Unit 4: Yeast Flashcards
What is ale yeast, its name, and its characteristics?
Saccharomyces cerevisiae
1) Works best at 65-75°F
2) Tends to collect on the top of the wort at the end of fermentation
What is lager yeast, its name, and its characteristics?
Saccharomyces pastorianus
1) Works best at 33-58°F
2) Stays distributed in the wort
3) Is capable of fermenting a wider range of sugars than ale yeast, including melibiose, a sugar that ale yeast cannot ferment
During the initial phase of fermentation, what occurs?
1) The yeast take up sugars and other nutrients from the wort to grow and reproduce.
2) This growth phase is aerobic, and the yeast use up all available oxygen in the wort.
During the second phase of fermentation, what occurs?
Actual fermentation is an anaerobic process where the yeast consumes sugars and processes them into carbon dioxide and ethanol.
What other compounds are created as a result of yeast metabolism?
A very complex and numerous array of compounds, including:
1) diacetyl - (buttery flavor)
2) acetaldehyde - (green apple flavor)
3) esters, such as:
a. isoamyl acetate - (banana flavor)
b. ethyl acetate - (light fruity, solvent-like)
c. ethyl caprylate - (apple-like)
d. ethyl caproate - (apple-like with a note of aniseed)
e. phenyl ethyl acetate (roses, honey)
4) phenols, such as:
a. 4-vinyl guaiacol - (clove flavor)
b. 4-Ethylphenol (4-EP) - is produced by the wild yeast type Brettanomyces, reminding people of farmyards, medicines, horse blankets, and mice. It is often seen as an off-flavor but a component of some Belgian ales and lambics.)
c. 4-Ethyl Guaiacol - (Smoked meat, clove-like spicy character, also has interesting woody and sweet vanilla nuances. Produced by Brettanomyces and seen as an off-flavor in most cases.)
What does the acronym POF stand for?
Phenolic off-flavor. Some yeast strains have what brewers call the POF-gene, which tends to create phenols. At times these flavors are desirable, so a new acronym PAD-gene, for Phenol Acrylate Decarboxylase, is used as a more neutral term.
Why are there so many more varieties of ale yeast available than lager yeast?
Lagers are characterized by their LACK of complex yeast flavors. Ales make tremendous use of those flavors and strains are cultivated to emphasize certain desirable flavors. Since lager yeast is expected to ultimately consume any of the flavors it creates in the lagering process, it serves well to have a simple selection.
When and where were pure yeast strains first developed?
Carlesburg developed a laboratory for beer research in 1875, and in 1883, one of the directors there, Emil Christian Hansen, was able to isolate and reproduce single yeast cultures. Further work allowed him to test and compare the various single strains and make a selection of the ones that performed best for making beer.
How does temperature affect yeast performance and the flavor compounds produced?
Within the general range of fermentation temperatures, each strain may have its own best temperature, and each strain may produce different flavors at various temperatures.
What is attenuation?
This refers to the amount of sugar that the type of yeast is able to consume. High attenuation leaves a very dry wort as nearly all available sugar has been converted to CO2 and alcohol. Hi attenuation also gives a lighter body to the wort. Low attenuation means that some of the sugars remain in the wort, making it sweeter in taste, and fuller-bodied.
What is flocculation?
This is something that occurs as fermentation nears the end, and proteins in the cell walls of yeast are attracted to receptor sites in the cell walls of others. The effect of this is that the yeast begins to clump or aggregate together. Lager yeasts tend to form a sediment on the bottom of the fermenter and occasionally need to be agitated to bring the yeast back into contact with the remaining sugars. Ales often flocculate forming a yeast layer on top. This top layer can be skimmed and repitched into a new batch of wort to start a new fermentation. In either case, there will still be yeasts suspended in the beer and without further clarification would produce a very cloudy beer.
What other type of fermentation can occur in brewing besides yeast?
Certain bacteria such as lactobacillus, Pediococcus and acetobacter, can be introduced.
