| Zein,the main corn protein,contains over 50% hydrophobic amino acids.Zein is insoluble in pure water and its usage was usually limited in food industry.Most efforts in enhancing the water-dipersibility of zein and improving its functional properties had concentrated on the structure modification using sodium dodecyl sulfate(SDS),alkali and heating treatments.However,the relationships between structural changes and functional properties improvement have not been explained clearly and the effect of solvent polarity change on the structure modification has been rarely studied.In this study,SDS or alkali combined with or without heating treatment was used to modify the structure of zein and ?/?-zein in different solvents(water and 70% ethanol aqueous solutions).The influence of solvent polarity on the structural characteristic and functional properties of modified zein was identified in system,and their relationships were determined via transmission electron microscopy(TEM),Circular dichroism(CD),dynamic light scattering(DLS)and High resolution-TEM.Fibril nucleus with higher activation energy was formed from whey protein concentration(WPC).Meanwhile,structural characteristic and functional properties of zein-fibril nucleaus colloidal particles which were made from zein and fibril nucleus by using a simple anti-solvent precipitation method were explored.The main results as follows:(1)The micromorphology and water-dispersibility of modified ?/?-zeinThe water-dispersibility improvement of modified ?/?-zein was correlated to the solvent polarity.Such as,in strong polarity solvent(water),the water-dispersibility of alkali or SDS combined with heating 10 h modified ?-zein increased 94.50% and 95.65% respectively compared with that of native ?-zein.While in weak polarity solvent(70% ethanol),the water-dispersibility of alkali or SDS combined with heating 10 h modified ?-zein increased 50.81% and 69.25% respectively in comparison with that of native ?-zein.These results suggested that strong polarity solvent was beneficial for the increase of water-dispersibility for alkali or SDS modified ?/?-zein.Moreover,in weak polarity solvent,large globular aggregates were formed from modified ?/?-zein,and the globular aggregates were the same as the globular aggregates formed from native ?/?-zein except their sizes.However,the globular aggregates were destroyed and irregular aggregates with small size were formed from modified ?/?-zein.(2)The amphiphilic property of modified ?/?-zeinThe amphiphilic property of alkali modified ?/?-zein in water was improved,while the amphiphilic property of SDS modified ?/?-zein in water was not improved.During the ethanol evaporation(ethanol concentration changing from 70% to 25%),alkali treated ?-zein in water was still high level dispersibility(> 95%),and the aggregates changed from globular shape to irregular morphology,and the particle size decreased from 353 nm to 237 nm;however,the dispersibility of the SDS treated in water ?-zein only changed from 43.61% to 67.89%,and the morphology changed from big aggregates(3326 nm)to small particle aggregates(295.40 nm).(3)The structural inversion ability of modified ?/?-zeinDuring the solvent polarity change,the relations between particle size and time for alkali or SDS modified ?-zein aggregates were fitted according to the kinetics formula of Wang and Padua.The hydrophilic constant K of alkali or SDS modified ?-zein were calculated by least square method,and they were 1.49×1013 and 3.22×10-5 respectively,indicating the hydrophilic ability of alkali treated ?-zein was higher than that of SDS treated ?-zein.The hydrophilic constant K of alkali or SDS modified ?-zein were 3.07×106 and 1.63×10-7 respectively.The properties difference of alkali and SDS treated ?/?-zein depended on the modification of structure.High recovery ability of ?-helix was obtained from alkali modified ?-zein.Such as,the ?-helix content of alkali modified ?-zein in water was only 8.06%,while,as solvent changing from water to 70% ethanol,the ?-helix content increased to 27.06% which was close to the ?-helix content(27.94%)of alkali modified ?-zein in 70% ethanol.On the contrary,the ?-helix content of SDS modified ?-zein in water was only 7.97%,while the ?-helix content further decreased to 2.45% as solvent changing from water to 70% ethanol,suggesting SDS modified ?-zein had poor recovery ability of ?-helix.Strong recovery ability of ?-helix may be responsible for the high structural inversion ability,which could also be proved from the results of high resolution transmission electron microscopy(HR-TEM).The ribbon re-curled into a ring or toroid for alkali modified ?-zein as the solvent changing from water to 70% ethanol.However,it was difficult for SDS modified ?-zein to recovery the ring or toroid structure from ribbon structure.(4)The propertyies of fibril nucleus-zein colloidsFibril nucleus which was formed during the formation of whey protein concentration(WPC)nano-fibril was special structure.Colloidal particles could be prepared from fibril nucleus and zein through anti-solvent precipatation,in which globular structure of zein aggregates was still found.But the water-dispersibility of colloids particles was greatly enhanced to 71.59%.Moreover,the gelation concentration of zein was dramatically decreased from 20%(w/v)to 12%(w/v)via the colloids particles.Rheological analysis results showed that the apparent viscosity,the elastic modulus(G?)and viscous modulus(G?)of the dispersions prepared by fibril nucleus-zein were higher than that of WPC-zein colloids dispersions.(5)Modified methods-properties-structure relationsBetter functional properties were found to be dependent on structural changes.Good water-dispersibility of ?/?-zein demanded lost its originally spherical structure and formed smaller irregular aggregates in which the ?-helix structure unfolded and lost.Nevertheless,well dipsersibility in ethanol aqueous solutions for ?/?-zein could be attributed to the formation of spherical particle structure in which the ?-helix structure was still contained.Different modification methods made a contribution to the improvement of structural inversion ability.It was easy for alkali modified ?/?-zein in water to recover or lost ?-helix structure as the solvent polarity changing,which endowed good amphiphilic properties.By contrary,it was difficult for SDS modified ?/?-zein to recover ?-helix structure,which made its amphiphilic properties weak.The functional properties of fibril nucleus-zein colloidal particle with intact globular particle structure were obviously different from that of alkali modified zein without globular particle structure.Fibril nucleus-zein colloidal particle had a potential advantage over alkali modified zein due to high apparent viscosity,elastic modulus(G?)and viscous modulus(G?)could be gained at low concentration(12%).On the other hand,the binding capacity,foam stability and emulsion stability of fibril nucleus modified zein were higher 65.21%?49.93% and 11.61% respectively than those of alkali modified zein.In conclusion,appropriate modified methods altered the structure of zein and improved their functional properties,thus broadened the applied scope and improved value of zein. |
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