Novel Galacto-oligosaccharides from Lactose: Chemical Synthesis, Structural Characterization, and in Vitro Assessment of Prebiotic Activity

Lactose is an underutilized byproduct of the dairy industry. Galacto-oligosaccharides (GOS) are established prebiotics with health-promoting properties. This study demonstrates a facile and high-yield chemical process to synthesize GOS from lactose as potential prebiotics in which lactose is hydrolyzed into galactose and glucose first, and they then undergo glycosylation to GOS in concentrated sulfuric acid. A GOS yield of 96% was obtained from 50% (w/w) initial lactose loading in 76% H2SO4 after reaction at 70 °C for 20 min, with 99% conversion of lactose and minimal side products (galactose, glucose, anhydrosugars, and sugar degradation products). The synthesized GOS were a mixture of di-, tri-, and tetrasaccharides (47%, 25%, and 16%, respectively) with a small portion of larger oligosaccharides (DP up to 6). Diverse glycosidic linkages including α/β-(1→6), α/β-(1→4), β-(1→3), α/β-(1→2), and α-(1→1) were identified between galactose and glucose units in the GOS. In vitro fermentation studies revealed that GOS promoted the growth of all tested Lactobacillus strains, with performance comparable to that of a commercial prebiotic GOS synthesized enzymatically.