What is the difference between light roast and dark roast caffeine content per water quality?

When brewing coffee, water quality is the most critical factor for flavor, but it has no impact on the total caffeine content. Caffeine is highly soluble in hot water, meaning that as long as your water is at the correct temperature and in contact with the grounds for the appropriate amount of time, the caffeine will be extracted regardless of the water's mineral composition.

Water quality—specifically the presence of magnesium, calcium, and bicarbonate—determines how well the water extracts flavor compounds from the coffee grounds. Soft water might lead to an under-extracted, sour cup, while very hard water can lead to over-extraction and bitterness. However, these minerals do not interact with the caffeine molecule in a way that would increase or decrease its presence in your cup.

Whether you are brewing a light roast, which requires higher extraction temperatures to pull out its delicate flavors, or a dark roast, which is more porous and extracts more easily, the caffeine will be released into the water. The roast level affects the bean's structure and solubility, but the water's role is simply to act as a solvent for the caffeine that is already present in the beans.

In practice, you should prioritize water quality to improve the taste of your coffee, not to manage your caffeine intake. Using filtered water with a balanced mineral profile will ensure that your light or dark roast tastes as intended. If you are concerned about caffeine levels, focus on the weight of the coffee beans you use and the brewing ratio, as these are the only variables that will truly change the amount of caffeine in your cup.