Quality Indicators of Northern Shrimp (Pandalus borealis)

Qingzhu2.3 Assessment methods of freshness and quality2.3.1 Sensory evaluationSensory evaluation is an important method for the assessment of freshness and quality,and is commonly used in the fish sector and fish inspection services (Martinsdottir 1997,Luten and Martinsdottir 1997). Sensory evaluation can be applied to all species of fishand laboratory facilities are not necessary. The evaluation is quick and non-destructiveunless the sample is being cooked, and moreover, the results often reflect the criteria theconsumer uses in evaluating acceptability (Connell 1990). Therefore, when chemical andphysical methods are being used for assessing the quality of fish, sensory evaluationshould be conducted to ensure that the results of the instrumental (objective) tests are inagreement with sensory analysis and thus indicating consumer perception (Alasalvar et al.2001). The quickest way, used by buyers and inspectors on the market, is to look at theappearance of the fish products, particularly the colour, luster of the shrimp. Thedisadvantages are that the evaluations of inspectors are difficult to standardize and theresults can be subject to the personal whims and biases of the assessors. However, mosttrade is based on sensory assessments, although measurements are not always objectiveand documented. The Quality Index Method (QIM), which as a method of sensoryevaluation, is a grading system based on adding demerit points for sensory attributes usedfor estimating the freshness and quality of seafood. The QIM has been demonstrated tobe rapid and more objective than sensory classification schemes often used by theindustry. QIM schemes have recently been developed for a number of fish speciesincluding: fresh herring, cod, red fish, Atlantic mackerel, mackerel, European sardine,brill, dab, haddock, pollock, sole, turbot, shrimp and farmed Atlantic salmon(Sveinsdottir et al. 2003).2.3.2 Chemical analysisSeveral chemical tests for freshness such as determination of amines, particularlytrimethylamine (TMA), and determination of hypoxanthine have been used for the pastdecades (Aitken et al. 1982). The former is related to bacterial activity while the latter isa measure of enzymic change. These two methods complement each other and havedifferent ranges of applicability and usefulness. A chemical test does not measurefreshness directly but the two are associated because the concentration of chemicalsmeasured is dependent on storage time and temperature, as freshness is.Trimethylamine, TMA, is formed in spoiling fish by the action of certain species ofbacteria on the substance trimethylamine oxide, TMAO. Therefore determination ofTMA content is a measure of bacterial activity and spoilage (Aitken et al. 1982). Increasein TMA during iced storage is similar to the increase in bacterial numbers. TMAO is notonly an important compound for maintenance of physiological functions in fish andshellfish but it is also a key substance in the spoilage of raw or processed seafood(Norman and Benjamin 2000). The TMAO content in the muscle of crustaceans is 9-28(mmole/kg wet weight) (Konosu and Yamagushi 1982).UNU-Fisheries Training Programme 8