With NASA planning to eventually get people to Mars, and with privately funded attempts such as Mars-One planning to do the same, and relatively soon, there is the issue of having to feed anyone you send for the 6 or so months journey.
(Obviously the issue extends to when you reach the planet but this question is focused on in flight food production as this deals more with the realm of long distance, manned space exploration)
What technologies currently exist or what research is being done focusing on sustainable food production for these long, manned, exploration trips?