From by-product to valuable components: Efficient enzymatic conversion of lactose in whey using β-galactosidase from Streptococcus thermophilus

β-Galactosidase from Streptococcus thermophilus was overexpressed in a food-grade organism, Lactobacillus plantarum WCFS1. Laboratory cultivations yielded 11,000 U of β-galactosidase activity per liter of culture corresponding to approximately 170 mg of enzyme. Crude cell-free enzyme extracts obtained by cell disruption and subsequent removal of cell debris showed high stability and were used for conversion of lactose in whey permeate. The enzyme showed high transgalactosylation activity. When using an initial concentration of whey permeate corresponding to 205 g L^-1 lactose, the maximum yield of galacto-oligosaccharides (GOS) obtained at 50°C reached approximately 50% of total sugar at 90% lactose conversion, meaning that efficient valorization of the whey lactose was obtained. GOS are of great interest for both human and animal nutrition; thus, efficient conversion of lactose in whey into GOS using an enzymatic approach will not only decrease the environmental impact of whey disposal, but also create additional value.

Microbial production and biotechnological applications of α-galactosidase

α-Galactosidase, (E.C. 3.2.1.22) is an exoglycosidase that target galactooligosaccharides such as raffinose, melibiose, stachyose and branched polysaccharides like galactomannans and galacto-glucomannans by catalysing the hydrolysis of α-1,6 linked terminal galactose residues. The enzyme has been isolated and characterized from microbial, plant and animal sources. This ubiquitous enzyme possesses physiological significance and immense industrial potential. Optimization of the growth conditions and efficient purification strategies can lead to a significant increase in the enzyme production. To boost commercial productivity, cloning of novel α-galactosidase genes and their heterologous expression in suitable host has gained popularity. Enzyme immobilization leads to its greater reutilization, superior thermostability, pH tolerance and increased activity. The enzyme is well explored in food industry in the removal of raffinose family oligosaccharides (RFOs) in soymilk and sugar crystallization process. It also improves animal feed quality and biomass processing. Applications of the enzyme is in the area of biomedicine includes therapeutic advances in treatment of Fabry disease, blood group conversion and removal of α-gal type immunogenic epitopes in xenotransplantation. With considerable biotechnological applications, this enzyme has been vastly commercialized and holds greater future prospects.

Whey permeate as the raw material in galacto-oligosaccharide synthesis using commercial enzymes

Galacto-oligosaccharides (GOS), molecules with prebiotic properties are considered promising in the food industry. Its synthesis can be performed by enzymatic pathway, using commercial microbial enzymes. The reaction, known as transgalactosylation, is mediated by the enzyme β-galactosidase and its catalysis is influenced during the process by substrate concentration present (in this case lactose), pH, and temperature, among others. The use of whey permeate, a by-product of the dairy industry, demonstrates the interest in making such processes viable from an economic and technological point of view. The main of this work was to use whey permeate as raw material in an enzymatic GOS synthesis, comparing three commercial enzymes of different microbial sources. For better performance, the results on lactose conversion, yield, and specific productivity were evaluated. The commercial enzyme of Kluyveromyces lactis (Lactozyme™ 2600 L) showed the best results for lactose conversion (89.27%), yield (25 g GOS/100 g lactose) and specific productivity (51 g GOS/g enzyme*h). Thus, it can be considered suitable for further technological development. Aspergillus oryzae commercial enzyme also showed good results and could be used for other studies either. However, the Escherichia coli commercial enzyme did not present good results in GOS synthesis, being more appropriate to lactose hydrolysis reactions. All the three enzymes showed a decrease in the production and even depletion of GOS molecules, and therefore, smaller reaction times should be established. New stages of optimization and processes development should be considered in future works, in order to obtain best yields and productivities.

Continuous synthesis of lactulose in an enzymatic membrane reactor reduces lactulose secondary hydrolysis

Newly developed parallel small-scale enzymatic membrane reactors (EMRs) were used to enhance the synthesis of lactulose using β-galactosidase. Under batch operation, the productivity of lactulose decreased abruptly from 2.72 down to 0.04 mg lactulose/(Uenzymeh) over 35 h of reaction. This was presumably caused by the action of β-galactosidase which performed secondary hydrolysis upon the produced lactulose. The continuous operations of an EMR system led to continuous removal of lactulose in the reactors restricting lactulose degradation caused by secondary hydrolysis. Therefore, continuous lactulose syntheses in the EMRs yielded significantly higher specific productivities under "steady state" conditions. Approximately 0.70 and 0.50 mg lactulose/(U enzyme h) for hydraulic residence times of 5 and 7h were reached, respectively. Continuous lactulose synthesis performed in an EMR system conclusively can circumvent the drawbacks (e.g., secondary hydrolysis) of lactulose synthesis encountered in batch operation. It is, therefore, beneficial in terms of enhanced lactulose productivity and reduced enzyme consumption.

