Of all the processes involved with coffee, the quickest and most critical is roasting. Roasting coffee is a terrible responsibility, because, in a few minutes, beans that cost hundreds of people time, effort and money can be completely ruined by ignorance or carelessness. Roasting coffee is an art, and like all true art, it takes years of practice before one becomes a master roaster; also, roasting is learned only by trial and error experience, and the errors may ruin many beans.

The main reason that roasting is difficult is that virtually every batch of beans is different from every other. In factories, a small sample of every shipment is "batch" roasted to anticipate every potential problem that could arise in a commercial-size roast of perhaps 114 kg/250 lb of beans. A bright rather than a dull roast is desired, as dullness can indicate over-drying or poor processing; also, a good quality sample of beans will appear even, since a variety of colors, particularly "pales", among the coffee beans can indicate immature picking, poor sorting of cherries, and varying degrees of fermentation and drying on the plantation. Misshapen beans, broken beans, or simply an overly wide variety in bean sizes constitutes a "ragged" roast, as different-sized beans roast to different colors in the same roast.

The physical changes that occur to beans during roasting are many; the obvious changes are that the beans get bigger, gaining up to a third more of their former "green" size; this is because the decomposing carbohydrates create carbon dioxide, which literally causes the cells of the bean to expand. Meanwhile, most of the moisture remaining in the green beans after the plantation de-pulping, drying, storing and shipping, will be evaporated by the heat of roasting, and the beans will thus lose weight. Although moisture content can account for up to 23 per cent of green bean density, dehydration during roasting is usually kept to a maximum of about 15 per cent of the beans' weight, lest they become tasteless, brittle objects that reduce to powder at the first touch of a grinder's blade. The other very obvious physical change is that beans change color during roasting.