Why the Coffee Valve also called One-way degassing valve?

The coffee valve need to be one-way degassing valve,because after you roasted the coffee bean,they will produce more and more co2,the coffee valve is playing an important role on it.The coffee valve will release the co2 outside the coffee bag,because while your coffee bag fill with co2,it will make the coffee bag explosion.

And the coffee valve called one-way degassing valve,Also because they only can release the co2 and would not get into the air throuhgt the valve.
You can understand the function of the coffee valve throught the picture.
Want to know more information about the coffee valve.
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