title_body: Why of the-shelf-coffee packages are aluminum from the inside
upvoted_answer: I used to think this was just a marketing exercise (when it was applied to crisps/potato chips in American-Speak) but it's to do with it's excellent properties for keeping things fresh and sealing properties.
  
  If we look at the Key features from this site:
  
  
  Aluminium foil in packaging helps keep foodstuffs fresh for months without refrigeration.
  With the outstanding barrier properties and contributions to the sealing process, aluminium foil guarantees the excellent material and filling performance of aseptic and retortable packaging for milk, juices and soups.
  It prevents the contents from losing nutritional value, vitamins or aroma and gives total protection against light, moisture, oxygen, foreign matter contamination and damage.
  
  
  This is why it's predominantly used in food packaging and not just for coffee packages, the foil is essentially a barrier to the outside environment to stop the beans from being exposed to oxygen and light, although some carbon dioxide gas will build up, there is a relief/degassing valve to allow this to escape