Galacto-oligosaccharides (GOS) are prebiotic compounds widely used for their health-promoting effects. Conventionally, GOS is produced by the enzymatic conversion of lactose in stirred tank reactors (STR). The high operational costs associated with enzyme inactivation and removal might be reduced by the application of enzyme membrane reactors (EMR). In this study, we aimed to assess the potential of continuous GOS production by EMR using soluble Biolacta N5, a Bacillus circulans-derived commercial enzyme preparation. The steady-state performance of the EMR equipped with an ultrafiltration module was investigated as function of residence time (1.1–2.8 h) and enzyme load (17–190 U·g−1) under fixed operational settings of temperature (50 °C), pH (6.0), lactose feed concentration (300 g·kg−1), and recirculation flow-rate (0.18 m3·h−1). Results indicate that the yield of oligosaccharides with higher degree of polymerization (DP3-6) in STR (approx. 38% on total carbohydrate basis) exceeds that measured in EMR (ranging from 24% to 33%). However, a stable catalytic performance without a significant deterioration in product quality was observed when operating the EMR for an extended period of time (>120 h). Approx. 1.4 kg of DP3-6 was produced per one gram of crude enzyme preparation over the long-term campaigns, indicating that EMR efficiently recovers enzyme activity.

Galacto-oligosaccharides (GOS) are prebiotic compounds, widely used as ingredients in various food, nutraceutical and pharmaceutical products. Enzymatic synthesis of GOS results in low-purity products that contain high amounts of glucose and lactose beside the valuable GOS. In this study, a systematic approach was used to develop yeast-based fermentation strategies to purify crude GOS. Potentially applicable yeast strains were identified based on an extensive search in literature databases followed by a series of laboratory-scale fermentation tests. Single-and two-step fermentation processes were designed for the removal of glucose alone or together with lactose from crude GOS syrup. Single-step fermentation trials with two strains of previously unreported species, Cyberlindnera jadinii NCAIM Y.00499 and Kluyveromyces nonfermentans NCAIM Y.01443, resulted in purified products free of both glucose and ethanol from a crude GOS syrup diluted to 15 and 10 w/v%, respectively. Simultaneous removal of glucose and lactose was achieved by Kluyveromyces marxianus DMB Km-RK in a single-step fermentation process with a yield of 97.5% and final purity of 100%. A two-step fermentation approach was designed to allow conversion of a glucose-free product into a high-purity GOS by removing glucose with C. jadinii Y.00499 in the first step, and lactose by Kluyveromyces lactis DMB Kl-RK in the second step, resulting in a final product with a yield of 100% and a final purity of 92.1%. These results indicate that the selected nonconventional yeasts are promising candidates for the removal of non-GOS components from commercial crude GOS products by selective fermentation.

This study presents a multistep process for the valorization of whey that involves membrane filtration techniques to fractionate whey compounds and the subsequent enzymatic conversion of lactose into galacto‐oligosaccharides with a degree of polymerization (DP) of 3–5. Partially demineralized whey was first concentrated and diafiltered by ultrafiltration (UF) to obtain whey protein isolates. Over 97% of the initial amount of lactose was recovered into the permeate which was then concentrated by nanofiltration (NF) up to 330 g L−1. Finally, a systematic study was performed to investigate the hydrolytic and transgalactosylation activity of Biolacta N5, a Bacillus circulans‐derived β‐galactosidase, on whey‐derived substrates consisting of lactose in various concentrations. Hydrolysis was especially pronounced for UF permeates with low lactose concentrations (< 50 g L−1), whereas transgalactosylation was dominant for streams concentrated by NF, obtaining comparable yields of the DP3‐5 fractions (∼ 35%) for moderate and high NF concentration factors. Practical applications The market for functional foods and dairy‐based functional beverages represents a growing segment of the global food industry. This article describes a technique for the valorization of whey that is based on the fractionation of whey constituents and the subsequent bioconversion of lactose into GOS. GOS are prebiotic substances with favorable technological properties such as low viscosity and good pH and temperature stability. These low‐calorie sweeteners can be added to a wide range of foods to increase product functionality. The here proposed multiproduct process has potential applications in whey management practice. The reported experimental data on the performance of the processing steps and the proposed approaches to the synchronization of the individual operations is useful for the design of whey valorization processes with reduced waste and energy consumption.

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Galacto-oligosaccharides (GOS) are used as prebiotic ingredients in various food and pharmaceutical formulations. Currently, production of GOS involves the enzymatic conversion of lactose by transgalactosylation using β-galactosidase. The purity of the resulting product is low, typically limited to up to 55% GOS on total carbohydrate basis due to the presence of non-reacted lactose, and the formation of by-products glucose and galactose. In industrial practice high-purity GOS is manufactured by removing the unwanted mono- and disaccharides from raw GOS with simulated moving bed (SMB) chromatography. This purification step is associated with high processing cost that increases the price of pure GOS and limits its marketability. The last decades have witnessed a growing interest in developing competitive biotechnological processes that could replace chromatography. This paper presents a comprehensive review on the recent advancements of microbial GOS purification, a process commonly referred to as selective fermentation or selective metabolism. Purification strategies include: (i) removal of glucose alone or together with galactose by lactose negative yeast species, that typically results in purity values below 60% due to remaining lactose; (ii) removal of both mono- and disaccharides by combining the fast monosaccharide metabolizing capacity of some yeast species with efficient lactose consumption by certain lactose positive microbes, reaching GOS purity in the range of 60–95%; and (iii) the application of selected strains of Kluyveromyces species with high lactose metabolizing activity to achieve high-purity GOS that is practically free from lactose and monosaccharides. Graphical abstract