Study of the color development during electro-activation of lactose solution for lactulose production

• Electro-activation showed a superior lactulose yield than the conventional process. • Brownish col is formed during electro-activation and relaxation time. • Kinetics of color formation were successfully assessed. • Results highlighted differences between electro-activation and chemical isomerization. • Advanced knowledge for possible color control or removal. Electro-activation (EA) for lactose isomerization into lactulose, a recognized prebiotic, has emerged as a promising and effective technology. However, despite its advantages, the yellowish colored electro-activated solution must be investigated to optimize lactulose production and new knowledge development. In this study, the color development of EA-lactose solution was studied as a function of electro-activation time (0–60 min), relaxation duration (0, 24, 48, and 72 h), and storage temperature for relaxation (5, 25, and 45 °C). The colorization of EA-lactose solution was compared with a chemically alkalinized lactose solution using KOH under equivalent alkalinity as that formed during electro-activation. The colorization was monitored through CIE L*a*b* color space and UV–vis absorption. The gradual increase of b* showed that the color was intense with the duration of electro-activation and the relaxation time. EA-lactose solutions absorbed light mostly in the UV-B region. To understand the color development, the kinetics were analyzed and were found to follow a zero-order reaction. The relatively lower activation energy showed better performance of the EA isomerization compared to chemical isomerization process. HPLC analyses showed that EA yielded higher lactulose (34.13 %) after 72 h relaxation and 19.44 % galactose, but with an intense yellowish color. Relaxation of 10 h was selected as the optimum, corresponding to lactulose and galactose of 30.39 % and 9.94 %, respectively. A similar result was obtained after 31 h for the chemical isomerization process. Moreover, the presence of oligosaccharides and unknown sugars was observed in HPLC, which may open future research on lactose electro-activation.