| As a non-thermal processing technology,ultra-high pressure(≥100 MPa)has the advantages of green and high efficiency.Heat moisture treatment is a physical modification method which can modify the aggregation structure of starch.There are differences in the structure,properties and pressure-resistant of starch nanocrystals with different crystalline types,however,the transformation of starch nanocrystals and ultra high pressure-resistance melting properties of starches is not clear.In this paper,starch nanocrystals was prepared by waxy maize starch with enzymatic hydrolysis and recrystallization method.Then,heat moisture treatment was used to obtain waxy maize starch nanocrystals with different crystalline packing.Finally,starch nanocrystals with different crystalline packing were treated under the same ultra-high pressure condition.In order to clarify the effect of ultra-high pressure on the structure and physicochemical properties of waxy maize starch nanocrystals with different crystalline structure during ultra-high pressure process,modern electronic instruments were used to examine the surface morphology,long-and short-range ordered structure and thermal properties of starch nanocrystals with different crystalline types.The main research contents,results and conclusions are as follows:(1)Starch nanocrystals were prepared by enzymatic hydrolysis and recrystallization method.Using higher relative crystallinity as the evaluation standard,the preparation of starch nanocrystals was efficient under the enzymatic hydrolysis condition of pullulanase addition amount of 8npun/ml and enzymolysis time 12 h,which was based on 0.05 g/mL starch solution,the pH of phosphoric acid buffer at 5.0 and the temperature of water bath at 58℃.Starch nanocrystals showed B-type X-ray diffraction pattern,and the relative crystallinity was 38.3%.Compared with native starch,the diameter of the starch sample changed from 5-20 μm to 50-300 nm,and both of them were irregular ellipsoid with smooth surface.Compared with native starch,the relative crystallinity,molecular order,double helix content and power law exponent of starch nanocrystals decreased by 3.0%,0.034,0.63%and 0.33,respectively,while the amorphous content and single helix content increased by 1.9%and 1.51%,respectively.Compared with native starch,the onset,peak and conclusion temperatures of starch nanocrystals increased by 3.8℃,14.5℃ and 19.7℃,respectively,while the enthalpy of starch nanocrystals decreasd for 1.7 J/g.(2)Using heat moisture treatment to promote the conversion of starch nanocrystals.After single heat moisture treatment,the starch nanocrystals still presented B-type X-ray diffraction pattern.However,the crystalline structure of starch nanocrystals gradually shifted from B-type to A-type after repeated heat moisture treatment for 4 times at the conditin of 35%-110℃-5 h.The morphology of B-type nanocrystals particles gradually agglomerated from irregular globular to irregular polygons,and the surface smoothness decreased with increasing cycles ofheat moisture treatment.Compared with B-type starch nanocrystals,the relative crystallinity,molecular order,double helix content and power law exponent of A-type nanocrystals increased by 16.9%-24.4%,0.045-0.1,0.19-0.5 and 3.42%-5.05%,respectively,while the amorphous content decreasd by 0.86%-2.51%.Meanwhile,the onset,peak and conclusion temperature of A-type nanocrystals increased by 14.4℃-25.9℃,16.9℃-19.8℃ and 14.9℃-21.3℃ in contrast to B-type nanocrystals,respectively.(3)Using ultra high pressure to induce melting phase transition of different crystalline starch nanocrystals.With the increase of pressure(300-600 MPa),the surface morphology of A-and B-type starch nanocrystals solution(20%)showed agglomeration and melting phenomenon,the particle size difference increased,and the surface became rough.The crystallization types of B-and A-type starch nanocrystals did not change after ultra-high pressure treatment,but the decrease ratio of relative crystallinity,molecular order,double helix content and power law exponent of B-and A-type nanocrystals were 21.4%-40.4%and 11.5%-20.3%,0.7%-2.8%and 4.3%-6.9%,5.9%-31.1%and 4.1%-14.1,1.1%-4.1%and 5.9%-9.2%,and the increase ratio of amorphous content were 1.1%-5.5%and 3.6%-5.8%,respectively.With the increase of ultra-high pressure,the onset,peak and conclusion temperature of B-and A-type nanocrystals decreasd.The enthalpy of B-and A-type nanocrystals were 7.6 J/g and 8.6 J/g treated at 600 MPa,indicating that the melting phase transition of B-and A-type starch nanocrystals did not completely occur under such a pressure.The surface morphology of B-type starch nanocrystals particles treated by 600 MPa was gelatinous,while that of A-type starch nanocrystals was irregular polygonal.Under the same conditions,the effect of ultra-high pressure on the ordered structure of A-type was more obvious than that of B-type nanocrystals.After ultra-high pressure treatment,the relative crystallinity,molecular order,double helix content and power law exponent of A-type nanocrystals were 19.6%-21.1%,0.054-0.065,1.08%-1.68%and 0.52-0.82 higher than that of B-type starch nanocrystals under the same pressure,and amorphous content was 1.48%-5.00%lower than B-type starch nanocrystals.Under same pressure,the onset,peak and conclusion temperature of A-type starch nanocrystals were 25.6℃-27.2℃,18.1℃-20.6℃,17.7℃-18.5℃ higher than B-type starch nanocrystals,and the enthalpy was 0.1 J/g-1 J/g lower than B-type starch nanocrystals.These results suggested that the pressure resistance of A-type starch nanocrystals was better than B-type starch nanocrystals under same pressure,which may be related to the relatively high proportion of crystalline structure in A-type starch nanocrystals. |
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