When baristas describe a coffee as “under-extracted,” they’re usually referring to a taste that feels thin, sharp, sour, or prematurely bright, flavors that seem incomplete. But beneath those sensory impressions lies a chemical reality. Coffee extraction is the dissolution of hundreds of compounds into water, each with its own solubility curve. When a brew is stopped too early, the water collects an uneven slice of the spectrum: the quick-to-dissolve acids and small molecules, but not the sugars, aromatics, and deeper flavor compounds that balance them. What you taste is chemistry cut short.
The earliest stage of extraction is dominated by organic acids. Chlorogenic acids, citric acid, malic acid, and phosphoric acid dissolve rapidly, leaping into solution long before the heavier flavor molecules do. At this point the brew smells promising, bright, fragrant, energetic, but the dissolved profile is chemically skewed. Chlorogenic acids in particular break down into compounds that register as intensely acidic, sometimes verging on harsh. Without sweetness or body to support them, they stand alone, sharp and unbuffered.
Next come the smaller aromatics and light volatile compounds. These include ester-driven floral notes, the sparkle of fruity aromatics, and some of the earliest Maillard products formed in roasting. They contribute complexity, but by themselves they are fleeting and unanchored. In a fully extracted cup, these aromas sit atop deeper flavors; in an under-extracted cup, they feel hollow, like top notes without a melody beneath them. This is why an under-extracted Ethiopian or Kenyan coffee may smell wonderful but taste startlingly sour.
The sugars and caramelized compounds responsible for sweetness take longer to dissolve. These include sucrose remnants, melanoidins, and the mid-stage Maillard products that give coffee its warm, rounded flavors. When the brew is cut short, these compounds never fully enter the cup. Without them, the palate loses its central axis—the part of the flavor spectrum that turns acidity into balance. Chemically, it is the difference between tasting citric acid alone and tasting citric acid blended with sweetness. The first is sharp; the second is lively.
The same is true of body. Many compounds responsible for texture, lipids, heavy melanoidins, and mid- to high-weight polysaccharides, extract more slowly. Without them, the mouthfeel becomes watery or thin. The coffee might hit your tongue intensely, then vanish before it develops any depth. This is part chemistry and part structure: lipids emulsify, melanoidins contribute viscosity, and polysaccharides coat the palate. Under-extraction denies the cup these building blocks, letting the acids dominate with no counterweight.
Bitters, often misunderstood, also emerge later in extraction, but not always in a negative way. Early bitterness comes from sharp, green-tasting compounds formed in underdeveloped beans or insufficient roasting. These dissolve quickly. But the later-stage bitters, complex, chocolate-like, or pleasantly roasted notes, arrive near the tail end of extraction. When a brew stops too soon, you lose the “good bitterness” that rounds out sweetness and accentuates depth, leaving only the early sharpness that contributes to the cup’s imbalance.
The result is a cup shaped like a lopsided curve: acidity spikes early, aromatics rise faintly, but sweetness, body, and deeper flavors never arrive. Chemically, the dissolved solids skew heavily toward organic acids and lightweight compounds rather than the full coffee matrix. You can taste the missing spectrum. The coffee feels unfinished, like fruit picked before ripening or a chord missing its lower notes.
The extraction spectrum is a timeline of chemical arrival. Under-extraction is simply a brew that ends before the important flavors have time to catch up. When balanced properly, acidity is the spark; sugars are the structure; melanoidins and aromatics are the color; and lipids are the body. When the chemistry stops early, the spark burns alone, and the cup tastes as incomplete as its chemical profile suggests.
Sources & Further Reading:
– Coffee Science Foundation: extraction chemistry and solubility curves
– “The Craft and Science of Coffee,” Elsevier (sections on organic acids and Maillard products)
– Journal of Agricultural and Food Chemistry: studies on melanoidins, lipids, and extraction kinetics
– Specialty Coffee Association brewing research on extraction yield and sensory balance
– Food chemistry analyses on polysaccharide contributions to beverage mouthfeel
(One of many stories shared by Headcount Coffee — where mystery, science, and late-night reading meet.)
