Can I take standard roasted instant coffee granules and further roast them in the oven?
Is this doable, or even advisable?
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No, I don’t think that’s feasible.
When you roast coffee beans, the beans undergo chemical changes that create the desirable flavor components. When you afterwards grind the beans and brew your coffee, the flavor components are extracted into the water. If (whole!) beans you bought are not roasted enough for your liking, you can try to re-roast, although the results may not be exactly what you were aiming for. Theoretically possible nevertheless.
Now, for instant coffee you need to remember that instant coffee is not finely-ground coffee beans, but prepared (=brewed and filtered) coffee that has been freeze-dried, clumps up into small granules and can be reconstituted by adding water. The beans that are changed in the regular roasting process simply aren’t there in instant coffee, which is why you won’t be able to develop the desired flavors by roasting instant coffee. You may be able to create some burnt flavors, but I don’t think that’s what you want. You should also note that the instant coffee granules are not very stable and likely to break up and, depending on your home-roasting method and tools, may clog up and damage your equipment.