1.A Grapefruit is sour. Grapefruit is not normally considered uncountable or a quality, though I could set up a context: 'Strawberry candy is sweet; grapefruit is sour'.
2.Grapefruits are sour.
If you add an article to the first, both are fine, both are common; just be consistent if you are writing a paragraph or essay on the topic.
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