I need to monitor biomass during microbial fermentation, but there is a catch. The nature of my bioprocess makes a non-invasive solution necessary. Any suggestions?

That is one heck of a catch, but you are in luck. For some years now there has been a family of sensors, available in hand held and mounted versions, made by the folks at Bug Lab that meet the requirements you mention. In your question you specify non-invasive, both the hand held and mounted versions of these sensors meet that requirement, so the link below will take you to more information about each.

Bug Lab