8. Assessment of fish quality

Contents - Previous - Next

8.1 Sensory methods
8.2 Biochemical and chemical methods
8.3 Physical methods
8.4 Microbiological methods

Most often "quality" refers to the aesthetic appearance and freshness or degree of spoilage which the fish has undergone. It may also involve safety aspects such as being free from harmful bacteria, parasites or chemicals. It is important to remember that "quality" implies different things to different people and is a term which must be defined in association with an individual product type. For example, it is often thought that the best quality is found in fish which are consumed within the first few hours post mortem. However, very fresh fish which are in rigor mortis are difficult to fillet and skin and are often unsuitable for smoking. Thus, for the processor, slightly older fish which have passed through the rigor process are more desirable.

The methods for evaluation of fresh fish quality may be conveniently divided into two categories: sensory and instrumental. Since the consumer is the ultimate judge of quality, most chemical or instrumental methods must be correlated with sensory evaluation before being used in the laboratory. However, sensory methods must be performed scientifically under carefully controlled conditions so that the effects of test environment, personal bias, etc., may be reduced.

8.1 Sensory methods

Sensory evaluation is defined as the scientific discipline used to evoke, measure, analyze and interpret reactions to characteristics of food as perceived through the senses of sight, smell, taste, touch and hearing.

Most sensory characteristics can only be measured meaningfully by humans. However, advances are being made in the development of instruments that can measure individual quality changes.

Instruments capable of measuring parameters included in the sensory profile are the Instron, Bohlin Rheometer for measuring texture and other rheologic properties. Microscopic methods combined with image analysis are used to assess structural changes and "the artificial nose" to evaluate odour profile (Nanto et al., 1993).

Sensory process

In sensory analysis appearance, odour, flavour and texture are evaluated using the human senses. Scientifically, the process can be divided into three steps. Detection of a stimulus by the human sense organs; evaluation and interpretation by a mental process; and then the response of the assessor to the stimuli. Variations among individuals in the response of the same level of stimuli can vary and can contribute to a non-conclusive answer of the test. People can, for instance, differ widely in their response to colour (colour blindness) and also in their sensitivity to chemical stimuli. Some people cannot taste rancid flavour and some have a very low response to cold- storage flavour. It is very important to be aware of these differences when selecting and training judges for sensory analysis. Interpretation of the stimulus and response must be trained very carefully in order to receive objective responses which describe features of the fish being evaluated.

It is very easy to give an objective answer to the question: is the fish in rigor (completely stiff), but more training is needed if the assessor has to decide whether the fish is past-or pre-rigor. Subjective assessment, where the response is based on the assessor's preference for a product, can be applied in the fields like market research and product development where the reaction of the consumer is needed. Assessment in quality control must be objective.

Sensory methods

The analytical objective test used in quality control can be divided into two groups: discriminative tests and descriptive tests. Discriminative testing is used to determine if a difference exists between samples (triangle test, ranking test). Descriptive tests are used to determine the nature and intensity of the differences (profiling and quality tests). The subjective test is an affective test based on a measure of preference or acceptance.

Figure 8.1 Methods of sensory analysis
Discriminative test
Is there a difference?
- Triangle test
- Ranking
Descriptive test What is the difference or the absolute value and how big is it?
- Quality index method
- Structured scaling
- Profiling
Affective test
Is the difference of any significance?
- Market test

In the following, examples of discriminative and descriptive testing will be given. For further information concerning market testing, see Meilgaard et al. (1991).

Quality assessment of fresh fish

Quality Index Method

During the last 50 years many schemes have been developed for sensory analysis of raw fish. The first modern and <retailed method was developed by Torry Research Station (Shewan et al., 1953). The fundamental idea was that each quality parameter is independent of other parameters. Later, the assessment was modified by collecting a group of characteristic features to be expressed in a score. This gives a single numerical value to a broad range of characteristics. In Europe today, the most commonly used method for quality assessment in the inspection service in the fishing industry is the EU scheme, introduced in the council decision No. 103/76 January 1976 (Table 5.2). There are three quality levels in the EU scheme, E (Extra), A, B where E is the highest quality and below B is the level where fish is discarded for human consumption. The EU scheme is commonly accepted in the EU countries for sensor assessment. There is, however, still some discrepancy as the scheme does not take account of differences between species into account as it only uses general parameters. A suggestion for renewal of the EU scheme can be seen in Multilingual Guide to EU Freshness Grades for Fishery Products (Howgate et al., 1992), where special schemes for whitefish, dogfish, herring and mackerel are developed (Appendix E).

A new method, the Quality Index Method (QIM) originally developed by the Tasmanian Food Research unit (Bremner et al., 1985), is now used by the Lyngby Laboratory (Jonsdottir, 1992) for fresh and frozen cod, herring and saithe. In the Nordic countries and Europe it has also been developed for redfish, sardines and flounder.

