Serious Eats - seriouseats.com

Without a doubt the microplane is the culprit. The "hot" flavor in garlic is produced when its cells are ruptured and exposed to air. You'd be hard pressed to invent a more efficient device for accomplishing this. One thing that may work is to put the whole batch in the blender with some parsley and/or lemon juice. You'll often find one or both paired with garlic because they mitigate that unpleasant rawness.