Coffee begins to lose about 40% of its aroma and essential oils within 15 minutes of grinding. Approximately 1000 different volatile flavors and aromatics concentrated inside a coffee bean are exposed by grinding, and begin to oxidize as soon as the seal is broken. Grinding just before brewing gives you the best chance of getting most of those flavors into your cup. Wait 24 hours and your coffee will taste drastically different. This is why it’s important to never store ground coffee and grind only what you need for each brew.