The Quality Index Technique
Within the developed scheme individual species are assessed using the score sheets presented in the Quality Index Manual. These have a scientifically predetermined number of scoring parameters which ALL must be assessed for the system to work correctly. The scores for each individual parameter are then added to determine the total Quality Index score for the product being assessed.
The parameters are scored using demerit points, with a small scoring range usually 0, 1, 2 or 3. A sufficient number of parameters have been chosen, based on experimental results, to cover the most important indicator attributes for the particular species; not too many and not too few. In this way no particular attribute dominates the overall result and the choices between the scores are thus based on substantial differences.
The scores of each parameter are added to provide the total, which is the Quality Index itself. The fundamental principle of the Quality Index technique is that all the schemes are developed to provide a straightline relationship between the total Quality Index score and the corresponding number of icedays.
The number of icedays since capture can then be determined by comparing the Quality Index score attained with that species’ icedays graph. A horizontal line drawn across from the score will intersect the figure line, and a perpendicular line dropped from this intersection will cross the lower axis at a value for equivalent icedays. Thus, the total time-temperature history can be expressed in icedays, irrespective of actual temperatures and clock time elapsed. Note that the number of icedays is often not the same as actual clock time particularly if the product has spent time at a temperature above 0˚C at some point prior to the Quality Index assessment. By the same process if the user knows where the appropriate shelf-life for his usage is on the figure line, the remaining shelf-life in icedays can be readily calculated.
The parameters, and the scores, used in the schemes need to be tailored to each species, although there is a high degree of similarity in the final schemes. More detail on fish spoilage, icedays, shelf-life and the development of the Quality Index scheme approach can be found within the Quality Index Manual. There is also a comprehensive list of references on the subject, for those interested in understanding more about the scientific principles regarding fish spoilage that underpin the development of the quality index technique.