Listeria monocytogenes is the causative agent of the disease listeriosis and was implicated in a multistate foodborne outbreak associated with... Show moreListeria monocytogenes is the causative agent of the disease listeriosis and was implicated in a multistate foodborne outbreak associated with ice cream spread over four years from 2010 to 2014 in Kansas, Oklahoma, Texas and Arizona. Ten illnesses were reported during the course of this outbreak, resulting in 100% hospitalization and a mortality rate of 33%; the individuals who died in the outbreak had consumed milkshakes made with the contaminated ice cream. The ice cream was found to be contaminated uniformly at 10 MPN/g. This study assessed the growth kinetics of L. monocytogenes in milkshakes made with naturally- and artificially-contaminated ice cream, and with or without flavoring agents. Artificial-contamination of ice cream samples included inoculation with a cocktail of rifampicin-resistant L. monocytogenes into the middle of the product using a wide-orifice pipet tip. Milkshake were prepared with or without strawberry, chocolate, or mint flavoring, using the recipe associated with the healthcare facility where the illnesses occurred. Milkshakes were stored in sterile plastic cups at 10ºC and an asymmetric (3 tubes of 100ml, 5 tubes of 10 ml, 8 tubes of 1ml and 8 tubes of 0.1ml sampling scheme) most probable number (MPN) enumeration was conducted after 0, 12, 24, 48, 72, 96 and 144 h. The Baranyi model was used to model L. monocytogenes growth rates, maximum populations, and lag phases. Compared to milkshakes made with no flavoring agents, the milkshakes with flavoring resulted in decreased growth rates for L. monocytogenes for both contamination states; the lowest growth rate of the pathogen was observed in strawberry flavored milkshakes made from artificially-contaminated ice cream (0.029 log CFU/ml per hour). Chocolate and mint flavorings resulted in significantly (P < 0.05) longer lag phases for both natural (68.2 and 83.0 h) and artificial-contamination (47.7 and 54.2 h, respectively). The highest maximum population (5.79 log CFU/ml) was achieved by L. monocytogenes in naturallycontaminated milkshakes without any flavoring agent. Since challenge studies often depend on the artificial contamination of a food product, this study evaluated the differences between artificial and natural contamination of L. monocytogenes in milkshakes. The data obtained can be used for risk assessment purposes. M.S. in Food Safety and Technology, December 2016 Show less
