During the roasting process, there are certain reactions that take place inside the coffee bean that transform it from a fairly tasteless pip (it's not really a bean, but the seed of the coffee cherry) into the wonderful coffee flavour that we know and love today.
A by-product of this process is the production of Carbon Dioxide inside the bean. This CO2 is trapped inside and is gradually released over time. This slow release of gas is called "de-gassing" and typically occurs over the space of two to three weeks after the coffee beans have been roasted.
Grinding the beans increases the total surface area and encourages the coffee to de-gas quicker. So if you want to maintain the freshness of your coffee for longer, buy them in whole bean form and grind only when needed. Then use the ground coffee immediately.
Now, you may ask, what is this "bloom" that is the subject of this blog post?
Well, this gas is released rapidly when the coffee beans come into contact with hot water. You can see this very easily if you're using a conical dripper, such as a V60, and pour a small amount of hot water onto the coffee grounds inside the cone of filter paper. Check out this short video demonstrating the bloom in action...
You can see why using freshly roasted coffee is extremely important. This release of gas when starting to brew your coffee brings out the full flavour and ensures you get a cup bursting with flavour.