What is the Maillard phase in coffee roasting?

The Maillard phase is the second of the three main roasting stages, beginning at yellowing and ending at first crack. During this window, the beans turn from yellow through to varying shades of brown as the Maillard reactions - the chemical interactions between amino acids and reducing sugars - begin producing the hundreds of flavour and aroma compounds that define roasted coffee.

It's sometimes called the 'nameless phase' or 'browning phase' in roasting literature, since unlike the drying phase and development phase it doesn't have a single universally used name. The rate of bean temperature rise (RoR) during the Maillard phase is one of the most important variables in shaping the final cup: a steadily declining RoR through this stage is associated with even flavour development; a crash or plateau can produce baked results.

For home roasters, the Maillard phase is where much of the craft of roasting plays out. The decisions made here - how much heat to apply, how fast to let the beans progress - determine how thoroughly flavours develop before first crack arrives and the development phase begins. Getting this stage right is the difference between a roast that sings and one that's flat.