Chapter 3: Cooked Fish and Fishery ProductsUpdated: 08/01/09
Pathogen survival through a cook step can cause consumer illness. Cooking is a relatively severe heat treatment, usually performed before the product is placed in the finished product container. Cooking procedures are often established to develop the desirable sensory attributes characteristic of cooked fish and fishery products, not specifically to eliminate pathogens. An important consequence of thorough cooking is the destruction of vegetative cells of pathogens (or reduction to an acceptable level) that may have been introduced in the process by the raw materials or by processing that occurs before the cook step. Cooking processes are not usually designed to eliminate spores of pathogens. (FDA, 2001; Rippen 1998).
Generally, after cooking, fishery products are referred to as cooked, ready-to-eat. Examples of cooked, ready-to-eat products are: crab meat, lobster meat, crayfish meat, cooked shrimp, surimi-based analog products, seafood salads, seafood soups and sauces and hot-smoked fish (FDA, 2001).
Undercooking may allow the survival of pathogens leading to several unintentional but potentially hazardous conditions: 1) direct contamination of a ready-to-eat product with pathogens, 2) elimination of other less heat resistant microorganisms that, if present, may suppress pathogen growth or cause spoilage prior to significant pathogen growth, and 3) thermal conditioning of pathogens and increasing their heat resistance to any subsequent cooking or reheating step. It is also possible for a sublethal heating step to trigger bacterial spores to germinate, producing vegetative cells that are more hazardous than spores, but also far more vulnerable to subsequent reheating (Rippen, 1998).
One of the purposes of cooking products that will be aerobically packaged is to eliminate vegetative cells of pathogens (or reduce them to an acceptable level) that may have been introduced to the process by the raw materials or by processing that occurs before the cook step. Selection of the target pathogen is critical. Generally, Listeria monocytogenes is selected, because it is regarded as the most heat tolerant, food-borne pathogen that does not form spores. Cooking processes are not usually designed to eliminate spores of pathogens. Determining the degree of destruction of the target pathogen is also critical. Generally, a reduction of six orders of magnitude (six logarithms) in the level of contamination is suitable. This is called a "6D" process. FDA's draft L. monocytogenes risk assessment indicates that approximately 7% of raw fish are contaminated with from 1 to 103 CFU/g, and that approximately 92% are contaminated at less than 1 CFU/g. Less than 1% of raw fish are contaminated at levels greater than 103 CFU/g, and none at levels greater than 106 CFU/g. FDA's action level for L. monocytogenes in ready-to-eat products, nondetectable, corresponds to a level of less than 1 CFU/25g (FDA, 2001).
Table A-3 provides 6D process times for a range of cooking temperatures, with L. monocytogenes as the target pathogen. Lower degrees of destruction may be acceptable if supported by a scientific study of the normal innoculum in the food. It is also possible that higher levels of destruction may be necessary in some foods, if there is an especially high normal innoculum (FDA, 2001).
When cooking is performed immediately before reduced oxygen packaging (e.g. vacuum packaging, modified atmosphere packaging), for product that will be marketed under refrigeration, it may be necessary for the cooking process to be sufficient to eliminate the spores of Clostridium botulinum type E and nonproteolytic types B and F. This is the case when the product does not contain other barriers that are sufficient to prevent growth and toxin formation by this pathogen (e.g. many refrigerated, vacuum packaged hot-filled soups and sauces). Generally, a 6D process is suitable. However, lower degrees of destruction may be acceptable if supported by a scientific study of the normal innoculum in the food. It is also possible that higher levels of destruction may be necessary in some foods, if there is an especially high normal innoculum. Table A-4 provides 6D process times for a range of cooking temperatures, with C. botulinum type B (the most heat resistant form of nonproteolytic C. botulinum) as the target pathogen. An example of a product that is properly cooked to eliminate nonproteolytic C. botulinum is a soup or sauce that is pasteurized at an internal temperature of 194°F (90°C) for at least 10 minutes. The lethal rates and process times provided in the table may not be sufficient for the destruction of nonproteolytic C. botulinum in soups or sauces containing dungeness crabmeat, because of the potential that naturally occuring substances, such as lysozyme, may enable the pathogen to more easily recover after damage (FDA, 2001).
