fermentation-science

name	fermentation-science
description	Mash temperature vs fermentability, yeast pitching rates, dry hop timing and biotransformation, and off-flavour diagnostics

Fermentation Science Reference

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Mash Temperature → Fermentability

Mash temp	Body	Fermentability	Typical attenuation	Best for
62–64°C (144–147°F)	Very light, thin	Very high	78–85%	Session lagers, dry/bone-dry styles
64–66°C (147–151°F)	Light-medium	High	74–80%	West Coast IPA, APA, Dry Stout, Saison
66–68°C (151–154°F)	Medium	Medium-high	68–75%	Most ales — balanced finish, standard target
68–70°C (154–158°F)	Medium-full	Medium	62–70%	Amber ales, Mild, ESB, English Porter
70–72°C (158–162°F)	Full, chewy, sweet	Low	55–65%	Wee Heavy, Imperial Stout, Oatmeal Stout

Notes:

• These ranges assume a single-temperature infusion mash with a well-modified modern malt
• Step mashing (protein rest + saccharification) allows more precise fermentability control
• Enzymatic limit: β-amylase (fermentable) denatures above ~65°C; α-amylase (dextrinous) active up to ~72°C
• Mash time: 60–90 min is standard; shorter times at optimal temp (66–68°C) are fine with well-modified malt

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Yeast Pitching Rates

Recommended rates

• Ales (OG 1.040–1.060): 0.75 million cells/mL/°Plato
• Ales (OG 1.060–1.080): 1.0 million cells/mL/°Plato
• Lagers (standard): 1.5 million cells/mL/°Plato
• Lagers (high gravity): 2.0 million cells/mL/°Plato
• Kveik: can underpitch to 0.1 M/mL/°P — compensates with high temp/fast fermentation

Worked example (20 L batch, OG 1.050 = 12.5°P)

• Ale pitch: 0.75 × 20,000 mL × 12.5°P = 187.5 billion cells
  • ≈ 2 packs dry yeast (95B cells each) or 1 fresh liquid pack + starter
• Lager pitch: 1.5 × 20,000 × 12.5 = 375 billion cells
  • ≈ 4 packs dry yeast or 2 fresh liquid packs (always use a starter for lagers)

Cell count reference

Format	Approx. viable cells
Dry yeast sachet (11.5 g)	80–100 billion
Fresh liquid pouch (Wyeast/White Labs)	100–150 billion
Starter (1 L, 24h)	+100–150 billion
Starter (2 L, 36h)	+200–300 billion

Underpitching effects

• Stressed yeast → higher ester and fusel alcohol production
• Longer lag phase → increased infection risk
• Lower attenuation → higher FG than expected

Overpitching effects

• Minimal ester development (unwanted in Hefeweizen, Belgian styles)
• Rapid fermentation → potential temperature spike
• Generally safe for neutral ale/lager styles

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Dry Hop Timing and Technique

Cold-side dry hop (standard)

• When: After fermentation reaches FG; before cold crash
• Temperature: 17–20°C (same as end of fermentation)
• Contact time: 3–5 days
• Result: Clean, stable hop aroma from essential oils; no biotransformation
• Best for: West Coast IPA, APA, Pale Ale, any style where clarity and stability matter

Biotransformation dry hop (warm-side)

• When: Day 2–4 of active fermentation (at peak krausen)
• Temperature: Fermentation temperature (17–22°C)
• Contact time: 3–5 days (through end of fermentation), then cold crash
• Result: Yeast converts bound hop glycosides into free volatile thiols — new tropical/citrus aroma compounds not present in hops alone
• Best for: NEIPA, Hazy IPA — requires high-biotransformation yeast (London Ale III, Conan, Verdant)

Double dry hop (DDH)

• First addition during active fermentation (biotransformation)
• Second addition after FG, before cold crash (cold-side)
• Maximum aroma complexity; standard for commercial NEIPA

Dose rates

See hop-pairing skill for the full dry hop rate table and NEIPA DDH worked example (all rates are total across all varieties, not per individual hop).

Cold crash timing

• Always cold crash after dry hop contact time is complete
• Biotransformation path: pitch → Day 2–4 add hops → FG reached → cold crash → package
• Cold-side path: pitch → fermentation complete → add hops → 3–5 days → cold crash → package
• Never add dry hops to a cold-crashed beer (oils don't extract well below 10°C)

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Off-Flavour Diagnostics

Fault	Descriptor	Root cause	Fix
Diacetyl	Butter, butterscotch, slick	Rushed conditioning, crashed before yeast cleaned up	Diacetyl rest: raise to 18–20°C for 48h before cold crash
Acetaldehyde	Green apple, latex, paint	Incomplete fermentation; yeast stressed	More conditioning time on yeast; raise temp 2°C for 24–48h at end
DMS	Cooked corn, creamed corn, vegetables	Poor boil vigour, covered kettle, slow chill	Vigorous uncovered rolling boil; rapid wort chilling
Astringency	Dry, puckering, harsh tannins	Over-sparging, sparge water >76°C, over-crushing, high mash pH	Lower sparge temp (<77°C), correct pH (5.2–5.4), check crush gap
Oxidation	Cardboard, paper, sherry, stale	O₂ introduction post-fermentation	Closed transfers, CO₂ purge, minimise splashing, avoid oxygen pickup
Fusel alcohols	Hot, solventy, harsh	Underpitching, fermenting too warm, nutrient deficiency	Lower fermentation temp; correct pitch rate; add yeast nutrient
Chlorophenol	Medicinal, plastic, band-aid	Chlorinated tap water reacting with wild yeast phenolics	Add 1 Campden tablet per 20 L to eliminate chlorine/chloramine
Acetic acid	Sharp vinegar	Acetobacter infection — O₂ + ethanol → acetic acid	Prevent O₂ exposure post-pitch; sanitise transfers; check airlocks
Phenolic (harsh)	Band-aid, smoky, medicinal	Wild yeast contamination, unsanitised equipment	Full sanitisation review; replace plastic equipment if scratched
Lactic acid	Sharp, mouth-puckering sourness	Lactobacillus infection	Ensure proper sanitation; no airlock issues
Light-struck	Skunky, cannabis, mercaptan	UV light exposure reducing iso-alpha acids	Use brown glass or cans; store in dark; avoid fluorescent light
Autolysis	Meaty, rubbery, yeast-like	Yeast autolysis from heat, age, or excessive pressure	Remove beer from yeast cake promptly; don't overheat

Common sources by stage

Mash/lauter faults:

• Astringency — over-sparging, high sparge temp, high pH
• DMS precursor (S-methyl methionine) — insufficient heat early in boil

Boil faults:

• DMS — covered kettle, low vigour, slow chill
• Cooked vegetable — early vigour too low (don't cover the kettle)

Fermentation faults:

• Diacetyl, acetaldehyde — underpitching, underconditioned
• Esters (too high) — too warm, overpitched in delicate styles
• Fusel alcohols — too warm especially early fermentation, underpitch, insufficient nutrients

Packaging/storage faults:

• Oxidation — splashing, poor sealing, overcarbonation blowoff
• Light-struck — clear glass bottles exposed to UV
• Flat beer — insufficient priming sugar, cold packaging temperature