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

In our previous study, we observed the sporadic presence of potentially heat-injured cells of Listeria innocua in ice cream mix following a selective enrichment protocol. Although injured cells have not yet been reported to cause any disease outbreaks, it is important to understand their presence in heat-treated food matrices. In this study, we propose a possible protective role of air pockets that may help explain the sporadic presence of potentially heat-injured cells following heat treatment. Challenge studies were conducted by inoculating ice cream mix samples (42% total solids, 16.3% fat, 22.2% total sugar, and 3.4% protein) with Listeria innocua (an established surrogate) at a mean spiking level of 4.0 log cfu/g. The inoculated samples were heat-treated at 69°C for 30 min and potentially heat-injured cells were detected using buffered Listeria enrichment broth, followed by plating on modified Oxford and Rapid'LMono agars. Scanning electron microscopy and atomic force microscopy were conducted on the air-dried, spiked ice cream mix samples, before and after the thermal treatment stages. Although direct plating did not reveal any intact cells in the heat-treated ice cream mix, a more sensitive enrichment protocol was able to identify cells that were potentially heat-injured. The scanning electron micrographs showed air pockets of different sizes in the ice cream mix samples. The spiked mix samples before heat treatment showed some Listeria cells unevenly distributed in the mix matrix and some entrapped within the larger air pockets. After heat treatment, scanning electron and atomic force micrographs showed cells entrapped only within the larger air pockets. The mix matrix, however, did not show any Listeria cells. Confirmation of Listeria at all stages of analysis was done using MALDI-TOF. These observations suggest that the Listeria cells could be entrapped within the larger air pockets and thus may undergo inadequate thermal effect. This could have resulted in their detection as potentially heat-injured cells, as evident under the conditions of the experiment. These results are preliminary observations and further studies are necessary to draw conclusions and understand the true implications of these findings.