Book 2 / Parts 2-3 — Thermodynamics & Roast Chemistry: How Heat Becomes Flavour

Heat is the only tool a roaster has. Everything else — flavour, sweetness, body, the crack you wait for — is just heat arriving in the right amount, at the right moment, in the right form.

Book 1 teaches you to roast. Book 2 explains why roasting works the way it does — and nowhere is that more useful than in the chemistry and physics of heat itself. Parts 2 and 3 of The Roasting Bible Book 2 form the scientific heart of the book: first the thermodynamics that move energy into the bean, then the cascade of reactions that turn that energy into flavour.

This post walks through both. By the end you'll see your temperature curve not as a line on a screen, but as a live readout of where your energy is going and which reactions are running. That shift — from following a profile to reading one — is what separates a roaster who repeats recipes from one who can fix a roast on the fly.

Part 2 · The Physics

Three Ways Heat Reaches the Bean

Every roast is a negotiation between three heat pathways. Conduction is direct contact — the bean tumbling against the hot drum wall, exactly like food searing on a pan. It dominates the first few minutes. Convection is heat carried by moving air, and it takes over through the browning phase. Radiation is the invisible infrared energy radiating from hot metal surfaces, quietly present throughout.

The book's most practical insight here is that coffee beans conduct heat more like wood than metal: "A metal spoon in hot soup burns your hand instantly; a wooden spoon stays cool. Beans behave the same way — surface heat travels slowly inward toward the centre." That slowness is a gift. It gives you time to develop the outside without charring the core — but it also means scorching is almost always a conduction problem (too much drum contact), while a stalled drying phase is too little.

Part 2 · The Energy Budget

Where Your Roaster's Energy Actually Goes

Book 2 asks you to think of the roaster as a bank account. Energy flows in through gas or electricity, and every unit gets spent in one of five places. The surprise for most roasters is the ranking: evaporating moisture is the biggest cost of the entire roast — around 40% of total energy — far more than heating the beans themselves (~25%). Exhaust losses, warming the machine's thermal mass, and heat lost to the room account for the rest.

That one fact explains a lot. It's why the drying phase looks stalled on your graph — energy is going into turning water to steam, not into raising temperature. It's why higher-moisture beans take longer. And it's why the rate of rise suddenly accelerates the moment drying completes: the biggest energy drain switches off, and the leftover heat finally goes into the bean.

Part 3 · Reading the Roast

The Temperature Curve Is an ECG for Coffee

"Imagine a doctor reading an ECG printout," the book begins. "Your roast profile is the same thing — an ECG for coffee." Every feature of that rising line is caused by a specific physical event: the charge dip as cold beans absorb drum heat, the turning point where the beans become warmer than their surroundings, the slow drying slope, the steep browning climb where Maillard and caramelization run hardest, and the flattening approach to first crack.

The true control variable isn't temperature — it's Rate of Rise (RoR), the degrees of change per minute. A smoothly declining RoR through development is the signature of a clean roast. A RoR that crashes can flatten the cup; one that flicks back upward can bake in harshness. One caution the book stresses: every temperature it quotes is the bean probe reading, not the true interior of the bean, whose core runs 10–30°C cooler early on.

Part 3 · The Chemistry of Flavour

Maillard and Caramelization: Where Flavour Is Born

If heat is the tool, the Maillard reaction is the masterpiece. "You already know this reaction — you just didn't know its name," the book notes. Brown butter, a seared steak, toast turning golden — all Maillard. In coffee it is the single most important chemistry of the roast, and it is not one reaction but a cascade of hundreds, unfolding in three stages: a flavourless Setup (121–160°C), an aromatic Action phase where fruity, honey and floral notes appear (149–196°C), and a Payoff where fragments link into melanoidin polymers that give coffee its nutty, caramel, roasty body.

Running alongside it is caramelization — sugar reacting with itself rather than with amino acids. The book's framing is memorable: "Maillard is a conversation between two molecules; caramelization is a monologue." It needs higher temperatures, evolves from buttery to caramel to bittersweet, and explains why medium roasts are often the sweetest — pushed too far, those same sugars burn away. Control the rate of these reactions and you control which flavour molecules dominate the cup.

Part 3 · The Turning Point

First Crack: A Pressure Problem

That popcorn-popping sound is not magic. It's a precise physical event: steam plus CO₂ from Maillard and caramelization, trapped inside a cellular structure that has been weakening for minutes, until the pressure — roughly 5–7 atmospheres — finally tears the bean open. At that instant the chemistry of the roast changes fundamentally. Cell walls rupture, the bean expands, trapped volatiles release in a burst of aroma, and a small exothermic kick means the bean briefly generates its own heat.

This is also where pyrolysis takes over — from the Greek for fire and loosening, the thermal breakdown of molecules in a low-oxygen environment. First crack is the boundary line between origin character and roast character. Everything before it preserves what the farm gave you; everything after is increasingly the roaster's signature. Understanding why it happens is what turns development time from guesswork into control.

Who This Post Is For

Roasters Ready to Go Under the Hood

If you can already produce a drinkable roast but want to understand why your adjustments work — why a gas change three minutes in reshapes the whole cup, why one bean scorches and another stalls — this is the part of Book 2 written for you. It assumes no chemistry background, building every concept from kitchen analogies before adding the science. It rewards the roaster who wants to diagnose, not just repeat.

Written by Wing Yuen, SCA Authorised Trainer and founder of Tasse Coffee Roastery in Shinjuku, Tokyo. Wing is the author of The Roasting Bible, a two-book series for home and professional coffee roasters.