Tracking Listeria survival at different stages of ice cream manufacture.


N. Neha
S. Anand

Document Type


Publication Date



American Dairy Science Association


Journal of Dairy Science




Suppl. 2






risk, listeria, ice cream


Recent recalls emphasize the need to find novel approaches for Listeria control in ice cream industry. Present study investigates survival of Listeria at different stages of ice cream manufacture. Listeria innocua ATCC 33090 (a surrogate) was spiked at mean log levels of 2, 3, and 4 log cfu/g in ice cream mix samples with total solid levels (32, 36, 42, 45%). The samples were pasteurized (69°C /30min) followed by aging (7°C), freezing (−4°C), and hardening (−40°C). Direct plating on selective agars was used to enumerate survivors, while any injured cells were resuscitated in Listeria enrichment broth, before plating. All trials were conducted in triplicates, and the data were statistically 124 J. Dairy Sci. Vol. 101, Suppl. 2 analyzed. Although direct plating did not show any survivors in pasteurized mix, the enrichment protocol randomly recovered some injured cells. It is important to note that random presence of heat-injured cells was observed only at higher spiking levels (4.36 ± 0.13 log cfu/g). The scanning electron micrographs showed entrapment of some Listeria cells in the air pockets of ice cream mix, which might have led to injured cells. Binary models further established the dose levels to be a predictor of the presence of heat-injured cells of Listeria. Significantly, the heat-injured cells did not recover within ice cream mix itself during the different processing steps. In a parallel study, spiked pasteurized mix samples (2.65 ± 0.07 log cfu/g) were subjected to aging, freezing and hardening steps, followed by direct plating. No detrimental effect of these stages was observed on the intact cells of Listeria. It can be concluded that although a higher contamination with Listeria may lead to a random presence of injured cells, these cells do not recover under normal mix handling and processing conditions. On the contrary, any post-pasteurization contamination of mix could pose a potential risk from intact cells of Listeria, thus making a case for more robust risk assessment protocols.