Table 8.1 Quality assessment scheme used to identify the quality index demerit score (Larsen et al. 1992)

QIM is based on the significant sensory parameters for raw fish when using many parameters and a score system from 0 to 4 demerit points (Jonsdottir, 1992). QIM is using a practical rating system, in which the fish is inspected and the fitting demerit point is recorded. The scores for all the characteristics are then summed to give an overall sensory score, the so-called quality index. QIM gives scores of zero for very fresh fish while increasingly larger totals result as fish deteriorate. The description of evaluation of each parameter is written in a guideline. For example, 0 demerit point for the appearance of the skin on herring means very bright skin only experienced in freshly caught herring. The appearance of the skin in a later state of decay turns less bright and dull and gives 2 demerit points. Most of the parameters chosen are equal to many other schemes. After the literal description, the scores are ranked for each description for all the parameters, giving scores 0-1, 0-2, 0-3 or 0-4. Parameters with less importance are given lower scores. The individual scores never exceed 4, so no parameter can excessively unbalance the score. A scheme for herring is shown in table 8.1; it is emphazised that it is neccessary to develop a new scheme for every species (the scheme for cod is seen in Appendix D).

There is a linear correlation between the sensory quality expressed as a demerit score and storage life on ice, which makes it possible to predict remaining storage life on ice. The theoretical demerit curve has a fixed point at (0,0) and its maximum has to be fixed as the point where the fish has been rejected by sensory evaluation of, e.g., the cooked product (see under structured scaling) or otherwise determined as the maximum keeping time. Using cooked evaluation the two parallel sensory tests demand an experienced sensory panel even though this is only required while developing the scheme, and later on it will not be necessary to assess cooked fish in order to predict the remaining shelf life. QIM does not follow the traditionally accepted S-curve pattern for deterioration of chilled fish during storage (Figure 5.1). The aim is a straight line which makes it possible to distiguish between fish at the start of the plateau phase and fish near the end of the plateau phase (Figure 8.2).

Figure 8.2 Combination of sensory curves for raw S(T) and cooked fish

When a batch of fish in Figure 8.2 reaches a sum of demerit points of 10, the remaining keeping time in ice will be 5 days. To predict remaining shelf life, the theoretical curve can be converted as shown in Figure 8.3.

Figure 8.3 A curve to predict the storage time remaining for herring stored in ice or sea water at 0°C

A fish merchant may want to know how long his purchase will remain saleable if the fish are stored on ice immediately. A buyer at a fish market might be interested in the equivalent number of days on ice where the fish have been stored since they were caught, and thus how much marketable time on ice is left. These condition indicators can be extracted for a fish sample with a known of change in demerit points using the quality index method.

Structured Scaling

Descriptive testing can also be used for quality determination and shelf life studies applying a structured scaling method. Structured scaling gives the panelist an actual scale showing several degrees of intensity. A few detailed attributes are chosen often based on work from a fully trained descriptive panel. Descriptive words must be carefully selected, and panelists trained so that they agree with the terms. Objective terms should be preferred rather than subjective terms. If possible, standards are included at various points of the scale. This can easily be done with different concentrations of salt but might be more difficult with conditions such as degree of spoilage. The most simple method (Table 8.2) can be 1. No off-odour/flavour, 2. Slight off-odour/flavour and 3 Severe off-odour/flavour, where the limit of acceptability is between 2 and 3. This has been further developed to an integrated assessment of cooked fish fillet of lean and fatty fish (see example in Appendix E).

A 10-point scale is used as described under 5.1 Sensory changes, and an overall impression of odour, flavour and texture is evaluated in an integrated way. For statistics, t-test and analysis of variance can be used (see example in Appendix F).

Table 8.2 Evaluation of cooked fish

Quality assessment of fish products

Assessment of fishers products can both be performed as a discriminative test and as a descriptive test.

Triangle test

The most used discriminative test in sensory analysis of fish is the triangle test (ISO standard 4120 1983), which indicates whether or not a detectable difference exists between two samples. The assessors receive three coded samples, are told that two of the samples are identical and one is different, and are asked to identify the odd sample.

Analysis of results from the triangle test is done by comparing the number of correct identifications with the number you would expect to obtain by chance alone. In order to test this the statistical chart in Appendix A must be consulted. The number of correct identifications is compared to the number expected by use of a statistical table, e.g., if the number of responses is 12, there must be 9 correct responses to achieve a significant answer (1% level).

Triangle tests are useful in determining, e.g., if ingredient substitution gives a detectable difference in a product. Triangle tests are often used when selecting assessors to a taste panel.

