Many people reach for the darkest roast for the biggest jolt, but a scoop of dark roast holds less caffeine than a scoop of light. Green coffee is dense, 10 to 12 percent water, and full of chlorogenic acids that make it grassy and astringent. In the roaster the beans first absorb heat, then around 175 degrees Celsius the reactions turn exothermic and the beans generate their own heat, forcing the operator to pull back the gas. Steam pressure swells them to nearly double their volume while they lose 15 to 18 percent of their mass, and Maillard reactions build aroma as chlorogenic acids break into caffeic and quinic acids.

First crack near 196 degrees Celsius marks a light roast that tastes of its farm and origin; second crack near 224 degrees pushes oils to the surface for French and Italian roasts dominated by roast character. Caffeine does not break down significantly until about 285 degrees Celsius, so weighed by mass the roasts match, but the puffier dark beans fit fewer per scoop. The episode then traces roasting from 15th century Yemen and Ottoman pans to Cairo’s cylinder roaster and today’s countertop machines.

• First crack sounds like popcorn, while second crack is a faster snapping like milk poured on Rice Krispies.
• Master roaster Gianni Frasi saw fire as a purifier and stopped roasts at a shade he called the color of a friar’s tunic.
• A tablespoon might hold 50 light roast beans but only 35 dark roast beans.
• Whole roasted beans peak for about two weeks, while ground coffee goes noticeably stale in roughly 15 minutes.
• One-way valves on coffee bags let carbon dioxide escape without letting oxygen in.