A method of reducing the level of RNA in a biomass comprising filamentous fungus involves the following steps: (i) heating the biomass, downstream of the fermenter to a first temperature preferably in the range 40- 69°C； (ii) heating the biomass to a second temperature which is preferably 6-20°C greater than the first temperature, such as by the application of steam - to facilitate release of RNA from cells of the filamentous fungus； and (iii) separating filamentous fungus having a reduced level of RNA from other components. The fluid removed in step (iii) may be recirculated to a heat exchanger, which is used to heat the biomass in step (i).
