I get asked this a lot for people building systems.
Microwave safe means it won’t melt in the microwave. It has NOTHING to do with food grade – it might be food grade… remember the chinese make things like poisonous baby formula, and toxic leather jackets… just because it is designed for food…. you get my meaning.
Food Grade Plastic is virgin plastic, in a form that does not react with foods, and specifically is fairly robust to general hydroponic use.
Food grade Plastic should not affect the pH as it does not leach any chemicals in normal pH ranges.
Food grade plastics don’t always like to be in contact with solvents – so only wash with soap, bleach or peroxide.
We also recommend using stabilised plastic so it won’t crack in the sunlight.
For information on the types of plastics go to wikipedia. http://en.wikipedia.org/wiki/Plastic_bottle.
Hope that helps.