BrewingTechniques
Starter Worts
Starter wort can be purchased in dried form, or you can make your own. I prefer 1.020-original gravity wort for starter tubes and 1.040 for pitching solutions. 1.020 wort gives better yeast multiplication, perhaps because it holds more dissolved oxygen than 1.040 starters. The 1.040 solution comes closer to gravity of the wort to be fermented. I believe that yeast pitched into the wort will start faster if it does not have to adjust to a significantly different gravity.

Boilover is a problem when making small batches of wort. I use a BB-size piece of hop pellet to reduce the problem. I have had excellent results using Wander brand dry malt extract for the fermentable base, plus a tiny pinch of yeast nutrient from William's Brewing (San Leandro, California). This mixture gives good growth and shows almost no tendency to boil over. The water used for mixing is very important. Never use high-carbonate water for these starters because high pH favors bacterial growth. If in doubt about your water, use distilled water. Starter pH should be no higher than 5.3. Start with about 20% more water than the volume you want because some will be lost during the 20-min boil.

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Typical Yeast Culturing Kit Components
Starter tube: A small, capped tube containing 10-15 mL of sterile wort. Used for expanding culture size.

Flask: A laboratory-grade (Erlenmeyer) flask, narrow at the mouth but wide at the bottom, usually 1/2 or 1 L in size. Used for expanding culture size.

Slant: A small, capped tube containing solidified (gelled) sterile wort. Used for long-term storage of yeast samples. Yeast is often supplied on slants when purchased.

Plate: A short, wide, covered dish to be half filled with solidified wort. Often called a petri dish. Used for isolating yeast colonies and short-term storage.

Inoculation loop: A metal rod with a wire extending axially from one end. The end of the wire forms a small loop. Used for transferring small amounts of yeast.

Growth media: Dry malt extract plus nutrients to feed growing yeast. Many specialized media are available, but this combination is easy for craft brewers to obtain, store, and use.

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Yeast Theory and its Application to Brewing

Yeast is what's known as a facultative anaerobe, which means that it grows both under aerobic and anaerobic conditions. Maximal yeast growth occurs under aerobic conditions or in the presence of oxygen. Other important factors influencing yeast growth are the pH, temperature, and growth medium. Actively growing yeast metabolize the sugar in wort at a much faster rate than nongrowing yeast. The purpose of a yeast starter is to accelerate the fermentation of wort.

     Yeast growth can be divided into five stages.

  • Lag phase occurs during the first few hours after inoculation. Although no signs of fermentation or growth are apparent, the yeast are busy becoming acclimated to their new environment. All the enzymes and other metabolic machinery necessary to ferment wort into beer are being synthesized. All the oxygen you introduced into the cooled wort is taken up by the yeast within the first 20 min and is being stored (as sterols and unsaturated fatty acids) for later use.
  • During the accelerating growth phase, yeast cells start to divide rapidly.
  • The actual number of yeast cells increases during the logarithmic phase. During this time the number of yeast cells may increase as much as 1000-fold (or 3.0 logs) within 24 h.
  • As the oxygen and nutrients are depleted, the yeast enter a phase of decelerating growth (approximately 12 h) and ultimately reach a stationary phase.
  • During the stationary phase, yeast growth ceases.

         Despite the rapid rate of yeast growth, a relatively large yeast starter or slurry of yeast is required for optimal beer production. Siebel Institute recommends one-sixth of the batch-size, one-tenth if you continuously aerate or agitate your starter. The scientific basis for this is currently unknown.

    Use of lower pitching rates such as those typically used by home brewers - 1/20 to 1/100 of the batch size - yield slightly higher terminal gravities because of an alteration in the metabolism of sugars in wort. Generally the order of sugar consumption is glucose (10% wort sugar), sucrose (2% wort sugar), maltose (40% sugar), then maltotriose (10% sugar). When underpitched, all of the later fermented sugars are not metabolized. The end result is a sweeter beer with terminal gravities above 1.010. Underpitching along with underaeration of the wort (oxygen levels should be about 8 ppm) can yield gravities above 1.015.

    As you increase the overall amount of yeast growth (such as occurs during underpitching), ester formation supposedly increases (according to the Siebel Institute). Theoretically, then, optimal fermentation does not involve significant yeast growth but rather more yeast metabolism of the wort and fermentation to alcohol. Optimal pitching rates should produce a balance between yeast growth and metabolism.

    The only practical way for home brewers to achieve these types of pitching rates is by repitching their yeasts. Otherwise you would be pitching a 1-gal starter (or its sediment) into a 5-gal batch! I recommend a compromise that is practical and routinely yields good results. Start with about 10 mL of wort, step it up at least once, and use 0.5-1 qt of starter/5 gal of wort. This is 1/20 to 1/40 of the total wort volume. Each of these steps is about 1-1.5 log dilutions and, according to the growth curves, should take less than 24 h to reach maximum growth. In general, strong ales and lagers require higher pitching rates (at least 1 qt/5 gal). Also, beware that using the standard starter conditions (1.040 O.G. hopped wort grown at room temperature) in these styles of beer may "shock" the yeast and increase the overall lag time. In this case, you may want to adjust your starter so that it more closely resembles the wort it is going to be pitched into.-Maribeth Raines

    Maribeth Raines received her Ph.D. in biochemistry and is currently assistant professor of radiation oncology at the UCLA School of Medicine. She also consults with Brewers Resource (Woodland Hills, California), for whom she developed a yeast culturing kit for home brewers.

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