Effect of freezing and hardening on injured versus intact cells of Listeria in ice cream mix.

Document Type


Publication Date



American Dairy Science Association


Journal of Dairy Science




Suppl. 2






injured, Listeria, ice cream


Ice cream manufacturing relies on pasteurization to eliminate any Listeria contamination. However, factors such as cross-contamination levels, entrapment in polymorpholeukocytes, and product matrices may influence cellular injury. Our previous studies demonstrated a dose-dependent, random presence of heat-injured cells of Listeria in pasteurized ice cream mix. Such cells did not show any recovery within the mix itself under normal handling conditions. The present study evaluates the effect of freezing and hardening of ice cream mix on heat-injured and intact cells of Listeria. Raw ice cream mix (42% TS) samples were spiked with 4.36 ± 0.13 logs per gram of Listeria innocua (a surrogate) and subjected to pasteurization (69°C for 30min), which resulted in the random presence of heat-injured cells. To simulate the post-pasteurization contamination of mix with intact cells, 2.65 ± 0.07 logs per gram of L. innocua were spiked in the pasteurized mix samples. The mixes containing injured and intact cells were followed through aging (72h at 7°C), freezing (−4.4°C), and hardening (12 h at −40°C) steps. Direct plating on Listeria selective agars enumerated intact cells, while Listeria enrichment broth (BLEB) was used to recover heat-injured cells before plating. All trials were conducted in triplicates and data were statistically analyzed. Although no post-pasteurization survivors were observed on direct plating, the enrichment protocol revealed heatinjured cells at all of the post-pasteurization stages of processing, tested. Freezing and hardening steps thus did not appear to have any detrimental effects on heat-injured cells. Injured cells have not been associated with any outbreaks; however, it would be interesting to study their ability to recover during any handling-abuse at retail or consumer end. In case of spiked intact cells, no detrimental effect of freezing and hardening was observed. This implies that post-pasteurization contamination of mix might pose a greater risk. Results from this study emphasize a need to design stage-specific critical control points to prevent any potential Listeria outbreaks.