Hi Matt,
I researched this quite extensively when I entered the arena of turning for profit.
First, Dave O's response is correct. The only disadvantage is that each time a bowl is used to eat out of tiny particles of the finish do detach and are ingested. I cant remember hearing of anyone being allergic to finishes though, and these particles simply pass thru the human digestive system unremarkably.
One product is made especially for this application. Behemen's Salad Bowl finish. I have used it on personal commissions for bowls that would be eaten from. Totally safe and non toxic, but also contrived to hold up to this type of use.
Most turners who provide food safe bowls, finish their bowls with an oil of some type. Tung oil or even vegtable oil. When a bowl is eaten from though, it must be washed thoroughly, dried completely, then treated with the same oil each time or the moisture will destroy the wood. This is more than most people will be willing to do consistently.
I do not prescribe to this. The repeated application of these oils can eventually build up within the outer surfaces after time and become rancid, spoiling the taste of the foods eaten from the bowl.
My favorite way of finishing a food bowl is by applying Walnut oil to the bowl, on the lathe, when it is ready for finish. You apply it liberally while it is still, let it soak a few minutes, then wipe up the excess. Then turn on the lathe and use a sturdy felt or cotton rag, and apply moderate pressure to all the treated surfaces. This will introduce friction, and "cure" the oil on the surface creating a hard shell.
Walnut oil is the only oil that will dry to a hard finish, and seal the wood naturally. No other oil will do this. I dries to a satin finish, but repeated "burn in's" will increase the gloss.
You will find this method to be very satisfactory, and will last a year or more if done in several coats.
One last thing. Spray Polyurethane is my default finish. I apply it to the bowl in coats while it spins. I lightly polish each coat to a high luster with felt rags while it spins. This is safe and will last a while, but repeated eating and washing from it will erode the poly. So while it is fairly durable you can see the shine from it and will detect when it has worn off.
So you have only to consider the long term benefits when choosing your method.
Good luck and happy turning.
cad
