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hardlikearmour is a trusted home cook.
Calamondins or limequats would be the best substitute in general, but depends on what you are making.
What are you making? Kumquats are delicious to me, but I've only eaten out of hand. They are quite tart. If you explain the application, maybe we can help. If it is something to use freshly sliced, maybe some kind of tangerine or mandarin, although they are sweeter, so maybe a bit of Meyer lemon mixed in.
Chris is a trusted source on General Cooking
I think of kumquats as more bitter than tart and would guess that in most recipes, substituting just some orange peel is what you might be thinking of.
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