The next time you have to go grocery shopping, leave her home. She's old enough to stay home if she's being a pain in the tush about this.
I grew up eating a lot of food that came in black and white wrappers and drank a lot of powdered milk. Sometimes, it sucked, but we all knew that things were tight. I worked in the cafeteria for free lunch for a few years, so I knew we were poor, and that I shouldn't complain if I got pb&j for dinner after the hot lunch because that was what we could afford.
If it were me, personally, I would address this in a few ways. You could offer that if she wants to buy the name brand food for herself, with her allowance (if she has one) that she's welcome to be the person to eat it all. You don't have the luxury of brand-name food, but if she wants to spend HER money on it, so be it.
If she doesn't have an allowance, let her choose how she wants to spend $10 a month on the foods of her choice. This will put her in an empathetic position of learning to budget and having to make choices.
She's also old enough to go down to the food bank with you to volunteer. Perhaps a small dose of reality might be helpful.
Also, read the article someone just posted a few questions back, about whining. There is a certain 'status' to name-brand foods. I will warn that it is very difficult to talk a youngster out of what they think is the 'best' or what they want to 'fit in', and I too went through some hard times when we couldn't afford a few items I thought I would die without. However, I'm happy to report that I did make it through life without those buckle-back jeans I so desperately wanted.
Sorry you are going through such a hard time with this. Hang in there and remember, the hard times can teach us a lot. Hopefully, she will learn to be very grateful for what she has. Maybe there are ways to help her focus on her blessings this season. Here's one you might like to do as a family:
http://www.shelterness.com/20-cool-thanksgiving-trees-you...
We will do this with our son; my sister did it with her three boys. Write a 'thank you' on a leaf each day and put it on the tree. Awesome, and keeps us focused on our blessings.
This hard time, too, shall pass.