Title: Quality Index Method (QIM) for frozen-thawed Atlantic mackerel (Scomber scombus) stored in ice: development and application in a shelf life study

The sensory methods are the most accurate and widely used to evaluate fish freshness and the Quality Index Method (QIM) is an objective, rapid and reliable sensory method. The aim of the present study was to develop and evaluate a QIM scheme for frozen-thawed Atlantic mackerel (Scomber scombrus) stored in ice. After thawing, the mackerel was stored at 0 °C on ice up to nine days. The QIM scheme and a vocabulary for generic descriptive analysis (DA) were developed during a pre-observation and panel training. During storage, changes of raw whole fish and cooked fillets were observed and analysed using the developed QIM scheme and DA, respectively. Moreover, the amount of histamine, total viable counts (TVC) and counts of H2S-producing bacteria were estimated. As a result of this study, a QIM scheme to evaluate freshness of frozen-thawed Atlantic mackerel storage in ice is proposed. A significant linear relationship between Quality Index (QI) and the storage time on ice was obtained. The maximum storage time on ice was four-six days according to DA of cooked fillets, mainly due to rancidity. The storage time could be estimated with an accuracy of ± 2.3 days using the QIM scheme. The TVC and H2S-producing bacteria increase during the storage time (nine days), but their values were low. The histamine content was below 5 ppm during the nine days of storage.