It is extremely important to minimise the time between the grinding and the brewing processes and there are several reasons why. Ground coffee reacts with oxygen and loses all its complex flavours. The more time passes the more tasteless and odourless coffee becomes. Moisture also significantly influences the taste of coffee. As soon as the oils in coffee beans are water-soluble (which is amazing for us to enjoy their taste from the cup of hot coffee), it is very easy to let water in the air dissolve the oil in ground coffee if it waits too long to be brewed. Another issue is that ground coffee can absorb all the smells around it because of delicate oils. Thus, it’s very possible you will feel foreign flavours in your cup after brewing if the grinding happened long time ago. Finally, CO2 is an important participant of transferring those precious coffee oils into your cup. Roasted beans are porous and insufficient storage of the beans can already affect the CO2 content in them. Grinding increases the surface area of the product and causes a major release of CO2. Thus, the longer ground coffee is exposed to the oxygen, the less CO2 can be kept in it, which again influences the intensity of the final taste.