Crema is the layer of foam/cream on top of an espresso and is the Italian word for 'cream'. It is a reaction of gases being forced out of the ground coffee forming bubbles and a compound called melanoidin forming a layer around these bubbles.
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 brown colour is usually the correct colour for crema. The colour does depend on the coffee you are using however as some coffees will result in a lighter or darker crema.
A light yellow and thin crema usually means that your coffee is not so fresh.
Fresh coffee contains more gases than older coffee and will therefore result in a different kind of crema. Very fresh coffee can give gassy crema and it will look more like brown froth than cream. This is why some coffees need to rest more than others which can take up to a couple of weeks sometimes.
Older coffee can still give crema on top of an espresso. The proof of freshness is seen however throughout the actual extraction. Fresh coffee will settle like a Guinness whilst older coffee will have crema appearing towards the end of the extraction.
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!
Poor quality coffee can produce a perfect crema as long as its fresh.
Crema does tell you various things about how the coffee was made as well such as the grind and the dose. An experienced barista should be able to identify these things.
Crema should stay on top for more than a minute. Correctly rested coffee will give a more consistent crema as well as a properly made espresso.
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. A perfect crema also has to do with the extraction so the better the extraction, the better the crema.
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