
Dont take food away when she growls over it, or she will learn not to trust you near her food either.
Do the other dogs respect her warnings and avoid her when shes got food? If so, I wouldnt panic about it too much. The problem is probably caused by the change in hormones and a phantom pregnancy following her season (Im told all bitches have them but not all do so 'obviously'), which leads her to value her food far more.
Keep other dogs away from her when shes eating so she doesnt feel the need to practice this so often, because as we all know... practice makes perfect!
If shes guarding food from YOU, or guarding food and you dont want her to right there and then.... instead of removing teh food from her.... remove HER from the food... and do so by providing a much more interesting distraction/offer.
This takes the focus away from what shes guarding and isnt confrontational so you wont be backing her into a situation where she feels her only option is to snap or bite (last thing you want to do). Long term if shes getting guardy with a lot of stuff, teach her a really solid 'leave it', whereby 'leave it' comes to mean 'ive got something better' (start out with outright bribery, and slowly move the goalposts until she is sure you DO have something better, even ifyou dont have it in your hand right this second).
For guarding it from other dogs though, there isnt really any kind way you can stop her doing this, but to remove her need to guard it, by stopping other dogs being close to her. It is after all, her food and shes got every right to want to eat it unmolested!