you coming back for more, anyways. Do you LIKE diacetyl? Sure, maybe in tiny quantities in a Scottish Ale, but in that Kölschbier? If you over-pitch that severely, you'll be learning to love it sooner or later.

So when someone goes off about these high pitching rates, you'll know what they're trying to do.

What you REALLY want is more along the lines of 250mL per 5 gallons. This and the above on why this discrepancy exists comes from a professional brewer friend answering my questions to him about this. I might over-pitch a bit, around 450mL for about a 6 gallon batch, but that's mostly out of habit....

Maintaining Proper Yeast Starter Temperatures

Unlike fermentation temperatures, when you are in the middle of starting and stepping up your yeast, the correct temperature is 75 deg.F. Depending on how much control you have over internal temperatures (in the tin can I'm currently stuck living in, it's nearly impossible), you may need some extra help. Using an insulating material based on bubble-wrap and aluminum foil (Reflectix) and a USPS Priority Mail box I just happened to have laying around, I came up with the following:

The IncuBox (front view)

The IncuBox - Front View

The IncuBox (side view)

The IncuBox - Side View

The lid, when closed, covers the remaining side with insulation as well.

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