Food grade is considered a purer grade for use in food products.
I started out using Tech Grade, was usually cheaper.
Then I tried Food Grade, because it was cheaper.
Last spring I ordered Tech Grade from AAA (cheaper) and received Food Grade instead... go figure.
This made me UNHAPPY, as I had observed some issues with the Food Grade that I did not have with the Tech Grade. I wanted the TECH grade.
Food Grade works, and cleans the iron, but...
1. I felt that the efficacy was less with Food Grade, and I had to add lye more often to bump it up
2. I observed 'rusting in the lye bath' phenomena that I had not seen with the Tech grade. Now, not serious 'bad' rust, it scrubs off. Just caused a little more scrub time and effort.
I called AAA and the chemist was supposed to call me, never did. Never got an explanation as to why I did not receive what I ordered... but the lady that answered the phone would really like me to send her a CI skillet!
Sniff... I'll send her a skillet when they return my call! ;D ;D ;D