Like brewers yeasts, lactobacillus metabolizes sugars as the main source of energy, but, unlike yeast, it produces lactic acid instead of alcohol. This is a desirable quality for an organism used in making such foods as yogurt, but notable lactic acidity is an off-flavor in most types of beer. Lactobacillus also produces other off-flavors, including diacetyl. Beer with a serious lactobacillus infection will often become hazy. Although undesirable in most beers, there are a few beer styles for which lactobacillus helps to create part of the beer’s characteristic flavor. These include most Belgian sour beers such as lambic, American sour beers, traditional Berliner Weisse, and even Belgian witbier, which traditionally showed at least a tinge of lactic acidity.
Pediococcus, aka pedio, is also a bacteria, not a yeast. Pedio is the other common lactic acid bacteria used in sour beers and in other culinary roles like the acidification of sauerkraut and traditional dried sausages. Pedio, unlike lacto, takes a long aging time to initiate a dramatic lowering in the pH of the beer, which works as an advantage because it allows time for the primary yeast strain to complete its fermentation before the substantial drop in pH occurs. The drawback to acidifying with pedio is that most strains produce concentrations of diacetyl above the taste threshold. Unlike brewer’s yeast, pedio doesn’t reduce diacetyl by converting it to less-flavorful by-products. Instead, it leaves the buttery popcorn flavor behind. A good remedy is to include Brett in beers pitched with pedio, so it can eliminate diacetyl. This takes time, so be patient if your beer tastes like movie theater popcorn when it’s young.
Acetic Acid Bacteria typically oxidize ethanol to form acetic acid (vinegar) and are used for commercial vinegar production. This family of bacteria (Acetobacteriaceae) encompasses some 10 types, of which the most common are Acetobacter, Gluconobacter, and Gluconacetobacter. It can only function in the presence of oxygen. If you do desire a low level of acetic acid, which is an important flavor component of traditional Flemish reds and Belgian lambics, add a small amount of unpasteurized vinegar (which contains live Acetobacter) when you pitch the other microbes.
What other yeasts are sometimes used besides ale and lager yeasts?
Brettanomyces, often called Brett, is a genus of yeast, not bacteria, as far too many brewers falsely believe. It is the principal wild yeast used in sour beer production. Specific flavors, aromas, esters, and phenols produced in the beer depend on the strain and species of brett, ranging from pineapple and hay to horse blanket and acrid smoke. The beer’s character is also influenced by the acids and alcohols available to be combined into esters during fermentation. Tonsmiere referred to Brett as the microbe that “doesn’t sour the beers you brew; it makes the sour beer you brew delicious.”
Brett also doesn’t contribute much to the acidity of sour beers, either. Acid production is the responsibility of bacteria. The only exception is when there is a large amount of oxygen available, which causes Brett to produce acetic acid, creating a vinegar-like sourness.
Brett serves the same function as Saccharomyces – it ferments beer. However, Brett works more slowly, meaning a beer that could have fermented in a few weeks might take months or years to display its full character when Brett is introduced. The great beer writer Michael Jackson once compared Saccharomyces to a dog and Brett to a cat. Saccharomyces is trainable, usually predictable, and comes back to you when called, while Brett will run away when it feels like it and probably scratch you when you pick it up. In other words, it can be hard to predict and manage Brett, but you’ll be rewarded if you can respect the yeast for what it is and what it can do.
Brett works well in tandem with the other microbes.
What is ‘wild fermentation?’
To begin, it’s critical to understand the difference between wild and single-culture fermentation. Sour beers undergo wild or mixed fermentation, meaning multiple yeasts and bacteria work together to create the funkiest brews. In the category of wild ales, beers can be either controlled, meaning the brewers have selected exactly which yeast and bacteria to pitch, or open-air fermented, where fermenters are left entirely or partially open to allow bacteria and wild yeast to enter.
Saccharomyces are the yeast responsible for fermenting all clean beers, while Brettanomyces (a yeast), Lactobacillus (a bacteria), and Pediococcus (a bacteria) are the “Three Stooges” of sour beer production. They produce funky and sometimes hard-to-predict beers.
What compounds are associated with Brettanomyces?
4-ethylguiacol (spicy, clovy, or smoky and perhaps even bacon-like)
4-ethylphenol (band-aid, antiseptic and horse stable)
4-vinylguiacol (clove, spices)
