Caffeine Chemistry

Caffeine (1,3,7-trimethylxanthine) is a purine alkaloid that occurs naturally in coffee, tea, cacao, and other plants. In coffee, it acts as a natural pesticide (paralyzing and killing insects). Caffeine is a crystalline compound (melting point 235-238 C) that is highly soluble in hot water (66 g/100 mL at 100 C) but less soluble in cold water (2 g/100 mL at 20 C). It is odorless, slightly bitter, and a white crystalline solid. Caffeine acts as an adenosine receptor antagonist in the human brain, preventing the sleep-inducing effects of adenosine.

Caffeine's chemical formula is C8H10N4O2 (molecular weight 194.19 g/mol). It is a methylxanthine, structurally related to the DNA/RNA base adenine. In coffee, caffeine is present at 0.8-2.5% by weight in Arabica and 2-4.5% in Robusta. Caffeine is relatively stable during roasting: only 5-10% is degraded at typical roast levels. During brewing, caffeine is one of the last compounds to extract, dissolving gradually throughout the brewing process. This means longer brew times and finer grinds extract more caffeine.

Caffeine's solubility is temperature-dependent: at 20 C, about 2 g dissolves per 100 mL of water; at 100 C, about 66 g per 100 mL. This means hot water extracts caffeine much more efficiently than cold water. However, cold brew compensates with longer steeping times (12-24 hours), resulting in similar or slightly higher total caffeine extraction. Caffeine has a melting point of 235-238 C, well above roasting temperatures, so it is not affected by the roasting process. Caffeine is a stable compound that does not degrade significantly during storage.

Caffeine extracts throughout the brewing process but is one of the slower-extracting compounds. In a 3-minute pour over, caffeine extraction is roughly complete by the end. In espresso, the 25-30 second extraction captures most but not all caffeine. In cold brew, the 12-24 hour steeping extracts caffeine efficiently despite the low temperature. Caffeine's extraction is less affected by grind size than other compounds because it dissolves readily in water. However, finer grinds still extract caffeine slightly faster due to increased surface area.

Caffeine contributes to the bitterness of coffee. The bitterness threshold for caffeine in water is approximately 60-200 ppm. In brewed coffee (1.15-1.45% TDS), caffeine concentration is typically 400-1000 ppm, well above the threshold. Caffeine bitterness is perceived on the back of the tongue and is one of the last tastes to register. Other bitter compounds in coffee (chlorogenic acid lactones, phenylindanes) also contribute to bitterness, especially in darker roasts. Caffeine's stimulant effects (increased alertness, reduced fatigue) are felt 15-45 minutes after consumption and last 3-5 hours.

Caffeine (1,3,7-trimethylxanthine) is a competitive antagonist at adenosine A1 and A2A receptors, preventing adenosine-mediated sedation. Its half-life in adults is 3-7 hours (average 5 hours), influenced by genetics, liver enzyme (CYP1A2) activity, pregnancy, smoking, and medications. A typical 250 mL cup of filter coffee contains 80-120 mg caffeine; a single espresso (30 mL) contains 60-80 mg. The FDA recommends a maximum of 400 mg caffeine per day for healthy adults. Caffeine tolerance develops with regular consumption; withdrawal symptoms (headache, fatigue, irritability) can occur 12-24 hours after last dose.

To maximize caffeine: use Robusta blends (2x the caffeine of Arabica), brew longer, use finer grinds, or use more coffee. To minimize caffeine: choose Arabica, use shorter brew times, or choose decaf (97% caffeine removed). Espresso actually has less total caffeine per serving than a cup of filter coffee (60-80 mg vs 80-120 mg), despite being more concentrated, because the serving size is much smaller. <a href="/coffee-encyclopedia/cold-brew-coffee-ratio-guide">Cold brew</a> can have higher caffeine than hot coffee due to the long steeping time and high coffee-to-water ratio.

• •Espresso has more caffeine than regular coffee. False per serving. A single espresso shot (30 mL) has 60-80 mg caffeine; a cup of filter coffee (250 mL) has 80-120 mg. Espresso is more concentrated but the serving is much smaller.
• •Dark roasts have more caffeine than light roasts. False. Caffeine is stable during roasting. By weight, light and dark roasts have similar caffeine. By volume (scoop), dark roasts may appear to have less because the beans are larger (less dense), but by weight they are equivalent.
• •Decaf is caffeine-free. False. Decaf coffee still contains 2-15 mg of caffeine per cup (97% removed). It is not completely caffeine-free.
• •Caffeine dehydrates you. False. While caffeine is a mild diuretic, the water in coffee more than compensates. Moderate coffee consumption does not cause dehydration.

• •The FDA recommends a maximum of 400 mg caffeine per day for healthy adults, based on extensive research on cardiovascular and neurological effects.
• •Caffeine's half-life averages 5 hours but varies from 3-7 hours based on CYP1A2 genetic polymorphism, pregnancy, and smoking.
• •Research shows that caffeine tolerance develops within 4-7 days of regular consumption, and withdrawal symptoms can occur 12-24 hours after last dose.
• •Studies show that Robusta contains 2-4.5% caffeine by weight, while Arabica contains 0.8-2.5%, explaining the difference in bitterness.
• •Research on cold brew shows that extended steeping (18-24 hours) extracts caffeine at levels comparable to or exceeding hot-brewed coffee despite lower water temperature.

• •Chapter 5: Extraction Science
• •Chapter 1: The History of Coffee

• •Nehlig, A. (2018). 'Caffeine and the Central Nervous System.' Handbook of Clinical Neurology.
• •Frary, C.D. et al. (2005). 'Food sources and intakes of caffeine in the diets of persons in the United States.' Journal of the American Dietetic Association.
• •Heckman, M.A. et al. (2010). 'Caffeine: A Review of Specific Benefits, Risks.' Journal of the American Dietetic Association.
• •US FDA (2018). 'Spilling the Beans: How Much Caffeine is Too Much?'

• •US FDA
• •Specialty Coffee Association
• •National Coffee Association