Synthesis of lactulose in batch and repeated-batch operation with immobilized β-galactosidase in different agarose functionalized supports

Synthesis of novel lactose-derived oligosaccharides by two-step enzymatic reactions and structural characterization of the oligomers

Exploitation of the added value of lactose by enzymatic synthesis of novel lactose-derived oligosaccharides has been a subject of increasing research interests. In this study, a strategy of two-step enzymatic reactions was designed to first synthesize galacto-oligosaccharide (GOS) from lactose by β-galactosidase, and then the novel oligosaccharides were synthesized by inulosucrase (IS) using the obtained GOS and sucrose as the substrates. The recombinant IS was initially expressed by employing the IS gene from Lactobacillus gasseri DSM 20604. The appropriate synthesis conditions by the IS were determined as: reaction time of 12 h, the concentration of IS at 25 U/mL, GOS to sucrose ratio of 1:2, and the total substrate concentration of 500 g/L. Purification of the synthesized oligosaccharides in the reaction mixture was done by fermentation with Kluyveromyces lactis CICC 1773 to remove the carbohydrate by-products, and HPLC on an oligosaccharide column. The purified oligosaccharides, namely, Oligo-1, Oligo-2, and Oligo-3 were determined for their monosaccharide components to be galactose and glucose in the molar ratios of 0.230:0.770, 0.522:0.478 and 0.637:0.363, respectively. These three oligosaccharides had relative molecular masses of 666.18, 990.26, and 1314.43, corresponding to the degree of polymerization (DP) value of 4, DP6, and DP8, respectively. NMR spectroscopic studies of the oligosaccharides revealed a common branched structure consisting of three glucosides, one of which was linked with one (Oligo-1), three (Oligo-2) and five galactosides (Oligo-3), respectively. These lactose-derived oligosaccharides with the structural patterns that were not reported earlier represented novel potential prebiotic agents for use in functional foods.

Lactulose production from lactose isomerization by chemo-catalysts and enzymes: Current status and future perspectives

Lactulose, a semisynthetic nondigestive disaccharide with versatile applications in the food and pharmaceutical industries, has received increasing interest due to its significant health-promoting effects. Currently, industrial lactulose production is exclusively carried out by chemical isomerization of lactose via the Lobry de Bruyn-Alberda van Ekenstein (LA) rearrangement, and much work has been directed toward improving the conversion efficiency in terms of lactulose yield and purity by using new chemo-catalysts and integrated catalytic-purification systems. Lactulose can also be produced by an enzymatic route offering a potentially greener alternative to chemo-catalysis with fewer side products. Compared to the controlled trans-galactosylation by β-galactosidase, directed isomerization of lactose with high isomerization efficiency catalyzed by the most efficient lactulose-producing enzyme, cellobiose 2-epimerase (CE), has gained much attention in recent decades. To further facilitate the industrial translation of CE-based lactulose biotransformation, numerous studies have been reported on improving biocatalytic performance through enzyme mediated molecular modification. This review summarizes recent developments in the chemical and enzymatic production of lactulose. Related catalytic mechanisms are also highlighted and described in detail. Emerging techniques that aimed at advancing lactulose production, such as the boronate affinity-based technique and molecular biological techniques, are reviewed. Finally, perspectives on challenges and opportunities in lactulose production and purification are also discussed.

Enzymatic production of lactulose by fed-batch and repeated fed-batch reactor

The effects of the most significant operational variables on reactor performance of fed-batch and repeated fed-batch were evaluated in the lactulose production by enzymatic transgalactosylation. Feed flowrate in the fed stage (F) and fructose to lactose molar ratio (Fr/L) were the variables that mostly affected the values ​​of lactulose yield (Y_Lu), lactulose productivity (π_Lu) and selectivity of transgalactosylation (S_Lu/_TOS). Maximum Y_Lu of 0.21 g lactulose per g lactose was obtained at 50% w/w inlet carbohydrates concentration (IC) of, 50 °C, Fr/L 8, F 1 mL⋅min^-1, 200 IU∙g_Lactose^-1 reactor enzyme load and pH 4.5. At these conditions the selectivity was 7.4, productivity was 0.71 g_Lu∙g^-1∙h^-1and lactose conversion was 0.66. The operation by repeated fed batch increases the efficiency of use of the biocatalysts (E_B) and the accumulated productivity compared to batch and fed batch operation with the same biocatalyst. E_B obtained was 4.13 g_Lu∙mg_{biocatalyst protein}^-1, 10.6 times higher than in fed-batch.

Chemical and physical Chitosan modification for designing enzymatic industrial biocatalysts: How to choose the best strategy?

