Histological discrimination of fresh and frozen/thawed fish meat: European hake (Merluccius merluccius) as a possible model for white meat fish species

The present study aimed at setting up a standard operating histological procedure to discriminate fresh from frozen-thawed fish products of the species Merluccius merluccius (European hake). A preliminary histological analysis of fresh M. merluccius muscle was performed to select the sampling site and highlight possible time-dependent tissue alterations during shelf-life. To set a suitable operational grid for discriminating the freezing process, morphological and morphometrical parameters were assessed on 90 muscle tissue samples collected from 30 fresh, 30 experimentally frozen at -20°31 C and 30 Individual Quick Frozen (IQF) specimens of M. merluccius. Structural score, presence of freezing vacuoles, a number of vacuoles per field higher than 1.12 and the presence of interstitial proteinaceous material, which had achieved statistical significance in group comparisons were chosen as freezing markers. Accuracy and repeatability, assessed on the analysis of two independent operators (on-training and expert), showed high analytical specificity and sensitivity and a concordant diagnostic performance regardless the operators expertise. The grid was finally validated by a single blind test on 30 additional M. merluccius commercial products and allowed the allocation of all the samples to fresh or frozen status without inconclusive results. The method could be profitably applied against fraudulent adulteration practices.