Espresso has a very powerful taste. It must be loaded with caffeine!
This is a very common misconception about espresso. There are several reasons why espresso has less caffeine than the ordinary cup of coffee.
First of all, espresso is consumed in smaller amounts. The amount of espresso used to make a drink is one or two ounces. A typical cup of coffee is approximately eight to ten ounces. Because espresso is much headier than coffee, less is used. It is like making a concentrate and using the concentrate as a base to flavour another liquid.
Second, espresso uses Arabica beans. A cup of coffee can also use Arabica beans. Remember that Arabica has less caffeine.
Third, espresso is made from dark-roasted coffee beans only. One can also brew a carafe of coffee from dark-roasted beans. Dark-roasting coffee burns off a lot of caffeine during the roasting process. But here’s the difference: when espresso is brewed, it requires a finer grind of coffee beans than regular brewed coffee. Coffee grounds for espresso are put into a basket and tamped down tightly. Because of this finer grind and tight compaction, more pressure is applied to the coffee grounds as hot water is forced through the coffee and the basket. The water at this point is actually cooking the grounds. This process burns off even more caffeine, but results in a higher concentration of flavour. When a carafe of coffee is brewed, the grinds are courser and loose and the hot water simply passes over them and into the carafe. The grounds are not under any pressure whatsoever and the caffeine has very little chance of being cooked out.
So as you see, it is continued heat and high water pressure that lowers the caffeine content of espresso. A one-ounce shot of espresso has half the caffeine of an eight ounce cup of coffee.