Where Coffee Acidity Comes From: Organic Acids and How They Taste
Coffee "acidity" isn't one taste but an impression made by a mix of acids. Lemon-like, apple-like and wine-like acidity feel different because the acids and their amounts differ. A meta-analysis by Yeager et al., drawing on hundreds of studies of coffee acids, summarized how each acid differs in concentration and in its effect on taste.
1. The main acids in coffee
| Acid | Where it comes from | Common associations |
|---|---|---|
| Citric | Naturally present in green coffee | Bright lemon or orange acidity |
| Malic | Naturally present in green coffee | Apple or pear acidity |
| Chlorogenic acids | Abundant in green coffee; break down in roasting | Contribute astringency and bitterness |
| Quinic | Formed as chlorogenic acids break down | Astringent, sour; the sourness of coffee left standing |
| Acetic and formic | Formed as sugars break down in roasting | Sharp, vinegar-like (lively in small amounts) |
| Lactic | Formed in roasting and fermentation processing | Soft, yogurt-like acidity |
| Phosphoric | An inorganic acid in green coffee | Thought to sharpen perceived acidity |
2. Roasting makes and destroys acids
Early in roasting, sucrose breaks down and forms new aliphatic acids such as formic, acetic, glycolic and lactic acid. Ginz et al. (2000) found that adding sugar to green coffee greatly increased these acids after roasting, showing sucrose is their main precursor. Meanwhile, acids already present such as citric and malic decline as roasting proceeds, and chlorogenic acids break down into quinic acid and others. Light roasts therefore have more and more varied acids, while darker roasts lose acidity and bitterness takes the lead.
3. pH is not acidity
Most coffee sits around pH 5, yet perceived acidity varies widely, because it depends not only on hydrogen-ion concentration (pH) but also on the total dissolved acid (titratable acidity) and which acids are present. Cold brew and hot coffee have similar pH, but hot coffee shows higher titratable acidity (Rao & Fuller, 2018). And just as more sweetness makes the same pH taste softer, acidity is perceived in balance with other tastes.
4. Extraction and water change acidity
Most acids dissolve readily and come out early in the brew. Under-extracted coffee has a lot of acid relative to sweetness and body, so it tastes sharply sour; with fuller extraction, sweetness follows and acidity rounds out. High-alkalinity water neutralizes acid with bicarbonate and mutes acidity; very low alkalinity makes it sharp.
Putting it into practice
- Sharply sour? Usually under-extracted. Grind finer or use hotter water so sweetness comes through too.
- No acidity at all? Try a lighter roast or a washed coffee, and check your water isn't high in alkalinity.
- Train your palate: taste lemon, apple and yogurt before your coffee to learn which kind of acidity it resembles.
References
- Yeager, S. E., Batali, M. E., Guinard, J.-X. & Ristenpart, W. D. (2021, online). Acids in coffee: A review of sensory measurements and meta-analysis of chemical composition. Critical Reviews in Food Science and Nutrition.
- Ginz, M., Balzer, H. H., Bradbury, A. G. W. & Maier, H. G. (2000). Formation of aliphatic acids by carbohydrate degradation during roasting of coffee. European Food Research and Technology, 211, 404–410.
- Rao, N. Z. & Fuller, M. (2018). Acidity and antioxidant activity of cold brew coffee. Scientific Reports, 8, 16030.