BJCP National · Lesson 5 · Characteristics question (C0)

C0: describe three characteristics, the way graders count

C0 asks you to describe three beer characteristics or off-flavors. It looks open-ended, but it is scored against a fixed checklist. The points are in the parts most people skimp: an exhaustive causes list and a full count of appropriate and inappropriate styles.

Aim. For any characteristic you are handed, write the same five-part skeleton without thinking about structure – so all your time goes to recall. Hit five-plus causes, all four perception senses, and three-plus styles on each side. Those three are exactly where the RTP said points were lost.

Compress: the five-part skeleton (write this every time)

For each characteristic: (1) Perception across all four senses → (2) Descriptor + compound/threshold → (3) Causes, exhaustive (aim 5+) → (4) Control / prevention → (5) Styles where APPROPRIATE (3+) and INAPPROPRIATE (3+). Structure from the vault off-flavor template; content source-gated to the BJCP Study Guide.

Graders literally count your causes and your style examples. One cause and one style is a half-answer. The skeleton forces you to keep going until you have the full set.

Perception – fill all four rows, even when one is "none". Sight · Smell · Taste · Mouthfeel. If a fault is not visible, write "not typically observable" – do not leave the row blank. The RTP flagged missing appearance/mouthfeel rows on acetaldehyde and sourness as dropped points.

The high-yield fault profiles (source-gated)

Each row: descriptor · causes (the exhaustive list is the point) · a low-level-OK example. Learn the causes cold – that is the graded column.

Fault	Perception / descriptor	Causes (exhaustive)
Diacetyl	Artificial butter / butterscotch / toffee; slickness on the palate. Many tasters are blind to it.	(1) beer prematurely separated from yeast; (2) oxygen exposure during fermentation; (3) low FAN; (4) yeast mutation; (5) lactic-bacteria contamination, notably Pediococcus damnosus.
DMS	Cooked vegetables – corn, celery, cabbage, onion, parsnip; shellfish at extremes.	(1) short/weak/closed boil (a long open boil evaporates it); (2) slow wort cooling; (3) low fermentation vigor (lagers scrub less); (4) wild yeast / Zymomonas; (5) Pilsner malt, which carries up to 8× the SMM of pale malt (the most common cause).
Acetaldehyde	Fresh-cut green apple; grass, green leaves, latex paint.	(1) green/young beer (normally reduced to ethanol in secondary); (2) beer pulled off the yeast too early; (3) oxidation reversing ethanol back to acetaldehyde; (4) bacterial spoilage (Zymomonas, Acetobacter); (5) inadequate wort oxygenation.
Estery / fruity	Banana, pear, apple, plum, papaya. Key esters: isoamyl acetate (banana), ethyl acetate (pear→acetone), ethyl butyrate (tropical/bubblegum), ethyl hexanoate (red apple).	(1) yeast strain; (2) high fermentation temperature; (3) high-gravity wort; (4) fatty-acid metabolism in the trub; (5) low pitching rate; (6) low wort aeration.
Phenolic – clove	Clove, spicy; 4-vinyl guaiacol.	Special S. cerevisiae strains convert ferulic acid; rises with fermentation temperature; enhanced by a ferulic-acid rest (~44 °C).
Phenolic – medicinal	Medicinal, plastic, band-aid, electrical tape; smoky.	(1) wild yeast; (2) chlorophenols from chlorinated water / sanitizer residue; (3) husk phenols from overcrushing, oversparging, or hot/alkaline sparge; (4) Brettanomyces (leather/horse-blanket/barnyard).
Sour / acidic	Sour taste; lactic (yogurt, sourdough) or acetic (vinegar).	Lactic from Lactobacillus / Pediococcus; acetic from Acetobacter, Zymomonas, Kloeckera/Brettanomyces. High levels usually mean a sanitation problem.
Oxidation – paper	Wet cardboard, recycled paper, cut cucumber, lipstick; (E)-2-nonenal, threshold ~0.1 ppb.	Oxidation (packaging + hot-side aeration). Pale, low-gravity beers show paper; dark, malty beers show sherry instead.
Oxidation – sherry	Dry sherry; hazelnut, almond, dried fruit (benzaldehyde). Winey/prune at high levels = past prime.	Oxidation of melanoidins (rich malt when fresh → sherry when oxidised); hot-side aeration.

