Please don't scold her or hold her by the scruff of her neck. As you can see, this only means that she growls more - the reason for that being that you are proving to her that there is a need to growl and protect herself, because you are a threat to her. If you continue to threaten her, she will continue to "warn" you. If you escalate your threat to her (by scruffing her), she will escalate her warning to you (by growling more).
Please also remove any thought of this being a dominance behaviour from your mind. Here is a link to help explain why: <a class='url' href='
http://www.clickersolutions.com/articles/2004/Debunking.pdf'>http://www.clickersolutions.com/articles/2004/Debunking.pdf</a>
If I understand right, she is not growling at you for touching her food or bowl - she is growling at you for attempting to stroke her, while she is waiting for food, is that right? And she growls regardless of whether it is the person preparing the food, or another person who strokes her?
Does she growl at you for stroking her at any other time? If you have a new toy that she wants, and someone attempts to stroke her while she is "waiting" for that, does she growl?
At what point does she growl? Does she growl when you approach her, when you extend your hand or when you actually touch her?
It sounds to me like she is saying "Go away and stop pestering me, I am waiting for my food and I don't want to play or do anything else right now". If I were you, I would throw her bits of her meal while she is sitting there, waiting, and while you are preparing it - while she is quiet. Then turn around and face her - if she still doesn't growl, throw her more food and turn back again. Keep doing that. Then you can face her, take a step towards her, and if she doesn't growl, throw her bits of her meal. Go back. Repeat several times. Walk right up to her. If she doesn't growl, give her several bits of her food. Repeat. Extend a hand (don't touch her). If she doesn't growl, give several bits of food. Repeat many times. Extend a hand and touch her (don't stroke, just touch). If she doesn't growl, give several bits of food.
And so on until you can actually stroke her.
Then you need to go back to step 1 with someone else approaching her, and you feeding her - just because she is fine with you doing it, doesn't mean she will be fine with anyone else, so you need to generalise it by going back to step 1 with another person.
Remember - she should not growl at any point, and if she does, it's your fault for pushing things too fast, too soon. Go back and progress more slowly.
You might want to feed her her entire meal like this until you've solved this problem.