Yeast basics

yeast cell

• bud formation: yeast cells multiply by budding. old yeast has many scars.
• with oxygen: use sugar to make CO2 and water, mainly multiplying
• without oxygen: use sugar to make CO2, alcohol, pleasant aromas. fermentation!
• yeast needs: sugar, protein, lipid(fat), vitamin(especially when re-use yeast), calcium, zinc, oxygen(up to 20 hours)

Saccharomyces cerevisiae

• top yeast, ale yeast, 13~33'c *pale ale, IPA, British ale under 23'c
• high fermentation temperature(ex. 20'c), fermentation is fast(ex. 3 days)
• skimming yeasts off the top of the beer for repitching at the end of fermentation
• can be clarified by the agent
• cannot ferment a sugar 'melibiose', so that have a full body
• in an open fermentor(British tradition), 100~200 re-use available
• in a cylindro conical tank, 8~12 re-use available

Saccharomyces carlsbergensis

• bottom yeast, lager yeast, 10~12'c
• low fermentation temperature(ex. 10'c) and fermentation is slower(ex. 10 days)
• usually not collecting for repitching but cultivate in tanks
• cannot be clarified by the agent
• can ferment a sugar 'melibiose'
• in a brewery: yeast pitching/cropping(for re-use)/storage/propagation(not usual)/waste

checklist before pitching

1. right kind of yeast for desired beer?
2. acid wash? (lower pH to prevent bacteria infection)
3. viability? (use a dye. healthy yeast cannot be colored) *uncolored under 75% is ok
4. yeast cell count
5. direct pitching or rehydration(save more yeasts from death)?-without rehydration, a half of pitched yeast will die

yeast cropping

• approximately at the end of fermentation
• yeast generate bubbles in 2~3days after the beginning of fermentation<- the right time to begin to crop
• in an open fermenter, skimming yeast off the top of the beer
• in a conical tank, waste trub(at the bottom, darker one) collect slurry
• storing in a freezer(2~4'c) or "con to con"(just pitching another tank without gap)
• rest at least 2 hours, let yeast rest more than 24 hours
• storing yeast must re-use in 7 days

yeast observation

• use a hemocytometer
• *cell counting with a hemocytometer link

Pitching rate

• desired pitching rate: 600,000~10million cells(ale)/1ml wort/ 12~16million cells/1ml(lager)
• over pitching: lower production of ester, unexpected flavor, bad foam stability
• less pitching: longer fermentation period(easy to be conteminated)
• yeast growth: pitching first 20 hours(ale) with 9~18ppm of O2(high target gravity, high oxygen)
• the amount of yeast needed=(600,000~)750,000*wort in ml*plato/yeast per ml
• ex. target gravity 1.050(=12.5 plato)for 1,000L
• 750,000*1,000,000(ml)*12.5=9,375,000,000,000,000/1,200,000,000*=781,250(ml)=7.8L
• *informed by yeast seller

Ferment control

• while ferment: temp up, (the amount of)yeast up, specific gravity down
• end of ferment: temp down, yeast down, specific gravity level off
• optimal ending pH: less than 4.4, 4.2~4.6(lager), 4.0~4.2 and some ale as low as 3.8(3.0 for sour)*
• *related article the principle of pH link

Beer flavor compounds

• ester: fruit flavor. high ester high temp and high sugar rate, Ethyl acetate(tropical fruit, strawberry, citrus, bubble gum), Iso-amyl acetate(banana, pear)
• ester=activated acid(limiting factor)+alcohol
• when a number of yeasts grow, activated acid will be absorbed=low ester
• when a number of yeasts level off, activated acid will be used=high ester
• carbonyls>aldehyde
• carbonyls>diketones
• Aldehyde>Acetaldehyde: green apple, immature beer or sudden temp down 
• Aldehyde>trans-2-noneal(T2N), furfural: cardboard, wet paper, oxydation 
• Aldehyde>propanol, 2-methyl butanol, pentanol: grass
• Alcohol(fusel oil): solvent, chemical, slight burn feel, at high temp, at lower pH
• carbonyls>diketones>Diacetyl, 2,3-pentanedione: butter scotch candy, immature beer, give yeast time to re-absorb. a parameter for deciding when to move from fermentation to an aging process.
• Sulphur compound: rotted egg(hydrogen sulfide), cautious cold crash process needed
• Phenols: clove, sanitizer, from bleach or wild yeast
• DMS or DMSO: vegetal, cooked corn

Yeast selection

• Belgian: suitable for high sugar wort, flower aroma
• British: high flocculation rate, malt, flower aroma and diacetyl warning
• American: slight yeast aroma, suitable for hoppy beer
• German: low flocculation rate, phenol warning 
• Brettanomyces: can digest dextrin, strong punk pungent. for traditional lambic, sour ale
• Lactobacillus: no oxygen needed(otherwise, baby vomit)
• Pediococcus: butter scotch, viscous, sourness. for sour ale

What to consider when select yeast

• attenuation: capability of digest sugar
• flavour profile
• flocculation: influence hazyness and capability of re-absorb diacetyl
• reliability of supply
• tank design, brewery design for cross contamination