Hot Smoking Content

In Summary

Hot smoking can be a critical control point, assuming the conditions are appropriately validated.
Different time/temperature combinations are indicated by competent authorities, emphasising the importance of individual products’ validation.
A table is provided below showing some validated time/temperature combinations in a range of products, which may be used as reference when carrying out critical control validations.
F- and D- values to reduce bacterial populations are discussed.

Temperature as a critical control point in hot smoking.

Hot smoking meets the FAO criteria for a critical control point (CCP) and so is an opportunity to improve the food safety of a fish process (Poysky et al. 1997). If hot smoking is achieved as planned, and a critical temperature is achieved for a set duration across all of the fillets in a process, it can result in a batch of fish being rendered completely free of L. monocytogenes (Jemmi and Keusch 1992; Branciari et al. 2016). With regard to L. monocytogenes, a successfully achieved CCP makes what has happened to the food prior to the thermal processing stage, largely unimportant (although there are other hazards in fish smoking that temperature does not effectively control – Fish Chemical Hazards and Fish Other Micro Hazards).

However, what the critical temperature should be for a L. monocytogenes CCP is contentious and there are a number of credible sources that advise different targets. In addition, the Scottish and UK Food Standards Agencies, European Salmon Smokers’ association guidance, FAO guidance, Canadian government and the USDA/US-FDA do not stipulate that a specific temperature should be achieved for the control of L. monocytogenes in hot smoked fish. Sikorski and Kolodziejska (2002) hypothesise a possible reason for not defining a specific CCP temperature might be that the thermal tolerance of L. monocytogenes in food products is dependent on many factors; including the L. monocytogenes strain and the composition of the food (Poysky et al. 1997). However, Food Standards Australia advises L. monocytogenes cannot survive >75oC, (with no hold duration specified) and some informed industry associations such as a working group created by the US National Fisheries Institute and US National Food Processors Association advise 145oF (62.8oC) for 30 minutes as sufficient for control.

A number of peer-reviewed publications state that it is a combination of temperature, the phenolic compounds in smoke and [salt] in the fish muscle acting synergistically that are responsible for L. monocytogenes injury and death (Poysky et al. 1997; Sikorski and Kolodziejska 2002; Hwang et al. 2009).

There are a number of scientific papers that have documented reductions in L. monocytogenes populations caused by an application of heat (with and without consideration of smoke and salt concentration), with some concluding some thermal processes can completely destroy all L. monocytogenes contamination from fish. A summary of these papers and their key findings are provided as Table 1.

Core temp. (oC)	Duration (min)	Product	Other considerations	Reference
62	F (calculated): 26 (Actual duration not specified)	Crab meat	Inoculated a single strain of L. monocytogenes at 107 cfu/g. Actual temperature with extrapolated duration from D values. Study criticism that the crab meat was blended and packed into sausage casing and heat was applied using a hot water bath.	Harrison and Huang (1990)
65	F (calculated): 3.2 (Actual duration not specified)	Crawfish tail meat	Inoculated a two-strain cocktail of L. monocytogenes at just over 103 cfu/g. Actual temperature with extrapolated duration from D values. Study criticism that the tail meat was previously cooked, frozen, thawed and then used for the experiments.	Dorsa et al. (1993)
62.7	F (calculated): 13.2 (Actual duration not specified)	Lobster	Inoculated a five-strain cocktail of L. monocytogenes at 107 cfu/g. Actual temperature with extrapolated duration from D values. Study criticism that heat was was applied to pouches of shredded lobster immersed in a hot water bath.	Budo-Amoako et al. (1992)
65	20	Trout	Inoculated single strain of L. monocytogenes at 106 MPN/g. Complete kill observed.	Jemmi and Keusch (1992)
62	18.5	Blended mussels	Inoculated a seven-strain cocktail of L. monocytogenes at 106 cfu/25g mussels, temperature and duration extrapolated from D values.	Bremer and Osborne (1995)
67.2	Unclear, there was stepwise increase of temperature; higher temperature meant longer duration	Salmon	Inoculated a two-strain cocktail of L. monocytogenes at 330-350 cfu/25g, effective killing at the temperature required exposure to smoke.	Poysky et al. (1997)
>82.8	Unclear, there was stepwise increase of temperature; higher temperature meant longer duration	Salmon	Inoculated a two-strain cocktail of L. monocytogenes at 330-350 cfu/25g, no smoke was required for effective kill at higher temperature.	Poysky et al. (1997)
≤60	85	Mackerel	Natural contamination of 102-104 aerobes/g fish, heat treatment was insufficient to destroy all aerobes.	Kolodziejska et al. (2002)
55	360	Salmon	Inoculated a six-strain cocktail of L. monocytogenes at 106 cfu/g, using 6% salt and 15ppm phenols, the fish were still contaminated with L. monocytogenes.	Hwang et al. (2009)
72	6	Tench	Inoculated a three-strain cocktail of L. innocua as a L. monocytogenes surrogate; inoculated at 105 cfu/g. Complete kill observed.	Branciari et al. (2016)

There are glossary entries for D-, Z- and F-values. FBOs may wish to review these glossary entries before reading this section. It is stressed that F is unrelated to the imperial Fahrenheit temperature scale. As stated above, it can be difficult to set absolute criteria for a target temperature and a hold duration because different strains of bacteria and the status of their stress response genes can introduce different susceptibilities to an application of temperature in foods of variable composition. It is an issue that is widespread in food microbiology and not confined to just L. monocytogenes in smoked fish. Partly for that reason, critical control points are defined as process stages that control an identified hazard absolutely or reduce the hazard to an acceptably low risk. A common, highly robust, strategy in food microbiology is to calculate an F-value from an experimentally derived D-value at a given temperature. F is widely defined as the duration at a specific temperature required for a 12-log reduction. However, how many log reductions reduce the L. monocytogenes hazard to an acceptably low risk is a decision to be made by an FBO, possibly in conjunction with their EHO. Tocmo et al. (2014) reviewed some of the surveillance undertaken for L. monocytogenes in smoked fish and report it was rare for L. monocytogenes in positive samples to exceed 1x104 cfu/g. A more comprehensive review was undertaken by Jami et al. (2014), although the summaries were made as presence absence and the actual papers need to be scrutinised for enumeration results. As for Tocmo et al. (2014), exceeding 104 cfu/g appears possible from the studies cited by Jami et al. (2014), but is unusual. The use of F as 12xD is very robust and was set originally for spore-forming Clostridium in canned/vacuum packed foods. Using 12xD for vegetative bacteria would substantially support a defence of due diligence. Some FBOs may consider that since L. monocytogenes rarely exceeds 104 cfu/g in samples purchased at retail, F set as 6xD may represent an acceptably low risk or completely control the hazard. The experimental data showing a six-log reduction in total mesophilic aerobe numbers during the cooking of surimi seafood (Shie and Park 1999), was considered by to be a CCP by (Himelbloom 2000).