Honey Jar Swabs in US Airport

[FairyNuf]

We were travelling back to the UK through Dulles and as part of the modified check-in process we had to weigh our bags, check them in and then security scan them before they left for the plane.
However, on the first or second case the security chap looked at the x-ray and asked if we had honey in there, then he opened the case and swabbed it. This happened for the other cases contaning honey, but not those containing some jelly (jam).
The question is twofold- how would he know that it was honey, and what would they be swabbing for, I guess it's explosives or drugs, but I'd like a definitive answer?

[heathfield]

Chinese honey is banned in the US due to it containing an unapproved food additive and a potentially harmful antibiotic - maybe that's why it was swabbed, to see if you were carrying it,. How jelly and honey were distinguished, I don't know.