"The acid content of green coffee bean is around 11%, mainly comprising citric, malic, chlorogenic and quinic acids, while roasted bean contains around 6% due to decreases in citric, malic and chlorogenic acids"¹

"It is still unknown which acids are imperative to recreate the acidity experienced in coffee. It is generally understood that citric acid, malic acid, and acetic acid are the most important because they exist in high proportions..."²

Citric - Tastes like citrus fruit

Malic - Tastes like green apples

Acetic - Tastes like vinegar

Coffee pH ≈ 5

Neutral pH = 7.0

Acids in Coffee

• Organic acids are early products of plant metabolism
• Also are produced through degradation of more reduced chemical entities in plants

Put simply:

• Photosynthesis produces sugars, which are converted to energy. In the conversion process, organic acids are produced as by-products.

Put even more simply:

• In order for plants to form acids, they must have sugars.

Acids in Coffee

Other sources of acids in coffee

• Coffee plant/development → photosynthesis
• Processing → fermentation = acetic acids
• Roasting
• Brewing?

Acids in Coffee - Roasting

Roasting coffee beans creates a temperature gradient in the bean

Acetic acid is formed by the breakdown of sugars. Other acids are formed by the breakdown of other pre-existing compounds.

Heat accelerates chemical reactions and in this case the formation of acids.

Larger gradients therefore provide a greater difference in acid development with the beans.

Put simply: faster + hotter roast = greater difference between ground and whole color = more complex acidity

Acids in Coffee

Citric and malic acids decompose at relatively high temps 176°C and 190°C respectively

Both are 'tart' but malic is smoother than citric.

Malic acid tends to be highly favored by coffee professionals when scoring a coffee.*

Malic Acid

• Decomposes: ~190°C
• High concentration in apples
• Perceived sourness—sensation on the sides of the tongue
• Formed by plant before cherry harvest
• Regional humidity for plant increases acidity in cherry
• Generally, more fruitiness (as with wine) = more malic acid
• Typical medium roast will lose ~30% of original malic acid content

Acetic Acid

• Boiling point: 118 - 119°C
• High concentration in vinegar
• Mostly formed during roasting process
• Also a by-product of fermentation
• Can increase up to 25 times from initial green bean content
• Formed by breakdown of carbs (sugar) — therefore more prevalent in arabica VS robusta
• Volatile - readily detected by smell
• Maximum concentration at light roast.

Citric Acid

• Decomposes: ~176C
• Decreases at medium and dark roasts
• High concentration in limes, lemons, oranges, etc.
• One of the most predominant acids
• Contributes to perceived acidity
• Not formed or increased during roasting proces

Acids in Coffee - Flavor Progression