My daughter said to me this morning (the context is irrelevant):
Er, it's all wet!
The interjection I have written here as Er was synonymous with Yuck. Its wetness did not cause great happiness.
But what's the right way to write it? The problem with the way I've formulated it above is that it looks like the Er of (genuine or mock) hesitation, synonymous with Um. In fact I think that would be as natural a reading of the sentence as the Yuck reading. This would significantly change the overall understanding, and change the response from disgust to something like confusion.
I could try Err, but I don't think that's any less ambiguous. And of course I could write Yuck, which would convey the right sense, but direct quotation ought to respect the original wording.
(I have a feeling that this is a British expression, so I'm tentatively tagging as BrE, but I'm uncertain about this. It's supported by noting that the answers to this question don't mention Er at all.)