5.3 Acidity Dynamics vs. Roasting & Fermentation

Core Knowledge: The acidity perceived in a cupping bowl is never static; it is the biochemical culmination of genetic potential, origin terroir, post-harvest fermentation, and roast modulation. A Q Grader must understand how processing creates or removes acids and how roasting shifts a coffee from fruit acidity toward roast bitterness.

---

Chlorogenic Acid (CGA) Pyrolysis & Roasting Degradation

In raw green Arabica coffee, the most abundant acid family is not citric or malic, but chlorogenic acids (CGAs), which constitute 6.0% to 8.0% of total dry bean mass (and up to 10% in Coffea canephora / Robusta). Chlorogenic acids are a family of phenolic esters formed between quinic acid and one or two trans-hydroxycinnamic acids (primarily caffeic acid, ferulic acid, and p-coumaric acid).

Chlorogenic Acid Breakdown Pathway During Roasting:

[ Chlorogenic Acids (CGAs) in Green Bean (~6–8% dry wt) ]
           │
           │  Thermal Hydrolysis & Pyrolysis (>160°C–200°C)
           ▼
[ Free Quinic Acid ]  +  [ Caffeic / Ferulic Acids ]
           │
           │  Continued Heating (Medium Roast, First Crack)
           ▼
[ Quinic Acid Lactones (e.g., 3-caffeoyl-1,5-quinide) ]
   (Contributes balanced, crisp medium-roast bitterness)
           │
           │  Dark Roasting / Second Crack (>220°C–230°C)
           ▼
[ Phenylindanes & Phenolic Oligomers ]
   (Contributes harsh, acrid, medicinal bitterness & lingering astringency)

CGA Isomer Fractions in Arabica

• Caffeoylquinic acids (CQA): Accounts for ~80% of total CGAs, dominated by 5-CQA (~4.0%–5.5% of green mass), alongside 3-CQA and 4-CQA.
• Dicaffeoylquinic acids (di-CQA): ~10% to 15% of total CGAs (3,4-diCQA, 3,5-diCQA, 4,5-diCQA).
• Feruloylquinic acids (FQA): ~5% to 8% of total CGAs.

Thermal Pyrolysis Mechanics

As green coffee is roasted and bean temperature surpasses 160°C to 190°C, the ester linkages in chlorogenic acids break down rapidly via hydrolysis and pyrolytic cleavage:

1. Liberation of Quinic Acid: The cleavage of 5-CQA releases stoichiometric amounts of free quinic acid and caffeic acid. Green Arabica contains only 0.3% to 0.5% free quinic acid, but roasted coffee contains 0.8% to 1.3% free quinic acid, representing an increase of up to 250%.
2. Formation of Quinic Acid Lactones: As roasting progresses toward the end of First Crack (Medium Roast), quinic acid molecules lose water (dehydration) to form intramolecular esters known as quinic acid lactones (predominantly 3-caffeoyl-1,5-quinide and 4-caffeoyl-1,5-quinide). These lactones are responsible for the pleasant, clean, mild bitterness that gives brewed coffee its structural backbone.
3. Degradation into Phenylindanes: In dark roasts (>220°C, Second Crack), quinic acid lactones decompose completely into phenylindanes. Phenylindanes impart a harsh, lingering, medicinal bitterness and severe oral astringency that masks all origin fruit characteristics.

---

The Roast Acidity Curve

A common misconception is that green coffee has the most acidity and that acidity simply falls as roasting goes on. Studies of titratable acidity in roasted coffee describe a rise and fall: total acidity tends to increase in light roasts, as sugar breakdown creates new acids such as acetic and formic acid, and then declines as roasting continues and acids degrade or volatilize.

Total titratable acidity across roast degree (schematic):

Acidity
  ▲
  │              ★ peak in light roasts
  │             / \
  │            /   \   cupping-level roast (ground Agtron about 63)
  │           /     \        │
  │          /       \───────┼──────┐
  │         /                │       \   acids degrade and volatilize
  │  ┌─────┘                 │        \
  │  │ green coffee          │         └──► dark roast: lower acidity, more bitterness
  └──┴───────────────────────┴───────────────────────────────►
      drying    browning    first crack    medium    dark

Phase 1: Drying and early browning

Most water leaves the bean, and the acid profile still resembles the green coffee: citric and malic acids plus intact chlorogenic acids.

Phase 2: Around first crack — the acidity peak

Sugar breakdown creates low-molecular-weight acids, including formic and acetic acid, while citric and malic acids are still largely present. Total acidity is therefore high. That is one reason very light roasts can taste sharply sour.

Phase 3: Cupping-level roast

At the medium roast SCA-102 specifies for cupping — ground L* 26–29, about Agtron Gourmet 63 (about 58 whole bean) — some volatile acids have been lost and citric and malic acids have partly degraded, while chlorogenic acid breakdown products add structure and bitterness. The aim of this roast level is a neutral window on the green coffee, not maximum acidity.

