Yeast: The Practical Guide to Beer Fermentation

  • Wort high in glucose produced more estery beer
  • Rice and corn also provide maltose but often lack nutrients needed by yeast
  • Optimal oxygen levels for wort is 8-10 ppm. Outside that range results in poor yeast health, longer fermentation times and fewer generations of usable yeast.
  • Using pure oxygen can over-oxygenate wort creating off flavors (but this is rare, especially outside of homebrewers)
  • Wort splashing rarely raises oxygen levels above 4 ppm
  • Using an aquarium pump and sintered some will not result in levels higher than 8 ppm
  • High gravity beers benefit from second aeration with oxygen after 12 hours which increases fermentation speed by 33% and decreases off flavors.
  • Temperature control is more important to flavor than whether to use a glass carboy, stainless conical or plastic bucket
  • Measurements in order of importance: temperature, specific gravity, pH, oxygen and carbon dioxide
  • Through selective evolutionary pressure by brewers, brewer’s yeast lost the ability to mate and form spores. It also became polyploidy (has multiple copies of each chromosome)
  • Yeast typically deplete wort oxygen levels in 30 minutes
  • Yeast get only 8% as much energy from sugar during anaerobic fermentation as they would in an oxygen rich environment
  • Brewer’s yeast can produce 500 different flavor and aroma compounds
  • All beer contains esters without which beer would taste bland. They are produced by combining and alcohol and an acid and have more flavor than each independently
  • Higher oxygenation reduces ester levels
  • Custer effect – Brettanomyces produces less alcohol without oxygen. Unlike other yeast, it will produce ethanol in aerobic fermentation
  • Brett can consume the wood sugar present in oak barrels
  • The unsaturated fatty acids in olive oil can make up for underaerated wort.
  • At 48 hours into fermentation, raising the temp to 77 for ale or 68 for lager keeps year working at their maximum.

Pitching

Temp:

  • Ale: 72-75° F (22-24° C) then dropping to 68° F (20° C) for fermentation can help makeup for a small pitch without off-flavors
  • Lager: 72-75° F (22-24° C) then dropping to 50-55° F (10-13° C)
  • Raising the temp by 4-10° F (2-5° C) during the last 24 hours of fermentation aids yeast activity, improves attenuation and drives of some volatile compounds.

Highlights

  • The easiest way to make small batches of starter wort is with metric measurements, using a 10 to 1 ratio. Add 1 gram of DME for every 10 milliliters of final wort volume.
  • Many brewers mistakenly focus on cell growth at the expense of yeast health. It is much better to have a smaller number of very healthy, young cells than it is to have a large number of weak cells. You should always make a starter if you suspect the viability or vitality of your yeast might be low.
  • Every time you make a starter, keep in mind the four main factors that affect yeast growth and health: nutrients, temperature, sugars, and pH
  • At a level of about 12 IBUs or higher, hops add some antimicrobial protection
  • Propagation Steps:
    • Make a starter with 100 grams DME, adding water to make 1 liter of finished volume.
    • Add the vial of yeast, and grow for 24 to 48 hours.
    • You should now have about 150 billion cells. You created 50 billion new cells.
    • Refrigerate until all of the yeast settles, and decant the spent wort.
    • Here is where people get confused. If you were to add another liter of starter wort, you would not create another 50 billion cells. Instead, you would create only 18 billion.
  • When harvesting early, the brewer usually discards the yeast or uses it as a nutrient in the brew kettle. The brewer does not retain it for re-use because repitching early-flocculating yeast often results in lower attenuation with every re-use
  • In homebrewing the concept of “secondary fermentation” was quite popular for a number of years. The belief was that transferring the beer from one fermentor to another would do a couple of things for the beer. The first was that it would get the beer off the yeast at the bottom of the fermentor, before the yeast broke down and caused off-flavors in the beer. The second was that transferring the beer made it clear faster. Both of these points are not completely valid
  • If you are planning on making sour beers or doing any dry-hopping, fruit additions, or oak aging (anything that will require either yeast-free beer or longer warm-storage time), then transferring the beer to a clean vessel is worthwhile.
  • Yeast repitched the same day as harvested is the goal. You should store yeast at 33 to 36° F (1 to 2° C) and use it within seven days. Consider 14 days the maximum storage time, discarding any older slurries.
  • A number of homebrewers have adopted the practice of transferring the beer from a fermentor at the end of fermentation and then adding a new batch of wort on top of the yeast cake. This is a bad practice.
  • In yeast rinsing, you are using dilution of a slurry to encourage better stratification of trub and yeast, allowing you to separate the two. In yeast washing, you are using acidification or other chemical means to reduce the number of active bacteria, while not damaging too many yeast cells.
  • Shut windows and turn off any fans in the area, including central heating and air. Even though you have now eliminated air movement, bacteria and wild yeasts are still descending constantly. To counter this, light a flame, such as an alcohol lamp or a Bunsen burner, and work near the updraft it creates (Figure 6.1). This is an inexpensive and effective barrier that pushes airborne bacteria and yeast up and away from sterile cultures and media
  • important aspects of your lab space:
    • Overall cleanliness
    • No or very low airflow
    • Minimal foot traffic, noise, and vibration
    • Wear appropriate clothing and keep hair back.
    • Adequate lighting and ambient temperatures
    • Wipe down surfaces before starting work.
    • Provide a microbe-free environment within the works
  • Any brewery, no matter how small, should run basic forced wort and forced ferment tests.
  • You can differentiate ale and lager strains by two methods, either the ability to grow at 99° F (37° C) or by the ability to grow on melibiose
  • Lager yeast can ferment the carbohydrate melibiose and ale yeast cannot.