Quality Index (QI) is an accurate method for measuring changes in chilled seafood over time and is readily applicable through the whole supply chain: from point of harvest; through transport; auction; distribution and sale. It provides a highly reliable basis for control of quality, management of product and business decisions on marketing pathways and opportunities.
It is a science based method that has developed and been refined through adaption from many applications and countries.
Potted history of development
A myriad of schemes have been developed over the years for the purpose of grading seafood and, ostensibly, for measuring the ‘quality’ and ‘freshness’ of product.
Individual companies and individual buyers have used their own schemes most of which were never formally recorded but passed on by practice and word of mouth. Most of these informal approaches did the job well enough, but the information was never in a form in which it could be passed on outside of the immediate circumstances and be recorded in ways that were both convenient and in which the results could be immediately understood. Few developers considered the characteristic that a scheme should possess beyond meeting the immediate use.
When a scheme is developed, consideration must be given to why it is required, what advantages it has over existing schemes, its accuracy and precision, whether it fits a need, what it will be used for, the circumstances in which it will be used, its robustness in different hands, its adaptability to changing circumstances, its potential in meeting future requirements, its communication value, its ease of use, its cost, its likelihood of adoption, its consistency with known theory and thus its predictive capacities.
The Torry Research Station in Aberdeen, Scotland was a leading technology institute that did consider many of these aspects and they developed a scheme which became widely adopted (Shewan et al. 1953) known as the Torry scheme. The main scheme was developed to deal mainly with cod and related gadoid whitefish but others were also developed for fatty fish.
The scheme was based on the results from their studies that during chilled storage fish went through a number of changes that could be broken down into discrete steps to which descending scores were given as the fish was stored from catch (score of 10) through to rotten (score 1) with 4 being about the limit at which it could reasonably be acceptable for human consumption. The major deficiencies in this scheme were that:
- considerable training by experts was required for the panellists,
- the schemes were not very applicable to the wide variety of orders and species used in commerce and as food by much of the Southern hemisphere and most of Asia,
- it is more logical to estimate demerit points as it is changes away from the ideal (recently caught fish) that are measured and observed, and
- there was no evidence that it gave the correct result when the temperature of the fish was not held constant at 0oC (this is very often the case) i.e. the scheme itself has temperature kinetics of change that are different from that of the fish itself and it may under- or over- estimate the score.
James and Olley 1971, at the CSIRO Tasmanian Food Research Unit (TFRU) plotted the rates of change versus the time taken for shark to spoil reported in the literature and discovered it was not a straight line, as had been commonly assumed, but that spoilage was slowed more than anticipated in the chill range near 0oC. Further reviews of the literature results for chemical, bacterial, organoleptic and physical spoilage of herring, cod, halibut, prawns, scampi, and beef resulted in the concept of the relative spoilage rate derived by dividing the rate of spoilage at any temperature by the rate at 0oC (Olley and Ratkowsky 1973a,b). The concept of relative spoilage rate meant that the storage period of seafoods with any known time-temperature history could be expressed on the common basis as equivalent number of days in ice, ‘icetime’, or ‘icedays’ .
Spoilage in chill stored seafoods is mainly caused by bacteria and attention turned to establishing their temperature response (Ratkowsky et al. 1982). Their response could be modelled using the square root form of the equation:
√r= 1+0.1t where r is the relative rate and t is degrees Celsius.
Or, more usefully as r = (1+0.1t)2 (Bremner, Olley and Vail 1987).
This equation was found to apply not only to spoilage by psychrotrophic bacteria but to degradation of ATP, to changes in myoglobin as well as to changes in bulk properties. This neat equation demonstrates that in comparison with 0o C, seafoods deteriorate twice as fast at 4oC, four times as fast at 10oC and about six times as fast at 16oC. This means that comparative storage lives can be expressed in terms of icedays. If a product is known to have a usable shelf-life of 12 icedays then at 4oC it will only have 6 days, at 100C it will have 3 days and at 16o C only 2 days. This vividly demonstrates the dramatic effects of temperature and the extreme importance of ensuring fish are chilled to a temperature of 0oC and maintained at it. The equation thus provides an excellent means of evaluating seafoods if temperature is measured along with storage period and as long as the units are traceable.
At the same time a scheme was developed by the TFRU which overcame the disadvantages of other schemes and which was designed to be capable of integrating time temperature effects.
The Quality Index Technique
Within the developed scheme individual species are assessed using the score sheets presented in the QI 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 QI 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 QI technique is that all the schemes are developed to provide a straightline relationship between the total QI score and the corresponding number of icedays.
The number of icedays since capture can then be determined by comparing the QI 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 QI 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 QI scheme approach can be found within the QI 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.