Impact of ultrasound on galactooligosaccharides and gluconic acid production throughout a multienzymatic system

Galactooligosaccharides (GOS), recognised prebiotic, can be industrially produced from lactose and commercial β-galactosidase (β-gal) from Kluyveromyces lactis. Residual lactose and glucose limit GOS applications. To handle this problem, a multienzymatic system, with β-gal and glucose oxidase (Gox), was proposed to reduce glucose content in reaction media through its oxidation to gluconic acid (GA). Besides, ultrasound (US) probe effect over the multienzymatic system to produce GOS and GA has been evaluated. A production around 40% of GOS was found in all treatments after the first hour of reaction. However, glucose consumption and GA production was significantly higher (P < 0.05) for sequential reaction assisted by US, obtaining the best production of GOS (49%) and GA (28%) after 2 h of reaction. The conformational and residual activity changes of enzymes under US conditions were also evaluated, Gox being positively affected whereas in β-gal hardly any change was found.

Prebiotic properties of Bacillus coagulans MA-13: production of galactoside hydrolyzing enzymes and characterization of the transglycosylation properties of a GH42 β-galactosidase

Background

The spore-forming lactic acid bacterium Bacillus coagulans MA-13 has been isolated from canned beans manufacturing and successfully employed for the sustainable production of lactic acid from lignocellulosic biomass. Among lactic acid bacteria, B. coagulans strains are generally recognized as safe (GRAS) for human consumption. Low-cost microbial production of industrially valuable products such as lactic acid and various enzymes devoted to the hydrolysis of oligosaccharides and lactose, is of great importance to the food industry. Specifically, α- and β-galactosidases are attractive for their ability to hydrolyze not-digestible galactosides present in the food matrix as well as in the human gastrointestinal tract.

Results

In this work we have explored the potential of B. coagulans MA-13 as a source of metabolites and enzymes to improve the digestibility and the nutritional value of food. A combination of mass spectrometry analysis with conventional biochemical approaches has been employed to unveil the intra- and extra- cellular glycosyl hydrolase (GH) repertoire of B. coagulans MA-13 under diverse growth conditions. The highest enzymatic activity was detected on β-1,4 and α-1,6-glycosidic linkages and the enzymes responsible for these activities were unambiguously identified as β-galactosidase (GH42) and α-galactosidase (GH36), respectively. Whilst the former has been found only in the cytosol, the latter is localized also extracellularly. The export of this enzyme may occur through a not yet identified secretion mechanism, since a typical signal peptide is missing in the α-galactosidase sequence. A full biochemical characterization of the recombinant β-galactosidase has been carried out and the ability of this enzyme to perform homo- and hetero-condensation reactions to produce galacto-oligosaccharides, has been demonstrated.

Conclusions

Probiotics which are safe for human use and are capable of producing high levels of both α-galactosidase and β-galactosidase are of great importance to the food industry. In this work we have proven the ability of B. coagulans MA-13 to over-produce these two enzymes thus paving the way for its potential use in treatment of gastrointestinal diseases.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12934-021-01553-y.

Production of galactooligosaccharides using various combinations of the commercial β-galactosidases

Galactooligosaccharides (GOS) are currently attracting considerable interest as prebiotic substances and can be prepared by transgalactosylation reactions from lactose using β-galactosidase. We applied various combinations of the commercial β-galactosidases, such as Nola Fit 5500, Saphera 2600 L, Maxilact LGI 5000 and Maxilact A4 MG to achieve the highest yield of GOS and reduced lactose content. The combination of the Maxilact LGI 5000 and Nola Fit 5500 resulted in amount of GOS 105 g L^-1 with lactose content lower than 5 g L^-1, whilst the combination of the Maxilact A4 MG and Maxilact LGI 5000 enzymes led to an increase in GOS to 141,1 g L^-1 and decrease of the lactose content to 46,9 g L^-1. The combination of enzymes produced a higher yield of GOS, reduced the concentration of lactose, eventually, increases the efficiency of galactooligosaccharides purification that could be potentially used in the further investigations.

