The step that changes everything
Green (unroasted) coffee doesn’t smell or taste like coffee. It smells like grass, hay, green wood. If you grind it and brew with hot water, you get a bitter, vegetal liquid without any of the characteristics we associate with coffee. Everything we recognize as “coffee flavor” — the aromas, sweetness, chocolate, fruit, body — is created during roasting.
Roasting is a violent chemical transformation. In 10-15 minutes, a hard green bean goes through over 800 different chemical reactions, loses 12-20% of its weight in water and gases, changes color from green to brown (or almost black), doubles in size and becomes fragile and brittle. It’s probably the most dramatic transformation made to any food.
Understanding what happens during those minutes gives you perspective on everything you’ve learned in previous phases: why certain coffees taste the way they do, why freshness matters and why roasting is so difficult to do well.
The three phases of roasting
Phase 1: Drying (0:00 - ~4:00 min)
Green coffee has 10-12% moisture. During the first minutes in the roaster (preheated to 180-220°C), that water evaporates. The bean absorbs heat but doesn’t change much visually — it transitions from green to pale yellow.
There’s no significant flavor development here. It’s preparation: the bean needs to dry before chemical reactions can start. If this phase is too short (too aggressive heat), the bean’s exterior dries before the interior, creating uneven roasting. If it’s too long, the bean “bakes” and develops flat, dull flavors.
Phase 2: Browning / Maillard (~4:00 - ~8:00 min)
The magic begins. When the bean’s internal temperature reaches ~140-170°C, two families of chemical reactions activate:
Maillard reaction: The same reaction that browns bread, sears meat and caramelizes onions. The bean’s amino acids react with reducing sugars, producing hundreds of new aromatic compounds. It’s the primary source of “roasted” aromas: bread, cereal, nuts, malt, biscuit.
Caramelization: The bean’s sugars (primarily sucrose) decompose from heat, producing compounds that contribute sweetness, brown color and caramel, butterscotch and toffee aromas. Caramelization requires higher temperatures than the Maillard reaction, so it starts slightly later.
The bean transitions from yellow to light brown during this phase. The aroma shifts from “toasting bread” to “recognizable coffee.” The speed at which the roaster applies heat during this phase profoundly affects the final flavor profile.
Phase 3: Development (~8:00 - end)
When the bean’s internal temperature reaches ~196°C, the first crack occurs: an audible popping sound, similar to popcorn. This happens because internal water vapor and CO2 pressure exceeds the bean’s cellular structure’s resistance, and the bean literally fractures from inside. It expands, loses weight (gases escape) and the surface smooths.
The first crack is the defining moment of roasting. It marks the start of the “development phase” — the period from first crack until the roaster decides to stop. This phase’s duration (measured as “development time ratio” or DTR) greatly determines the coffee’s character.
Short development (short DTR, ~15-18%): Origin flavors are preserved — bright acidity, fruity and floral notes, complexity. But if stopped too soon, under-developed flavors remain: astringency, herbaceous notes, sour acidity. This is what people call “underdeveloped” or “baked.”
Medium development (DTR ~18-22%): Balance between origin character and roasting flavors. Present acidity but softened, developed sweetness, medium body. This is the range most specialty roasters work in.
Long development (long DTR, >22%): Roasting flavors dominate: dark chocolate, smoke, ash, spices. Acidity almost completely disappears. Coffee loses its origin and tastes primarily of “roast.” If you continue past the second crack (~224°C), you reach dark/French/Italian roast where carbonization dominates.
First crack and second crack
First crack (~196°C)
A dramatic event: beans explode, expand, release gases and moisture. The sound is like popcorn — rhythmic, frequent pops lasting 1-2 minutes. The bean transitions from light brown to medium brown.
First crack isn’t a discrete point but a range. It starts with sporadic pops, reaches peak activity then decreases. Most specialty roasts end during or just after first crack.
Second crack (~224°C)
If you continue roasting after first crack, temperature keeps rising. At ~224°C, a second audible event occurs: the second crack. It sounds different — more like crackling than popping, more subtle and rapid. At this point, the bean’s oils migrate to the surface (the bean becomes visibly oily), cellular structure significantly degrades and carbonization begins.
Roasts reaching second crack or beyond (French, Italian) produce coffees with dominant carbon, smoke and bitter flavors. Origin’s complexity and acidity are completely destroyed. There’s nothing intrinsically wrong with dark roast — some people prefer it — but from a specialty coffee perspective, you lose what makes the bean special.
