Coffee: Roast Levels — Temperature, Color, and Chemical Changes

Coffee roast level is the single most visible variable in the supply chain — it shapes flavor, body, acidity, aroma, and shelf life more dramatically than almost any other post-harvest decision. Understanding how temperature drives chemical change is essential for both roasters and buyers.

The Agtron Color Scale

The Specialty Coffee Association uses the Agtron M-Basic spectrophotometer to assign roast color numbers. The device measures near-infrared reflectance of ground coffee. Higher numbers indicate lighter roasts; lower numbers indicate darker roasts.

Agtron Range	Classification	Bean Surface
75–95	Very Light / Cinnamon	Dry, pale tan
60–74	Light	Dry, medium brown
45–59	Medium	Dry, warm brown
35–44	Medium-Dark	Dry to faintly moist
25–34	Dark	Oily sheen present
<25	Very Dark / Italian	Heavy oil, near-black

Roast Level Temperature and Flavor Reference

Roast Level	Drop Temp (°C)	Agtron #	Moisture Loss	Key Flavors
Light	196–205	70–95	15–16%	Floral, citrus, berry, high acidity
Medium-Light	205–210	60–70	16–17%	Stone fruit, bright acidity, tea-like
Medium	210–220	45–60	17–18%	Caramel, hazelnut, balanced acidity
Medium-Dark	220–225	35–45	18–19%	Bittersweet chocolate, low acidity
Dark	225–230	25–35	19–22%	Smoky, roasty, bold, oils visible

Light Roast: Origin-Forward Character

Light roasting drops the bean between first crack and the early stages of development. The bean retains most of its moisture-loss differential and preserves the volatile aromatic compounds produced during fermentation, drying, and milling. This is why a washed Ethiopian light roast can exhibit jasmine and bergamot — those aromatic esters survive only when heat application is brief. Acidity is highest in light roasts because chlorogenic acids have not yet fully degraded into quinic acid and other breakdown products.

Medium Roast: The Balance Point

Medium roasting continues past the peak of first crack development. Sugar browning (Maillard reaction and caramelization) proceeds further, generating melanoidins responsible for the characteristic caramel and nutty sweetness. Acidity softens as more chlorogenic acid degrades. Body increases as soluble compounds concentration shifts. This level suits coffees with inherent sweetness — Brazilian naturals, Colombian washed lots — where balance is the selling point.

Medium-Dark Roast: Bittersweet Transition

Between roughly 220°C and the onset of second crack, the roast enters a bittersweet zone. Sugars continue to caramelize toward bitter breakdown products. Acidity is notably reduced. Chocolate and tobacco notes emerge. This level historically dominated American “diner coffee” culture and remains popular in espresso blends where the reduced acidity aids extraction balance.

Dark Roast: Roast-Dominant Flavor

At 225°C and above — particularly beyond second crack — roast-derived flavor compounds overwhelm origin character almost entirely. Pyrazines, furans, and guaiacol dominate the sensory profile with smoky, ashy, and bold notes. Bean lipids migrate to the surface. The CO₂ network within the bean is heavily disrupted. Shelf life decreases as oxidation begins immediately on roasted oil surfaces.

The Caffeine Myth

A persistent misconception holds that dark roast coffee contains more caffeine. In fact, caffeine is remarkably thermostable, with negligible degradation across all standard roast temperatures. The slight difference between light and dark roast caffeine content arises from mass loss: because dark roasting evaporates more water and dry matter, a gram of dark roast coffee may contain fractionally more caffeine by mass than a gram of light roast — but the effect is so small (<1%) it is irrelevant in practice. Volume-measured brewing (scoops rather than grams) can produce measurable differences, with lighter roasts producing a slightly more caffeine-dense cup per scoop due to less puffing.