## 1 Introduction  
By 2019, cheese whey production globally reached a total of 21,600,000 tons, and even though several strategies are applied to take advantage of this whey, there is still a concern about environmental problems generated by its unproper disposition, especially due to the high biological oxygen demand (BOD) of this stream [1]. Among the components in cheese whey, whey protein, lactose, fat, and minerals represent the majority of solids present. Fat is normally removed in equipment to defat milk, which is common in dairy-processing plants; however, separation of protein, lactose and minerals represent a bigger challenge. Having in mind the technological and economic advantages of fractioning the components in whey, tangential filtration gives the opportunity to separate said components, from lab scale to industrial set-ups. Using ultrafiltration membranes, with a molecular weight cut-off (MWCO) of 10 kDa, protein concentration in whey became possible, enabling opportunities to use this stream as an ingredient in dairy industries or as raw material for production of protein concentrates such as whey protein concentrate (WPC), whey protein isolate (WPI) or whey protein hydrolysate (WPH) [2]. The permeate from ultrafiltration, rich in lactose (4-5%) and minerals can be concentrated by nanofiltration, where the size of the pores in the membranes allows us to retain the lactose and some of the minerals, increasing the concentration of the carbohydrate up to 20-25% [3,4]. This stream is underutilized by many dairy industries; however, lactose can be used as a substrate for multiple fermentative and enzymatic processes to valorize this by-product. One of the options explored for utilization of whey permeate is the production of galacto-oligosaccharides (GOS) from lactose [5; 6]. GOS are oligomers of glucose connected to galactose chains of different lengths, normally between 3 and 10 monomers per molecule [7]. Production of this oligomers has been evaluated through chemical synthesis [8] with poor opportunities for industrial scale-up; hence, the use of \(\beta\)-galactosidase has become the most popular method to generate GOS. \(\beta\)-galactosidaseases are enzymes commonly used by the food industry to hydrolyze lactose in dairy products, looking to reach lactose intolerant consumers. However, these enzymes have an alternative transferase activity known as transgalactosylation, able to use lactose, or other carbohydrate, as an acceptor of galactosyl units, forming oligomers with different degrees of polymerization (DP) [9]. Different microorganisms have been studied as a source of \(\beta\)-galactosidaseases for GOS production, including strains of _Kluyveromyces_[10], _Aspergillus_[11], _Bacillus_[12], and _Bifidobacterium_[13]. However, commercial enzymes designed for GOS production (Novozymes, IFF) are now available to be used in dairy products, aiming to reduce lactose content and generate prebiotic fiber. A prebiotic effect has been attributed to GOS molecules given the specific effect on growth promotion of probiotic bacteria, primarily strains from the _Bifidobacterium_ and _Lactobacillus_ genus [14]. Several studies have evaluated different prebiotic effects generated by the consumption of GOS, including the improvement of colonic health in breast-fed infants, and reduction in colon cancer risk and enhanced immunity in older consumers [15; 16; 17; 18]. Specifically, infant food products such as formulas, include a mixture of GOS and FOS (fructo-oligosaccharides) given the resemblance on the gut microbiota formation when compared to breastfed infants [19]. Besides the prebiotic benefits of GOS, some technological advantages when applying GOS ingredients in food products have been identified, including improvements in mouth feel, creaniness, and sweetness [20; 21].