Instant coffee usually has the lowest carbon footprint because it uses the least amount of coffee beans. The at home machines and filters get about 1/4 of the coffee bean into the cup. In the factory we use high temperature and pressure to squeeze out about 1/2 of the coffee bean flavours.
But actually a lot of the impact is in how we make coffee. Adding milk can easily double the carbon footprint. Boiling too much hot water or wasting the coffee is also not good.
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