Quality Control & Off-Flavor Diagnostics

Sensory evaluation, common off-flavor fault signatures, root-cause matrix, and how to use Brewfather batch data to accelerate diagnosis.

Consistent, high-quality beer begins with a structured approach to evaluating what you produce and a systematic method for tracing faults back to their origin. This page covers the sensory evaluation process, a catalog of common off-flavors with their perceptual descriptors and root causes, and a process-stage root-cause matrix to guide corrective action.

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Sensory Evaluation: A Structured Approach

A repeatable tasting protocol separates objective observation from guesswork.

1. Environment

• Taste in a neutral, well-lit space away from cooking smells, perfume, or smoke.

• Use clean, room-temperature, odor-free glassware with no detergent residue.

• Taste without food for at least 30 minutes beforehand.

2. Appearance

• Note clarity (clear, hazy, turbid), color (compare to your target SRM/EBC), and head formation and retention.

• Haze can signal chill haze (temporary, clears when warm), protein haze, yeast in suspension, or biological contamination.

3. Aroma

• Swirl and sniff immediately after pouring. Note the dominant notes (malt, hops, fruity esters, sulfur, off-notes).

• The olfactory system adapts quickly — make your first impression count.

4. Taste

• Take a small sip, let it coat the whole palate. Note sweetness, bitterness, acidity, saltiness, and any off-notes.

• Evaluate balance: malt sweetness vs hop bitterness (related to your IBU:gravity ratio).

5. Mouthfeel & Finish

• Note body (thin, medium, full), carbonation level, warmth (alcohol), astringency, and lingering aftertaste.

6. Document Everything

• Write tasting notes immediately. Memory degrades within minutes.

Brewfather Tip: Use the Tasting Notes field in the Batch's Completed step to record structured sensory observations. Over time, comparing notes across batches reveals patterns — for example, a recurring diacetyl note only in colder-weather batches may point to an insufficient diacetyl rest.

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Common Off-Flavors: Descriptors, Thresholds & Root Causes

Diacetyl

Attribute	Detail
Descriptor	Butter, butterscotch, toffee
Threshold	Often cited around ~0.05–0.15 mg/L (varies by beer matrix and taster sensitivity)
Root causes	Insufficient diacetyl rest; premature cold crash; severe under-pitching; high fermentation temperature followed by rapid chilling; Pediococcus or Lactobacillus contamination
Fix	Raise temperature to 18–20 °C (64–68 °F) for 48–72 h before crashing; ensure adequate pitch rate; verify sanitation

Diacetyl is a normal yeast by-product (from acetolactate) reabsorbed during conditioning. Crashing before reabsorption is complete is the most common cause in homebrewing.

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Acetaldehyde

Attribute	Detail
Descriptor	Green apple, fresh-cut pumpkin, cidery
Threshold	~10–25 mg/L
Root causes	Incomplete fermentation; beer removed from yeast too soon; insufficient conditioning time; yeast stress
Fix	Allow more conditioning time in contact with yeast; slightly raise temperature (diacetyl rest helps here too); do not rack off yeast prematurely

Acetaldehyde is an intermediate metabolite. Healthy, active yeast will typically reduce it further — patience is the primary remedy.

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DMS (Dimethyl Sulfide)

Attribute	Detail
Descriptor	Cooked corn, creamed corn, cooked vegetables
Threshold	~30–50 µg/L (style-dependent; lagers are more sensitive)
Root causes	Covered kettle during boil (prevents evaporation of DMS precursor, S-methylmethionine); insufficient boil vigor; slow chilling after knockout; use of high-SMM malt (pale lager malt) without adequate boil-off
Fix	Boil uncovered with vigorous rolling boil; target 10–12% evaporation per hour; chill wort rapidly; avoid covering kettle during whirlpool stand

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Hydrogen Sulfide (H₂S)

Attribute	Detail
Descriptor	Rotten egg, struck match (at high levels)
Root causes	Yeast stress (nutrient deficiency, temperature shock, low FAN); reduced yeast viability; sulfur-producing strains (common in many lager strains)
Fix	Ensure adequate yeast nutrition and healthy pitch rates; avoid temperature shocks; give beer time for natural off-gassing/conditioning; avoid prolonged oxygen exposure while attempting to degas

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Fusel Alcohols

Attribute	Detail
Descriptor	Solventy, hot, harsh, nail polish remover
Threshold	Variable; n-propanol, isobutanol, isoamyl alcohol each have distinct thresholds
Root causes	High fermentation temperatures (especially during the first 48–72 h); severe under-pitching; insufficient wort oxygenation; nutrient deficiency
Fix	Ferment at the low end of the yeast's recommended range for the first 48–72 h; maintain adequate pitch rate; oxygenate wort properly

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Phenols (Undesirable)

Attribute	Detail
Descriptor	Medicinal, plastic, band-aid, TCP
Root causes	Chlorophenols: chlorine/chloramine in water reacting with wort phenolics; wild yeast (Brettanomyces); POF+ yeast outside intended style; infected equipment
Fix	Treat brewing water for chlorine/chloramine (common rule of thumb: ~1 Campden tablet per ~75 L / 20 US gal; verify with your water report and product label); verify no wild yeast contamination; thorough cleaning and sanitation

Note: Clove/spicy phenols are desirable in German Hefeweizen and many Belgian styles when produced by intentionally selected POF+ brewing yeast strains.

