Just tell her plainly, "We have certain standards for health reasons. If you want, I can teach you more sanitary food preparation, and it'll serve you well for the rest of your life! Please don't take this the wrong way, I'm just guessing no one ever taught you any different, so here's your chance to learn, okay? Feel free to ask all the questions you want, and if I do something you don't understand, I'll be glad to explain it."

You can mention that, to give an excuse, at your age you need to be careful about things, as your immune system is not as strong as hers.