Generally kefir should not be in contact with metals as metals are prone to leach under acidic conditions (kefir is acidic).
However as our mixing ball is stainless steel, there is no issue using it. Stainless steel is considered safe and inert and should not leach into the kefir. The kit is designed to have the ball left inside, so you can simply shake up the kefir each time before consuming.
It's usually metals such as copper, brass, zinc, iron and aluminium that are reactive at a low pH (acidic environments) and are considered unsafe.