Immobilization of β-Galactosidase From Aspergillus oryzae on Electrospun Gelatin Nanofiber Mats for the Production of Galactooligosaccharides

Two simple and easily reproducible methods for the immobilization of β-galactosidase (β-gal) from Aspergillus oryzae on electrospun gelatin nanofiber mats (GFM) were developed. The process was optimized regarding the electrospinning solvent system and the subsequent cross-linking of GFM in order to increase their stability in water. β-Gal was covalently immobilized on activated gelatin nanofiber mats with hexamethylenediamine (HMDA) as a bifunctional linker and secondly via entrapment into the gelatin nanofibers during the electrospinning process (suspension electrospinning). Optimal immobilization parameters for covalent immobilization were determined to be at pH 7.5, 40 °C, β-gal concentration of 1 mg/mL and immobilization time of 24.5 h. For suspension electrospinning, the optimal immobilization parameters were identified at pH 4.5 and β-gal concentration of 0.027 wt.% in the electrospinning solution. The pH and temperature optima of immobilized β-gal shifted from 30 °C, pH 4.5 (free enzyme) to pH 3.5, 50 °C (covalent immobilization) and pH 3.5, 40 °C (suspension electrospinning). Striking differences in the Michaelis constant (KM) of immobilized β-gal compared with free enzyme were observed with a reduction of KM up to 50% for immobilized enzyme. The maximum velocity (vmax) of immobilization by suspension electrospinning was almost 20 times higher than that of covalent immobilization. The maximum GOS yield for free β-gal was found to be 27.7% and 31% for immobilized β-gal.

Application of Transgalactosylation Activity of β-Galactosidase from Kluyveromyces lactis for the Synthesis of Ascorbic Acid Galactoside

In view of a commonly known beneficial role and low stability of ascorbic acid, many efforts are constantly undertaken to produce its improved derivatives. This paper presents results on the synthesis of ascorbic acid galactoside using transgalactosylation properties of β-galactosidase from Kluyveromyces lactis and lactose as a donor of galactosyl moiety. The purpose of this study was to determine the influence of selected factors (concentration and molar ratio of substrates, amount of the enzyme preparation, pH of the solution, presence of different ions) on the course of transgalactosylation reaction. Research has shown that approx. 2.5% dry matter (d.m.; 12.7 g/L) of ascorbic acid galactoside is formed under favourable conditions (50% (w/v) substrates, sodium ascorbate and lactose at the molar ratio of 1.9:1, enzyme dose of 28,600 U/100 g lactose, pH = 7.0). The addition of Mg²⁺ or K⁺ ions to the reaction medium caused an increase in the final product content (even up to approx. 3.4% d.m., 17.2 g/L), while Na⁺ or Mn²⁺ had an adverse impact on the yield. The gathered data may be valuable for cosmetic or food industry.

Hydrolysis and transgalactosylation catalysed by β-galactosidase from brush border membrane vesicles isolated from pig small intestine: A study using lactulose and its mixtures with lactose or galactose as substrates

Enzymatic transgalactosylation, in different concentrated carbohydrate solutions, was investigated using brush border membrane vesicles (BBMV) from the pig small intestine. When lactulose was incubated with BBMV, the hydrolytic activity of the enzyme towards the disaccharide was observed to be very low compared to that towards the lactose, but the linkage specificity β-(1 → 3), previously observed in lactose solutions, was not significantly affected. As in the case of lactose, lactulose transgalactosylation by BBMV synthesizes the corresponding 3'-galactosyl derivative (β-Gal-(1 → 3)-β-Gal-(1 → 4)-β-Fru). Fructose released during lactulose hydrolysis was found to be good acceptor for the transgalactosylation reaction, giving rise to the synthesis of the disaccharide β-Gal-(1 → 5)-Fru. When incubating an 80/20 mixture of lactulose/galactose, the presence of galactose did not affect the qualitative composition of the transglycosylated substrate but enhanced the synthesis of β-Gal-(1 → 5)-Fru and decreased the synthesis of β-(1 → 3) glycosidic bonds. The marked tendency for synthesizing this linkage indicates that under hydrolytic conditions, β-Gal-(1 → 3)-Gal- and β-Gal-(1 → 5)-Fru glycosidic bonds would be preferentially digested.

