Editors' ChoicePUBLIC HEALTH

The curious case of the caramel apples

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Science Translational Medicine  28 Oct 2015:
Vol. 7, Issue 311, pp. 311ec184
DOI: 10.1126/scitranslmed.aad5500

The bacterium Listeria monocytogenes causes a serious foodborne illness, particularly in infants, the elderly, and pregnant women. Food commonly harbors low levels of Listeria, but fortunately a high inoculum— around a million colony-forming units (CFU)—must be ingested to initiate disease, even in high-risk persons. However, one peculiarity of the organism is that it can reproduce at relatively cold temperatures; thus, even in properly refrigerated foods, a small Listeria contamination can grow into a big public health problem. In November 2014, a multistate Listeria outbreak was linked to an unexpected food source: caramel-coated apples. Listeria with the same DNA “fingerprint” as the human isolates was eventually found at a fruit-packing facility that supplied several caramel apple producers. Ultimately, L. monocytogenes sickened at least 35 people in 12 U.S. states and Canada, and three deaths—including one fetal loss—were directly attributed to it.

At first glance, caramel apples would not seem to make an ideal substrate for Listeria growth. Apples are too acidic and caramel too dry for Listeria, which grow best in a moist, moderately acidic or neutral environment. Also, caramel is heated above 90°C for apple dipping; Listeria’s heat tolerance is about half that. So how did this outbreak occur? In a new study, Glass et al. sought to answer that question with 144 Granny Smith apples, a commercial caramel apple dip, wooden popsicle sticks, and a cocktail of L. monocytogenes isolates, including three from the 2014 outbreak.

As expected, apple flesh was acidic (pH 3.2) but had a high moisture content; the opposite was true of the caramel dip. Each apple was spiked with ~16,000 CFU of Listeria, and half of the apples had a wooden stick inserted next to the stem, while the other half did not. The apples were then dipped into hot caramel (95°C), allowed to cool, and stored in the refrigerator (7°C) or at room temperature (25°C) for up to 4 weeks. About 90% of the Listeria survived the hot-caramel dip, but no further bacterial growth was observed in refrigerated caramel apples without sticks. However, those with sticks supported an additional 2 logs of bacterial growth over 4 weeks in the refrigerator. At room temperature, the Listeria in caramel apples without sticks multiplied by 200-fold over 11 days, whereas those with sticks saw bacterial counts jump almost 4000-fold in just 3 days. The authors hypothesize that acidic apple juice tracks up along the stick to the caramel-apple interface, where it both moistens the caramel and is neutralized by it. Thus the wooden stick, so integral to the caramel apple eating experience, is the connection that helps Listeria thrive at the junction of two otherwise inhospitable environments.

K. A. Glass et al., Growth of Listeria monocytogenes within a caramel-coated apple microenvironment. mBio 6, e01232-15 (2015). [Full Text]

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