Smoked fish has been processed and eaten for centuries in many parts of the world. Until commercial refrigeration became mainstream, smoking of fish was done for preservation without refrigeration and likely tasted much different than the smoked fish that most people are accustomed to eating today. With the introduction of refrigeration, the smoking of fish became nuanced by complex flavor profiles based on the type of wood used for smoke, the ratio of sugar used with salt for brining/dry curing, and a drive to include as little salt as possible to meet changing consumer taste preferences.

Over time, these consumer-driven modifications in processing presented their own set of food safety hazards and unique challenges to control them. Two of the most prevalent food safety hazards are non-proteolytic Clostridium botulinum (NPCB), comprising C. botulinum type E and non-proteolytic types B and F; and Listeria monocytogenes. This article focuses on understanding the science behind the controls required to ensure consistent, safe production of both hot and cold smoked fish, using generated wood smoke and liquid smoke.

Smoked fish is defined as "fish prepared by treating fish with salt (sodium chloride) and subjecting it to the direct action of smoke from burning wood, sawdust, or similar material and/or imparting to it the flavor of smoke by means such as immersing it in a solution of wood smoke with or without heat."¹ Critical moisture measurements used for smoked fish are water phase salt (WPS) and/or water activity (A_w).

For each finished smoked fish product, a scheduled process or validation study must be present to document defined controls (minimum and/or maximum values) for identified hazards at each processing step to establish critical limits in the processor's Hazard Analysis and Critical Control Points (HACCP) Plan.^1,2,3

The steps in the processing of hot and cold smoked fish are somewhat similar until reaching the smokehouse. The following steps walk through a rudimentary narrative of the flow of smoked fish production.

Receiving

All raw fish that is received must be properly refrigerated at 40 °F/4.4 °C or below, in most scenarios, to control food safety hazards such as pathogenic bacteria growth and/or histamine formation. Processors of fish to be cold smoked must monitor refrigerated transport conditions to control pathogenic bacteria growth, generally on the fish surface. If the product is a histamine-forming species (such as tuna, mahi-mahi, mackerel, etc.), then monitoring refrigerated transport will be required for the control of histamine formation, regardless of whether the fish will be hot or cold smoked.

If the fish is a parasite-containing species and is to be cold smoked, then the processor must have written assurance that a previous processor has destroyed all viable parasites with a proper freezing regime. If written assurance cannot be secured, then the fish will need to be properly frozen in-house to destroy all viable parasites before cold smoking. Hot smoking procedures are sufficient to destroy all viable parasites.

Refrigerated Storage

All raw fish must be stored under refrigeration (generally below 40 °F/4.4 °C) and/or frozen until further processing takes place as required by Good Manufacturing Practices (GMPs) and as a critical control for various food safety hazards (such as pathogenic bacteria growth, histamine formation, etc.). Refrigerated storage must be monitored for fish to be cold smoked for control of pathogenic bacteria, which may not be destroyed at later steps. If the product is a histamine-forming species, then refrigerated storage must be monitored to prevent histamine formation that cannot be destroyed at later processing steps.

Removal of Scales and Evisceration

Scales are a harborage point for surface pathogens, which hot and cold smoked fish processors aim to destroy. Removing the scales makes this process easier and ensures that salt or salt/sugar penetration through the fish side(s) with skin will not be impeded.

Evisceration is required for all fish five or more inches in total length to be smoked. Proper evisceration (gills and viscera removal) ensures that as many C. botulinum spores are safely removed as possible, since gills and viscera naturally harbor these bacterium spores. Uneviscerated fish less than five inches in total length can be smoked, but must have defined moisture control (≥10 percent WPS or ≤0.935 A_w) to inhibit growth and toxin formation of both proteolytic C. botulinum and NPCB in the finished product. For a detailed video of proper fish evisceration, see the "Fish Evisceration Training Video" (Figure 1) from the Association of Food and Drug Officials (AFDO).⁴

Surface Pathogen Elimination Procedures for Cold Smoked Fish

Cold smoking generally does not provide a 6D lethality (99.9999 percent reduction) for Listeria and vegetative microorganisms of public health significance, which hot smoking readily achieves. As such, ensuring control of pathogens from the harvest area, as well as sanitation from the boat to the smokehouse, becomes of utmost importance. This is because the harvest waters, harvest vessels, transportation vehicles, and in-plant areas can all be sources of pathogenic bacteria. Numerous industry sanitation failures and increasing natural levels of surface pathogens on raw fish have led many smokehouses to employ Surface Pathogen Elimination Procedures (SPEP)⁵ to achieve a 6D lethality for Listeria.

