The drop in pressure that pulls out the unwanted CO2 gas also pulls out the delicate, volatile compounds responsible for the good aromas (the fruity, floral, or chocolatey notes).
The vibrant, premium aromas in specialty coffee come down to about 25 highly volatile âkey odorantsâ that our noses detect in microscopic amounts. These compounds form distinct chemical families: thiols create the quintessential âfresh coffeeâ smell; pyrazines deliver nutty and cocoa notes; furans provide sweet caramel dimensions; and esters and aldehydes supply the delicate, fruity, and floral top-notes prized in light roasts. Because these fruit and floral compounds have the lowest molecular weights, they are incredibly fragile; applying a harsh mechanical vacuum lowers their boiling points and forces them out of the bean matrix first, meaning your coffee is saved from going stale but at the physical cost of losing its most complex and vibrant flavors.
Once those flavors are sucked out of the bean into the empty space of the container, you lose them.