Reduced oxygen packaged soups or sauces that are cooked immediately before packaging to control nonproteolytic C. botulinum, but not proteolytic C. botulinum, and that do not contain barriers to its growth, must be refrigerated or frozen to control proteolytic C. botulinum. Control of refrigeration is critical to the safety of these products (FDA, 2001).
Cooking processes that target nonproteolytic C. botulinum have much in common with pasteurization processes. Like products that are pasteurized in the final container, products that are cooked and then placed in the final container also are at risk for recontamination after they are placed in the finished product container. Controls, such as container seal integrity and protection from contamination by cooling water, are critical to the safety of these products. Additionally, because these products are cooked before they are packaged, they are at risk for recontamination between cooking and packaging. The risk of this recontamination must be minimized by filling the container in a continuous filling system while the product is still hot (i.e. hot filling), another critical step for the safety of these products. This control strategy is suitable for products that are filled directly from the cooking kettle, where the risk of recontamination is minimized. It is not ordinarily suitable for products such as crabmeat, lobster meat, or crayfish meat, or other products that are handled between cooking and filling (FDA, 2001).
Controlling pathogen survival through the cook step is accomplished by:
FDA’s recommendations for cooking fish and fishery products to destroy organisms of public health concern in food service, retail food stores, and food vending operations include:
Death of bacteria subjected to moist heat is logarithmic. A D-value (decimal reduction time) is the time required to kill 90% of the spores or vegetative cells of a given microorganism at a specific temperature in a specific medium. A 90% reduction in bacteria is equivalent to a reduction from 10,000 bacteria/g to 1,000 bacteria/g or 1 log cycle.
D-values can be determined from survivor curves when the log of population is plotted against time ( Figure 3-1), or by the formula:
Where T = time of heating, a = the initial number of microbial cells, and b = the number of surviving microbial cells after heating time T (Stumbo, 1965; Rippen et al., 1993).
For example, if a suspension containing 10,000 microbial cells/ml is heated for 4 min at 140ºF (60ºC) and only 293 microbial cells survive:
Harrison and Huang (1990) determined D-values for L. monocytogenes (Scott A) in crabmeat (Table 3-2).
Table 3-2. D-values for L. monocytogenes (Scott A) in blue crabmeat.
Where T 1 and T2 are temperatures and D1 and D2 are D-values at temperatures T1 and T2 (Rippen et al., 1993).
For example, using the D values (D122ºF = 40.43 min and D140ºF = 2.61 min) for L. monocytogenes:
z = 15.1ºF
Z-values are used to determine D-values at different temperatures using the formula:
Using the known D-values for L. monocytogenes (Scott A) in blue crabmeat and the z-value, D-values can be calculated for any given temperature. For example, substituting D and z values for Listeria in blue crabmeat (D140 = 2.61 min, z = 15.1ºF) from the Harrison and Huang (1990) study, the equivalent D-value at 185ºF is 0.16 s.
D185 = 0.0027 min or 0.16 s
D-values vary with product type and pubished D-values are rarely determined at the temperatures encountered during commercial processing.. Equivalent D-values should not be calculated for temperatures far hotter or cooler than those used in the original laboratory studies or errors may result due to the non-linerarity of some survivor curves (Rippen, 1998).
Adequate cooking processes are generally 6-D to 7-D processes at the geometric center of the thickest product or container being processed. Table 3.3 gives 1D-, 6-D, and 7-D-values for L. monocytogenes (Scott A) calculated from the Harrison and Huang (1990) study with blue crabmeat.
All cooking processes are product and equipment specific and must be evaluated independently. Any changes in the critical aspects of processes will effect the adequacy of the cook.
