Abstract

Galacto-oligosaccharides (GOS) are prebiotic carbohydrates that mimic the biological activity of human milk oligosaccharides (HMOs), providing several health benefits to consumers. Because of their technological and functional properties, GOS have attracted the attention of academia and industry. In addition, their manufacturing allows adding value to lactose, a low-cost, fully available, and underutilized carbohydrate. GOS are synthesized almost exclusively by transgalactosylation reactions catalyzed by β-galactosidases, the same enzymes that have been used traditionally for lactose hydrolysis in the dairy industry. In order to depress the hydrolytic activity while enhancing the transgalactosylation activity of β-galactosidases, the synthesis of GOS is usually carried out at very high lactose concentrations. Since transgalactosylation activity is highly dependent on the origin of the enzyme, research has been focused on the characterization and engineering of already available β-galactosidases, as well as the search for new sources of enzymes. Most of the reported β-galactosidases catalyze GOS synthesis with a moderate efficiency, resulting in low reaction yields and moderate lactose conversion. Consequently, the downstream processing of the reacted mixture is costly and complex. For this reason, the development of economical and scalable purification processes is a topic of research and industrial interest. Alternatively, with the aim of avoiding the costs associated with downstream processing, intensive research has been carried out in recent years to optimize the enzyme source, substrate, and reaction conditions for the in situ synthesis of GOS in milk, yogurt, and other dairy products. Regarding GOS applications, significant efforts have been made to improve the understanding of the prebiotic activity exerted by these carbohydrates in humans, pets, and farm animals. Recent studies conclusively demonstrate that the prebiotic effect of GOS is not limited to the intestine axis, but, indirectly, positive effects on the immune system, bones, brain, and skin have also been demonstrated. These new, exiting discoveries allow us to envision an increase in GOS applications in food, nutraceuticals, and pharmaceuticals. © 2026 Elsevier Inc. All rights reserved.