I can't find any guidelines for use of LS25 - the closest I can get is the manufacturer's web page:
http://www.gewuerzmueller.com/en/produc ... erculturesBut that doesn't have any instructions. Did you get instructions for use with the culture?
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My own feeling is that humidity will be less important for starter culture development than temperature (the reason being that there is plenty of moisture inside the sausage for the starter culture to grow). So I wouldn't worry about the effect of low humidity on culture growth during those first 24 hours.
I imagine that the need for high RH during the first few days has more to do with avoiding case-hardening; the higher temperatures (20-25oC) required for starter culture establishment, combined with a low RH might promote rapid drying of the surface if the sausage.
Also, if you are hoping to achieve a surface mould, high RH is good.
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Speculation on the mould/fat relationship:
your LS25 rapid-acidification culture has rapidly acidified the muscle portion of the sausage while the fat portion retains enough moisture to allow a surface mould to grow but - as the LS25 doesn't grow so well on the fat - the fat has not experienced such a rapid PH drop and the surface mould can grow there.
Like Phil, I spray my salami with a mould culture and so get mould development all over the surface.