Conducting an in-plant process establishment study may result in a lower temperature process resulting in improved quality or yields. A process authority can usually identify alternative cook schedules that achieve equivalent pathogen kill (Rippen, 1998).
Table 3-3. 1-D and 7-D values for L. monocytogenes (Scott A) in blue crabmeat.
F-values and D-values are related in that a process F-value usually represents multiple D-values. If a research study determined that an organism’s D-value was 1 min at 185ºF (D185 = 1 min), then a process with a F185 = 10 min would achieve 10 decimal reductions for the target microorganism, or a 99.99999999 % kill (Rippen, 1998).
This curve can be divided into small sections, the F-value calculated for each, then added together. The formula for each time interval is:
where, T = the midpoint of two crabmeat temperatures over a period of time, 185 = the reference temperature and 16 = the z-value (a factor related to the sensitivity of bacteria to heat).
Using the formula, if the midpoint temperature of the crabmeat during a 5 minute period of heating was 175° F, then;
If the measured temperatures were 174° F and 5 minutes later, 176° F then the midpoint temperature for the time interval is 175° F – half way in between.)
Using data for crabmeat temperatures from a production trial;Elapsed Time Measured Temperature Midpoint Temperature Calculated F-value
0 152 --- ---
5 160 156 0.08
10 167 163 0.23
15 172 170 0.54
20 176 174 1.03
25 177 176 1.47
30 179 178 1.82
35 181 180 2.43
40 182 182 3.02
45 186 184 4.33
50 186 186 5.77
55 187 186 5.77
60 187 187 6.67
65 187 187 6.67
70 186 186 5.77
75 186 186 5.77
80 186 186 5.77
85 184 185 5.0
90 170 178 1.83
95 152 161 0.16
Total F = 64.13 min
F-value may be reported as F185 to identify 185° F as the reference temperature.
Notice in the example above that the F-value at 180° F is only half that at 185° F. In other words, crabmeat held for 10 minutes at 185° F would require more than 20 minutes at 180° F to achieve the same process.
An F-value based on one reference temperature can be converted to an equivalent F-value at another temperature by applying the formula:
Where, T1 = reference temperature, and T2
= another temperature for which you wish to know the equivalent process.
For example, if a process produces F185 = 30 minutes, the time needed at 170° F to achieve the same lethality is 260 minutes. That is:
TheoryAdequate cooking of crawfish eliminates proteolytic enzymes. Gelatin liquefaction is used to indicate the presence of proteolytic enzymes in crawfish after cooking.
Equipment and materials
Note: gelatin-raw fat mixture should have a texture = 1; plain gelatin should have a texture = 4, and should serve as a reference when checking for lack of enzyme activity
*Minimum cook time required to eliminate enzymes.
Cooked Dungeness crab sections IRemove crab carapace cleanly. Cut the crabs in half. Brush off intestinal content as completely as possible. Wash and rinse the halves in running water or with spray. Cook the crab sections at 100ºC (212ºF) for at least 15 min (Lee and Hilderbrand, 1992).
Cooked Dungeness crab sections IIButcher live crab by holding the crab by the legs on each side while bringing the belly down shapely on a knife edge. Shake to jar out viscera that cling to the body cavities. Clean off gills and remaining viscera with revolving nylon brush. Cook crab sections in unsalted boiling water for 10-12 min (Babbitt, 1981).
Cooked whole Dungeness crabFor crab planned for fresh sales, cook whole crab in salted boiling water for 20-25 min. Begin timing cook when water returns to a boil after crab have been added. Use 4-5% salt (16-20º salimeter) in the cook water. (Babbitt, 1981).
Cooked whole Dungeness crabLive male Dungeness crabs, weighing about 2 pounds (907 g) each, were cooked using either a minimal input of steam (less than 212ºF [100ºC]) or an excess input of steam. After a 23 min cooking period, crabs processes with a minimal input of steam had an internal temperature of 60ºC (140ºF); crabs processed with an excess input of steam had an internal temperature of 77.8ºC (172ºF). All parts of crabs cooked with excess steam input were cooked adequately (Barnett and Nelson, 1966).