Food contact antimicrobials approved by the U.S. Food and Drug Administration (FDA) for use on seafood that are used in the smoked fish industry include acidified sodium chlorite, peroxyacetic acid, and ozone. A food-grade antimicrobial used is calcium hydroxide. These antimicrobial dips must have a scientifically validated SPEP to achieve a 6D lethality for Listeria. In addition, these antimicrobial dips are very effective at reducing spoilage microorganisms, which are required for control of NPCB. If the SPEP significantly reduces spoilage microorganisms, then the entire cold smoked process must be developed to control NPCB without the presence of spoilage microorganisms.

Filleting/Trimming Fish

Fish must have a defined maximum thickness based on the scheduled process or validation study. If that thickness cannot be ensured, then each fillet larger than the maximum thickness must be trimmed to less than or equal to the defined maximum thickness.

Brining

The most common ways of brining fish to be either hot or cold smoked are wet brining and dry curing. Brining must drive salt or salt/sugar (sodium nitrite, when allowed) into the flesh and remove moisture. A scheduled process or validation study for each product must outline all defined parameters required to achieve a minimum amount of moisture reduction and the addition of salt, salt/sugar, and/or other ingredients during wet brining or dry curing of a batch of brined fish. This ensures a predetermined minimum WPS or predetermined maximum A_w prior to a structured smoking/drying process. Cold smoked fish processors generally use sugar during brining to provide a nutrient source for spoilage microorganisms (if present) to flourish during smoking as a control for NPCB and to ensure safety with lower salt concentrations.

Wet brining is done when a defined maximum volume (pounds) of fish with a maximum thickness (inches) is placed into a brine solution with a defined minimum volume of brine (gallons) with a defined minimum amount (pounds) of dissolved salt or salt/sugar, carried out under refrigeration at less than 38 °F/3.3 °C for a defined minimum amount of time. Brine strength (salt concentration) is measured with a Salometer, yielding measurements in degrees Salometer. Measurements are standardized at 60 °F, and with every 10 °F drop in brine temperature, the Salometer reading will drop by 1 degree Salometer. Knowing the temperature of the brine when measuring is essential to having the proper concentration of salt in the brine solution.

Dry curing is when a defined maximum thickness of fish is coated with salt or salt/sugar (predetermined ratio) at a minimum quantifiable thickness of salt or salt/sugar, carried out under refrigeration at a predetermined minimum amount of time while the liquid drains from the fish.

Rinsing

Wet brined or dry cured fish must be quickly rinsed (without any discernable changes in salt content) with potable water to freshen the surface and remove any adhered surface particles or extraneous matter. This provides a smooth, clean surface for the pellicle to form on. The pellicle is glossy proteinaceous matter that forms on the surface of brined fish once the surface moisture starts to evaporate. This pellicle allows for uniform surface deposition of smoke to achieve the desired visual characteristics associated with smoked fish.

Liquid Smoke Dip

Processors choosing to use liquid smoke in lieu of (or in addition to) generated smoke often dip their rinsed fish in liquid smoke. This is done at a predetermined volumetric ratio for a predetermined minimum amount of time to allow for the penetration of liquid smoke into the surface of the fish at a defined minimum concentration of phenolic compounds. This ensures that the phenolic compounds in the liquid smoke are applied before pellicle formation.

Hanging on Racks

Rinsed and/or liquid smoke dipped fish must be hung up or laid on smoking racks without touching one another. A predetermined maximum volume of fish must be set per smoking load to ensure adequate airflow around each piece of fish during the smoking/drying process in the smokehouse. This is essential for structured removal of moisture. During this time, excess surface moisture from rinsing can drain off before the product enters the smokehouse.

Smoking/Drying

A scheduled process or validation study for a specific product at must outline all defined parameters required to achieve:

• A minimum amount of moisture reduction (based on previous brining/dry curing)
• Proper application of smoke at minimum phenolic concentrations
• Minimum internal temperature, and minimum time at this minimum internal temperature (hot smoked); or maximum smokehouse ambient temperature and minimum/maximum times at this maximum smokehouse ambient temperature (cold smoked)
• Any other parameter(s) required at the smoking/drying step.

Hot smoking requires that a smoked fish must reach a minimum internal temperature of 145 °F/62.8 °C and be held there for a minimum of 30 minutes with a minimum finished product WPS of 3.5 percent (3 percent WPS with 100–200 ppm sodium nitrite for salmon, sablefish, shad, and chubs only) when sold in a vacuum package. Many states require a minimum 2.5 percent WPS when the product is air packaged. The heat treatment provides more than a 6D lethality for Listeria, yet only provides a partial destruction of NPCB spores. The heat treatment must be combined with a predetermined concentration of phenolic compounds from smoke (under, in, and above) the pellicle, adequate WPS levels, and refrigeration at 38 °F/3.3 °C or less to ensure control of NPCB.