Controls (the fourth part)

Diacetyl: diacetyl rest (60–65 °F / 15–18 °C for a few days after racking); or kräusening (fresh yeast rapidly removes it).
DMS: long, open, rolling boil; fast wort cooling; healthy vigorous fermentation.
Oxidation (paper/sherry): avoid splashing when racking/bottling; fill with <5 cm headspace; oxygen-scavenging caps; store cool; avoid hot-side aeration.
Chill haze / cloudiness: protein rest for high-protein malt; adequate calcium; kettle finings (Irish Moss / Whirlfloc in the last 10–15 min); vorlauf before lautering.
Medicinal phenol / sour: sanitation; dechlorinate water; proper crush and sparge (pH <6.0, <77 °C).

Styles – three-plus each side (the counted column)

Fault	Appropriate (low level)	Inappropriate
Diacetyl	Czech lagers, English Bitter, British brown/dark ales, Irish Red Ale	German lagers (Kellerbier excepted), Irish Stout, American ales, Belgian ales
DMS	Pale lagers: American Lager, Cream Ale, International Lager, Munich Helles, German Pils	Ales, Czech lagers, amber lagers, dark lagers
Estery/fruity	Most ales, especially Belgian and British styles	Lager styles (should be clean – malt + hops)
Clove phenol	German Weissbier, some Belgian styles, Saison (peppery)	Clean lagers, most non-Weizen ales
Brett phenol	Lambic, American Wild Ale	Any clean style (indicates contamination)
Sour/acidic	Berliner Weisse, Flanders Red, Oud Bruin, Lambic, Gose	Weissbier, Imperial Stout, Baltic Porter
Sherry-like	Old Ale, English Barleywine, aged Weizenbock, Oud Bruin	Light, low-gravity styles (any level)
Paper/cardboard	None – inappropriate in any style	Every style

You are asked to describe diacetyl for C0. You have written the descriptor and one cause. What most improves your score next?

Say it out loud in full sentences before you reveal. Half-answers count as misses.

Four more causes and 3+ styles each side

Graders count causes and style examples. Exhaustive causes (5+) and 3+ appropriate / 3+ inappropriate styles is where the marks are.

Body & mouthfeel (often one of the three)

If a "characteristic" is body, astringency, alcohol or carbonation, use the same skeleton.

Body = fullness/viscosity, set by dextrins + medium-length proteins. High alcohol and high carbonation both lighten apparent body. Thin body ← low mash temp, too many fermentable adjuncts, over-attenuative yeast. Fix with dextrin malt or flaked barley/oats/wheat. Light body OK in American Light Lager, Lambic; wrong in Barleywine, Scotch Ale, Doppelbock.
Astringency = mouth-puckering (grape skins, over-steeped tea). From husk tannins (overcrush, oversparge, sparge >pH 6.0 or >77 °C), high sulfate/magnesium, oxidation, over-attenuation. Low level OK in American IPA, American Porter, Flanders Red; undesirable in most.
Alcoholic warmth = spicy/vinous, warm/prickly. Fusel alcohols rise with underpitching, low O₂, low FAN, high gravity, high ferment temp. Desired in Eisbock, Imperial Stout, Barleywine.

Feynman teach-back

Cover the screen. Pick DMS. Out loud, deliver the full five-part answer: perception (all four senses), descriptor + source compound, five causes, control, and 3+ appropriate / 3+ inappropriate styles. Then reveal and check you hit every part.

Perception: Sight – none. Smell & taste – cooked corn, celery, cabbage, onion, parsnip; shellfish at extremes. Mouthfeel – not typically observable.

Descriptor / source: dimethyl sulfide, from heat-induced conversion of SMM (S-methylmethionine) in malt.

Causes: (1) short/weak/closed boil; (2) slow wort cooling; (3) low fermentation vigor; (4) wild yeast / Zymomonas; (5) Pilsner malt (up to 8× the SMM of pale malt).

Control: long open rolling boil; rapid cooling; healthy vigorous fermentation.

Styles: Appropriate (low) – American Lager, Cream Ale, International Lager, Munich Helles, German Pils. Inappropriate – ales, Czech lagers, amber lagers, dark lagers.

Run: today's scoring artifact. Add to your C0 drill scoresheet:

"Per characteristic: 4 perception rows filled (none blank) · ≥5 causes · control · ≥3 appropriate AND ≥3 inappropriate styles. Count them before moving on."

Trap watch. Do not stop at "buttery = diacetyl, appears in some lagers." That is a 3/10 answer. The compound is easy; the causes list and the two style columns are where National-level points live. Keep writing until each list is full.

Primary source

Every descriptor, cause and style example above is from the BJCP Exam Study Guide (off-flavor and body/mouthfeel sections), verified card-by-card against the source. The Study Guide is the ground truth for C0 – not general brewing lore.