Cooked lobster IHeat lobster for a period of time such that the thermal center of the product reaches a temperature adequate to coagulate the protein (FAO, 1978a).
Cooked lobster IIPlace lobsters in 1 layer on racks immersed in fresh boiling water. For whole lobster that will be frozen up to 1 month, cook 1 pound (454 g) lobsters 1-2 min and 1½ pound (680 g) lobsters 2-3 min. For lobsters that will be kept in frozen storage for 3 months or longer, cook 1 pound (454 g) lobsters 8-10 min and 1½ pound (680 g) lobsters 12-14 min. After cooking, cool lobsters in clean cold water for about 10 min, drain 5-10 min, and commence freezing within 1 h (Wojtowicz, 1974).
Cooked lobster IIILobsters (about 1¼ pounds [567 g]) were cooked in boiling water. With lobster 1, temperature changes were monitored from the time the lobster was placed in the boiling water. For lobsters, 2-4, temperature changes were monitored when the pot began to boil a second time (Table 3-1). Thermocouples were inserted into the lobster’s crusher claw by punching a small hole in the top of the claw, and inserted into the lobster’s tail through the first joint in the carapace. The researchers concluded that 12-15 min cooking time was sufficient to kill disease-causing bacteria (Bushway and Bayer, 1996).
Table 3-1. Temperature changes in lobster claw and tail muscle during cooking in boiling water.
Cooked Pacific shrimpControlled experiments with a pilot scale mechanical peeler gave 23.5% yields for untreated shrimp and 28.6% yields for shrimp treated with 1.5% condensed phosphate for 5 min prior to steam precooking. The shrimp were fed onto a steam pre-cooker no more than 1 body layer thick and cooked for 90 s in steam at 101ºC (213.8ºF) (Crawford, 1980).
Cooked shrimpBoil shrimp in potable water, clean sea water, or brine or heat in steam for a period of time sufficient for the thermal center of the shrimp to reach a temperature adequate to coagulate the protein (FAO, 1976; FAO, 1978b).
Inactivation of C. botulinum toxinCooking to an internal temperature of 79ºC (174.2ºF) for 20 min or to an internal temperature of 85ºC (185ºF) for 5 min inactivates any C. botulinum toxin at concentrations up to 105 LD50/g in foods (Woodburn et al., 1979).
Note: LD50 is an abbreviation for the dose (expressed in milligrams per kilogram of body weight of the test animal) that is lethal to 50 per cent of the group of test animals (Ali, 1995).
Ali, S. (Ed.). 1995. Pesticide toxicity, hazard and risk. http://itsd-s3.agric.gov.ab.ca/pests/pestcide/toxicity.html#LD50 (25 June, 1998).
Babbitt, J.K. 1981. Improving the Quality of Commercially Processed Dungeness Crab. SG 65, Oregon State University, Extension Marine Advisory Program, Corvallis, OR.
Barnett and Nelson, 1966. Recent technological studies of Dungeness crab processing. Part 4 – Preliminary report on salt uptake and heat penetration in whole cooked crab. Fishery Industrial Research 3(3):13-16.
Bushway, A.A. and Bayer, R. 1996. Lobster processing temperature recordings needed for HACCP plans. MSG-E-96-11. Maine/New Hampshire Sea Grant College Program and the Lobster Institute, 5715 Coburn Hall #22, University of Maine, Orono, ME.
Crawford, D.L. 1980. Meat yield and shell removal functions of shrimp processing. Special Report 597, Oregon State University, Extension Marine Advisory Program, Seafoods Laboratory, Astoria, OR.
FAO. 1976. Recommended international standard for quick frozen shrimps and prawns. CAC/RS 92-1976. Joint FAO/WHO Food Standards Programme, Codex Alimentarius Commission, Food and Agriculture Organization of the United Nations, World Health Organization, Rome.
