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I had gone completely cold-turkey on caffeine for a while due to anxiety issues, and I'm slowly reintroducing caffeine to my diet. I have read in many places that freeze-dried instant coffee has less caffeine than the equivalent amount of fresh-brewed, but I haven't seen any explanations as to why. Is it something inherent to the freeze-drying process which causes this change?
And, of course, how much lower is the caffeine content in general?
Caffeine is water soluble, but it takes a bit of time to be absorbed by water.
What determines the caffeine content of a coffee process is how much time it spends with/in water.
For example, a bodum French press will pick up more caffeine from the coffee since the grounds stay soaked for a while. Drip coffee will have more than a single espresso (common misconception is to think the opposite).
It's likely that the instant coffee process passes the water through the coffee under pressure and faster than drip. This will naturally prevent the caffeine to be absorbed in water as much.
Besides using decaf roast, what you may wish to do is get an americano coffee made with one or two Ristretto shots of espresso. A Ristretto shot is usually pull away after 10 seconds.
Take a look at this analysis from the Mayo Clinic for the actual Caffeine values.
Instant coffee is not dried beans, its dried coffee. Water is added to finely ground roasted beans and then one of many methods is used to preserve it. As you are adding water to coffee that has already had a small amount of water added to it, it is a lower concentration of coffee, and thus less caffeinated. It is also often a dark roast, which has had more caffeine roasted off.
Results vary as to how much less caffeinated it is from half as caffeinated as drip coffee to 3/4 as caffeinated.
Instant coffee may not be lost during processing and just have a lower caffeine content simply because less coffee is used in making the cup. One tablespoon of instant coffee in 8 oz of water gives a brew with TDS of .75%, about 30 - 45% lower than the recommended TDS for coffee. https://www.scaa.org/PDF/resources/golden-cup-standard.pdf An 8 oz cup normally brewed with one tablespoon and containing 55 mg of caffeine, would contain 80-85 mg when brewed with matching TDS.
At similar TDS, instant coffee might have less caffeine, because it is often extracted at higher temperatures and pressures than drip-brewed coffee. This would make it easier to extract more solids less soluble than caffeine than in normal conditions, resulting in an extract with a lower caffeine to total solids ratio than in drip brew.
In regards to process related losses: If freeze drying, caffeine could be lost due to sublimation. However, the rate at which it does is might not be significant, since caffeine seems to be normally heated above 100 C, in a vacuum, when intentionally sublimated.
If the coffee was spray-dried instead, caffeine could be lost with evaporating water vapor. A major loss of caffeine during roasting has been attributed to this. http://www.sciencedirect.com/science/article/pii/S030881461100762X
Instant coffee has lesser caffeine because it was processed with lower heat.
The caffeine content is also dependent on how long the coffee steeps. So, since it is instant, it will have less time to create more caffeine. Unless you have VERY hot water and let the instant coffee steep for a long time :p which may be gross.
