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Advanced methods for identification and quality monitoring of (heat) processed fish


The aims of the project are:

- development and evaluation of methods for identification of species in heated fishery products.
- testing the suitability of image analysis for interpretation and comparison of electrophoresis gels.
- development of a data base containing physical parameters (isoelectric point and/or molecular weight) of proteins for fish species identification. This reference data base will contain data for raw and heated fish and products.
- evaluation of electrophoretic methods to monitor processing parameters of fishery products.

The project will help:
- to improve quality monitoring and quality assurance for fish and fishery products;
- to reduce health risks to consumers
(i) by detecting allergy inducing fish species
(ii) by establishing heat treatment efficiency to destroy nematodes;
- to improve control specifications of products;
- to prevent fraud;
- to control correct customs tariff rates;
- to formulate regulations for labelling composite fishery products;
- to optimise existing process technologies by determination of the time-temperature profile in the product during heat processing;
- to improve the exchange of information among research institutes, food inspection laboratories, customs control laboratories, and industry being engaged in the identification of fish species;
- to harmonise and standardise methods for analysis of fish species authenticity

In the case of raw fishery products the identification of species by protein electrophoresis is now a well-established procedure (Sotelo et al., 1993). The suitability and reliability of isoelectric focusing (IEF) of sarcoplasmic proteins for fish speciation has been demonstrated in the EU project UP 3.783 "Identification and quantitation of species in marine products" (Rehbein et al., 1995).
For heated fish, however, the situation is different. The conditions of processing (cooking, frying, smoking) may vary considerably, resulting in more or less pronounced denaturation and degradation of muscle proteins. Techniques for determination of species in heated products have been described (Rehbein, 1990; Mackie, 1994), but they were applied only for a few species and products.
Two different routes have been followed for analysis of heated products:
(i) The denatured proteins are solubilized by means of urea and/or SDS in the presence of a thiol compound like 2-mercaptoethanol. Dissolved proteins are separated by IEF (in gels containing urea) or by SDS-PAGE.
(ii) Only a few heat-stable proteins are used for species identification.

Parvalbumins may be suitable for many fish species (Rehbein, 1992a); some myofibrillar proteins like the myosin light chains and tropomyosin, which are very resistant against denaturation by heat, have been used also for species identification.
It depends on the species of fish or shellfish and on the type of product, which of the two ways outlined above will give better results. For example, in the case of products made from surimi, analysis of structural proteins may be the method of choice, because the concentration of water soluble proteins is very low in these products due to the extensive washing procedures. Prefried fish sticks, on the other side, may be successfully analysed by IEF of parvalbumins.
The application of chemometric methods in the analysis of seafoods is steadily increasing (Tennyson, 1995). Image analysis systems for evaluation of protein patterns obtained by IEF or SDS-PAGE are commercially available. The suitability of these systems for fish species identification by means of data bases, which had to be established at first, has not been published up to now.
In the last years alternative techniques to electrophoresis in gels have been proposed for protein analysis. Capillary electrophoresis (CE) and high performance liquid chromatography (HPLC) have been applied to fish species identification (Sotelo et al., 1993), but the power of discrimination of closely related fish species, and the reproducibility among laboratories have not been tested. At the moment the widespread applicability of CE and HPLC in food control is hampered by the high capital investment necessary, and the great expense of time and labour for running large numbers of samples.
Some techniques of DNA analysis as hybridisation methods, polymerase chain reaction, and others, may be useful for species identification of heated products (Leighton Jones, 1991), but can be performed at the moment only in a few specialised laboratories.
Knowledge of the temperature, which has occurred in the interior of fishery products during heating, is important in several respects:
(i) One must be sure that the product has experienced temperatures high enough to kill pathogenic organisms like nematode larvae.
(ii) The quality of products depends on the temperature regime during processing: Protein analysis of different parts of a product may offer a direct measure of process control.
(iii) The customs tariffs for imported blanched or cooked fishery products are different in Europe.
Food control and customs laboratories need methods for subsequent determination of the heating temperature of fishery products. It has been shown previously for a few species that the same technique, which is used for fish species identification (IEF of sarcoplasmic proteins), can be used for this purpose (Rehbein, 1992b).

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