Cold smoking requires that ambient temperatures of the smokehouse do not exceed 90 °F/32.2 °C (120 °F/48.9 °C for sablefish)³ to ensure that adequate natural spoilage microorganisms remain present in the finished product. This ensures that these spoilage microorganisms will create a foul odor and provide inhibitory competition for NPCB in the event of temperature abuse. When adequate spoilage microorganisms are present, finished product vacuum packaged cold smoked fish must have a minimum 3.5 percent WPS (3 percent WPS with 100–200 ppm sodium nitrite for salmon, sablefish, shad, and chubs only).^1,2,3

Certain states, such as New York, require a minimum 2.5 percent WPS when the product is air packaged.² As such, any processor without validation that their SPEP does not reduce spoilage microorganisms will need to have their finished product smoked fish achieve a minimum 5 percent WPS, or an A_w of 0.97 or lower, as a barrier combined with refrigeration to control NPCB toxin formation. WPS of ≥5 percent or A_w of ≤0.97 are sufficient to control NPCB without the presence of spoilage microorganisms.³ Cold smoking does not completely inhibit nor destroy histamine-forming bacteria and, as such, cold smoking of histamine-forming species will require control of cumulative time at ambient temperatures above 40 °F/4.4 °C before smoking, during smoking, and after smoking to prevent histamine formation. A WPS of 5 percent or greater is very salty for smoked fish, resulting in some processors using more sugar in the product to achieve A_w of 0.97 or less to ensure consumer palatability while making a safe product.

Most smoked fish are not brined to achieve the required finished product WPS or A_w levels and must be dried in a smokehouse to achieve these levels. Scheduling all the critical factors of drying is required to ensure uniform moisture removal during the smoking/drying cycle. Removal of moisture is achieved with air flowing over the surface of each fish at speeds that will allow for the moisture to saturate the flowing air and be removed from the smokehouse.

Critical factors of drying include, but are not limited to:

• Volume of fish
• Surface area of fish
• Fat content of fish
• Thickness of fish
• Air temperature
• Rate of airflow (cubic feet per minute, or cfm)
• Input humidity levels
• Output humidity levels
• Smoke density in smokehouse
• Minimum WPS or maximum A_w before drying begins.

When a cubic foot of air at a defined humidity level (X) enters a smokehouse at a defined velocity (cfm) and exits the smokehouse at desired humidity level (Y), the volume of moisture existing between the difference of humidity level X and humidity level Y will be the volume of moisture being removed from the batch of fish per cfm of exiting air. This value is not a static number and can change depending on the speed of the air, how much moisture is left in the fish, etc.

Critical drying phases, called periods, are used to control the rate and amount of moisture removal:

• The constant rate drying period is where moisture is diffusing to the surface of the fish at a rate where a steady layer of moisture evaporates from the fish surface into the air flowing over the fish. As soon as a molecule of water reaches the surface, it is evaporated into the drier air, and another molecule of water comes to the surface to take its place. The Scottish Ministry of Agriculture, Fisheries, and Food notes in its Fish Handling and Processing guide, "The duration of the constant rate period of drying depends upon the rate of water loss. More water can be evaporated from fish during the constant rate drying period if drying is slow than if drying is rapid. This situation can be complicated, however, by the fat content of the fish. For samples of the same size under the same drying conditions, the constant rate drying period is shorter for fatty fish than for lean fish."⁶ When possible, prudent processors establish their smoking/drying process to reach their required WPS or A_w level during this drying period.
• The falling rate drying period is where moisture is diffusing to the surface of the fish at a rate where there is not a constant layer of moisture on the surface of the fish to be evaporated by air flowing over the fish surface. The Fish Handling and Processing guide notes, "During falling rate drying, after surface water has been evaporated, the rate of evaporation is controlled by the speed at which water can diffuse; that is, move to the surface from layers beneath. The speed of diffusion, and hence rate of drying, is governed by the nature of the fish, its thickness, and its temperature, becoming faster the higher the temperature."⁶ This drying period is the most difficult to remove moisture and must be carefully controlled if utilized to achieve the required WPS or A_w levels.

All smoked fish must form a pellicle for smoke to achieve uniform smoke deposition on the outer surfaces of the fish. Once formed, the pellicle will seal in C. botulinum spores and pathogenic bacteria residing on the surface of the fish. Phenolic compounds in wood smoke provide modest inhibition/inactivation of pathogenic bacteria and spores, which are crucial for safe smoked fish production. Applying smoke directly to those spores provides assurance that they will have an additional inhibitory barrier controlling NPCB toxin formation in the event of temperature abuse, regardless of whether the fish is hot or cold smoked.

