Survival of Listeria monocytogenes in whole egg and egg yolk powders and in liquid whole eggs

Abstract

A mixture of five strains of L. monocytogenes was inoculated at two populations (approximately 10 cfu g −1 and 10 5 cfu g −1 ) into commercially dried powdered whole egg and egg yolk, and liquid whole egg. Inoculated dried egg products were stored at 5 and 20°C for 6 months and liquid whole eggs were stored at 0 and −18°C for 2 and 24 weeks, respectively. The presence and populations of viable cells were determined periodically throughout storage. Presence of L. monocytogenes was determined by subjecting egg products to primary enrichment in tryptic soy broth followed by plating the broth on Lee Modified Oxford (MOX) agar. Enumeration was done by directly plating serially diluted egg products directly on MOX agar. All egg products inoculated with 10 cfu g −1 yielded viable Listeria cells throughout storage. L. monocytogenes decreased about 1 and 1·5 log 10 cfu g −1 in powdered whole egg and egg yolk, respectively, when inoculated with 10 5 cfu g −1 and stored at 5°C. Inactivation occurred more rapidly at 20°C. The number of viable L. monocytogenes in frozen liquid whole egg initially containing 10 5 cfu g −1 remained unchanged throughout storage. These results indicate that L. monocytogenes can survive throughout the normal shelf life of powdered and frozen egg products.