Researchers found that Listeria grew on caramel apples when inoculated on the stem end, but for fresh apples, there was no growth but only survival. So the caramel and along with the stick help to form a microenvironment within the apple.
This is a good reason why we have not seen Listeria outbreaks associated with fresh apples. However, since it survives, there must be controls in place for apples that will be used as an ingredient in foods that may support growth.
The Packer
http://www.thepacker.com/news/report-listeria-spreads-faster-caramel-apples-fresh-apples?
Report: listeria spreads faster on caramel apples than fresh apples
By Doug Ohlemeier May 31, 2016 | 5:15 pm EDT
Listeria proliferates faster on caramel apples than fresh apples, according to research.
In a study published in the May issue of Journal of Food Protection, “Fate of Listeria monocytogenes in fresh apples and caramel apples,” researchers wanted to determine information about the survival and growth of L. monocytogenes in fresh apples and caramel apples.
A listeriosis outbreak in late 2014 and early 2015 associated with caramel apples prompted questions about how the product became a vector for Listeria monocytogenes.
The researchers were associated with the Food and Drug Administration’s Division of Food Processing Science and Technology in Bedford Park, Ill., and examined how the site and level of inoculation, inoculum drying conditions and storage temperature affects listeria growth.
At a high inoculation level (7 log colony-forming unit per apple), L. monocytogenes inoculated at the stem end proliferated on gala caramel apples at both 41 degrees and 77 degrees and on granny smith caramel apples at 77 degrees by as much as 3 to 5 logs CFU per apple, according to the study.
Fresh apples and caramel apples inoculated at the equatorial surface supported survival but not growth of the pathogen, according to the study.
Growth rates for apples inoculated at the stem end registerd at 1.64 ± 0.27 and 1.38 ± 0.20 log CFU per apple per day for gala and granny smith caramel apples, respectively, stored at 77 degrees.
At a low inoculation level (3 log CFU per apple), L. monocytogenes inoculated at the stem end and the equatorial surface survived but did not grow on fresh gala and granny smith apples stored at 77 degrees for 49 days.
On caramel apples inoculated at the stem end, L. monocytogenes had significant growth under the same conditions, according to the study.
When the inoculum was allowed to dry for 24 hours at 41 degrees, growth was significantly slowed compared with inoculum allowed to dry for 2 hours at 77 degrees.
Variation in stick materials did affect L. monocytogenes survival, but the differences were diminished once sticks were placed into caramel apples.
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