Influence of copper catalyst on the mechanism of carbohydrate radicals generation in oxidized potato starch

Abstract

Using EPR technique it was stated that copper ions introduced into potato starch, as catalyst of its oxidation with H2O2, was the active reagent in the generation of stable carbohydrate radicals. EPR parameters indicated that radicals, thermally generated in starch oxidized with H2O2 in the presence of Cu(II), were formed by abstraction of hydrogen atom from C(5) atom of glucose unit. It was found that generation of radicals was correlated with the reduction of Cu(II) to Cu(I) and formation of glucose-copper complexes. The reduction of copper occurring during thermal treatment was confirmed by colour parameters (a* increased from 0.96 for native starch to 9.10 for starch oxidized with H2O2 in the presence of copper catalyst). In the starch not containing copper (native and oxidized with H2O2) different types of radicals were generated. The mechanism of their creation consisted in abstraction of hydrogen from C(6), C(1) and C(3) carbon atoms. Formation of radicals upon dehydration of starch was also considered. Generation of radicals was correlated with the amount of copper catalyst used to oxidation process (0.1–0.3%) and with the number of carboxyl groups which was equal to 0.12% in starch oxidized with H2O2 and increased to 0.22% in starch oxidized in the presence of copper catalyst (0.3% of Cu). The stability of carbohydrate radicals was influenced mainly by distortion of the starch structure, as well as the presence of copper ions. Increase of the content of carbonyl groups upon oxidation (to 0.13% for starch oxidized with H2O2 and 1.31% for starch oxidized with H2O2 in the presence 0.3% copper catalyst) pointed to the progressive degradation of starch structure during oxidation.