Grounded coffee can be preserved vacuum packed, but pressurization extracts the air from the can and replaces it with inert gases at a higher-than-atmospheric level of pressure. This method guarantees a longer preservation time, maintaining the coffee for over 2 years until it is used. At the same time it 'ages' the coffee, improving its aroma over time by causing its volatile aroma compounds to bind with the oils that are contained in the beans.
The positive pressure exerted by the pressurization method actually forces these aroma compounds back into the coffee so they can be released in the cup. The effect of the ageing process is similar in many ways to the development of complex flavors when a fine red wine is aged under proper conditions.