Chair of Microwave Engineering

SEQUID - SEafood QUality IDentification

A New Method for the Objective Measurement of the Quality of Seafoods


Zwei Studenten und eine Tasse Kaffee


Description of the project


1. Introduction
There exists no entirely satisfactory rapid physical method to determine the quality or freshness of fish and fish products. As an example, it is increasingly common in the international fish trade that products have been frozen at least twice. Frozen fish blocks produced from whole fresh fish onboard fish processing trawlers are thawed for further processing (filleting, skinning, trimming), then re-frozen as blocks of fish fillets. Thus, blocks of both single and double frozen fish fillets can be the raw material for the production of frozen battered and breaded products for example. The questions arising from this are:
  • Is the quality of frozen fish fillets influenced by re-freezing?
  • Is there a significant influence on the quality of the final product?
  • Can the processing industry differentiate the refrozen raw material from that which is frozen only once, by using the analytical methods currently available?

At the start of this project the answers to these questions were yes, yes and no! As a result of other studies performed during the last four years it can be stated that some quality differences could be measured which depend on both re-freezing and the rigor state of the raw material. However, no existing method was found to be accurate enough to be used for differentiating between single and double frozen fish fillets and their products.
Further problems relate to time and temperature of frozen storage and to the changes that occur during storage on ice of freshly caught/slaughtered fish.
2. State of the art
A concerted action on the evaluation of fish freshness in the AIR programme of the 3rd Framework of the European Community (AIR3 CT94 2283) reported much work in this general area. That project not only concerned itself with defining "quality" as opposed to "freshness" but also examined in detail the various methods available for its estimation. Amongst the methods considered were physical methods such as:
  • Time temperature indicators
  • Texture measurement
  • Electrical properties
  • Colour measurement
  • Near infrared spectroscopy

and sensory tests such as:
  • EU-scheme
  • Quality Index Method (QIM)

It was concluded amongst other things that, more effort has to be put into making reliable instrumentation for measuring freshness. The need for developing methods for measuring freshness is real, but for all mentioned techniques more research is needed.

It was also concluded that all physical methods need to be significantly improved to fulfil the expectations of the users. The new method being developed in SEQUID is such a physical method and lies in the domain of electrical properties.

A sensory test for freshness evaluation in the fish sector has been developed and is known as the Quality Index Measurement (QIM). This method has been adopted to some extent in the EU. The drawbacks with it are that it is rather time consuming, only skilled persons can use it and that to some extent, it is still a subjective method.

Multivariate analysis for combining results from several measurement techniques has been discussed as a potentially useful approach and a new project was begun under the 4th Framework FAIR programme (CT98 4076). This however was concerned with optimising the use of existing methods (mainly chemical and organoleptic) in a multivariate approach to the problem of determining fish freshness. It has no direct connection with SEQUID, which relates to the development of a completely new technique.

Another project in the area of fish quality is FAIR PL98-4174 dealing with "Fish Quality: Labelling and Monitoring". This project is mostly concerned with the existing methods and their ability to meet the demands of quality labelling.

Recently a preliminary test of a broadband microwave approach has been carried out in an entirely new approach to electrical measurement of quality. This work was in fact a spin-off from a FAIR project in the 4th Framework programme (CT97 3020). Although that project was principally concerned with the measurement of added water in foods, the methodology used was able to distinguish different aspects of quality in seafood products, notably whether raw materials had been frozen and how well they had been stored. These results were unique. It is therefore probable that the method proposed here might be capable of distinguishing between single and multiple frozen raw materials.

The new approach relied on measurement of the microwave dielectric properties of food materials as a function of frequency. The ability to store and dissipate electromagnetic energy is dependent on the magnetic and electrical properties of a substance. The dielectric properties are highly dependent on the nature and strength of molecular electric dipoles, for example water. Using an approach for analysis of microwave dielectric properties, (common in other fields such as IR spectroscopy, pattern recognition) multivariate measurements can be carried out to determine the condition of substances. This is based on the observation, that many of the characteristic properties of materials influence the frequency behaviour of the complex dielectric spectrum in subtle ways.

SEQUID therefore aims, by measuring and analysing different seafoods during different storage conditions, to produce data, which will be dependent on seafood quality. This is achieved by transforming the microwave complex dielectric spectra of samples, (and any other relevant variable such as temperature) using the method of Principal Component Analysis (PCA). This applies a linear transformation to a set of correlated variables to produce a new set of uncorrelated and standardised variables called "Principal Components" or scores. These PC scores can then be used in discriminant functions to allocate samples to particular groups. The original variables, which areusually the complex spectral values and the temperature, can also include any other variable believed to have an effect on the spectra. Thus it allows the addition of variables from other methodologies, physical or otherwise, and has the potential to be independent of prior treatment e.g. mechanical handling or other interfering factors.

As an example of the quality related results being obtained, different "qualities" of fresh, whole, gutted Baltic cod defined in this case as days kept on ice, were allocated to groups separated by intervals of two to three days by the method of discriminant analysis. The overall success rate was 100% for minced flesh. Using both PCR and PLSR to create a calibration showed that the number of days on ice of an unknown sample could be estimated to within ±1.5 days.
3. Innovation aspects of new approach
This project aims to provide a new means of quality assessment, using a physical method for measuring freshness and history of fish and fish products. The method will have potential to be non-destructive, instantaneous and with capability of being implemented on-line. It should go a long way towards satisfying the demands expressed in the above conclusions of the AIR project.



Project Partners


  • CAU
    Christian-Albrechts-Universität zu Kiel - Lehrstuhl für Hochfrequenztechnik
  • SIK
    The swedish institute for food and biotechnology
  • CSIC
    Instituto del frio consejo superior de investigaciones cientificas
    Insituto de investigação das pescas e do mar - departamento de inovação tecnológica e valorização dos produtos da pesca
    Bundesforschungsanstalt für Fischerei - Institut für Biochemie & Technologie



Dr.-Ing. Frank Daschner
Phone: 0431 / 880-6169

If interested in publications or other information materials for this project please contact the secretary or to the persons indicated above.