| Porous starch is a functional biomaterial with a typical pore structure and is widely used as an adsorbent and retarder in the food,pharmaceutical,tissue engineering,agricultural and environmental industries.Porous starch can be obtained by physical,chemical,or enzymatic actions and their combinations.In recent decades,with the increasing demand for "clean label" ingredients in food products,the preparation of porous starch by some physical methods(extrusion,microwave,ultrasonic,and pulsed electric field)has gradually gained much attention.However,porous starch prepared by some traditionally used physical methods has many limitations,such as limited porosity,difficulty in industrialization,and poor stability of the granular structure.Although the typically used chemical or enzymatic methods can successfully prepare porous starch with ideal pore size.However,it easily causes damage to or reduces the thermal stability of starch granules.In terms of the practical application of porous starch,a certain degree of thermal stability and mechanical strength is critical.Therefore,in this study,normal maize starch(NMS)and waxy maize starch(WMS)were used as research materials and critical melting(based on the onset of starch from the ordered structure to disordered)combined with freeze-thawing("critical melting and freezethawing",CMFT)was used for porous starch preparation.In this work,our aim is to prepare porous starch using the comprehensive effects of hydrothermal energy and combining with different freezing rates and freeze-thawing cycles to effectively exploit the mechanical force of ice crystals for expanding the internal channels of starch granules,without the damage of the intact granule morphology of porous starch.In addition,the release of soluble starch chains and their rearrangement during the treatment can promote the formation of a more ordered double-helical structure to increase the thermal stability of porous starch granules.(1)Firstly,the possibility of CMFT treatment for porous starch preparation was investigated.Compared with native starch and freeze-thawing treatment alone(FT),CMFT showed more obvious grooves and deep holes in the starch granules and significantly improved the pore-forming characteristics.In which,CM combined with the slow freezing rate(-20℃)can prepare porous starch with grooves and cavities,while combined with the rapid freezing rate(-80℃)can be prepared with holes and channels,and these pore-forming features became more obvious with increasing FT cycles.Furthermore,the pore characteristics values(specific surface area,pore volume,and average pore size)and particle size of the CMFT-prepared porous starch granules increased significantly.It indicated that the porous starch can be successfully prepared by CMFT treatment.(2)Secondly,the effects of CMFT treatment on the physicochemical properties and multiscale structure of porous starch were investigated.①Physicochemical properties:compared with FT treatment.CMFT significantly increased the release of soluble starches,peak time(PT1),peak temperature(PT2).as well as the water/oil absorption of starch.In addition,the water solubility index(WSI),swelling power(SP),water holding capacity(WHC),light transmission(LT),peak viscosity(PV),and breakdown(BD)were significantly decreased.All of these changes became more significant with increasing FT cycles.It indicated that the porous starch prepared by CMFT has significantly improved the heat/shear properties,mechanical strength,and adsorption ability to favor its applications.②Multiscale structures:compared with native and FT,the relative crystallinity,single/double helix structure content,shortrange ordering degree,semicrystalline lamella thickness,and the thermal stability of CMFT-prepared porous starch were also increased significantly.All of the changes for CMFT-prepared porous starch indicated that the release and reassociation of soluble starch chains,as well as the contraction and rearrangement of the overall starch granules during the treatment,were critical for preparing the structural stabilized porous starch.(3)In addition,to further enhance the pore-forming properties of porous starch,the optimum technological parameters of CMFT-prepared porous starch and the porous starch prepared by the combination of CMFT with enzymatic hydrolysis(CMFT+EN)were investigated;then prepared the porous starch under the obtained optimal conditions,and finally investigated the pore-forming,physicochemical,and multiscale structure of porous starch.①Pore-forming properties:compared with enzymatic hydrolysis alone(OEN:the optimal enzymatic hydrolysis of native starch,EN:the optimal enzymatic conditions of CMFT and enzymatic hydrolysis)and FT+EN,porous starch prepared by CMFT+EN showed much denser with deeper pores on the surface,and channels with cavities inside.These changes in the surface and internal structure of starch made the porous starch has a higher specific surface area,pore volume,and average pore sizes.In which,the specific surface area of CMFT+EN-prepared porous starch granule was increased to about 23～26 times as compared with native starch,while EN-prepared increased only 5～6 times.②Physicochemical properties:CMFT+EN treatment reduced the PV,FV,and BD with significantly increased water,oil,and vitamin C absorption of starch.③Multiscale structures:CMFT+EN treatment did not change the crystallinity type of starch but significantly increased the relative crystallinity,the degree of ordered structure,the thickness of the semicrystalline layer,and the melting temperature(To、Tp、Tc)of starch.All of these changes indicated that the formed grooves and pores on the surface of granules after CMFT treatment can improve the enzyme sensitivity of starch and promote the enzymatic hydrolysis in the starch amorphous region,thus significantly improving the pore-forming properties and adsorption capacities.Overall,the CMFT+EN-prepared porous starch has more ordered structures against higher processing temperatures.(4)Finally,the effect of prepared porous starches used as an adsorbent on the improvement of the storage stability,solubility,and slow release of curcumin was investigated.Compared with OEN,EN,and FT+EN-prepared porous starches,the CMFT+EN significantly increased the encapsulation of curcumin to 86%～88%.In addition,CMFT+EN-prepared porous starch can adsorb more curcumin into the pore’s structures or adsorb on the outside surface,which can be absorbed by comparing the color of the encapsulated samples.moreover,the color after encapsulated samples of CMFT was the closest to the color of curcumin itself.In addition,the CMFT+ENprepared porous starch encapsulated with curcumin showed good dispersion stability in the aqueous phase,which can be used to protect some unstable substances under higher temperatures or long-term storage conditions.CMFT+EN-prepared porous starch encapsulated with curcumin significantly reduced the curcumin release rate to 22%～26%as compared with native starch encapsulated with curcumin.These results indicated that the porous starch prepared by CMFT+EN can be used as an adsorbent to effectively improve the encapsulation,storage stability,solubility,and slow release of curcumin. |
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