Assistant to Office manager/HR here. Gonna give OP and rest of GAF some perspective.
There are three parts where he could have over reacted.
OP complaining to co-worker about sniffing his food
OP asking for him to buy replacing the food he sniffed
OP placing the food co-worker sniffed on his desk
Well let's go through it.
Did OP buy his food for his coworker to sniff it in close proximity? No
Did OP ask him to sniff his food before he ate it? No
Did coworker ask for permission to touch OP's food? No
Did coworker ask for permission to sniff OP's food? No
Should OP be forced to eat food he's not comfortable eating? Of course not
Yes, the people who prepared his food may have germs, but said people and establishment has to make sure the kitchen, cookware, and cooking practices are up to code in order to serve food, his coworker is not held to the same standard, so OP shouldn't assume that his face, follicles, hair, skin are perfectly clean and harmless. OP already said he doesn't know him.
Is it customary in OP's office to touch things that doesn't belong to them? Probably not
Is it customary in OP's office to sniff things that doesn't belong to them? Probably not
Is it customary in OP's office to not ask for permission before doing any of the above two? Probably not
Serah admitted that she would not eat food that a person she doesn't know already sniffed from so at the very least she understand why OP was upset. OP isn't 100% in the wrong. If a person does something you don't approve of, which Serah and OP has acknowledged, then why would you not tell said person to stop it? If not, they will think that behavior is acceptable. But not telling them, you are reinforcing something you find unacceptable to YOUR belongings.
What kind of tolerance people have with their own stuff is on them, but people have to respect how you feel about your own stuff. Especially if they didn't pay for it and/or is not theirs to begin with. Serah is completely wrong and thank goodness she isn't in HR.
This is probably gonna fall on deaf ears, but OP you were right. I can't believe 100% of the people who finds strangers putting their faces in close proximity to their food is comfortable to eat it afterwards and to think it's acceptable after the fact.
If the coworker thinks you are an asshole for it, he's probably not worth getting chummy with anyhow. Respect people's personal space and their belongings. If you didn't ask for permission, don't assume that it's okay.
Glad you're not doing hr in my organization. You've provided zero perspective or insight beyond having officious versions of the OPs overreactions and you have his and other posters absurd penchant for overstating the "danger" and"inavsive behavior"
And your post was overlong and redundant.