I'm getting ready to farm some yeast for freezing. My question is regarding the glycerin. I know it needs to be "food grade", so can I use any Vegetable Glycerin as opposed to petroleum-based? Walgreens has VG that has a nutrition label complete with calories, but I don't see where it says "Safe for Consumption", or anything like that. There's also different grades... What are you guys using and where do you get it? Thanks.