You may be right, here's what I found in several places:
"There are at least two different grades of CO2 available, (I found
out during a similar problem) Food grade, which is used in soda
machines and such, and commercial grade, which is used in some
welding applications from what I can tell. The food grade is what
The only difference between these two grades of CO2 is that the
bottle that it comes in. Both gases are 99.97% CO2, the food
grade bottles are internally glass coated, whereas the industrial
cylinders are straight steel.
The reason that there is a difference, is that by law, the food
systems have the possibility of back-flowing into the bottle
when the gas runs out. If the bottle was a standard industrial
bottle, the system would have the possibility of becoming
contaminated with rust from the internals of the bottle. (CO2
disolved in liquid from beverages is quite acidic and rusts the
inside of gas cylinders. "