Optimizing Conditions for TEMPO/NaOCl-Mediated Chemoselective Oxidation of Primary Alcohols in Sweet Potato Residue

Sweet potato residue (SPR), a by-product of the sweet potato-based beverage industry, contains health-beneficial dietary fiber, but it has been limited to secondary-use applications due to poor water solubility. The water solubility of polysaccharides in SPR increases following chemoselective oxidation of the primary alcohol group through 2,2,6,6-tetramethyl-1-piperidinyl oxoammonium ion (TEMPO)/sodium hypochlorite (NaOCl)-mediated oxidation. In the present study, a central composite experimental design was used to optimize conditions for chemoselective oxidation of SPR with three variables, including TEMPO concentration (0.05–0.25 mmol, X1), NaOCl concentration (4.0–12.0 mmol, X2), and pH of the reactants (9.5–11.5, X3). The quadratic polynomial model equation describing this relationship was as follows: \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$ {\text{degree of oxidation}}\left( \% \right) = - {871}.{929} + {975}.{4}0{5}{X_{{1}}} + {2}0.{7181}{X_{{2}}} + {143}.{358}{X_{{3}}}-{12}0.{682}X_1^2-0.{59}0{426}X_2^2-{5}.{78182}X_3^2-{7}.{725}00{X_{{1}}}{X_{{2}}}-{76}.{4}000{X_{{1}}}{X_{{3}}}-0.{55}0000{X_{{2}}}{X_{{3}}}\left( {{R^{{2}}} = 0.{9633}} \right) $$\end{document}. The maximum degree of oxidation (98.3%) was consistent with the expected value (100%) and was obtained from a TEMPO concentration, NaOCl concentration, and reactant pH of 0.1980 mmol, 9.6236 mmol, and 10.6271, respectively. The introduction of a C6 carboxyl group without oxidation of the secondary alcohols was confirmed by 13C nuclear magnetic resonance and Fourier transform infrared spectroscopy. The water solubility of the oxidized products was 99.95%, whereas that of native SPR was <26.45%. The initial velocity of the oxidation showed a strong linear relationship (R2 = 0.9239) with the degree of oxidation, indicating that initial velocity could be another quantitative index for determining the degree of oxidation and productivity of oxidized SPR. These results suggest that oxidized SPR could be used as a new water-soluble dietary fiber in the food industry.