fermentation-science - SKILL.md Agent Skill

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

Fermentation Science Reference

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

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

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)

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