FAO. 1978a. Recommended international standard for quick frozen lobsters. CAC/RS 95-1978. Joint FAO/WHO Food Standards Programme, Codex Alimentarius Commission, Food and Agriculture Organization of the United Nations, World Health Organization, Rome.
FAO. 1978b. Recommended international code of practice for shrimps and prawns. CAC/RCP 17-1978. Joint FAO/WHO Food Standards Programme, Codex Alimentarius Commission, Food and Agriculture Organization of the United Nations, World Health Organization, Rome.
FDA. 1998. Pathogen survival through cooking. Ch. 16. In Fish and Fishery Products Hazards and Controls Guide, 2nd ed., p. 189-196. Department of Health and Human Services, Public Health Service, Food and Drug Administration, Center for Food Safety and Applied Nutrition, Office of Seafood, Washington, DC.
FDA. 1999a. Destruction of organisms of public health concern: Cooking (raw fish). p. 53. Section 3-401.11(A)(1). 1995. Food Code, United States Public Health Service, Food and Drug Administration, Washington, DC.
FDA. 1999b. Destruction of organisms of public health concern: Cooking (comminuted fish). p. 54. Section 3-401.11(A)(2). 1995. Food Code, United States Public Health Service, Food and Drug Administration, Washington, DC.
FDA. 1999c. Destruction of organisms of public health concern: Cooking (stuffed fish). p. 54. Section 3-401.11(A)(4). 1995. Food Code, United States Public Health Service, Food and Drug Administration, Washington, DC.
FDA. 2001. Pathogen survival through cooking. Ch. 16. In Fish and Fishery Products Hazards and Controls Guidance, 3rd ed., p. 209-218. Food and Drug Administration, Center for Food Safety and Applied Nutrition, Office of Seafood, Washington, DC.
Harrison M.A. and Huang, Y. 1990. Thermal death times for Listeria monocytogenes (Scott A) in crabmeat. J. Food Protect. 53:878-880.
Lee, J.S. and Hilderbrand, K.S. 1992. Hazard analysis & critical control point applications to the seafood industry. Oregon Sea Grant Publication ORESU-H-92-001. Oregon State University, Corvallis, OR.
Lind, J. 1965. Determination of activity of acid phosphatase in canned hams. Danish Meat Products Laboratory, The Royal Veterinary and Agriculture College, September 23, 1965.
Moody, M. 1999. The gelatin test. Louisiana State University, Baton Rouge, LA.
North Carolina. 1997. Handling, packing and shipping of crustacea meat. North Carolina Administrative Code, Title 15A, Department of Environment, Health and Natural Resources, Chapter 18, Environmental Health, Subchapter 18A - Sanitation, Section .0100 - (April, 1997).
Rippen, T.E. 1998. Personal communication. University of Maryland, Princess Anne, MD.
Rippen, T.E. 2002. Personal communication. University of Maryland, Princess Anne, MD.
Rippen, T.E., Hackney, C.R., Flick, G.J., Knobl, G.M., Ward, D.R., Martin, R.E., and Croonenberghs, R. 1993. Seafood Pasteurization and Minimal Processing Manual. Virginia Cooperative Extension Publication 600-061 (1993), Virginia Sea Grant Publication VSG 93-09, Virginia Polytechnic Institute and State University, Blacksburg, VA. 173 p.
Stumbo, C.R. 1965. Thermobacteriology in Food Processing. Academic Press, New York, NY.
USDA. 1993. Internal cooking temperature determination (ICT1-2). In FSIS Analytical Chemical Laboratory Guidebook. Chemistry Division, Food Safety and Inspection Service, U.S. Department of Agriculture, Washington, DC.
Woodburn, M.J., Somers, E., Rodriguez, J. and Shantz, E.J. 1979. Heat inactivation rates of botulinum toxins A, B, E, and F in some foods and buffers. J. Food Sci. 44: 1658-1661.
Wojtowicz, M.B. 1974. Information on production of whole, frozen lobsters. New Series Circular No. 74. Environment Canada, Fisheries and Marine Service, Halifax Laboratory, Halifax, Nova Scotia.