Selection of Histological Parameters for the Development of an Analytical Method for Discriminating Fresh and Frozen/Thawed Common Octopus (Octopus vulgaris) and Preventing Frauds along the Seafood Chain

This study aimed at selecting effective histological indicators of the freezing process, for the discrimination of fresh and frozen common octopus Octopus vulgaris. Histological indices of freezing process were selected in mantle and arm muscle and axial nerve tissues. Seven histological parameters were chosen: overall muscle tissue structural organization (a); gaping among muscle bundles (b); presence of optically empty spaces between and within muscle bundles (c); white spaces percentage between and within muscle bundles (d); overall nerve structural organization (e); presence of linear fissures and/or empty spaces within neuropil and axonal tract (f); presence of empty spaces within connective tissue matrix surrounding the nerve (g); empty space percentage within arm axial nerve region (h). The parameters were assessed on 150 mantle muscle sections (a, b, c, d), 150 arm muscle (b, c) and nervous (e, f, g, h) tissue sections belonging to 20 fresh exemplars further subdued to conventional freezing procedure at − 20 °C, 25 fresh curled exemplars, 25 exemplars industrially frozen at − 80 °C, and 20 thawed, curled, and individually quick frozen exemplars. Overall structural organization (a, e), gaping (b), and presence of optically empty spaces (c, f) confirmed significant morphological freezing indices in mantle and arm sections. In mantle, two d values (d < 25% and 25% < d < 33%) were proposed as complementary thresholds to be applied in association with b and c indices for freezing discrimination. These parameters appear eligible to set a method for discriminating fresh/thawed octopus products to be applied both in official control and self-check activities.