Man tells GF to 'stop f*****g eating,' GF says, 'you're fat shaming me.' AITA?

When this man is annoyed with his girlfriend, he asks Reddit:

'AITA for yelling at my girlfriend to “stop f***ing eating?”

My (M26) sister (F23) runs a bakery business and she’s been struggling lately to keep up with orders because she’s been short staffed. She does a lot of orders for wedding cakes that require custard or marmalade fillings, and I offered to help her out by making these fillings at home and bringing them to her so she has less work to do.

Unfortunately, the past four times I’ve made these fillings, my girlfriend (F24) has literally dipped her fingers into the filling jars and contaminated them because, in her words, she “just wanted to try some.” I’ve tried explaining to her that she can’t dip her fingers in and contaminate the entire batch, because then I have to remake it. I said she should use a spoon and take some out if she wants to try so bad, but she just pouts and says that she likes using her fingers because it takes her back to her childhood.