## 4 Discussion  
By-products of the dairy industry are used as raw materials for the generation of added-value ingredients containing whey protein, phospholipids, minerals, among others. Thanks to separation technologies such as tangential filtration, these elements can be easily concentrated at industrial level based on molecular size, widening the possibilities of exploitation. Lactose is the main carbohydrate in milk and can be found in cheese whey (4-5% _w/w_), and even though its commercial value is not as attractive as that of protein and fat, biotechnology tools can be applied to transform and valorize it [30]. Different studies have focused on the production of GOS from by-products generated by the dairy industry, specifically milk and cheese whey and its derivatives from filtration processes, evaluating different enzymes in suspension or immobilized, as well as different process parameters such as pH, temperature and reaction time, always looking to increase GOS yields [29; 30; 31; 32; 33]. In this research, a GOS yield of 74% (g GOS/g lactose*100) was achieved after 0.5 h of reaction, converting 63% of the lactose available. DP distribution of the oligomers generated includes 57% GOS DP4, 42% GOS DP3, and 1% GOS DP2. After 0.5 h, GOS yields decrease to 71% (1 h), 68% (1.5 h), and 62% (2 h). The reduction in the GOS yield can be attributed to the hydrolase activity of the \(\beta\)-galactosidase used, able to break down GOS molecules formed initially, if not inactivated promptly [34]. The work presented by [PERSON] et al. [35] evaluated the production of GOS from lactose (20% _w/v_) in dairy whey with a final yield of 44.7%, using a \(\beta\)-galactosidase from _Lactobacillus bulgaricus_. In a different study, [PERSON] et al. [5] used a continuous flow packed-bed reactor to convert lactose in whey permeate into GOS, immobilizing a \(\beta\)-galactosidase from _Aspergillus oryzae_ in glass beads. According to their results, a maximum GOS yield of 39.3% was achieved after the 2 nd cycle of passing through the packed-bed reactor. An additional study, carried out by [PERSON] et al. [36], evaluated GOS production in a lactose solution at 40% _w/v_, resulting in a conversion yield of 34%, utilizing 96% of the lactose available. As observed in the previous paragraphs, most of the methods evaluated for GOS production, under different process conditions as source of enzyme, substrate matrix, type of process (batch or continuous) resulted in GOS yields lower than that obtained in this study. It is worth noting that the enzyme used here is a commercial preparation design for maximum GOS production, with optimum temperature and pH previously determined by the manufacturer. In this case, optimum pH and temperature are 6.5 and 55 \({}^{\circ}\)C, respectively. For the tests, the pH was not adjusted in the CWP; however, its natural pH of 6.1, was  
Figure 4: Final products of control and GOS-added porridge. not far from the optimum. On the other hand, the temperature of the enzymatic process was adjusted at 55 \({}^{\circ}\)C in the bioreactor to meet the optimum conditions of the enzyme. These conditions, plus the additional concentration of the CWP to increase the lactose concentration from 20% to 30%, enhanced the transgalactosylation activity, increasing GOS yields compared to other studies where new enzymes are being tested under process conditions that can be distant from the optimum ones. The molecular size distribution of the GOS generated during this study shows that short chain oligomers (DP2-DP4) with \(\beta\)(1->3) and \(\beta\)(1->6) linkages are formed preferentially, which is similar to the results reported by [PERSON] et al. [37], where a \(\beta\)-galactosidase from _B. bifidum_ was used to produced GOS from a lactose solution at 40% \(w/v\). The results indicated a GOS yield of 27% with a lactose conversion of 90%, and a majority of GOS DP2 formed, followed by GOS DP3 and GOS DP4, with a tendency for linkages \(\beta\)(1->3) and \(\beta\)(1->6), and, in a lesser proportion, \(\beta\)(1->4).