Acids in Coffee
Acidity is considered the signature trait of coffee. Let’s explore how the main acids in coffee are formed, develop, and manifest in flavor.
What Is Acidity?
Acids are complex chemical compounds containing hydrogen. The Latin root of the word means “sour.” Acid solutions in water have a pH below 7—the lower the pH, the higher the acidity.
Coffee beans contain a variety of acids that are naturally formed during multiple organic chemical reactions. These acids are also present in everyday foods like fruits, vegetables, and dairy.
Main Acids in Coffee
Lactic Acid
Development: A fermentation byproduct, also found in kimchi, sauerkraut, and spoiled milk. In coffee, it’s formed from sugar breakdown during roasting.
Taste Profile: Milder than malic acid.
Acetic Acid
Development: Formed from carbohydrate breakdown during roasting. Often found in washed coffees due to fermentation. Peaks during roasting, then declines.
Taste Profile: Adds aroma in small amounts but can create unpleasant “fermented” notes if too concentrated.
Malic Acid
Development: One of the main acids in green coffee; decreases during roasting. Begins to degrade at around 190°C.
Taste Profile: Sharper than citric acid; felt on the sides of the tongue.
Citric Acid
Development: Found in higher concentration in unripe beans. Starts breaking down at 176°C. Medium roasts lose about 50% of it.
Taste Profile: Tastes like unripe fruit—sharp on the palate. In moderate amounts, it adds pleasant brightness.
Phosphoric Acid
Development: Comes from inorganic compounds in soil. CQI researcher Joseph Rivera found high levels in East African coffee (1998/1999).
Taste Profile: Distinctive for enhancing brightness and perceived sweetness in coffee.
Tartaric Acid
Development: Common in grapes and wine, but present in coffee in undetectable concentrations.
Taste Profile: Not applicable.
How Acids Are Formed in Coffee
1. Plant Growth & Development
Cellular respiration plays a key role—photosynthesis acts as “inhalation,” and respiration as “exhalation.”
The plant uses sugars from photosynthesis and converts them into byproducts like citric acid. As the cycle continues, malic and acetic acids form.
Phosphoric acid may also form if phosphate is present in the soil.
Factors affecting respiration:
Temperature
Plant age
Photosynthesis presence
Healthy leaves
Temperature matters:
Low night temperatures slow respiration, allowing more sugars to remain and potentially convert to acids during roasting.
2. Processing Methods
Washed (wet) process:
Anaerobic environment
Soaking causes water to extract soluble compounds
Natural process:
Higher acetic acid levels
Less uniform flavor due to berry individuality
Mechanically depulped:
No pulp, sugars, or fermentation environment
3. Roasting Effects on Acids
Citric & Malic Acids: Form early in roasting; reduce significantly at the first crack.
Acetic Acid: Forms early, peaks, then decreases after 240°C.
Lactic Acid: Slowly builds from start to finish of roast.
Chlorogenic Acid: Breaks down into quinic and caffeic acids.
4. Brewing Effects
High temperature breaks down chlorogenic acid.
Extraction ≠ acidity level.
After brewing, acids continue to degrade into quinic and caffeic acids.
Key Characteristics of Coffee Acids
Malic & Citric Acids
Formed through respiration
More abundant in Arabica than Robusta
Citric dominates in green coffee
Malic acid requires long maturation
Malic is more delicate
Acetic Acid
Formed during respiration, fermentation, roasting
Peaks during roast, degrades above 240°C
Small amounts = positive; excess = over-fermented notes
Lactic Acid
Formed during wet processing or fermentation
Increases throughout roasting
Quinic Acid
Protective compound in green coffee
Slightly increases during roasting
Result of chlorogenic acid breakdown
Continues to form after brewing
Example flavor: cranberry
Phosphoric Acid (inorganic)
Can be lost during soaking
Phytic acid can degrade into phosphoric during roasting
Robusta generally has more than Arabica
