Purpose: A method that achieves homogenous and stable inoculation of powdered infant formula (PIF) samples at realistic levels for use in quality assurance (QA) studies is summarized. This method was used to inoculate PIF with pathogens of concern including Salmonella and Cronobacter.
Methods: Dry inocula were prepared by spraying liquid inocula of target organisms over PIF using an ultrasonic atomizing spray nozzle while mixing. Serial dilutions of dry inocula with uninoculated PIF were then made to prepare final samples. Homogeneity and stability of target organisms in dry inocula as well as final samples were confirmed by quantitative testing and statistical analysis.
Results: Within test portions, the mean analytical standard deviation for PIF inocula was 0.37 log CFU/g and 0.25 log CFU/g for final samples. Mean heterogeneity standard deviation between test portions for PIF inocula was 0.02 log CFU/g and 0.05 log CFU/g for final inoculated samples. Heterogeneity standard deviation in both inocula and final samples were not significantly different than zero according to F-tests, indicating sufficient homogeneity. Total aerobic counts in Cronobacter inocula PIF decreased by 0.25 log CFU/g over one month and fulfilled the expanded criteria for stability according to ISO 13528.
Significance: Statistical analysis of homogeneity and stability data supports that the inoculation method used in these studies is a reliable technique to artificially contaminate PIF. This method may be used to prepare reference materials and check samples for QA studies in microbiological laboratories.