Short communication: Entrapment of Listeria cells within air pockets of ice cream mix matrix may lead to potentially heat-injured cells

摘要

In our previous study, we observed the sporadicpresence of potentially heat-injured cells of Listeriainnocua in ice cream mix following a selective enrichmentprotocol. Although injured cells have not yet beenreported to cause any disease outbreaks, it is importantto understand their presence in heat-treated food matrices.In this study, we propose a possible protectiverole of air pockets that may help explain the sporadicpresence of potentially heat-injured cells following heattreatment. Challenge studies were conducted by inoculatingice cream mix samples (42% total solids, 16.3%fat, 22.2% total sugar, and 3.4% protein) with Listeriainnocua (an established surrogate) at a mean spikinglevel of 4.0 log cfu/g. The inoculated samples wereheat-treated at 69°C for 30 min and potentially heatinjuredcells were detected using buffered Listeria enrichmentbroth, followed by plating on modified Oxfordand Rapid’LMono agars. Scanning electron microscopyand atomic force microscopy were conducted on the airdried,spiked ice cream mix samples, before and afterthe thermal treatment stages. Although direct platingdid not reveal any intact cells in the heat-treated icecream mix, a more sensitive enrichment protocol wasable to identify cells that were potentially heat-injured.The scanning electron micrographs showed air pocketsof different sizes in the ice cream mix samples. Thespiked mix samples before heat treatment showed someListeria cells unevenly distributed in the mix matrixand some entrapped within the larger air pockets. Afterheat treatment, scanning electron and atomic force micrographsshowed cells entrapped only within the largerair pockets. The mix matrix, however, did not show anyListeria cells. Confirmation of Listeria at all stages ofanalysis was done using MALDI-TOF. These observationssuggest that the Listeria cells could be entrappedwithin the larger air pockets and thus may undergoinadequate thermal effect. This could have resultedin their detection as potentially heat-injured cells, asevident under the conditions of the experiment. Theseresults are preliminary observations and further studiesare necessary to draw conclusions and understand thetrue implications of these findings.