It is essential that phenolic compounds from smoke are applied before, during, and after pellicle formation at predetermined phenol concentrations. To ensure that phenolic compounds from smoke are deposited under the pellicle (when not using a liquid smoke dip after rinsing), it is essential that brined fish are placed in the smokehouse without prior refrigerated drying, to ensure that phenolic compounds from the smoke adhere to the surface of the fish at a minimum concentration before the pellicle forms. Applying smoke during and after pellicle formation ensures the smoke will deposit phenolic compounds. This is why state smoked fish regulations, such as those of New York,² as well as AFDO's "Cured, Salted, and Smoked Fish Establishments Good Manufacturing Practicies"¹ guidance require both hot and cold smoked fish to have smoke applied during the first half of the smoking cycle when using generated smoke. Liquid smoke can also be applied in the smokehouse with an atomizer in lieu of generated smoke or in combination with generated smoke, but this process is no longer frequently used.

Table 1 shows WPS and A_w requirements for vacuum-packaged smoked fish finished products.

Cooling

Cooling smoked fish under structured cooling conditions is essential for control of C. botulinum, histamine, and other food safety hazards that were not eliminated at previous steps. Large batches of smoked fish do not cool rapidly. The maximum time to reach 40 °F/4.4 °C for cold smoked histamine-forming fish species must be controlled based upon the cumulative time above 40 °F/4.4 °C in all steps before, during, and after cooling. When histamine is not a hazard of concern, smoked fish must be cooled to a maximum temperature of 50 °F/10 °C within 5 hours of smoking and to 38 °F/3.3 °C within 12 hours of smoking.^1,2

Labeling

Packaging materials are generally labeled before smoked fish are placed in them. These labels must have a production code, indicate the instructions to "Keep Refrigerated at 38 °F/3.3 °C or less," all allergens listed as per the FDA Food Allergen Labeling and Consumer Protection Act (FALCPA),⁷ and any other relevant information related to the safety and identification of the product.

Vacuum Packaging

If a smoked fish product is to be vacuum packaged, it must be vacuum packaged under a well-structured GMP program to ensure the product is not re-contaminated with pathogenic bacteria and/or pathogenic spores. A final label review is required to ensure each package has a label attached.

Refrigerated Storage

Refrigerated storage is a vital last step to ensure the safety of smoked fish finished products. When WPS is less than 5 percent and/or A_w is above 0.97, smoked fish must be stored at 38 °F/3.3 °C because NPCB toxin formation has not been completely inhibited. When WPS is above 5 percent and/or A_w is below 0.97, storage at less than 40 °F/4.4 °C is effective for control of pathogens and histamine formation; yet, certain states and AFDO guidance require 38 °F/3.3 °C.^1,2

Smoked fish processing is an incredibly scientific art form that takes a team of well-trained and dedicated employees, led by a technical expert, to perform correctly on a consistent basis. Developing a comprehensive understanding of the science associated with smoked fish processing will set the foundation for anyone from the regulatory community, industry, or academia to pursue technical expertise in the field of smoked fish processing.

References

1. Association of Food and Drug Officials (AFDO). "Cured, Salted, and Smoked Fish Establishments Good Manufacturing Practices." 2019. https://www.afdo.org/wp-content/uploads/2020/11/Cured_salted_and_smoked_fish_establishments_good_manufacturing_practices_acc_updated_2019.pdf.
2. New York Codes, Rules, and Regulations. "NYCRR Part 262: Fish Processing and Smoking Establishments."
3. U.S. Food and Drug Administration (FDA). "Fish and Fishery Products Hazards and Controls." June 2022. https://www.fda.gov/food/seafood-guidance-documents-regulatory-information/fish-and-fishery-products-hazards-and-controls.
4. Association of Food and Drug Officials (AFDO). "Fish Evisceration Training Video." November 16, 2012. https://www.afdo.org/resources/fish-evisceration/.
5. Evans, Eugene E. "Surface Pathogen Elimination Practices for Ready-to-Eat Fishery Products." Journal of the Association of Food and Drug Officials 75, no. 1 (2015). https://www.afdo.org/wp-content/uploads/2021/02/Volume-75-%E2%80%93-No.-1.pdf.
6. Scotland Ministry of Agriculture, Fisheries, and Food. Fish Handling and Processing. 1982. Edinburgh, Scotland: Torry Research Station.
7. FDA. "Food Allergen Labeling and Consumer Protection Act of 2004 (FALCPA)." 2004. https://www.fda.gov/food/food-allergensgluten-free-guidance-documents-regulatory-information/food-allergen-labeling-and-consumer-protection-act-2004-falcpa.