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7  Summary

Effect of evisceration on the sensory and hygienic condition of ice-stored pike-perch ( Sander lucioperca) and rainbow trout (Oncorhynchus mykiss)

In order to compare the hygienic and sensory quality of whole gutted and ungutted fresh water fishes, freshness and spoilage were monitored by measuring sensory and microbiological indicators. Gutted and ungutted rainbow trout (Oncorhynchus mykiss) and pike-perch (Sander lucioperca) were stored in melting ice until spoilage. Examinations were carried out every 2nd or 3rd day, or at the beginning and end of each storage period. Moreover, the content of TVB-N in muscle and the number of nematodes in the intestines and muscle were determined in the 195 pike-perches examined. Investigations were conducted from September 1999 until September 2001. In agreement with the EU quality grading scheme for marine fish (Council Regulation (EC) No. 2406/96 of 26 November 1996 laying down common marketing standards for certain fishery products) new grading schemes and more detailed sensory assessment sheets for cooked samples were developed for the two fresh water fish species.

The results have shown that, in most cases, there were no significant differences between gutted and ungutted fishes. Differences were found in the freshness categories only. After 7 days of storage, freshness of the ungutted trouts was assessed to be significantly below that of the gutted ones due to autolytic processes in their guts. Their shelf life was reduced by 1 to 2 days. Particularly the group of rainbow trouts fed shortly before slaughter and whose guts were filled was rejected by sensory assessment after 2 to 3 days of storage as a result of the disgusting organoleptic properties of their belly cavity. The ungutted pike-perches were rejected after 8, the gutted ones after 10 days. Between days 3 and 8/10 of storage, their freshness factors did not vary considerably. The examination of the cooked flesh samples did not reveal any significant sensory differences between gutted and ungutted fish for the entire storage period of 10 days.

On the basis of the sensory assessment, gutted and ungutted fish was rejected while the total viable aerobic count on the fish surface and in muscle was still below the [Seite 171↓]sensorily critical count of 106 to 107 cfu/g (colony forming units). From the microbiological point of view, early gutting of the fish is not required because the marketable part of the ungutted fish, the filet, did not show higher counts than that of the gutted fish. Furthermore, there was no massive contamination of the peritoneum by enteric pathogens in ungutted fish even when the decomposition of the guts through autolytic processes was at an advanced stage. The peritoneum of ungutted fish seemed to remain germ-free. In contrast, microbiological testing revealed that gutted fish had a significantly higher colony count on the peritoneum. Results have shown that the hygienic conditions prevailing during gutting and the quality of the water used for rinsing the abdominal cavity have a decisive influence on the contamination of the peritoneum. Therefore the fish should not be gutted in cases, where good hygienic conditions during slaughter and gutting cannot be guaranteed.

There was no evidence that gutting of fish would have an influence on the incidence of organisms potentially pathogenic to humans. Salmonella spp., Clostridia spp. as well as Vibrio spp. were not detected in trouts and pike-perch. The quantitative method for the enumeration of Aeromonas spp. on a selective medium did not produce clear results due to the presence of high numbers of concomitant microorganism. In the examinations conducted in winter 1999 almost all trouts were found to be contaminated with Listeria innocua. These organisms may have been carried over during the slaughter process and subsequent handling, since they were found to be present in the environment and on the tools used. In summer 2001, these findings were not confirmed since all trouts as well as the pike-perches were free from Listeria spp. Enterobacteriaceae were found to be present on the skin and in the intestines in small quantities only. The ISO/FDIS 5552 1997 method used for the detection of Enterobacteriaceae in pike-perch proved to be more sensitive for their identification in fish tissue than the § 35 method (LMBG) used in trout. The ISO method requires that findings of organisms suspected of belonging to the group of Enterobacteriaceae be confirmed using biochemical tests such as the oxidase reaction and glucose fermentation.

Larvae of the human pathogenic nematode Anisakis simplex were found to be present in the abdominal cavity of pike-perch which, in this respect, has so far been considered as safe. Some nematodes were found to stuck inside the belly flaps, [Seite 172↓]some were found even in the filets thus presenting a potential risk for the consumer, especially if fish is consumed raw like Sushi. In the German Fish Hygiene Regulations fresh water fishes are excluded from the examination for nematodes. However, with regard to fresh water fishes living in brackish water like perk-perch, an examination for parasites appears to be meaningful.

Measurement of the total viable basic nitrogen (TVB-N) in the muscle of pike-perch did not reveal any significant differences between gutted and ungutted fish. The content of TVB-N remained stable during the whole storage period with levels far below the limit of 30-35 mg/100g for marine fish.

It can be concluded from the results that, provided that optimal storage conditions are guaranteed, storage of ungutted fishes is to be recommended and preferred. Moreover, microbiological results do not support early gutting. Although late gutting results in a faster quality loss of the whole fish there is no increased health hazard for the consumer from the hygienic point of view.

In addition, the quality grading scheme for freshness proved to be suitable for use in gutted, ungutted and cooked rainbow trout and pike-perch samples in our examinations and can be recommended for further use.

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