[Avodah] Microbes and Kashrus

[Joe Slater]

A while ago I had some very nice goat milk kefir, a fermented milk
drink. Kefir is produced by the action of a mixture of bacteria and
yeasts on milk and you need a live culture of these microorganisms to
produce it - you can't simply leave some milk out overnight and hope
for the best. The usual way of innoculating a new batch of milk is to
use some kefir from the last batch.

It occurred to me that there are many such products which derive from
a chain of earlier ones: yoghurt, koji (the bacterial culture used to
make Japanese foods like miso), fermented pickles, and sourdough
bread. Do the cultures used to make these foods need to be kosher? I
would have assumed yes, but I'm not aware of distinctively kosher
varieties of (e.g.) sourdough bread culture that can be traced to some
ancient loaf produced near Mount Sinai. Perhaps you would say that we
presume that these cultures are intrinsically kosher, but what about
the problem of chalav akum? And can we really assume that the culture
was never raised on a treif medium?

If non-kosher cultures are used, could they be made kosher via bitul?
Could we say that everything is ultimately batul in microscopic
amounts, even something which has such a profound effect on the food?
Because all you need to revive a culture is a (literally) microscopic
amount. What if there's no actual contact of the old culture and the
new growing mechanism; what if the treif culture is left to grow
spores which waft across and innoculate the new  culture? Even if we
wouldn't otherwise recognise reahh in such a context, shouldn't we
recognise the substantiality of a connection with such a profound
physical effect?

I'd be very interested in hearing the list members' thoughts on this.

Joe Slater