Abstract
This study assessed the growth of Listeria monocytogenes in milkshakes made using the process-contaminated ice cream associated with a listeriosis outbreak in comparison to milkshakes made with artificially contaminated ice cream. For all temperatures, growth kinetics including growth rates, lag phases, maximum populations, and population increases were determined for the naturally and artificially derived contaminants at 5, 10, 15, and 25°C storage for 144 h. The artificially inoculated L. monocytogenes presented lower growth rates and shorter lag phases than the naturally contaminated populations at all temperatures except for 5°C, where the reverse was observed. At 25°C, lag phases of the naturally and artificially contaminated L. monocytogenes were 11.6 and 7.8 h, respectively. The highest increase in population was observed for the artificially inoculated pathogen at 15°C after 96 h (6.16 log CFU/mL) of storage. Growth models for both contamination states in milkshakes were determined. In addition, this study evaluated the antimicrobial effectiveness of flavoring agents, including strawberry, chocolate and mint, on the growth of the pathogen in milkshakes during 10°C storage. All flavor additions resulted in decreased growth rates of L. monocytogenes for both contamination states. The addition of chocolate and mint flavoring also resulted in significantly longer lag phases for both contamination states. This study provides insight into the differences in growth between naturally and artificially contaminated L. monocytogenes in a food product.
Highlights
Listeria monocytogenes is commonly present in bulk tank milk (Jackson et al, 2012) and has long been recognized to be able to survive in facilities where dairy products are processed, including frozen dairy products such as ice cream
A listeriosis outbreak, which occurred in the United States between the years 2010 and 2015 and resulted in 10 total cases, 100% hospitalization rate, and three deaths, was linked to the consumption of ice cream products, which became contaminated post-pasteurization by L. monocytogenes present in the processing environment (CDC, 2015; FDA, 2015)
Individual 80-g ice cream product samples (“scoops”), which were naturally contaminated with approximately 10 most probable number (MPN)/g L. monocytogenes during manufacture (Chen, 2015), were divided into four groups (Figure 1): one group remained as naturally contaminated (N), the second group was artificially contaminated with the four-strain cocktail (A), and the third group was artificially contaminated with JKS-1 (AJ)
Summary
Listeria monocytogenes is commonly present in bulk tank milk (Jackson et al, 2012) and has long been recognized to be able to survive in facilities where dairy products are processed, including frozen dairy products such as ice cream. A listeriosis outbreak, which occurred in the United States between the years 2010 and 2015 and resulted in 10 total cases, 100% hospitalization rate, and three deaths, was linked to the consumption of ice cream products, which became contaminated post-pasteurization by L. monocytogenes present in the processing environment (CDC, 2015; FDA, 2015). Four of the cases resulted from consuming milkshakes prepared from the contaminated ice cream at a healthcare facility. The availability of uniformly and naturally contaminated samples of the ice cream product provided an unusual opportunity to study the behavior of L. monocytogenes contaminants in a realistic condition. One study involved a naturally contaminated dairy product, i.e., the ice cream associated with the 2010–2015 listeriosis outbreak (Chen et al, 2016a)
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