Roast levels
Light roast: Ends during or just after first crack. Light brown color, no visible surface oil. High acidity, pronounced origin flavors (fruity, floral), light body, subtle sweetness. Requires high-quality coffee because roasting flavors don’t mask defects.
Medium roast: Ends 1-3 minutes after first crack, before second crack. Medium brown color, no visible oil. Balanced acidity and body, developed sweetness, chocolate and caramel notes. The most versatile and popular range in specialty.
Medium-dark roast: Near second crack. Dark brown color, possibly oil droplets. Dense body, low acidity, prominent roasting flavors (dark chocolate, spices, light smoke).
Dark roast: Second crack or beyond. Nearly black color, oily surface. Dominant bitterness, heavy body, carbonization flavors. Origin is irrelevant — everything tastes similar.
Freshness: why roast date matters
After roasting, coffee begins to degrade. The roasting process created volatile compounds that gradually escape. The process:
Days 0-3 (degassing): The bean releases large amounts of CO2. If you brew coffee the same day it’s roasted, the amount of gas interferes with extraction — the bloom will be explosive but the cup inconsistent. Not ideal for brewing.
Days 3-14 (optimal window): CO2 stabilizes, flavors settle. This is when coffee is at its best for most brewing methods. For espresso, many roasters recommend waiting 7-10 days because the method is sensitive to gases.
Days 14-30: Still good, but beginning to lose delicate notes (floral, fruity). “Base” flavors (chocolate, nuts, caramel) persist longer.
Days 30+: Notable decline. Complex notes are gone. Left with flat, boring, possibly rancid coffee. Oxidized oils produce unpleasant flavors.
3+ months: Old coffee. Aromatic compounds have almost completely dissipated. May taste like cardboard, old wood, or simply “nothing.” Ground coffee degrades much faster (days, not weeks) because exposed surface area is enormously greater.
This is why the roast date on the bag is so important. If there’s no roast date, it’s probably old. Large commercial brands use “best before” (consume by) dates that can be 12-24 months after roasting — by then, the coffee is sensorially dead.
The roaster as an instrument
A coffee roaster is basically an oven with a rotating drum and heat control. But the variables it can manipulate are many:
Charge temperature: The drum’s temperature when beans enter. Hotter = faster drying. Typically 180-220°C.
Gas/power: The amount of heat applied during roasting. Can vary during the process (increase, decrease, maintain).
Airflow: Controls convection and smoke removal. More air = more convection = more uniform roasting but potentially faster.
Drum speed: Affects heat conduction and uniformity.
Total duration: An 8-minute roast produces a very different profile than a 14-minute roast, even if they end at the same color.
The roaster’s art is manipulating these variables to guide chemical reactions in the desired direction. Two roasters can produce coffees with the same final color but completely different flavors, depending on how they got there.
What you need for this lesson
- This is a theory lesson — you don’t need roasting equipment
- It’s helpful to have 2-3 coffees of the same origin but different roast levels to compare
- Access to a specialty coffee store where you can see bags with roasting information
Practical exercise
Roast comparison: If you can find the same coffee in two different roast levels (some roasters offer this), prepare both with the same method and compare side by side. Note differences in acidity, sweetness, body and complexity. This shows you directly the roasting effect.
Bag reading: Go to a specialty store and examine 5-6 bags. Look for roast date (not “best before”). Look for roast level information. Calculate how many days old each coffee is. Which one is in its optimal window?
Freshness experiment: If you have a freshly roasted coffee, prepare a cup on day 3, another on day 10, and another on day 21 (keep the coffee in its sealed bag). Taste the differences. This teaches you empirically how freshness affects flavor.
Key concepts from this lesson
- Green coffee doesn’t taste like coffee; all flavor is created during roasting through Maillard reactions and caramelization
- Roasting’s three phases are: drying (remove moisture), browning (flavor development) and development (post-first crack)
- First crack (~196°C) is the defining event; time after the crack determines roast level
- Light roast preserves acidity and origin; dark roast produces carbonization flavors that mask origin
- Optimal freshness window is 3-14 days post-roast for filtered, 7-14 for espresso
- The same green bean can produce completely different flavor profiles depending on how it’s roasted
- Roast date on the bag is more important than “consume by” date