I've posted on this sunject a few times before, here goes again. I was once a delivery manager for a caterer at LGW, responsible for getting the aircraft catered for around a dozen scheduled & charter airlines, including some of the biggest names in the business.
I can tell you categorically that catering kitchens (which incidentally are all within a couple of miles of the airport) go to very great lengths to ensure that they are clean. They also go to even greater lengths to ensure that temperature regimes are maintained on caters & in most cases, that carts are sealed & not handled prior to crew service.
If any problems occur, it is generally not because the crew have contaminated things, nor in most cases the pax. It is usually because something has gone wrong at the kitchen. But I will say this, practically every airline that I ever had dealings with in respect of catering was so incredibly parsimonious that it remains a constant source of amazement to me that kitchens even bother to supply.
All caterers involve themselves in supplying newspapers, booze, duty free & anything else it would be possible to turn a buck on, since food on planes does not pay. I saw the writing on the wall & got out, after a meeting with a well known charter airline whose name is a number somewhere above 1999 at which we were told to go off & create a menu including cutlery & trays etc. for 65p per pax.
The crews fare better, but not much. You gets what you pays for & that includes hygiene.