Enzymatic generation of lactulose in sweet and acid whey: Optimization of feed composition and structural elucidation of 1-lactulose

Prebiotics are rising in interest in commercial scale productions due to increasing health awareness of consumers. Under bio-economic aspects, sweet and acid whey provide a suitable feed medium for the enzymatic generation of prebiotic lactulose. Since whey has a broad variation in composition, the influence of the feed composition on the concentration of generated lactulose was investigated. The influence of lactose and fructose concentration as well as enzymatic activity of two commercially available β-galactosidases were investigated. The results were evaluated via response surface analysis with a quadratic model containing pairwise interaction terms. The optimal feed composition yielding a theoretical maximal amount of lactulose was determined as 1.28 or 0.74 mol/kg fructose and 0.17 or 0.19 mol/kg lactose with an enzymatic activity of 2.0 or 2.8 μkat/kg for acid (pH 4.4) or sweet (pH 6.6) whey. Furthermore, the major reaction product was isolated and subsequently, the structural identity was elucidated and verified via extensive NMR analysis.

Design of immobilized biocatalyst and optimal conditions for tyrosol β-galactoside production

Tyrosol β-galactoside (TG) is a phenylethanoid glycoside with proven neuroprotective properties. This work deals with its biocatalytic production from tyrosol and lactose using Aspergillus oryzae β-galactosidase in immobilized form. Six commercial carriers were examined to find the optimal biocatalyst. Besides standard biocatalyst performance characteristics, adsorption of the hydrophobic substrate on immobilization carrier matrices was also investigated. The adsorption of tyrosol was significant, but it did not have adverse effects on TG production. On the contrary, TG yield was improved for some biocatalysts. A biocatalyst prepared by covalent binding of β-galactosidase on an epoxy-activated carrier was used for detailed investigation of the effect of reaction conditions on glycoside production. Temperature had a surprisingly weak effect on the overall process rate. A lactose concentration of 0.83 M was found to be optimal to enhance TG formation. The impact of tyrosol concentration was rather complex. This substrate caused inhibition of all reactions. Its concentration had a strong effect on the hydrolysis of lactose and all products. Higher tyrosol concentrations, 30-40 g/L, were favorable as pseudo-equilibrium concentrations of TG and galactooligosaccharide were reached. Repeated batch results revealed excellent operational stability of the biocatalyst.

An acid-tolerant and cold-active β-galactosidase potentially suitable to process milk and whey samples

A novel β-galactosidase gene (gal_M) was cloned from an aquatic habitat metagenome. The analysis of its translated sequence (Gal_M) revealed its phylogenetic closeness towards Verrucomicrobia sp. The sequence comparison and homology structure analysis designated it a member of GH42 family. The three-dimensional homology model of Gal_M depicted a typical (β/α)8 TIM-barrel containing the catalytic core. The gene (gal_M) was expressed in a heterologous host, Escherichia coli, and the purified protein (Gal_M) was subjected to biochemical characterization. It displayed β-galactosidase activity in a wide range of pH (2.0 to 9.0) and temperature (4 to 60 °C). The heat exposed protein showed considerable stability at 40 and 50 °C, with the half-life of about 100 h and 35 h, respectively. The presence of Na, Mg, K, Ca, and Mn metals was favorable to the catalytic efficiency of Gal_M, which is a desirable catalytic feature, as these metals exist in milk. It showed remarkable tolerance of glucose and galactose in the reaction. Furthermore, Gal_M discerned transglycosylation activity that is useful in galacto-oligosaccharides' production. These biochemical properties specify the suitability of this biocatalyst for milk and whey processing applications. KEY POINTS: • A novel β-galactosidase gene was identified and characterized from an aquatic habitat. • It was active in extreme acidic to mild alkaline pH and at cold to moderate temperatures. • The β-galactosidase was capable to hydrolyze lactose in milk and whey.

