D CONTROL (aquaculture)

D CONTROL (aquaculture)

Summary

Significant hazards a r Algal toxins. r Bacterial and viral pathogens as for “fresh fish” (when consumed raw).

r Antibiotic residues. r Parasites (in some areas; not in reared salmon and other fish fed with dry

feeds).

Control measures

Initial level (H 0 )

r Avoid harvesting during/after algal blooms. r Avoid animal/human fertilizers. r Holding periods after antibiotic treatments.

Reduction (Σ R)

r Freezing. r Cooking.

Increase (Σ I)

r Time × −temperature.

Testing

r surveying pond water for fecal contamination.

Spoilage

r Time × −temperature. r Sensory evaluation.

a Under particular circumstances, other hazards may need to be considered.

Hazards to be considered. Aquaculture products differ in some ways from wild caught fish, but overall the same rationale in terms of identification of hazards as used for marine and freshwater fish applies. However, aquatic toxins like ciguatera is not a safety issue being associated with tropical reefs, whereas shellfish toxins can accumulate if algal blooms occur in the ponds or ocean areas where the net pens are placed. Parasites are not tranferred by dried fish feed and is only a potential risk if live fish (or fish offal) is used as feed. The same types of indigenous, potentially pathogenic, bacteria, as described for wild fish, will be present on reared fish. Enteric organisms may occur in higher numbers, particularly in areas where ponds are situated in fields with agricultural run off or if manure is used to fertilise the

211 ponds. L. monocytogenes is more common in fresh water ponds with agricultural run-off. Antibiotic

FISH AND FISH PRODUCTS

residues can occur and inappropriate use of antibiotics can lead to the selection of antibiotic-resistant bacteria.

Control measures Intial level of hazard (H 0 ). Algal shellfish toxins are controlled by limiting initial concentrations

(H 0 ) through surveillance of culture waters for blooms. Antibiotic residues can be avoided by allowing the animals a clearing period without treatment before slaughter. The exact time will depend on the size of the animal and the temperature of the water. Most legislation will provide guidance on time × temperature requirements for antibiotic clearance.

Reduction of hazard (Σ R). In cooked products, potential enteric pathogens and vibrios are con- trolled by the reduction step (ΣR) during cooking. Maintaining the microbiological quality of the rearing sites, limits the presence of pathogens. Use of animal or human excreta for enriching rearing-ponds should be discouraged unless subsequent controls insure that there is no increased risk of the prod- uct. This includes increased care to prevent cross-contamination of cooked (or otherwise processed) products produced from aquaculture sources known to have a high incidence of pathogenic enteric mi- croorganisms. Presence of low levels of Vibrio spp. – and from some areas also Salmonella spp. – is to

be expected and, as for indigenous bacteria, such as Cl. botulinum, processing procedures must ensure that these organisms do not become a hazard. Raw seafood does not differ from other raw commodities in that a low level of potential human pathogenic organisms are present and that the product should be handled accordingly.

Increase of hazard (Σ I). Controls for psychrotrophic spore formers in packed fresh fish are similar to wild fish, e.g. controlling increase (ΣI) during storage by temperature control.

Testing. Periodic sampling of pond waters or sediment for indicators of faecal pollution (e.g. En- terobacteriaceae, E. coli) or Vibrio spp. may be useful for assessing the classes (e.g. vibrios versus enterics) and relative extent of pathogens that are likely to be encountered with aquaculture products harvested from those waters (Leung et al., 1992). In freshwater ponds it may be useful to know if L. monocytogenes are present in large numbers so that further treatment and processing can be designed to reduce numbers and/or ensure that growth does not occur in the product.

Spoilage. Products from aquaculture do not differ from products caught from a wild population and time–temperature control is the most important controlling factor limiting increase (ΣI) of spoilage bacteria.

V Frozen raw seafood

A Freezing process Raw seafood may be frozen as whole fish or clams in the shell, eviscerated and butchered fish, tuna loins,

fillets, steaks or portions, shucked shellfish, etc. Some of these products are breaded prior to freezing. In most cases, seafood are frozen, unwrapped to facilitate rapid freezing, but for some purposes products may be packaged before freezing. Fish are frozen on-board fishing vessels, processing ships, and in land-based processing plants. Finfish destined for shaped products (e.g. fish sticks) are frozen in blocks and later cut with saws. All types of freezing systems are used for seafood including contact plate or

MICROORGANISMS IN FOODS 6

shelves, brine, and other direct contact refrigerant systems, continuous moving belt air freezing systems and passive air blast freezers as well as traditional sharp freezers (Pigoff and Tucker, 1990b). The rate of freezing is as rapid as possible, but with large whole fish such as tuna and shark freezing this may take several days. Frozen seafood is taken down to a temperature below −18 ◦

C and more commonly with modern practices to even lower temperatures. The faster the initial freezing, the lesser the damage to the protein fraction and the better is the product. Whole tuna destined for secondary processing are frozen on shipboard in brine tanks where the system cannot reach such low temperatures. Storage of frozen seafood is at −20 ◦

C or lower to maintain product quality. Fish frozen before rigor mortis are often held at −7 ◦

C for a few days to enhance quality.

