In Summary
- Cold smoking of fish can reduce numbers of L. monocytogenes, however significant reductions only occurred when smoking dry-salted or brine-injected fillets.
- Freezing fish does not affect the survival of L. monocytogenes.
- Lower ambient temperature smoking (between 17.2 – 21.1°C) can significantly reduce the levels of L. monocytogenes compared to higher temperature cold smoking. However it is important to note that this process is not considered a critical control.
- High hydrostatic pressure, is a newer treatment which (when incorporated with other factors) may cause a reduction of L. monocytogenes to be considered a critical control. However, such a control has not be authorised by the relevant competent authorities currently.
Treatment
The process of cold smoking was shown to reduce L. monocytogenes numbers from 1000 to 100 CFU/CM² by Porsby et al. (2008). However, the only significant reductions in L. monocytogenes numbers were observed when smoking dry-salted or brine-injected fillets, (compared with fish which had not been salted at all). The cold smoking of unsalted fresh fillets did not result in significant reductions to the numbers of L. monocytogenes associated with the fish (Porsby et al. 2008). For that reason, it is important to periodically check that the curing stage of processing has completed as expected.
In addition, Kang et al. (2012) report on the effect of cure method on L. monocytogenes lag time on smoked salmon stored at 7°C. (Lag time is the amount of time taken by bacteria to adapt to a new nutrient source before they begin to grow exponentially). The lag time for a lab-cultured cocktail of four L. monocytogenes strains used to inoculate freeze-thawed dry cured salmon fillets ranged from 4-11 days. Growth in brine cured salmon was observed in under 24h.
Freezing fish
Ice crystal formation can rupture cells in the fish muscle (Love, 1955) and L. monocytogenes and other bacteria can take advantage of released cellular nutrients very rapidly (Gram, 2001). EU regulation 1276/2011 requires the freezing of some wild fish species including salmon to kill parasites that are common in wild fish. The risk from parasites is higher in ocean caught fish compared with the risk from L. monocytogenes. Furthermore, in contrast to some bacterial species, the process of freezing does not appear to significantly reduce the numbers of L. monocytogenes present on fish fillets. There is little evidence that prolonged frozen storage, either of the raw fish or packed product results in significant reductions to L. monocytogenes numbers (Gram, 2001). Furthermore, sub-zero temperatures do not cause sufficient injury to L. monocytogenes cells to affect their subsequent regrowth at chill temperatures after thawing fish (Gram, 2001, El-Kest et al, 1991). Gram (2001) hypothesised that the high lipid concentrations found in fish flesh may protect L. monocytogenes cells against sub-zero temperatures and ice crystal formation damage. Therefore, freezing cannot be thought of as a control point for L. monocytogenes.
It took longer (more than four days) for L. monocytogenes to grow on freeze-thawed dry cured cold smoked fish compared with freeze-thawed wet cured (Gram 2001, El-Kest et al. 1991, Kang et al. 2012). How freeze-thawed fish were cured in terms of brining or dry salting can also be an important measure when considering the shelf life of previously frozen smoked product.
Temperature
Ecklund et al. (1995) have reported that at lower ambient temperatures (17.2- 21.1oC), a smoke treatment significantly reduced the numbers of L. monocytogenes on fish compared with an unsmoked control i. e. cold smoking can reduce but did not eliminate L. monocytogenes within the assessed temperature range.
Rørvik (1997, 2000) summarised the findings from a number of cold smoking studies as: “cold-smoking has been shown to eliminate L. monocytogenes in challenge tests at temperatures from 17.1 to 21.1oC while from 22.2 to 30oC the bacteria survived [the cold smoking process]”. Rørvik (2000) makes clear there should be distinctions made between inoculated and naturally contaminated fish and also notes that for natural contamination “cold-smoking (19 to 22oC) [caused] the frequency and numbers of L. monocytogenes to decrease.”
It is important to note that cold smoking between 19oC and 22oC can reduce, but not eliminate L. monocytogenes populations on fish (Rørvik, 1997, Porsby et al. 2008, Rørvik 2000). Equally important is that there is a knowledge gap as to the fate of cold smoked L. monocytogenes. It is possible that injury of the L. monocytogenes cells is sub-lethal and that there could be recovery during subsequent refrigerated storage of the fish. Furthermore, the chemical composition of smoke is variable and influenced by factors such as wood type and growth conditions. It is also important to make clear that the results for each of the studies reporting a reduction in L. monocytogenes numbers are applicable only for the smoke compositions used by these studies.
High Hydrostatic Pressure
A list of potential interventions is available on the product interventions page. Recently, Ekonomou et al. (2020) evaluated various combinations of decontamination treatments of high hydrostatic pressure (HHP), liquid smoking and freezing on the numbers of L. monocytogenes. A synergistic effect of liquid smoke and HHP was observed and was further enhanced by freezing overnight prior to HHP. A widely accepted convention is that a process intervention can be considered a critical control point if it causes a six log reduction in the numbers of a bacterial hazard. The Ekonomou et al. (2020) paper is important because it describes a treatment that can significantly reduce L. monocytogenes numbers by more than five logs, and which may be legal, if the intervention was authorised by an EU member state government. Please note that although the UK has left the EU, the EU regulations continue to apply in the UK (and devolved national governments such as Holyrood) as a consequence of the EU-UK withdrawal agreement, which has been passed into UK and devolved national laws.
Other authors (Gudbjornsdottir et al. 2010) have reported no significant changes to the organoleptic properties of the fish after an application of HHP.
