There are two common and conflicting misconceptions about caffeine levels in coffee. Many people belive that darker roasted coffee which seems to pack a bold flavour punch must contain more caffeine. There are also those that believe that the darker roasted the coffee the less caffeine it must contain because it will have been ‘burnt off’ during the roasting process.
If we assume a coffee is roasted professionally, if you roast the same coffee bean light or dark, the caffeine content will be the same. However, what does happen is that the darker we roast a coffee, the less unique origin flavours remain, effectively the coffee goes past being unique, balanced and sweet, and as we go darker, we minimise fruit acids and ultimately ‘cook’ the bean to such a degree that we begin to make smokey, baked, bitter and ashy the main flavours we will experience.
While the roast degree has a minimal, if any, impact to the caffeine level, there are however other factors which can greatly affect the level of caffeine in your drink.
Many dark roasted coffees (especially those purchased from supermarkets) and nearly all instant coffees contain Robusta coffee. Robusta is a type (varietal) of coffee which is more hardy (easier to grow), tends to be of a lower grade, and is therefore cheaper to use in coffee manufactured for mass consumption. Robusta also contains nearly twice as much as Arabica coffee (the main varietal of coffee used by specialty roasters).
When it comes to higher grade specialty coffee, the biggest factor affecting caffeine content is the amount of coffee being used compared to the amount of water used when brewing (the brewing ratio).
As a coffee drinker, the decision you need to make is whether you would rather drink specialty coffee, and experience floral aromas and flavours that range from complex, intense sweetness, and fruity tanginess that is representative of this style of coffee, or experience the typically more-bitter, less-complex, roast-flavoured dark-roast coffee found on the supermarket shelf.