Chitosan is one of the most abundant natural polymer worldwide, and due to its inherent characteristics, its use in industrial processes has been extensively explored. Because it is biodegradable, biocompatible, non-toxic, hydrophilic, cheap, and has good physical-chemical stability, it is seen as an excellent alternative for the replacement of synthetic materials in the search for more sustainable production methodologies. Thus being, a possible biotechnological application of Chitosan is as a direct support for enzyme immobilization. However, its applicability is quite specific, and to overcome this issue, alternative pretreatments are required, such as chemical and physical modifications to its structure, enabling its use in a wider array of applications. This review aims to present the topic in detail, by exploring and discussing methods of employment of Chitosan in enzymatic immobilization processes with various enzymes, presenting its advantages and disadvantages, as well as listing possible chemical modifications and combinations with other compounds for formulating an ideal support for this purpose. First, we will present Chitosan emphasizing its characteristics that allow its use as enzyme support. Furthermore, we will discuss possible physicochemical modifications that can be made to Chitosan, mentioning the improvements obtained in each process. These discussions will enable a comprehensive comparison between, and an informed choice of, the best technologies concerning enzyme immobilization and the application conditions of the biocatalyst.

Process intensification using immobilized enzymes for the development of white biotechnology

Process intensification of biocatalysed reactions using different techniques such as microwaves, ultrasound, hydrodynamic cavitation, ionic liquids, microreactors and flow chemistry in various industries is critically analysed and future directions provided.

Immobilization of Aspergillus oryzae β-galactosidase in cation functionalized agarose matrix and its application in the synthesis of lactulose

Aspergillus oryzae β-galactosidase was immobilized in in-house quaternary ammonium agarose (QAA) and used for the first time in the synthesis of lactulose. A biocatalyst was obtained with a specific activity of 24,690 IU_H∙g^-1; protein immobilization yield of 97% and enzyme immobilization yield of 76% were obtained at 30 °C in 10 mM phosphate buffer pH 7 for standard size agarose at 100 mg_protein∙g_support^-1 which the maximum protein load of QAA. Highest yield and specific productivity of lactulose were 0.24 g∙g^-1 and 9.78 g∙g^-1 h^-1 respectively, obtained at pH 6, 100 IU_H∙g lactose^-1 enzyme/lactose ratio and 12 lactose/fructose molar ratio. In repeated-batch operation with the immobilized enzyme, the cumulative mass of lactulose per unit mass of contacted protein and cumulative specific productivity were higher than obtained with the soluble enzyme since the first batch. After enzyme activity exhaustion, the enzyme was desorbed and QAA support was reused without alteration in its maximum enzyme load capacity and without detriment in yield, productivity and selectivity in the batch synthesis of lactulose with the resulting biocatalyst. This significantly decreases the economic impact of the support, presenting itself as a distinctive advantage of immobilization by ionic interaction.

Synthesis of Butyl-β-D-Galactoside in the Ternary System: Acetone/1-Butanol/Aqueous Solution

The enzymatic synthesis of short-tailed alkyl glucosides is generally carried out in an aqueous-organic biphasic reaction medium with a rather low fatty alcohol concentration in the aqueous phase (where the synthesis occurs). Thus, hydrolytic reactions have a significant impact on the synthesis performance. Given this background, the use of acetone as cosolvent was studied for the synthesis of butyl-β-galactoside with Aspergillus oryzae β-galactosidase. The liquid-liquid equilibrium of the reaction mixture components (acetone/1-butanol/aqueous solution) was determined and the single- and two-phase regions were defined at 30, 40, and 50°C. It was observed that the liquid-liquid equilibrium of the ternary system acetone/1-butanol/water differs significantly from the one obtained using an aqueous solution (50 mM McIlvaine buffer pH 4.5; 5 g L^-1) instead of water. This is mainly because of the salting-out effect of the buffer; nevertheless, the presence of lactose also altered the equilibrium. Having this in mind, the effects of temperature (30 and 50°C) and reaction mixture composition were assessed. Three general conditions were evaluated: single-phase ternary system (30% acetone), two-phase ternary system (10% acetone) and two-phase binary system (0% acetone). Acetone had a deleterious effect on enzyme stability at 50°C, leading to low reaction yields. However, no enzyme deactivation was detected at 30°C. Moreover, a reaction yield of 0.98 mol mol^-1 was attained in the 30/50/20% (w/w) mixture of acetone/1-butanol/aqueous solution. This very high yield can be explained by the huge increase in the concentration of 1-butanol and the reduction of water activity. The synthesis was carried out using also the β-galactosidase immobilized in glyoxal-agarose and amino-glyoxal-agarose, and by aggregation and crosslinking. In the case of agarose-derived catalysts, two average particle diameters were assessed to evaluate the presence of internal mass transfer limitations. Best yield (0.88 mol mol^-1) was obtained with glyoxal-agarose derivatives and the particle size had non-effect on yield. The chemical structure of butyl-β-galactoside was determined by NMR and FT-IR.

