How to tell a colleague to keep his mouth closed when chewing without being offensive?

It is not up to you to decide if he feels offended or not, and frankly he can do whatever he wants.

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You can share how you honestly feel when you see him chew with his mouth open, make a request that he closes his mouth when he chews, and indicate that it is also okay if he doesn't agree to your request. After that, simply let this matter go and see if he decides to agree or not.

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The best part here is that it is also up to you to decide if you want to see him chew his food again, and you can also do whatever you want. :)

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Talk to the nicely or bring it up in a joking

Dont dine with him

I think just don't eat lunch with said colleague