After a few busy days, we found time to write a blog about something that I found very important in my coffee: the crema. Crema is the italian name for the espresso foam and the presence of this foam is an indication of freshly roasted coffee, proper extraction and proper grinding. So, the lack of crema should warn you not to even try that coffee!
Crema is one of the two parts of the composite beverage called espresso; the other part being the underlying liquid. Espresso is unstable and the most rapidly varying property is the gas content, hence the crema. When the beverage comes out of the espresso machine there is a sudden drop of pressure in the fluid, and this drop causes gas release. The dissolved gases (mainly CO2) fizz into the cup and build up a layer of froth. This layer should be at least 1/10th of the volume of the espresso and the crema should be persistent: it must survive at least a couple of minutes before breaking up and it should be solid: capable of bearing for some time the weight of a spoonful of sugar.
The beautiful aspect of the foam is formed by tiny gas bubbles inside the viscous espresso liquid where minute bubble fragments floating producing a “tiger skin”effect on the surface. But why sometimes we do not have that espresso with a thick, smooth, tiger skin effect? Here are some of the reasons:
- The beans are not fresh. The CO2 in the beans releases with the ageing of the beans and none is left to make the crema. On the other hand, coffee brewed immediately after roasting generally produces too much of a lot of light-coloured crema. Therefore we prefer to let the coffee rest for at least 24 hours before brewing it.
- The type of beans. Some Robusta beans generate more crema, but usually this crema fades sooner than with Arabica.
- The grinding and the tamping. The beans should be ground to the precise size and the coffee should be packed even and tampered properly in the portafilter. If the ground coffee is too coarse, the water will pass through too quickly and little to no crema will be produced. The same effect will happened when coffee is not properly tampered and thus being too loose. You can see it in the picture below: the crema is very light.
On the other hand if the ground coffee is too fine or if the coffee is tampered with too much pressure the machine would not be able to force the water through without straining. The crema will have a dark brown color, or in extreme cases, there will be a dark crema with large oily spots (see below). When heavily over extracted, the crema may not be more than just some foam along the glass.
To achieve a good crema and a good espresso the flow of the coffee, when being extracted, should be steady and even and it should flow from both holes in the portafilter. This creates a thick crema with a slight striped pattern at the surface.
The brewing temperature and pressure. To produce crema the water should be warm enough to caramelize the sugars in the coffee and the optimal temperature for this is between 92 to 96 C. A low temperature produces light crema too. The brewing pressure should be between 8 and 10 bars to achieve good crema.
The foam layer not only fascinates our eyes but also acts as an aroma-sealing lid, preserving the volatile compounds that make the aroma of the espresso. Good crema is the result of all the variables working together and maybe also a good amount of practice!