Have you ever wondered why there is sometimes a Crema on top of your coffee.
The crema is a layer foam/cream on top of an espresso and comes from the Italian word for cream.
When coffee beans are roasted it creates carbon dioxide in the beans and over time this de gases out of the beans, this is why our coffee bags have a one way directional valve to let the gases out and not let oxygen in. Oxygen will turn your coffee stale.
However, despite the gases escaping as soon as the coffee is roasted, there is still some carbon dioxide left in the coffee bean.
When we make an espresso there is a reaction of gasses being forced out of the freshly ground coffee. This creates tiny bubbles that emulsifies the coffee oils. This result is a creamy looking layer on the top of your espresso.
Crema should always appear on an espresso as it is a sign that you are using fresh coffee. Fresh coffee contains gases but over a period of time these gases will disappear. Your coffee will most likely be stale when there is no crema on top of your espresso. The crema should be visible during the extraction and the first drop should contain it.
A light yellow and thin crema usually means that your coffee is not so fresh.
Crema is mainly a sign of freshness and certainly not a proof of a perfect espresso. An espresso without a crema will most likely not be that tasty though!
How can I make sure we have good crema all day long?
Keep your ground coffee fresh - a grind on demand grinder really helps with this. Also ensure that the grind, the dose and the tamping is correct.