Modifications in proximate composition, fatty acids, amino acids and –SH groups content, as well as changes in solubility and nutritional quality of protein, were studied in fillets of sardine (Clupea pilchardus) that had been successively pan‐fried, frozen stored and reheated by two different means, namely conventional oven and microwave oven. Upon pan‐frying in olive oil the sardine absorbed C18: 1(n‐9) and C18: 2(n‐6) and lost saturated (SFA) and polyunsaturated fatty acids (PUFA); this loss continued upon reheating, and as a consequence the percentage of MUFA increased compared to the just‐fried sardine. A loss of water was observed during all processes. Upon frying there was a decrease of cyst(e)ine. Upon reheating by both microwave and conventional oven, methionine decreased; however, cyst(e)ine only decreased with the use of a conventional oven. A loss of –SH groups was recorded during frying and this phenomenon continued upon reheating. Biological value (BV) together with net protein utilisation (NPU) decreased upon both frying and reheating. © 1997 SCI.

Studies were carried out on the changes in solubility in SDS+β‐mercaptoethanol, the –SH group content, the amino acid and the fatty acid compositions, and nutritional quality of the muscle protein following freezing, storage at −20°C and conventional defrosting of sardines (Clupea pilchardus). During the processes, 8% of the total SH– group content was lost and there was a slight drop in solubility in SDS+β‐mercaptoethanol. The amino acids which dropped to the greatest extent were the S‐amino acids, followed by histidine, tyrosine, leucine, lysine and phenylalanin. The C22:6/C16:0 ratio dropped and this was considered to be related to oxidation phenomena. The digestibility of protein, biological value and net protein utilization (NPU) also dropped as a result of the processes studied.

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.