Freezing/defrosting/frying of sardine fillets. Influence of slow and quick defrosting on protein quality

The effect of sequential freezing/defrosting/frying on protein quality is not well known. With this in mind, fillets of fresh sardine were stored frozen, then thawed, either conventionally at 4 °C in a refrigerator or with the use of a microwave oven, and subsequently deep-fried. Proximate and amino acid compositions, protein solubility in sodium dodecyl sulphate/β-mercaptoethanol (SDS/β-ME), total SH group content and amino acid chemical score were determined. The lowest protein concentration was observed in frozen/4 °C-thawed sardines (CR), whilst the lowest fat content was found in both fresh/fried sardines (F) and 4 °C-thawed/fried sardines (CF). Every step of each process studied caused a decrease in cyst(e)ine; the most important loss was recorded in CF samples and in frozen sardines fried without defrosting (Fro-F). The lowest solubility in SDS/β-ME and the lowest total SH group content were observed for Fro-F samples and microwave-thawed/fried sardines (MF). On the other hand, the lowest chemical score was found for Fro-F, CF and MF samples. Although weight loss and proximate composition seemed to change less when defrosting sardine fillets using a microwave oven rather than at 4 °C, the results for SDS/β-ME solubility and total SH group content suggest that a slow defrosting process (refrigerator at 4 °C) is preferable to a much quicker process (microwave oven) for thawing frozen sardine fillets before frying. Copyright © 2003 Society of Chemical Industry