I know that TK's food to IST is not by DO&CO for long haul flights - have flown to many destinations and none of it was from DO&CO. It wasn't the case when we first flew with them - I think they changed a couple of years ago.
The food wasn't bad to IST from HKG but it's not nice compared to ex-IST flights. Normally I sleep through that leg and have food in the IST lounge instead.