| Common carp(Cyprinus carpio) was taken as raw material, from the view of novel protein ingredients development, mechenisim of fish protein isolating based on p H-shifting was explored comparing with conventional surimi methods. p H-shifting induced changes in the structure and properties of myosin, factors affecting protein isolation, functional properties of protein isolates, and digestion properties of protein isolates were investigated systematically.Firstly, distribuation of muscle proteins of common carp were analyzed according to protein solubility. The results showed that fractions of total nitrogen compounds of common carp muscle were as follows: salt-soluble protein 48.22%, and water-soluble protein 32.73%, alkali-soluble protein 13.44%, insoluble protein 3.58%, non protein nitrogen 6.49%.This is no surprising that salt-soluble protein were majority components of common carp muscle, nearly 50%. It is of interest to understand water-soluble proteins were nearly one third in the common carp.Secondly, p H-shifting was suitable for the recovery of common carp protein. Recovery of alkali-aided process(p H 12.0-13.0)was higher than acid-aided process(p H 1.5-2.5), maxium recovery was 87.56% and 76.30%, respectively. SDS-PAGE indicated that most of protein components of protein isolate were myofibrillar proteins and part of sarcoplasmic proteins.Thirdly, p H-shifting induced changes in the structure and properties of myosin from common carp were investigated. Results suggest that the solubility of myosin was obvious increased under the extreme acidic and alkaline condition, whether in 0.5 M KCl ionic strength medium or 0.05 M KCl ionic strength medium. And there was a drastic loss in Ca2+-ATPase activity when acid and alkali unfolding, which indicated the molecular structure in the globular head region led to significant conformational changes. Furthermore, even though a few recovery on Ca2+-ATPase activity on p H readjustment to neutrality, it had led to the irreversible change in the globular part of the myosin molecule.Subjecting myosin to either acidic p H or alkaline p H lead to no significant changes in the secondary structure of the protein compared to the native protein at p H 7.5,which indicated the helical structure of myosin rod may not be much affected at low and high p H. Furthermore, changes in the environment of myosin tryptophan residues showed the hydrophobic group of myosin was exposed and a increased surface hydrophobicity, decreased sulfhydryl content which suggesting the significant changes in the tertiary structure of myosin.In addition, myosin was cleaved into HMM and LMM by chymotrypsin digestion during extreme acidic and alkaline condition. These results also suggest that myosin takes on a distinctly different conformation in p H-shifting process.Finally, common carp protein isolates had the features of high protein and low-fat.Protein content were upon 80%. And protein isolate proteins have an advantage on digestion and absorption by using α-chymotrypsin digestion. Furthermore, functional properties of common carp protein isolates were investigated, including solubility, water holding capacity, oil holding capacity, emulsifying capacity, emulsifying stability, foaming capacity, foaming stability and gel properties. Results showed that common carp protein isolates had obvious advantage in water holding capacity, oil holding capacity, emulsifying capacity, emulsifying stability, foaming stability, and no advantage in foaming capacity. Textural characteristics of gels prepared from common carp acid- and alkali-aided protein isolates were investigated and compared to the surimi gels derived by conventional methods. The results showed the texture profile parameters(hardness, chewiness and gumminess) of gel samples prepared by acid-aided protein isolates was significantly(p<0.05) greater than those from alkali-aided protein isolates. However, there is no significantly different(p>0.05) on the springiness between Surimi gels and acid-and alkali-aided protein isolates. |
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