Aggregation of myofibrillar proteins in hake, sardine, and mixed minces during frozen storage

Abstract

The effect of frozen storage on the loss of extractability of natural actomyosin (NAM) with 0.6 M NaCl (P1) was studied in hake (Merluccius merluccius), sardine (Sardina pilchardus), and mixed minces in varying proportions stored for up to 1 year at -20 °C. The objective was to ascertain the types of bonds established among the proteins. The unextracted NAM was treated with different extracting agents to cleave secondary bonds (2% SDS, 2% SDS + 8 M urea) and secondary and disulfide bonds [2% SDS + 5% β-mercaptoethanol (ME)]. There was more NAM extracted from the mixes with higher percentages of sardine, although this effect was weaker than might have been expected from the percentages of sardine in the mixes. In hake, for up to 8 months of storage, the bulk of the proteins not extracted in 0.6 M NaCl was extracted with SDS + urea or SDS + ME, with extraction declining as storage progressed. This suggests that the amount of protein linked by covalent bonds increased with time. In sardine and mixed lots, a high percentage of proteins linked by covalent bonds was detected earlier, although no clear pattern emerged in terms of hake/ sardine ratios. SDS-PAGE showed that treatment with SDS + urea and SDS + ME extracted protein in the form of small aggregates that were retained in the stacking gel. These aggregates were also found in the SDS extracts from the sardine and mixed minces.