I want to make the most caffeinated coffee possible. What method should I use to yield the most caffeine content per liter?
The first thing you want to do is choose the right bean and the right roast.
Longer roasting removes some of the caffeine from the bean, so a lighter roast would be preferred. Additionally you should chose Robusta beans, since their inherent caffeine content is higher than that of arabica beans.
The next step is the grinding. The smaller you grind, the easier it is to extract "material" from the ground. This is due to the additional surface area you expose.
Then the last step is the extraction. The longer you expose the ground to water, the more substances you absorb. The extraction roughly has three steps:
- Extraction of Acidity
- Extraction of Sweetness
- Extraction of Bitterness
Usually you try to stop extraction after the second step, to have the coffee taste nicely. By that point in time most of the caffeine is already extracted.
If you'd want to get all the caffeine out of a ground, you'd have to go through with extracting the bitterness, too. But for general purpose drinkable coffee it's definitely preferrable to stop after extracting the sweetness
Brew method doesn't make any difference in caffeine content.
Caffeine generally extracts very quickly from the grounds... as fast or faster than the acids, so you will have most of it in your cup regardless of brew method. Continuing to over-extract the coffee isn't going to yield any more caffeine, either, at least not in any significant measure. I certainly wouldn't brew over-extracted coffee just in hopes of a little extra caffeine. (source)
Darkness of the roast does affect caffeine content in way, because it affects the weight of each bean as it roasts. If you measure your grounds by weight (as you should), dark roasts have more caffeine. Though if you measure by volume (as is more common), then light roasts will have more (source). So, for more caffeine, favor dark roasts, but be sure you measure your coffee by weight, not volume, if you're comparing to light roasts.
Some people talk about whether espresso or drip coffee has more caffeine. The problem with this is it's not an apples-to-apples comparison. A cup of coffee has more caffeine than a shot of espresso... but of course it does, it's eight fluid ounces of coffee compared to one fluid ounce of espresso. And if you were to compare equal volumes, the espresso has far more... because it uses about eight grams of beans per ounce where drip coffee is around two and a half.
Which, in a roundabout way, gets to the point: the best thing you can do, if you want more caffeine, is brew it stronger, using any method. And by that, I mean use more grounds.