Methods used to assess fish quality
Several methods are used to determine the quality of fish. These can be classified into sensory and instrumental methods. The latter comprise chemical, physical and microbiological methods.
Sensory evaluation of fish quality is 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. Sensory methods have the advantage of being simple, cheap and rapid. However, they can be very subjective, as they are based on the assessment of individuals, their likes and dislikes. The subjectivity and bias can be reduced significantly by proper training and the use of proper descriptors and structured scaling. Also, advances are being made in the development of instruments capable of measuring parameters such as texture and other rheologic properties, and microscopic methods combined with image analysis are used to assess structural changes and "the artificial nose" is now used to evaluate odour profile.
Chemical methods rely on the measurement of metabolite(s) produced during fish storage or distribution to obtain a quantitative fish index. The most widely used chemical test is Total volatile bases (TVB), which measures the content of trimethylamine (TMA) + dimethylamine + ammonia + other basic nitrogenous compounds associated with seafood spoilage. Other tests target the separate measurement of TMA, DMA, nucleotide catabolites (known as the %K-value) or biogenic amines contents (BA). The K or "freshness" index gives an indication of fish freshness during the early stages after capture, whereas TMA, TVB or BA gives this indication at later stages when the bacterial spoilage starts. DMA is used to measure the quality of frozen fish. Also, oxidative rancidity is measured by evaluating the peroxide value (PV) during the early stages or the thiobarbituric acid-related substances (TBA-RS) during later stages. Fish authentication relies on electrophoretic or DNA sequencing methods. Chemical methods are rapid, quantitative and reproducible. However, no one test is capable by itself of providing a picture of the full spectrum of changes that take place in fish and lead to its spoilage.
Physical methods involve the measurement of fish pH, texture or electrical properties. These methods are rarely used because they are either not sufficiently reliable or require calibration depending on the fish species.
Microbiological examination of fish aims at evaluating hygienic quality of fish, including temperature abuse, and the possible presence of pathogenic microorganisms in the fish. They consist in the measurement of total aerobic bacteria also called total plate count (TPC), spoilage bacteria, and various pathogenic bacteria. They should be kept to the minimum as they are time consuming, costly and require technical skills. Several new rapid methods are being developed and marketed. They are based on immunological reactions (ELISA, monoclonal antibodies) or genetic engineering (PCR, DNA-probes).