The increased number of revertants was not concentrationrelated,
but was assumed by the applicant to be due to histidine present in the food enzyme. For that
reason, the experiment was repeated applying a “treat and plate assay”, in which the Salmonella
typhimurium strain was exposed to six concentrations of the food enzyme in a liquid medium for 3
hours before plating the centrifuged cells. No increase of revertant colony numbers was observed.