Oligosaccharide Production Using β-galactosidase from Lactobacillus bulgaricus and Kluyveromyces lactis in Sweetened Reconstituted Nonfat Dry Milk

β-galactosidase (B-gal) is a vital enzyme used in the food industry to reduce lactose from dairy ingredients through hydrolysis and to synthesize galacto-oligosaccharides (GOS), a known bifidogenic prebiotic, from galactose moieties through transgalactosylation. To favor transgalactosylation, high carbohydrate concentrations must be available to accept the galactose moiety. Since many dairy products rely on sweeteners for flavor and texture, B-gal from GRAS status Lactobacillus bulgaricus strains LB11, YB1, L. helveticus B548, or Kluyveromyces lactis (KL) in the commercial product Lactozyme® 2600L were used to determine if transgalactosylation can be driven in reconstituted nonfat dry milk (NDM, 10%, wt/wt) with commercial sweeteners compared with unsweetened NDM at 50°C for 18 h. Sucrose (14.4%, wt/wt), corn syrup solids (14.4%, wt/wt), or a mixture of sucrose (9.9%, wt/wt) and corn syrup solids (4.5%, wt/wt) were dissolved with NDM (n = 3), then heated with stirring for 30 min at 70°C for batch pasteurization. For Lactobacillus strains, whole cells were inoculated at 7 log cfu/mL, or the same quantity of cells were lysed before adding to the milk and carbohydrate mixtures, or 1.85 U of KL B-gal was used. The sucrose mixture released the highest glucose from all B-gal preparations, with LB11 lysate releasing the highest (245.57 ± 25.16 mM). High-Performance Anion-Exchange Chromatography with Pulsed Amperometric Detection was used to visualize the presence of oligosaccharides with degrees of polymerization of 3 to 7. Oligosaccharides from YB1 lysate treated sucrose mixtures also increased the anaerobic growth of Bifidobacterium breve 2141 at 37°C compared with untreated or KL B-gal treated sucrose mixtures. B-gal treatment of sucrose in nonfat milk with and without corn syrup can be a source of prebiotic oligosaccharides, although more work is needed to determine the composition of the oligosaccharides.

Targeted gene manipulation of Leloir pathway genes for the constitutive expression of β-galactosidase and its transgalactosylation product galacto-oligosaccharides from Kluyveromyces lactis GG799 and knockout strains

Galacto-oligosaccharides (GOS) are used as prebiotic ingredients in various food and pharmaceutical industry. At present, production of GOS involves the enzymatic transformation of lactose by transgalactosylation using β-galactosidase. The yeast Kluyveromyces lactis can utilize lactose as its carbon and energy source. In this species lactose is hydrolyzed by an intracellular β-galactosidase (EC 3.2.1.23) which is induced by its substrate and related compounds like galactose. The molecular details of gene regulation in kluyveromyces lactis, we have used multiple knockout approaches to study the constitutive expression by which galactose induces β-galactosidase. The present study involved carrying out to a method of enhancing the constitutive expression of β-galactosidase through galactose induction and its trans-galactosylation reaction for the production of galacto-oligosaccharides (GOS) in Kluyveromyces lactis (K. Lactis) by applying a knockout based approach on Leloir pathway genes based on fusion-overlap extension polymerase chain reaction and transformation into its genome. The k.lactis strain subjected to Leloir pathway genes knockout, resulted in the accumulation of galactose intracellularly and this internal galactose acts as an inducer of galactose regulon for constitutive expression of β-galactosidase at early stationary phase was due to the positive regulatory function of mutant gal1p, gal7p and both. These resulted strains used for trans-galactosylation of lactose by β - galactosidase is characterized for the production of galacto-oligosaccharides. Galactose-induced constitutive expression of β-galactosidase during the early stationary phase of knockout strains was analysed qualitatively & quantitatively. The activity of β-galactosidase of wild type, gal1z, gal7k and gal1z & gal7k strains were 7, 8, 9 and 11 U/ml respectively using high cell density cultivation medium. Based on these expression differences in β-galactosidase, the trans-galactosylation reaction for GOS production and percentage yield of GOS were compared at 25% w/v of lactose. The percentage yield of GOS production of wild type, Δgal1z Lac4+, Δgal7k Lac4++ and Δgal1z Δgal7k Lac4+++mutants strains were 6.3, 13, 17 and 22 U/ml, respectively. Therefore, we propose that the availability of galactose can be used for constitutive over expression of β - galactosidase in Leloir pathway engineering applications and also for GOS production. Further, increased expression of β - galactosidases can be used in dairy industry by-products like whey to produce added value products such as galacto-oligosaccharides.