دانلود رایگان مقاله خواص فیزیکوشیمیایی و کریستالی هیدروترمال نشاسته ذرت

abstract

The changes in physicochemical properties of standard maize starch (SMS) by three hydrothermal treatments; DV-HMT (Direct Vapor-Heat Moisture Treatment), RP-HMT (Reduced Pressurized-Heat Moisture Treatment) and DIC (instantaneous controlled pressure drop) were investigated at different processing conditions; steam pressure (SP) varied from 1 to 3 bar during 20 min. Starch was steamed by direct contact, whose interest was to intensify the heat transfer phenomenon but also the water transfer. The physicochemical changes of SMS depended on process conditions and their extent followed this order: DIC > RP-HMT > DV-HMT. All treatments significantly increased gelatinization temperatures and decreased the enthalpies, confirmed by loss of granules birefringence. From 2 bar, the crystalline structure changed from A-type to Vh-type, revealing formation of amylose-lipid complexes during steaming. The results clearly showed that the particle size distribution depends on the melting extent of crystalline structure during treatment. At severe processing conditions the melted fraction increased and more complex aggregates of different sizes have been formed.

4. Conclusion

The physicochemical and structural properties of SMS depended on the type of process and steaming conditions. The intensity of modifications followed this order: DIC > RP-HMT > DV-HMT. The extent of the physicochemical changes is due to the presence of initial vacuum for RP-HMT and DIC processes, contributing to the intensification of heat and mass transfers compared to DV-HMT in which this step is not present. Moreover, at the end of DIC process, the mechanical action induced by the sudden decompression towards vacuum contributed to alter crystalline structure and weaken the starch granules.The results showed that the general effects on starch granules were the decrease of the relative cristallinity, the decrease of the enthalpy and the temperature range of gelatinization as well as an increase in the gelatinization temperatures. Therefore,the hydrothermaltreatments contributed to increase the thermal stability of starch granules by promoting the strengthening of the ties which making the structure more cohesive. From 2 bar, the decrease in relative cristallinity was accompanied by the change from A-type crystalline structure toVhtype, revealing the formation of amylose-lipid complexes during the steaming.The treatments modified the shape and size distribution of starch granules. The increasing of size was due both to the improvement of swelling capacity of granules but also, for the intense conditions, to the formation of aggregates of various sizes, favoured by the direct contact of particles and steam. The number of melted granules had a direct impact on the aggregate sizes. For a processing pressure lower than 2 bar, the melted fraction of granules was low, therefore the glued particles was limited, leading to the formation of small-diameter aggregates. Beyond 2 bar, the melting phenomenon of starch granules progressed and aggregates of large size appeared mainly for starch treated by DIC at 3 bar for which a total fusion of granules occurred.