Effect Of Ultra-high Pressure Technology On Key Enzymes And Structural Proteins In Silver Carp Processing

China is the largest producer of freshwater fish in the world, and silver carp(Hypophthalmichthys molitrix) is one of the main freshwater fish species. However, it issusceptible to lipid oxidation and the subsequent development of rancid and fishy odourduring processing and storage. It has been reported that Lipoxygenase (LOX) primarilyaccounting for peroxidation participated in enzymatic reaction and resulted in lipid oxidationand the rapid formation of fishy odour in silver carp processing. High hydrostatic pressure(HHP) processing is an alternative method for enzyme inactivation. Inactivation kinetics ofendogenous lipoxygenase in crude silver carpextract was studied for thermal (50–80°C,ambient pressure) and high hydrostatic pressure combined with heat treatment (300–700MPaat50–80°C) process conditions. LOX inactivation followed first-order kinetics at allpressure–temperature combinations used. Moreover, LOX inactivation rates became lesspressure dependent with increasing pressure at all temperatures studied and also showed lesstemperature dependence of the enzyme inactivation rate at various high pressure levels tested.It is evident that inactivation kinetics is complex affected by both pressure and temperature.Therefore, response surface methodology (RSM) was attempted to investigate combinedtemperature–pressure processing conditions on LOX inactivation kinetics in crude silver carpextract samples. In conclusion, the LOX inactivation rate constant was modelled as a functionof both temperature and pressure conditions by a quadratic polynomial equation as follows:k0.2780.3991E004X0.01X6.167E006X X3.608E007X29.525E005X2121212Silver carp is a popular cultured freshwater fish in China, and becomes a potential rawmaterial for surimi product. However, silver carp surimi exhibits considerable gel softening(modori) and myofibril-bound serine proteinases (MBSP) involved in the degradation ofmyofibrillar proteins have been assumed to be crucial to the modori phenomenon.High-pressure technology has demonstrated its potential application to control enzyme-relatedseafood texture deterioration. Herefrom, it could be further applied to the freshwater fishprocessing and the effect of high pressure treatments on the activity of MBSP implicated intexture deterioration in myofibrils and crude enzyme extracts was evaluated. The inactivationkinetics of MBSP pressured from200MPa to500MPa at room temperature (20°C) werefitted first-order kinetics, The extent of enzyme inactivation was lower in silver carpmyofibrils in comparison with the crude enzyme extracts from myofibrils showing aprotective effect against the high pressure treatment. The effect of HHP on MBSP myofibrillardegradation shown by sodium dodecyl sulphate-polyacrylamide gel electrophoresis(SDS-PAGE) was associated with a corresponding decrease coincided with the elevated highpressure levels in intensity of myosin heavy chain (MHC). Dynamic rheological measurements indicated that high-pressure treatment exerted considerable influence on MBSPactivities as well as proteins (mainly myofibrils), resulting in structural modifications andtexture changes. In this study,300MPa (?10min) was the process condition (pressure andtime) that caused apparent MBSP inactivation derived from the first order kinetic model andhad the efficacy in controlling silver carp texture deterioration both verified by SDS-PAGEand dynamic rheological measurements.Silver carp is an abundant protein resource and is widely used as a material for processedfoods. Myofibrillar proteins play an important role in the quality of fish products because ofits responsibility for imparting textural attributes to fish muscle and endowing functionalproperties of fish meat such as gel-forming ability, emulsifying property, and water-holdingability. Conformational changes of myofibrillar proteins have an enormous potential tomanufacture desirably processed fish meats with lower cost and more desirable nutritionalproperties. Modification of myofibrillar proteins induced by high pressure processing (HPP)has been investigated at pressures ranging from200to500MPa for10min at20°C. Influenceof high pressure on conformational changes of myofibrils was studied by means of differentialscanning calorimetry (DSC), surface hydrophobicity measurement, ultraviolet absorptionspectra and second-derivative spectroscopy, intrinsic spectrofluorimetry, Raman spectroscopicanalysis and circular dichroism (CD). DSC analyses revealed that the conformational stabilityof myofibrils is reduced to different extents when they are exposed to a pressure over200MPa. More sulfydryl groups as well as hydrophobic regions and amino acid residues whichhad ultraviolet absorbance had been found significantly after HPP (?300MPa). Changes inthe Raman spectra were interpreted as the occurrence of secondary structural changes inpressure-induced myofibrillar proteins. CD analysis results demonstrated that ?-helix wasdestroyed after processed at400MPa for10min. The results implicated that HPP can be usedas a possible means of improving the functional properties for growing demand for healthierfish products.The comparative study between HHP and thermal treatment on silver carp surimi gelproperties of silver carp surimi confirmed that pressured surimi products with smallerunpleasant fishy taste are better in gel strength, elasticity and so on than the traditionalthermal processed, and SEM showed that surimi pressured at300MPa for20min is morecompacted in reticular structure and refined in gel, which are consistent with theoreticalresearch. But in terms of hardness the pressured surimi gel is the hardness smaller than theheat treated, which is contradictory to our study.