Comparison of dilution schemes for enumerating fungi in ground pepper: a collaborative study

Ten lots of ground pepper were analyzed for fungal populations using 1:5 and 1:10 dilutions in sterile 0.1% peptone. Duplicate samples (0.1 ml) diluted 1:5 were surface plated on dichloran rose bengal chloramphenicol (DRBC) agar whereas quadruplicate samples (0.1 ml) diluted 1:10 were surface plated on DRBC agar. Nine collaborators from six countries participated in the study. Significantly (p < 0.05) higher fungal populations were detected in 7 of the 10 lots of pepper diluted using the 1:5 dilution scheme; the other three showed no significant difference. Using 1:5 and 1:10 dilution schemes, among-laboratory variability in mean fungal populations detected was not significant (p < 0.05) for 7 of 10 and 2 of 10 lots of peppers, respectively. Coefficients of variation for reproducibility (among-laboratory variation) were 2.8% and 6.2%, respectively, using 1:5 and 1:10 dilution schemes. Analysis of collapsed data from all ten lots of pepper revealed that significantly higher populations of fungi were detected using a 1:5 dilution scheme.