Freezing-induced myofibrillar protein denaturation: Role of pH change and freezing rate

The purpose of this study was to investigate the role of pH change and freezing rate on myofibrillar protein (MFP) denaturation during freezing. To demonstrate that the pH change was a cause of freezing-induced MFP denaturation, the sodium phosphate buffer was added to the MFP solution utilizing selective crystallization of Na2HPO4•12H2O that resulted in a pH decrease. Freezing MFP solution from 4 to −20 °C caused pH decreases of ~2.80 and ~3.91 units when the buffer concentration was 20 mM and 200 mM, respectively. The pH change disrupted the secondary and tertiary structures of MFP. As the pH decreased, the unfolding of protein molecules progressively increased. The exposure of internal hydrophobic groups resulted in aggregation and larger size formation of MFP. Compared to slow freezing, part of the protons was trapped in the small ice crystals formed by fast freezing, which attenuated the pH shift (decrease of ~3.26 unit) and reduced protein denaturation. These results suggested that pH change was an important factor for freezing-induced MFP denaturation, and the increase of freezing rate could diminish the pH decrease, which was conducive to maintain the stability of MFP.