Have you ever ordered an espresso at a cafe and it comes with a little rind of lemon beside your cup? Well here is a little tip: don't go there anymore because they probably serve shit espresso. The only reason they encourage you to rub some lemon rind on the cup is because they don't pull a good espresso, and the lemon oil will cut the bitterness of the coffee. Which is why I added it to my espresso biscotti - to cut the bitterness of the dry espresso powder! Duh.