Effect Of Repeated/continuous Dry Heat Treatments On Structural And Physicochemical Properties Of Different Crystal Types Of Starches

Dry heat treatment(DHT)is a physical modification method that changes the properties of starch by heating starch at 120 ~oC to 200 ~oC under dry conditions(moisture content is less than 10%).At present,the effect of CDHT on the structure and physicochemical properties of starch and the mechanism of CDHT are not clear.According to previous studies,the modification of CDHT on starch is limited.Thus,the starch after dry heat treatment is dry heat treated again,and repeat this way,it is assumed that if the effect of this treatment on the structure and physicochemical properties of starch will be more significant.Compared with short-term CDHT,repeated dry heat treatment(RDHT)on starch can be said to be a new type of dry heat treatment method.It is expected to provide a new idea and new method for the full development and utilization of starch resources,and provide a theoretical basis for the physical modification on starch.In this study,three starches with different crystal types,including A-type(wheat starch,corn starch,sweet potato starch),B-type(potato starch)and C-type(mung bean starch)were used as materials,and were dried to a moisture content of<10%.The starches was heat treated at 130 ~oC.At the same time,the starch was taken out and cooled at room temperature after each dry heat treatment,and the starches were repeatly treated of 1 to 6times,respectively.In this paper,the effects of RDHT/CDHT methods on the structure properties and physicochemical properties of different crystalline starches were compared under the same dry heat treatment time.The relationship between the structure properties and physicochemical properties of different crystalline starches was discussed.The main conclusions are as follows:After RDHT/CDHT,the granular morphology of normal wheat starch,partial waxy wheat starch,potato starch,sweet potato starch and mung bean starch had no significant difference from that of the native starch.However,under RDHT/CDHT for a long time,the surface of a few holes appeared on the surface of waxy wheat starch granules,and the pores on the surface of normal corn and waxy corn starch granules increased with the increase of treatment time.The position,sharpness and brightness of the polarized cross of different crystalline starch granules did not change visually after RDHT/CDHT,and the structure and clarity of the growth rings,channels and the like of the starch granules did not change.After RDHT/CDHT,the molecular weight of normal wheat starch and partial waxy wheat starch increased gradually with the increase of repeated times and continuous treatment time,while the molecular weight of waxy wheat starch decreased overall;the molecular weight of normal corn starch also increased gradually,whereas that of waxy corn starch decreased;the molecular weights of sweet potato,potato and mung bean starch all increased first and then decreased.Under the same dry heat treatment time,the starch molecular weight of normal wheat,partial waxy wheat,normal corn,sweet potato and mung bean after RDHT were significantly lower than that of CDHT.For waxy wheat,waxy corn and potato starch,the molecular weight of starches after RDHT were slightly higher than that of CDHT.After RDHT/CDHT,the pasting properties and swelling power of starches decreased with the increase of repeated cycles and continuous treated time.Under the same dry heat treatment time,the pasting properties and swelling power of starches after RDHT were higher than those of CDHT.The solubility of starches showed an opposite change from the above.The transparency of A-type and C-type starches increased,and the transparency of A-type and C-type starch after RDHT were lower than that of CDHT.The transparency of B-type starch exhibited an opposite change from the above.Under the same hydrolysis time,the enzymatic hydrolysis rate of normal wheat starch,normal corn starch and potato starch decreased first and then increased.The enzymatic hydrolysis rate of partial waxy wheat starch,waxy corn starch,sweet potato starch and mung bean starch increased first and then decreased.The enzymatic hydrolysis rate of waxy wheat starch increased gradually.The content of RS+SDS of normal wheat starch and partial waxy wheat starch decreased gradually,the content of RS+SDS of waxy wheat starch,potato starch and mung bean starch increased first and then decreased,the content of RS+SDS of corn starch decreased first and then increased,and the content of RS+SDS of sweet potato starch increased gradually.After RDHT/CDHT,the solubility,gelatinization,digestion and enzymatic properties of different crystalline starches were significantly improved,which showed enhanced solubility,improved hot paste stability and cold paste stability.resistant starch and slow digested starch content increased.In addition,the effect of RDHT/CDHT on the characteristics of different crystalline starches is more obvious.The solubility,hot paste stability and cold paste stability of the RDHT starch can be better improved than the CDHT starch.