B Saprophytes and spoilage Freezing halts bacterial growth and metabolism and the major cause of quality changes during frozen

storage is non-microbial changes in the protein and lipid fractions of the fish flesh. Bacterial counts on frozen products to some extend reflect the bacteriological quantity of the raw material and contamination or its removal during processing. Additional bacteria may be introduced as the result of batter and breading (Surkiewicz et al., 1967; Duran et al., 1983). The reduction in count resulting from freezing and storage in the frozen state is highly variable, and this makes the assessment of prefreezing quality difficult in some cases (DiGirolamo et al., 1970). The psychrotrophic bacteria in fish are not particularly resistant to freezing stress, but response is so strain specific that no general rule can be given. Freeze injury is generally more pronounced with Gram-negative bacteria than with Gram-positive species.

Spoilage microorganisms may grow in raw fish products if held too long before freezing, frozen at a grossly slow rate, thawed too slowly, or held thawed too long. No microorganisms grow below − 10 to −12 ◦

C. Because thawing without cooking is an inherently slow process, this step selects for a psychrotrophic population. Once thawed, the biochemical and microbiological changes are similar to those described earlier for raw refrigerated seafood (see Section III).

C, for example) are likely to support very slow mould growth. A few moulds, and possibly yeasts can grow in that range, whereas bacteria only grow at somewhat higher temperatures. While bacteria do not grow in frozen foods, they are able to varying degrees survive extended frozen storage. Frozen thawed fish spoil about as fast as

Seafood held at improperly elevated frozen storage temperatures (−10 to −5 ◦

fish that has never been frozen. One exception is CO 2 storage of frozen, thawed marine fish. CO 2 storage selects, as described above, for the CO 2 resistant spoilage bacterium Photobacterium phosphoreum (Dalgaard et al., 1993). However, this organism is freeze-sensitive and subsequent CO 2 storage of frozen-thawed fish may extend shelf-life dramatically (Bøknæs et al., 2000). Bacteria populations tend to decline over time during frozen storage, though the rate is highly dependent on bacterial species. For example, enterococci are highly resistant to extended frozen storage. In general Gram-negative bacteria die off more rapidly during frozen storage than Gram-positive. Spores are even more resistant.

C Pathogens As noted above, freezing will bring about a general reduction of the bacterial populations on seafood.

This is true for pathogens as well as psychrotrophic spoilage organisms. Generally Gram-negative pathogens such as Salmonella and other Enterobacteriaceae are sensitive to freezing injury and there is also some mortality of mesophilic vibrios. However, because products are generally maintained at refrigeration temperatures prior to freezing, it is unlikely that the extent of freeze injury will be great. In all cases there is measurable survival and the extent of actual mortality is highly variable. Spores are unaffected by freezing and vegetative cells of Gram-positive bacteria including Staphylococcus and

213 Listeria usually survive well. During storage of frozen seafood, there is a continued die off of vegetative

FISH AND FISH PRODUCTS

bacteria at rates corresponding to the specific species’ sensitivity and the temperature regime in the storage chamber.

Freezing followed by frozen storage will normally destroy all fish parasites dangerous to man. This procedure is recommended for raw products to be eaten as sushi. Freezing does not affect marine toxins accumulated in the living animal nor bacterial toxins produced during improper storage before freezing.

Survival of bacteria in seafood during frozen storage can have importance for infective organisms such as Salmonella, Shigella, Listeria, V. cholerae, and other Vibrio species since these may be transmitted without further growth and infectivity is dose related. Most reports suggest that V. cholerae tends to be reduced to very low levels after about 3–6 weeks of storage, but V. parahaemolyticus can persist for several months (Johnson and Liston, 1973).

It is important to note again that although freezing halts the production of histidine decarboxylase by bacteria, any pre-formed enzyme continues to be active. This can result in significant elevation of histamine levels during long term frozen storage, particularly when freezer temperatures are too high. The enzyme activity observed would be sufficient to raise the histamine levels to above the spoilage limit of 5 ppm or even the danger level of 50 ppm.