You should always grind your coffee beans just before brewing it. It can be convenient to grind a large batch of beans, but oxydation will be faster with ground coffee beans. Weight your beans and grind what you are going to consume, nothing more, nothing less.
- Extraction: Dissolving coffee solubles into water. Just like tea infusion.
- Degassing: Once beans are roasted, CO2 forms inside beans and begin to leave them. A few days of degassing is necessary before grinding coffee beans.
- Oxydation: Solubles degradation by oxygen. Oxydation gives coffee a stale taste because soluble compounds of interest are degraded. Beans should be sealed as soon as possible.