How to brew Field Espresso
Espresso is all about flavour balance, and this is acheived by understanding what you need to balance exactly - sweetness, acidity, and bitterness.
Acidity comes out first. Sweetness second. Bitterness third.
The trick is to get the perfect grind size that allows the water to pass through the ground coffee, dissolve the acidity, sweetness, and a small amount of bitterness and arrive in your cup.
The Golden Rule is that you should get double the amount of coffee that you put in, in 30 seconds.
15g of finely ground coffee
30g of espresso
30 seconds (from when you push the button, to when the espresso is stopped).
If you get your espresso in less time it may be sour, as it's the acidity that comes out first. You got all the acidity, but the water didn't dessolve any sugar. If this happens, make your grind finer and try again.
If your espresso is taking a lot longer than 30 seconds, then it may become very bitter, or even more sour! This is caused by either the grind size being too fine - a good way to think of it is like sand, pepples, and rock and how water passes through them, or because you have a lot more than 30g of espresso.
It may be bitter, because you have taken more out of the coffee than you need to. This is called over-extraction, and it is a very bitter cup.
Ideally, you press the button, and within 10 seconds you have espresso coming into your cup. By 30 seconds you should have an espresso that weighs around 30g, has a golden crema, and is delicious.