Any food grade chemical is a chemical that has been analyzed and found not to have poisonous impurities that would render it unsafe for use in the manufacture of food. Nothing is ever really 100% pure. Food grade really means that the chemical is safe for use in food production; it meets the Food Chemical Codex standards established by the FDA. Not all lye are created equal and if the one you have does not say food grade or its label has the warning "not safe for food use", it is not to be used in making any food.
It doesn't have any impurities. Some lye sold doesn't need to be processed as well.
However, if you're making bagels or pretzels consider using a trick with baking soda.
Sodium Bicarbonate will change to Sodium Carbonate if heat it.
spread a 3/4 cup on a sheet pan and bake 2-3 hours at 300.
Dissolve in a quart of water. By baking the soda you make a much more alkaline product.
http://www.nytimes.com/2010/09/15/dining/15curious.html
I've done this with pretzels and they turn out wonderful, very brown.
Baked baking soda works pretty well, but the lye really is worth getting, at least for pretzels. I got a significantly better pretzel flavor with a lye dip than a baked baking soda dip.
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However, if you're making bagels or pretzels consider using a trick with baking soda.
Sodium Bicarbonate will change to Sodium Carbonate if heat it.
spread a 3/4 cup on a sheet pan and bake 2-3 hours at 300.
Dissolve in a quart of water. By baking the soda you make a much more alkaline product.
http://www.nytimes.com/2010/09/15/dining/15curious.html
I've done this with pretzels and they turn out wonderful, very brown.