Coffee is one of the most chemically complex beverages humans consume. Over 1,000 volatile compounds contribute to its aroma alone. Understanding the science of caffeine, acids, extraction, and antioxidants gives every cup more meaning.
A double espresso contains 60-100mg of caffeine in 36-40ml of liquid. Despite the small volume, the concentration is high. The total caffeine per serving is often lower than a full mug of drip coffee. Single-origin Ethiopian espresso often has higher caffeine than Brazilian blends due to higher-altitude, slower-maturing beans with greater caffeine accumulation.
A 240ml cup of drip coffee contains 80-140mg of caffeine depending on the coffee and brewing parameters. The longer contact time and larger volume of water extract more caffeine than espresso despite the lower pressure. An 8-cup drip pot can deliver 600-800mg of caffeine total — relevant information for anyone sharing a pot over a morning.
Cold brew concentrate can contain 200-300mg of caffeine per 60ml serving before dilution. As served at 1:4 dilution in a 240ml drink, this equates to roughly 100-150mg. The high caffeine content comes from the large coffee-to-water ratio used in cold brew production (typically 1:5 to 1:8 compared to 1:15 to 1:17 for hot filter).
Decaf is not caffeine-free. The Swiss Water Process and CO2 method remove over 99.9 percent of caffeine, leaving around 1-7mg per 240ml cup. The solvent-based process (ethyl acetate or methylene chloride) leaves similar trace amounts. Swiss Water and CO2 processes are considered cleaner and are preferred by specialty roasters for their ability to preserve more original flavour compounds.
The most abundant acid family in green coffee, comprising 6 to 12 percent of green bean weight. During roasting, chlorogenic acids degrade into quinic and caffeic acid. Light roasts retain more chlorogenic acids, contributing both antioxidant activity and a clean, mild bitterness. Dark roasting destroys most chlorogenic acids and increases quinic acid, which creates the sharp, astringent bitterness associated with over-roasted coffee.
Responsible for the bright, lemon or orange-like acidity in many washed African coffees. Present in the fruit itself and retained in the green bean. Citric acid degrades with increasing roast temperature, which is why light roasts from Ethiopia or Kenya taste vibrant and bright while dark roasts from the same beans taste comparatively flat.
The acid responsible for the green apple, pear, or stone fruit tartness in many coffees. Malic acid is common in coffees from Yemen, Ethiopia, and some Central American origins. It is more heat-stable than citric acid and survives into medium roast levels. Malic acid produces a softer, longer-lasting tartness compared to the sharp brightness of citric acid.
Found in high concentrations in Kenyan coffees and responsible for their distinctive clean, almost cola-like tartness. Phosphoric acid produces an intensely bright, mouth-watering sensation without the sharp edge of citric acid. It is present in very small quantities but has an outsized perceptual effect — one reason why top Kenyan AA coffees are so polarising and sought after simultaneously.
Ground coffee contains approximately 28 to 30 percent soluble compounds. The SCA ideal extraction yield is 18 to 22 percent of the available solubles. Under-extraction (below 18 percent) produces sour, salty, or weak cups. Over-extraction (above 22 percent) produces bitter, harsh, or astringent cups. Extraction is controlled by grind size, water temperature, time, and agitation.
The pH of brewed coffee (4.85-5.10) represents the combined effect of all dissolved acids. Perceived acidity in the mouth is not identical to measured pH — the types of acids present matter as much as the quantity. Phosphoric and citric acids are perceived as bright and clean at the same pH that quinic acid would be perceived as harsh and bitter.
Total dissolved solids percentage (TDS%) measures the concentration of coffee solubles in the brewed liquid. The SCA ideal for filter coffee is 1.15 to 1.35 percent TDS. Espresso typically sits at 8 to 12 percent TDS. A refractometer measures TDS% optically and allows precise, repeatable dialling in of extraction without relying on taste alone.
The Maillard reaction during roasting produces melanoidins — large, brown, polymeric molecules that contribute bitterness, sweetness, and a significant portion of coffee's antioxidant activity. They are also responsible for coffee's distinctive dark colour and part of its body. Melanoidins extract slowly, which is why very short extraction times tend to produce brighter, less complex cups.
Coffee is the largest source of antioxidants in the average Western diet — not because it is uniquely rich per gram but because of consumption volume. The primary antioxidants are chlorogenic acids and their degradation products. These compounds scavenge free radicals, reduce inflammation markers, and are associated with reduced risk of type 2 diabetes and neurodegenerative diseases in observational studies.
Chlorogenic acids are polyphenols — plant-derived compounds with demonstrated bioactivity. Other polyphenols in coffee include caffeic acid, ferulic acid, and p-coumaric acid. These compounds survive partially through roasting and have shown anti-inflammatory, hepatoprotective, and neuroprotective properties in laboratory research, though clinical translation remains an active area of study.
Cafestol and kahweol are diterpene alcohols found in coffee oils that have complex health effects. They raise LDL cholesterol in some individuals, which is why filtered coffee is considered cardiovascularly neutral while unfiltered coffee (French press, moka pot, Turkish) can elevate LDL with frequent consumption. Paper filters remove virtually all diterpenes from the final cup.
A alkaloid present in green coffee at 0.3 to 1.3 percent concentration that degrades to nicotinic acid (niacin, vitamin B3) during roasting. Trigonelline itself contributes bitterness and aroma compounds. It is responsible for some of the characteristic roasted coffee aroma produced in the first minutes of roasting and may have mild anti-diabetic effects according to preliminary research.
