Can you mix the FOS or other prebiotics with the kefir?
FOS can be added to the kefir. I would not add other fibers though.
How long does it take to ferment the kefir to reduce the lactose as much as possible? Thanks.
That depends a lot on temperature. Warmer temperatures will speed up the fermentation. You can try a taste test to see how acidic the taste is. The more the lactose is fermented the more acid is formed. Or if you want to avoid the lactose altogether you could use coconut milk or make water kefir.