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I have felt that percolated/french pressed coffee keeps me awake more than instant coffee brew.
EDIT: Since it was (validly) pointed out it will depend on the amount, so the amount, that I am comparing is basically what is mentioned on the packet of instant or ground coffee (whichever is the case). As in if my instant coffee sachet says it makes 2 cups of coffee, I use half of it. And if the ground coffee says to use the filtrate of 2 tablespoons for each cup, I do the same.
Caffeine content varies greatly by preparation and brew method. Instant coffee is generally designed to produce something close to a cup of drip coffee. "Cups" is unfortunately not well defined and may vary based on it's usage. Beyond that, percolated and french press coffee is generally prepared stronger (more concentrated) that regular drip coffee (and probably much stronger than the watered down concentration instant is designed for). It's also very likely that some amount of caffeine is lost during the freeze dry process since it is so water soluble.
TL;DR Yes. Real brewed coffee will generally have a higher caffeine concentration than coffee that has been brewed, dehydrated, then re-hydrated.
This is a bit more complex than it seems at first blush. The reason why instant coffee may have more caffeine than your typical brewed coffee is that it's usually Robusta instead of Arabica coffee. Or some blend of the two.
http://www.barkingdogcoffee.com/our-coffee/arabica-vs-robusta/
A cup of joe at Peets, Starbucks, etc, will be made with Arabica beans exclusively, and therefore have a lot less caffeine than the Robusta instant coffee.
It also depends on how finely you grind your french press grounds. You may be getting more solids in your cup of coffee with a french press (and it's hard to compare french press vs instant coffee by weight since instant is so processed). Also, some Arabica beans have more caffeine content than others. Thus, you may well be getting more caffeine from the dosage of french press coffee than your instant coffee.
There are a lot of variables to consider, and the answer is unclear.
I may be wrong but I think it's because instant has already been processed.
If you're using ground coffee it's much more likely that any substances that coffee would normally come with would be better retained in the grounds. Whereas with instant coffee, the process of creating these easily dissolving granules would have most likely caused shedding of a lot of the original substance content, which includes caffeine. Of course in this process manufacturers can add their own extras which might include extra caffeine but assuming that they don't, you would have lost some of the chemicals that keep you awake.
