Electro-isomerization of lactose into lactulose with simultaneous purification from coloring molecules (color) in situ of the electro-activation reactor: comparative study between anion exchange resin and activated carbon

This study investigated an in situ purification as a color-removal strategy by simultaneous electro-activation and decolorization during lactose isomerization into lactulose. Anion-exchange resin and granular activated carbon (AC) were incorporated directly into the cathodic compartment during electro-activation of 5–30% (w/v) lactose solutions at 600 mA current intensity for 60 min, followed by a 10 h relaxation period. Decolorization performance was monitored using UV–Vis absorbance at 296 and 420 nm, corresponding to low-molecular-weight degradation products and high-molecular-weight colored compounds, respectively. The anion-exchange resin exhibited markedly superior decolorization efficiency compared with AC. At 7.5% resin loading, absorbance at 296 nm decreased by 80.25% and 78.13% for 5% and 10% lactose solutions, respectively, while at 420 nm it ranged from 88.43% to 93.46% across the studied lactose concentrations. Resin-based purification eliminated color development during relaxation, indicating its ability to limit both already formed coloring compounds and reactive color precursors. Kinetic analysis showed that second-order behavior best described the resin-mediated decolorization, particularly during relaxation, with R² values of 0.94 and 0.97 at 296 and 420 nm, respectively. Isotherm analysis indicated a Langmuir-type adsorption at 296 nm and Freundlich-type adsorption at 420 nm, suggesting different interaction mechanisms for the low and high-molecular-weight coloring compounds. HPLC analysis confirmed that the in situ process affected sugar profile in a concentration-dependent manner. The anion-exchange resin provided an effective integrated purification process to reduce the color intensity of electro-activated lactose solution in lactulose production.