Synthesis of Lactulose in Continuous Stirred Tank Reactor With β-Galactosidase of Apergillus oryzae Immobilized in Monofunctional Glyoxyl Agarose Support

Lactulose synthesis from fructose and lactose in continuous stirred tank (CSTR) reactor operation with glyoxyl-agarose immobilized Aspergillus oryzae β-galactosidase is reported for the first time. The effect of operational variables: inlet concentrations of sugar substrates, temperature, feed substrate molar ratio, enzyme loading and feed flow rate was studied on reactor performance. Even though the variation of each one affected to a certain extent lactulose yield (Y_Lactulose), specific productivity (π_Lactulose) and selectivity of the reaction (lactulose/transgalactosylated oligosaccharides molar ratio) (S_Lu/TOS), the most significant effects were obtained by varying the inlet concentrations of sugar substrates and the feed substrate molar ratio. Maximum Y_Lactulose of 0.54 g⋅g^–1 was obtained at 50°C, pH 4.5, 50% w/w inlet concentrations of sugar substrates, feed flowrate of 12 mL⋅min^–1, fructose/lactose molar ratio of 8 and reactor enzyme load of 29.06 IU_H⋅mL^–1. At such conditions S_Lu/TOS was 3.7, lactose conversion (X_Lactose) was 0.39 and total transgalactosylation yield was 0.762 g⋅g^–1, meaning that 76% of the reacted lactose corresponded to transgalactosylation and 24% to hydrolysis, which is a definite advantage of this mode of operation. Even though X_Lactose in CSTR was lower than in other reported modes of operation for lactulose synthesis, transgalactosylation was more favored over hydrolysis which reduced the inhibitory effect of galactose on β-galactosidase.

Improvements in the production of Aspergillus oryzae β-galactosidase crosslinked aggregates and their use in repeated-batch synthesis of lactulose

Aspergillus oryzae β-galactosidase was immobilized by aggregation and crosslinking, obtaining catalysts (CLAGs) well-endowed for lactulose synthesis. Type and concentration of the precipitating agent were determinants of immobilization yield, specific activity and thermal stability. CLAGs with specific activities of 64,007, 48,374 and 44,560 IU_H g^-1 were obtained using 50% v/v methanol, ethanol and propanol as precipitating agents respectively, with immobilization yields over 90%. Lactulose synthesis was conducted at 50 °C, pH 4.5, 50% w/w total sugars, 200 IU_H g^-1 of enzyme and fructose/lactose molar ratio of 8 in batch and repeated-batch operation. Lactulose yields were 0.19 g g^-1 and 0.24 g g^-1 for fructose to lactose molar ratios of 4 mol mol^-1 and 8 mol mol^-1 while selectivities were 3.3 mol mol^-1 and 6.6 mol mol^-1 respectively for CLAGs obtained by ethanol and propanol precipitation. Based on these results, both CLAGs were selected for the synthesis in repeated-batch mode. The cumulative mass of lactulose in repeated-batch was higher with CLAGs produced by ethanol and propanol precipitation than with the free enzyme. 86 and 93 repeated-batches could have been respectively performed with those CLAGs considering a catalyst replacement criterion of 50% of residual activity, as determined by simulation.

Naturally-derived biopolymers: Potential platforms for enzyme immobilization

Naturally-derived biopolymers such as alginate, chitosan, cellulose, agarose, guar gum/guaran, agar, carrageenan, gelatin, dextran, xanthan, and pectins, etc. have appealed significant attention over the past several years owing to their natural abundance and availability all over the years, around the globe. In addition, their versatile properties such as non-toxicity, biocompatibility, biodegradability, flexibility, renewability, and the availability of numerous reactive sites offer significant functionalities with multipurpose applications. At present, intensive research efforts have been focused on engineering enzymes using natural biopolymers as novel support/composite materials for diverse applications in biomedical, environmental, pharmaceutical, food and biofuel/energy sectors. Immobilization appears as a straightforward and promising approach to developing biocatalysts with improved catalytic properties as compared to their free counterparts. Biopolymers-assisted enzymes are more stable, robust, and recoverable than that of free forms, and can be employed for continuous biocatalytic reactions. The present review highlights the recent developments and use of biopolymers and their advanced composites as support carriers for the immobilization of a variety of different enzymes to develop biocatalysts with desired catalytic activity and stability characteristics for emerging applications.

d-Tagatose production by permeabilized and immobilized Lactobacillus plantarum using whey permeate

The aim of the work is to produce d-Tagatose by direct addition of alginate immobilized Lactobacillus plantarum cells to lactose hydrolysed whey permeate. The cells were untreated and immobilized (UIC), permeabilized and immobilized (PIC) and the relative activities were compared with purified l-arabinose isomerase (l-AI) for d-galactose isomerization. Successive lactose hydrolysis by β-galactosidase from Escherichia coli and d-galactose isomerization using l-AI from Lactobacillus plantarum was performed to investigate the in vivo production of d-tagatose in whey permeate. In whey permeate, maximum conversion of 38% and 33% (w/w) d-galactose isomerization by PIC and UIC has been obtained. 162mg/g and 141mg/g of d-tagatose production was recorded in a 48h reaction time at 50°C, pH 7.0 with 5mM Mn²⁺ ion concentration in the initial substrate mixture.