Comparison of methods for estimating the biomass of three food-borne fungi with different growth patterns.

To evaluate the effectiveness of steps taken to reduce the growth of molds in food and feed, methods that can accurately quantify the degree of fungal contamination of solid substrates are needed. In this study, the ergosterol assay has been evaluated by comparing the results of this assay with spore counts and hyphal length measurements made with a microscope and with CFU counts. Three fungi with different growth patterns during cultivation on a synthetic agar substrate were used in these experiments. For the nonsporulating Fusarium culmorum, there was good agreement between changes in hyphal length, CFU, and ergosterol content. Penicillium rugulosum and Rhizopus stolonifer produced many spores, and the production of spores coincided with large increases in CFU but not with increases in hyphal length or ergosterol content. Spores constituted between 3 and 5% of the total fungal mass. Changes in ergosterol level were closely related to changes in hyphal length. It was concluded that ergosterol level is a suitable marker for use in quantitatively monitoring fungal growth in solid substrates.