Phase 4: Dark roasting

Further heating degrades more of the fruit acids and drives off volatile acids. Measured acidity falls and bitterness from chlorogenic-acid breakdown products rises, so origin acidity is muted.

---

Post-Harvest Fermentation Dynamics: Engineering the Acid Profile

Post-harvest processing at farm level fundamentally dictates the biological precursors and raw acid ratios delivered to the roaster:

Loading diagram...

1. Washed (Wet) Processing: Transparency and Intrinsic Terroir

• Mechanics: The skin and outer fruit pulp are mechanically stripped within hours of harvest. The seeds, coated in mucilage (pectin, cellulose, and sugars), are fermented in concrete tanks (submerged under water or dry) for 12 to 36 hours. Ambient pectinolytic enzymes and yeasts digest the pectin.
• Acid Impact: The parchment is thoroughly washed with clean water and frequently soaked in fresh spring water for an additional 12 to 24 hours. This clean-water rinsing purges water-soluble fermentation byproducts (excess acetic acid, ethanol, and foul short-chain fatty acids).
• Cup Character: Washed processing highlights the coffee's intrinsic botanical and terroir acids: pristine citric and crisp malic acids. It produces the highest cup clarity, translucent sweetness, and sparkling brightness.

2. Natural (Dry) Processing: Winey Complexity and Acetic Elevation

• Mechanics: Intact whole cherries are laid out on raised drying beds or brick patios for 15 to 30 days. The coffee seed remains in intimate contact with the moist, sugary mesocarp (Brix 18°–24°) throughout the drying cycle.
• Acid Impact: Microorganisms (Saccharomyces, wild yeasts, and aerobic Acetobacter) proliferate within the moist pulp. Yeasts convert glucose and fructose into ethanol, which Acetobacter subsequently oxidizes into acetic acid and ethyl esters (ethyl acetate, ethyl hexanoate).
• Cup Character: Naturals possess significantly higher baseline concentrations of acetic acid, lactic acid, and fruit esters. At balanced levels, this creates luscious strawberry, blueberry, and winey acidity with heavy body. However, if drying is delayed or ambient humidity is high, acetic acid concentrations spike above 1.2 g/L, producing the dreaded "sour/ferment" defect.

3. Anaerobic Fermentation & Carbonic Maceration: The Lactic Acid Revolution

• Mechanics: Harvested whole cherries (Carbonic Maceration) or depulped seeds (Anaerobic) are sealed inside food-grade stainless steel tanks or plastic drums fitted with one-way gas release valves. In Carbonic Maceration, the tank is flushed with pressurized carbon dioxide ($CO_2$) to instantly displace all atmospheric oxygen.
• Microbial Ecology: The absence of oxygen creates a strict anaerobic environment that suppresses obligate aerobic bacteria, specifically Acetobacter. This prevents the runaway synthesis of volatile vinegar-like acetic acid.
• The LAB Pathway: Anaerobic conditions selectively foster Lactic Acid Bacteria (LAB), primarily Lactobacillus plantarum, Leuconostoc mesenteroides, and Pediococcus. These bacteria metabolize simple sugars via homofermentative and heterofermentative pathways, producing immense concentrations of lactic acid.
• Sensory Transformation: Lactic acid fundamentally reshapes the cupping profile. It imparts a creamy, yogurt-like, silky acidity that cushions sharp citric notes and dramatically increases tactile mouthfeel and perceived body. Concurrently, intracellular enzymatic breakdown generates exotic tropical fruit precursors (passionfruit, mango, candied pineapple), yielding vibrant experimental cupping scores.

---

Acidity, Sweetness, and Mouthfeel Together

Acidity is never perceived in isolation:

1. Acidity as structure. Acids give lift, freshness, and clarity.
2. Sweetness as balance. Sweet taste and sweet-smelling aromatics soften how sharp acidity seems. A crisp, apple-like acidity in a sweet coffee can read as juicy; the same acidity in an unripe coffee reads as sour.
3. Mouthfeel as carrier. Thickness and texture change how acidity lingers, and astringency can make acidity feel harsher.

On the CVA forms these interactions show up in two places. The Descriptive Form records acidity, sweetness, and mouthfeel intensities separately, with free descriptors for acidity and sweetness. The Affective Form lets you rate your impression of each, and the overall section captures balance.

Practical warnings

• Underdeveloped or unripe coffee: low sweetness with sharp, grassy acidity. Describe it accurately (for example, high acidity intensity, low sweetness, "green, tart") and let it lower the relevant affective ratings.
• Over-fermented coffee: acetic, vinegar-like sourness that separates from sweetness. Record it under Sour/Fermented and in the acidity description, and rate it in the affective sections. It is not a CVA sensory defect — SCA-104 limits those to potato, moldy, and phenolic. Do not confuse it with the green-grading defect "full sour," which is a physical bean defect counted on the Physical Form.