Why do coffee bags have valves?

Ever bought a bag of specialty coffee and wondered what the little valve in the bag is for? Well, worry not, you’re in the right place.

Chances are, when you go to pick a bag of good coffee off the shelf in a supermarket, you hold the valve to your nose, give a little squeeze and decide whether it’s the stuff for you. And while this experience of ‘smelling’ before buying is valuable for coffee companies, it’s not the intention of the valve itself. There’s a far more pragmatic reason for that little hole in the top of your bag.

So, why the valve?

Coffee (especially freshly roasted speciality coffee) has, like almost anything, a shelf life. Eat by Date claim that whole bean coffee lasts for 6-9 months in the pantry, but if you speak to any independent coffee roaster, you’ll likely get a much shorter time frame. Like a fine wine or a juicy steak, timeframe is important. The longer you leave it, the worse (or in wine’s case, the better), it gets.

What the valve does, then, is release essential air molecules from the bag to help preserve the life expectation of the beans inside.

When green coffee beans are roasted, they begin the process of releasing carbon dioxide. The fresher the roast, the more carbon dioxide is released from the bean. Sealing fresh beans inside an unventilated bag would result in the bag expanding, and eventually, the bag would rupture.

The science behind it all

When you introduce heat to green coffee beans, the amino acids and sugar in the bean combine and begin to ‘react’. This reaction process is what makes up the array of smells, tastes and colours that the roasted bean is characterised by.

This process is known as ‘oxidisation’, and essentially, the more oxygen coffee beans are exposed to, the less fresh the coffee. It’s why coffee bags often come with resealable zip-lock pouches, and it’s why the valve on the bags don’t let oxygen in, they only let carbon dioxide out.