Acids in Coffee

Coffee acids are classified as: 1. Chlorogenic acids (CGA): the most abundant acids in green coffee (5 to 8% of dry weight). CGA is a family of esters between caffeic acid and quinic acid. Major CGA compounds: 3-caffeoylquinic acid (3-CQA), 4-CQA, 5-CQA, 3,4-dicaffeoylquinic acid, 3,5-diCQA, 4,5-diCQA. CGA contributes astringency, bitterness, and acidity. During roasting, CGA degrades: ~50% lost by medium roast, ~70% by dark roast. CGA degrades to caffeic acid and quinic acid, and forms lactones that contribute to bitterness. 2. Citric acid: present in green coffee at ~0.5 to 1.5%. Provides bright, citrusy acidity. Degrades during roasting (~50% loss by medium roast). Higher in Arabica than Robusta. 3. Malic acid: present at ~0.1 to 0.4%. Provides green apple, pear-like acidity. Degrases during roasting. 4. Acetic acid: present at ~0.1 to 0.3% in roasted coffee. Produced during roasting from sugar degradation. Contributes vinegar-like acidity. 5. Quinic acid: produced during roasting from CGA degradation. Quinic acid is bitter and astringent. Increases during roasting. 6. Phosphoric acid: present in small amounts but contributes significantly to perceived acidity, especially in Kenyan coffees. Phosphoric acid is notable for its bright, clean acidity.

Key acid reactions during roasting: 1. Chlorogenic acid degradation: 3-CQA -> caffeic acid + quinic acid (hydrolysis). 3-CQA -> 3-CQA lactone (intramolecular esterification). CGA lactones are bitter, contributing to the bitterness of medium and dark roasts. 2. Citric acid degradation: citric acid decomposes at roasting temperatures, producing citraconic, itaconic, and mesaconic acid, and various volatile compounds. 3. Acetic acid formation: acetic acid is produced during roasting from degradation of sugars and polysaccharides. Acetic acid is volatile and contributes to the aroma of roasted coffee. 4. Formic acid: produced similarly to acetic acid. 5. Quinic acid increase: quinic acid increases during roasting as CGA degrades. Quinic acid is more bitter than CGA but less astringent. The perceived acidity of coffee is not solely determined by the total acid content. The pH of brewed coffee ranges from 4.8 to 5.5. Titratable acidity (the total acid content measured by titration) correlates better with perceived acidity than pH. Buffering (from bicarbonate in water and coffee's own buffers) affects how acidity is perceived.

The extraction of acids from coffee is a physical process of mass transfer governed by the principles of solubility, diffusion, and thermodynamics. When hot water contacts ground coffee, it must penetrate the cellular matrix of the bean to dissolve the targeted compounds. The rate at which these acids enter the solution is primarily determined by their molecular weight and the diffusion coefficient. Small-molecule acids, such as acetic acid (approx. 60 g/mol) and formic acid (approx. 46 g/mol), have higher diffusion rates and lower hydrodynamic radii than larger phenolic acids like chlorogenic acids (approx. 354 g/mol). This physical disparity causes the brighter, volatile acids to be extracted earlier in the brewing cycle than the heavier, more complex acids. Additionally, the solubility of coffee acids is highly temperature-dependent; increasing the brewing temperature increases the kinetic energy of the water molecules, facilitating the breaking of the intermolecular bonds that hold the acids within the bean's cellulose structure. The physical state of the roasted bean—specifically its increased porosity and internal surface area—decreases the diffusion path length, allowing for a rapid release of acids compared to green coffee, which is dense and physically resistant to water penetration.

pH of brewed coffee: 4.8-5.5. Titratable acidity: 0.5-1.0 meq/100mL. Chlorogenic acid content: green ~5-8%, medium roast ~2-3%, dark roast ~1%. Citric acid: green ~0.5-1.5%, roasted ~0.2-0.5%. Malic acid: ~0.1-0.4%. Acetic acid: ~0.1-0.3% in roasted. Phosphoric acid: trace but significant in Kenyan coffees. CGA degradation during roasting: ~50% lost by medium, ~70% by dark roast. CGA lactones (bitter) increase during roasting. Perceived acidity depends on: acid concentration, acid type (phosphoric > citric > malic > acetic for brightness), buffering (bicarbonate neutralizes), and temperature (hotter = more acid perception). Light roasts: more CGA, citric, malic (brighter, fruitier). Dark roasts: more quinic, acetic (duller, more bitter). Origin effects: Kenyan coffees have high phosphoric acid (bright, clean). Brazilian coffees have lower acid content.

If you prefer bright, acidic coffee, choose light roasts and use water with low alkalinity (under 50 ppm). If you prefer lower acidity, choose medium or dark roasts or use water with higher alkalinity (50 to 75 ppm). Pour over and AeroPress methods highlight acidity; French press and espresso can mask some acidity with body. Extraction temperature affects acid extraction: lower temperatures extract fewer acids (try 88 to 90 degrees C for low acidity). Shorter brew times also extract fewer acids.

• •Dark roasts have more acid. In reality, dark roasts have significantly less acid than light roasts. CGA degrades during roasting, reducing acidity while increasing bitterness.
• •Acidity is bad in coffee. In reality, acidity is a desirable quality in specialty coffee, providing brightness, complexity, and liveliness. The key is balance: too little is flat, too much is sour.
• •pH determines perceived acidity. In reality, titratable acidity (total acid content) and acid type (phosphoric vs citric vs acetic) are more important than pH for perceived acidity.

• •Research has shown that chlorogenic acid content decreases by approximately 50% during medium roasting and 70% during dark roasting.
• •Studies have demonstrated that phosphoric acid, though present in trace amounts, contributes significantly to the bright acidity of Kenyan coffees.
• •Research on acid perception has shown that titratable acidity correlates better with perceived acidity than pH.
• •Studies on CGA lactones have identified them as key bitter compounds in medium and dark roasts.

• •Chapter 5: Coffee Chemistry
• •Chapter 3: Acids in Coffee

• •Clifford, M.N. (1985). 'Chlorogenic Acids in Coffee.' Journal of the Science of Food and Agriculture.
• •Illy, A. & Viani, R. (2005). 'Espresso Coffee: The Science of Quality.' Academic Press.
• •Ginz, M. et al. (2000). 'Formation of Aliphatic Acids During Coffee Roasting.' European Food Research and Technology.
• •Woodman, J. et al. (2020). 'Coffee Acids and Perception.' Journal of Food Science.

• •Coffee Science Foundation
• •Illy Coffee Quality Book
• •Journal of the Science of Food and Agriculture