PHYSICOCHEMICAL CHARACTERIZATION OF DIFFERENT CHEMICALLY MODIFIED CORN STARCHES

LÓPEZ OV; GARCIA MA; ZARITZKY NE

Monterrey, California, USA

Congreso; Scanning 2007, The Eighteenth Annual International Scientific Meeting on Scanning Microscopies; 2007

Starch modifications, which involve the alteration of the physical and chemical characteristics of the native starches to improve their functional characteristics, can be used to tailor starch to specific food applications. The most common types of chemical starch modifications are: etherification, esterification, crosslinking, grafting and decomposition (acid or enzymatic hydrolysis and oxidization).  The introduction of different functional groups alters markedly the physicochemical properties of native starch. The aim of the present work was to evaluate the effect of chemical modifications of corn starch on the physicochemical, morphological, thermal and rheological properties. Native and modified corn starches (acetylated, acetylated-crosslinked, hydroxypropylated-crosslinked and acid modified) were used in this study. Amylose content was determined spectrophometrically with iodine. Morphological characterization of starch granules was performed by SEM using a JEOL 35 CF electron microscope. Rheological characterization of 4% starch suspensions was performed with a Rheo Stress 600 ThermoHaake (Haake, Germany) rheometer using a plate-plate sensor system PP35 at controlled temperature (25°C).  X ray diffraction analysis were performed in a X-ray diffractometer X’Pert Pro PANalytical Model PW3040/60, (Almelo, The Netherlands). Thermal behavior was evaluated by DSC in a Polymer Laboratories (Rheometric Scientific Surrey, UK) calorimeter using a PL-V5.41 software program. Modified starches exhibited lower values of amylose content, quantified by the spectrophotometric method, than native starch, being substituted and crosslinked starches more affected than acid modified one. Similar results were reported by Lawal and Adebowale. Chemical modifications led to a considerable change in the rheological properties of starches. With regard to the rotational assays, acid modified starch showed a Newtonian rheological curve while native corn starch, substituted and cross-linked starches exhibited a pseudoplastic slightly thixotropic behavior. Rheological data were fitted by the Ostwald de Waele model, in the cases of pseudoplastic behavior, and a linear model was used for the Newtonian equation. Starch substitution and cross-linking modified the molecular structure; this was evidenced by the decrease in the viscoelastic linear properties such as the elastic dynamic modulus (G´) plotted as a function of frequency (ω). SEM analysis showed that starch granule morphology changed due to the chemical modifications. In the case of hydroxypropylated starches, most of the structural changes were observed at the central core region of the granule in agreement with published results. Acetylation treatment also altered the granule morphology, although to a lesser extent. DSC analysis showed that chemical modification altered their thermal behavior. Compare with native starch, acid modification increased enthalpy and gelatinization temperature values while acetylation and dual modification decreased them. When substituents were introduced they interfere in the polymer chain associations thus weaker granules and fewer crystallites were obtained facilitating their fusion at lower temperature.  All starch granules exhibited a typical cereal (A) diffraction pattern with three high intensity peaks at 2θ: 15, 18 and 23 º. The reduction in the crystallinity degree observed in the case of modified starches was in agreement with the thermal analysis results .n conclusion, chemical starch modifications lead to changes in the rheological behavior, crystallinity degree and thermal properties, opening new applications avenues.Starch modifications, which involve the alteration of the physical and chemical characteristics of the native starches to improve their functional characteristics, can be used to tailor starch to specific food applications. The most common types of chemical starch modifications are: etherification, esterification, crosslinking, grafting and decomposition (acid or enzymatic hydrolysis and oxidization).  The introduction of different functional groups alters markedly the physicochemical properties of native starch. The aim of the present work was to evaluate the effect of chemical modifications of corn starch on the physicochemical, morphological, thermal and rheological properties. Native and modified corn starches (acetylated, acetylated-crosslinked, hydroxypropylated-crosslinked and acid modified) were used in this study. Amylose content was determined spectrophometrically with iodine. Morphological characterization of starch granules was performed by SEM using a JEOL 35 CF electron microscope. Rheological characterization of 4% starch suspensions was performed with a Rheo Stress 600 ThermoHaake (Haake, Germany) rheometer using a plate-plate sensor system PP35 at controlled temperature (25°C).  X ray diffraction analysis were performed in a X-ray diffractometer X’Pert Pro PANalytical Model PW3040/60, (Almelo, The Netherlands). Thermal behavior was evaluated by DSC in a Polymer Laboratories (Rheometric Scientific Surrey, UK) calorimeter using a PL-V5.41 software program.Modified starches exhibited lower values of amylose content, quantified by the spectrophotometric method, than native starch, being substituted and crosslinked starches more affected than acid modified one. Similar results were reported by Lawal and Adebowale. Chemical modifications led to a considerable change in the rheological properties of starches. With regard to the rotational assays, acid modified starch showed a Newtonian rheological curve while native corn starch, substituted and cross-linked starches exhibited a pseudoplastic slightly thixotropic behavior. Rheological data were fitted by the Ostwald de Waele model, in the cases of pseudoplastic behavior, and a linear model was used for the Newtonian equation. Starch substitution and cross-linking modified the molecular structure; this was evidenced by the decrease in the viscoelastic linear properties such as the elastic dynamic modulus (G´) plotted as a function of frequency (ω). SEM analysis showed that starch granule morphology changed due to the chemical modifications. In the case of hydroxypropylated starches, most of the structural changes were observed at the central core region of the granule in agreement with published results. Acetylation treatment also altered the granule morphology, although to a lesser extent.DSC analysis showed that chemical modification altered their thermal behavior. Compare with native starch, acid modification increased enthalpy and gelatinization temperature values while acetylation and dual modification decreased them. When substituents were introduced they interfere in the polymer chain associations thus weaker granules and fewer crystallites were obtained facilitating their fusion at lower temperature.  All starch granules exhibited a typical cereal (A) diffraction pattern with three high intensity peaks at 2θ: 15, 18 and 23 º. The reduction in the crystallinity degree observed in the case of modified starches was in agreement with the thermal analysis results .In conclusion, chemical starch modifications lead to changes in the rheological behavior, crystallinity degree and thermal properties, opening new applications avenues.