Kefir and blender/metal blade?

[1love4ever]

I've heard you are not supposed to expose kefir to metal except stainless steel, but the blade on my blender does not say what it is made of?  It is an Oster.  I would like to make my green smoothies with kefir but is it bad if the blade is metal?  Or is it just the kefir Grains I am not supposed to expose to metal?

Thanks!

[CoBabyMaker]

I don't think the relatively short period of time it is in the blender is an issue.