How much weight do beans lose during roasting?

Weight loss is a fundamental aspect of the coffee roasting process, and it is a key metric that roasters use to track their progress. When green coffee beans enter the roaster, they contain a significant amount of water—usually around 10% to 12%. As the beans are heated, this water evaporates, which accounts for a large portion of the initial weight loss. This is the drying phase, where the beans shrink and their color begins to shift.

As the roast continues, the beans undergo further chemical changes, including the breakdown of organic compounds and the release of gases like carbon dioxide. This process, known as pyrolysis, continues to reduce the weight of the beans. The longer the roast, the more material is lost. A light roast will have a lower weight loss percentage, while a dark roast, which has been subjected to more intense heat for a longer period, will have a higher weight loss percentage.

For a commercial roaster, tracking weight loss is essential for calculating the final yield and pricing. If a roaster buys 100 pounds of green coffee, they will end up with significantly less than 100 pounds of roasted coffee. This 'shrinkage' must be accounted for in the business model. For the home roaster, it is a fascinating way to see the transformation of the bean in real-time.

In practice, you can track this yourself by weighing your beans before and after the roast. You will notice that the beans not only lose weight but also increase in volume, becoming larger and more porous. This expansion is what makes the beans easier to grind and extract. Understanding this weight loss helps you appreciate the concentration of flavor that occurs during roasting; you are essentially removing the 'filler' (water) to leave behind the concentrated essence of the coffee.