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Oxidation

Attribute	Detail
Descriptor	Cardboard, papery, stale, sherry-like (trans-2-nonenal), honey-like
Root causes	Oxygen pickup post-fermentation: splashing during transfer, headspace in packaging, improper purging of kegs/bottles, repeated dry-hopping with O₂ exposure
Fix	Minimize splashing at all cold-side transfers; purge vessels with CO₂; minimize headspace; use closed transfers; consider oxygen-scavenging caps for bottling

Oxidation is largely irreversible — prevention is the only effective strategy.

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Astringency

Attribute	Detail
Descriptor	Harsh, puckering, dry, tea-like, grape-skin
Root causes	Tannin/polyphenol over-extraction: aggressive sparging with high runoff pH (typically risk increases as runoff pH rises toward/above ~6.0); very hot sparge water; overly fine crush; high-pH contact with husk material
Fix	Limit sparge water to 75–77 °C (167–170 °F); stop sparging when gravity of runnings drops to ~1.008–1.010; check grain crush; check mash/sparge pH

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Acidity / Sourness

Attribute	Detail
Descriptor	Sharp acidity, vinegar (acetic acid), lactic tartness
Root causes	Lactobacillus or Pediococcus contamination → lactic acid; Acetobacter contamination + oxygen → acetic acid (vinegar); wild yeast (Brettanomyces) producing acetic and other acids
Fix	Rigorous sanitation (see Cleaning, Sanitation & Safety); identify and replace contaminated equipment (plastic scratches harbor bacteria); prevent oxygen exposure during fermentation

Note: Intentional souring (kettle sours, mixed fermentation) is distinct from accidental contamination and is managed accordingly.

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Lightstruck (Skunky)

Attribute	Detail
Descriptor	Skunk, mercaptan, cat musk
Root causes	UV and short-wavelength visible light breaks down isomerized alpha acids (iso-alpha acids) in the presence of riboflavin, generating 3-methyl-2-butene-1-thiol (3-MBT) — the same compound produced by skunks
Fix	Store and ferment in opaque or amber vessels; avoid clear/green glass bottles; minimize light exposure in the brewery

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Autolysis

Attribute	Detail
Descriptor	Meaty, yeasty broth, rubbery, vegemite-like
Root causes	Yeast cell death and rupture releasing intracellular contents; extended storage on yeast cake at elevated temperatures
Fix	Avoid prolonged warm storage on a large yeast cake; package or cool beer in a reasonable timeframe. In typical homebrew timescales, autolysis is uncommon if temperature is controlled and yeast is healthy

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Root-Cause Matrix by Process Stage

Process Stage	Likely Off-Flavor	Key Variables to Check
Water treatment	Chlorophenols (medicinal), sulfur faults	Chloramine/chlorine removal (campden/metabisulfite dose), water report, and dosing accuracy
Mash	Astringency, starchy/grainy	Sparge temp & pH; crush quality; conversion completeness
Boil	DMS (cooked corn)	Boil vigor; evaporation rate; cover off; chill speed
Fermentation — early	Fusel alcohols	Pitch rate; wort oxygenation; fermentation temp (first 48 h)
Fermentation — late	Diacetyl, acetaldehyde	Diacetyl rest; conditioning time; yeast health
Packaging	Oxidation, flat beer, over-carbonation	O₂ exposure; priming sugar calc; FG confirmed stable
Storage/serving	Lightstruck, oxidation, staling	Light exposure; storage temperature; O₂ ingress
Sanitation failure	Acidity, lactic, acetic, medicinal, rope	Cleaning efficacy; sanitizer contact time; plastic equipment condition

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Sensory Threshold Reference (Selected Compounds)

Compound	Off-Flavor Note	Approximate Threshold
Diacetyl	Butter	~0.05–0.15 mg/L
Acetaldehyde	Green apple	10–25 mg/L
DMS	Cooked corn	30–50 µg/L
Isoamyl acetate (high)	Banana (desirable in Hefeweizen, off in others)	~1.2 mg/L
3-MBT (lightstruck)	Skunk	~4 ng/L
Trans-2-nonenal	Cardboard	~0.1 µg/L (often cited range around 0.03–0.15 µg/L)

Thresholds vary by individual, training status, and beer matrix effects (masking/synergy). Treat these values as order-of-magnitude guidance, not hard cutoffs. Training with standardized off-flavor kits (Siebel, FlavorActiV, Aroxa) remains one of the most reliable ways to calibrate sensory detection.

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Sources

• Bamforth, C.W. (2003). Beer: Tap into the Art and Science of Brewing (2nd ed.). Oxford University Press.

• Briggs, D.E., Boulton, C.A., Brookes, P.A., & Stevens, R. (2004). Brewing: Science and Practice. Woodhead Publishing.

• Palmer, J. & Kaminski, C. (2013). Water: A Comprehensive Guide for Brewers. Brewers Publications.

• White, C. & Zainasheff, J. (2010). Yeast: The Practical Guide to Beer Fermentation. Brewers Publications.

• Meilgaard, M., Civille, G.V., & Carr, B.T. (1999). Sensory Evaluation Techniques (3rd ed.). CRC Press.

• ASBC Methods of Analysis (sensory and flavor-related methods).

• Meilgaard, M.C. et al. (1979). Beer Flavor Terminology / Beer Flavor Wheel (ASBC Technical Committee).

• Shellhammer, T. (Ed.) (2009). Hop Flavor and Aroma. Master Brewers Association of the Americas.