The samples marked A and B can be presented in six different ways:

Equal numbers of the six possible combinations are prepared and served to the panel members. They must be served randomly, preferably as duplicates. The number of panel members should be no less than 12 (an example of a triangle test from the ISO standard can be seen in Appendix B).

Table 8.3 Example of score sheet: triangle test

Ranking

In a ranking exercise, a number of samples are presented to the taste panel. Their task is to arrange them in order according to the degree to which they exhibit some specified characteristics, e.g. downward concentration of salt. Usually ranking can be done more quickly and with less training than evaluation by other methods. Thus ranking is often used for preliminary screening. The method gives no individual differences among samples and it is not suited for sessions where many criteria have to be judged simultaneously.

Profiling

Descriptive testing can be very simple and used for assessment of a single attribute of texture, flavour and appearance. Methods of descriptive analysis which can be used to generate a complete description of the fish product have also been developed. An excellent way of describing a product can be done by using flavour profiling (Meilgaard et al., 1991). Quantitative Descriptive Analysis provides a detailed description of all flavour characteristics in a qualitative and quantitative way. The method can also be used for texture. The panel members are handed a broad selection of reference samples and use the samples for creating a terminology that describes the product.

In Lyngby a descriptive sensory analysis for fish oil using QDA has been developed. A trained panel of 16 judges is used. Descriptive terms such as paint, nutty, grassy, metallic are used for describing the oil on an intensity scale. A moderately oxidized fish oil is given fixed scores and used as a reference.

Table 8.4 Profile of fish oil

Advanced multivariate analysis is used for statistics and makes it possible to correlate single attributes to oxidative deterioration in the fish oil. The results can be reported in a "spider's web" (se Figure 8.5). The panel uses an intensity scale normally ranging from 0 to 9.

Profiling can be used for all kinds of fishery products, even for fresh fish when special attention is placed on a single attribute.

The results of QDA can be analyzed statistically using analysis of variance or multivariate analysis (O'Mahony, 1986).

Statistics

In any experiment including sensory analysis the experimental design (e.g., number of panel members, number of samples, time aspects, hypotheses to test) and statistical principles should be planned beforehand. Failure to do so may often lead to insufficient data and non-conclusive experiments. A guide to the most used statistical methods can be seen in Meilgaard et al. (1991). A panel used for descriptive testing shall preferably consist of no less than 8-10 persons, and it should be remembered that the test becomes statistically much stronger if it is done in duplicate. This can often be difficult using sensory analysis on small fish. Thus the experiment must include a sufficient number of samples to remove the sources of variability, and the testing must be properly randomized. For further information see O'Mahony (1986) and Smith (1989).

Figure 8.4 Flavour profiles of a fish oil after 2 weeks of storage at various temperatures (Rorbaek et al., 1993)

Training of assessors

Training of assessors for sensory evaluation is necessary in almost all sensory methods. The degree of training depends on the difficulty and complexity of the assessment. For example, for profiling a thorough training with presentation of a large range of samples is necessary in order to obtain proper definitions of the descriptors an equal use of the scoring system. The triangle test normally requires a minor degree of training.

Sensory quality control is often done by a few persons either at the fish market when buying fish or at quality inspection. The experience of these persons allows them to grade the fish. Starting as a fish inspector it is not necessary to know all the different methods of sensory assessment described in textbooks (Meilgaard et al., 1991), but some of the basic principles must be known. The assessor must be trained in basic tastes, the most common fish taste and must learn the difference between off- flavour and taints. This knowledge can be provided in a 2- day basic training course.

In bigger companies and for experimental work a further training of a sensory panel is necessary m order to have an objective panel. A laboratory panel must have 8-10 members, and the training and testing of panel members must be repeated regularly.

Facilities

The facilities required for sensory evaluation is described in textbooks on sensory evaluation.

The minimum requirement for evaluation is a preparation room and a room where the samples are served. The rooms should be well ventilated and provided with a good light (Howgate, 1994). There must be enough space on the tables for inspection of raw samples of fish.

Cooking and serving

The samples of fishery products should not be less than 50-100 g per person. Fillets can be served in loins and should be cooked to an internal temperature of 65°C. The samples should be kept warm when served, i.e., in insulated containers or on a hot plate. The fish can be heat treated by steaming in a water bath, packed as boiled-in-the- bag in a plastic pouche or in alufoil. An oven (microwave or steam-oven) can also be used for heat treatment. The fish can be packed in plastic or put on a small porcelain plate covered with alufoil. For cod loins (2,5x1,5x6cm) on a porcelain plate covered with alufoil the heating time in a steam-oven (convectomate) at 100°C must be 10 minutes. The samples should be coded before serving.

Continue

Contents - Previous - Next