Study On The Effects Of Protein Crosslinking Induced By Freezing-thawing On In Vitro Digestion And Texture Properties Of Tofu

Protein crosslinking refers to the formation of covalent bonds between polypeptide chains within a protein(intramolecular crosslinks) or between proteins(intermolecular crosslinks). Protein crosslinking induced by freezing-thawing includes disulfide bonds formed by the oxidative coupling of two cysteine residues, isopeptide bonds formed via condensation of p henolic hydroxyl oxidation and the amide group and strengthened hydrophobic effect of covalent crosslinks. As a kind of traditional C hinese food, frozen tofu received growing popularity in Japan and Korea. In the freezing process, water turns into ice to expand in volume which squeezes the protein molecules, making protein intramolecular and intermolecular crosslinking. In the thawing process, the moisture loss makes tofu porous network structure. With increasing freezing-thawing cycles, the water content and driage decreased and protein content increased. Protein crosslinking could significantly enhance texture properties of tofu, promoting hardness, cohesiveness, gumminess, chewiness and resilience. However, the freezing-thawing process not only brings the tofu with yummy taste, but also some bad effects. It is shown that protein crosslinking like isopeptide bonds reduces the generation of amino acids, especially amino acids involving covalent crosslinks, because they could inhibit the further enzyme hydrolysis and the denser structure of protein which could reduce the action sites of enzyme hydrolysis. Furthermore, protein crosslinking influenced the secondary structures of protein including an increase in ?-sheets and random coil and an decrease of ?-helix and ?-turns, which could be used to deduce the possible model of protein intramolecular and intermolecular crosslinking. By scanning electron microscope(SEM), it is clear to show that the protein crosslinking increased markedly with increasing number of freezing-thawing cycles. SDS-PAGE analysis suggested that the distribution of proteins of the hydrolysate of fozen tofu became more concentrated compared with that of original tofu. With increasing freezing-thawing cycles, distribution showed no obvious change but the molecular weight distribution decreased. Especially, the peptides under 27 kDa increased with increasing freezing- thawing cycles. Besides, MALDI-TOF-MS analysis indicated that the structure of the peptide in the hydrolysate of fozen tofu became more complicated.