Why does an espresso need a crema and how should it look like?

The video is based on the articles:

• Espresso – A Feast for the Senses,
  Klaus Roth, translated by E. Russey,
  ChemistryViews 2010.
  https://doi.org/10.1002/chemv.201000016
  
  Klaus Roth concludes his look at espresso with a scrutinizing glance at the foamy, consolidated surface layer, the crema