Catalytic isomerization of galactose into tagatose in the presence of bases and Lewis acids

Biobased Chemicals from d-Galactose: An Efficient Route to 5-Hydroxymethylfurfural Using a Water/MIBK System in Combination with an HCl/AlCl3 Catalyst

5-Hydroxymethylfurfural (HMF) is an attractive building block for biobased chemicals. Typically, ketoses like d-fructose (FRC) are suitable starting materials and give good yields of HMF in a simple aqueous phase process with a Bro̷nsted acid catalyst. With aldoses, such as d-glucose (GLU), much lower yields were reported in the literature. Here, we report an experimental and modeling study on the use of d-galactose (GAL) for HMF synthesis, using a liquid-liquid system (water/MIBK) in combination with an HCl/AlCl3 catalyst. Experiments were conducted in a batch system with temperatures between 112 and 153 °C, HCl and AlCl3 concentrations ranging from 0.02 to 0.04 M, and initial GAL concentrations between 0.1 and 1.0 M. The highest HMF yield was 49 mol % obtained for a batch time of 90 min at 135 °C. This value is much higher than in experiments with GAL in a monophasic aqueous system with HCl as the catalyst (2 mol % HMF yield) under similar reaction conditions. Based on detailed product analyses, a reaction scheme is proposed in which the isomerization of GAL to tagatose (TAG), catalyzed by the Lewis acid AlCl3, is the first and key step. TAG is then converted to HMF by Bro̷nsted acid HCl. The experimental data were modeled using a statistical approach as well as a kinetic approach. The kinetic model demonstrates a good agreement between the experimental and modeled data. Our findings reveal that temperature is the reaction variable with the most significant influence on the HMF yield. The use of a biphasic system appears to be a promising method for HMF production from GAL.

Efficient synthesis of rare sugars from galactose in hot compressed water using eggshells as an environmentally friendly catalyst

Aqueous galactose solutions containing eggshell was heated at 120 °C to produce calcium supplements containing rare sugars. Galactose was isomerized to rare sugars with improving rare sugar yields compared to those without eggshell. Organic acids were also formed as byproducts during the reaction. These acids were neutralized by dissolving eggshells with increasing the calcium ion concentration in the solution. When eggshell components (calcium carbonate, magnesium carbonate, or calcium phosphate) were used for the treatment, rare sugars were also formed. Especially, addition of magnesium carbonate improved rare sugar yield, but byproduct formation became more pronounced. Eggshells used in the treatment were used for repeated treatments. When eggshells were used three times, rare sugar yield changed only slightly but the selectivity of rare sugars improved significantly. By these processes, we obtained an aqueous solution of rare sugars containing calcium ion at 295 mg/L, which has potential as ingredients for dietary supplements.

Effect of phosphate buffer concentration on the isomerization of galactose to rare sugars under subcritical water conditions

Galactose was treated in sodium phosphate buffer at various concentrations (0.1-500 mmol/L) under subcritical water conditions (140 °C), and the effects of the buffer concentration and reaction time (0-300 s) on the reaction behavior were evaluated. The reaction proceeded rapidly at higher buffer concentrations. Rare sugars (tagatose, talose, and sorbose) were formed from galactose by isomerization. The highest yield of the main product, tagatose, was approximately 14 % in 50 mmol/L buffer. However, the tagatose yield did not increase further with increasing buffer concentration. On the other hand, the formation of talose and sorbose was accelerated at higher buffer concentrations, with the highest yields of approximately 5 % and 12 %, respectively, in 500 mmol/L buffer. At the same time, the formation of byproducts (organic acids and colored substances) was also accelerated in high-concentration buffers. These results suggest that phosphate buffer promoted all reactions occurring under subcritical water conditions.

Highly efficient production and simultaneous purification of d-tagatose through one-pot extraction-assisted isomerization of d-galactose

A one-pot extraction-assisted d-galactose-to-d-tagatose isomerization strategy was proposed based on the selective extraction of d-tagatose by phenylborate anions. 4-Vinylphenylboronic acid was selected with high extraction efficiency and selectivity towards d-tagatose. The extracted sugars could be desorbed through a two-staged stripping process with the purity of d-tagatose significantly increased. In-situ extraction-assisted d-galactose-to-d-tagatose isomerization was implemented for the first time ever reported, and the effect of boron-to-sugar ratio (boron: sugar) was investigated. The conversion yield of d-tagatose at 60 °C increased from ∼ 39 % (boron: sugar = 0.5) to ∼ 56 % (boron: sugar = 1) but then decreased to ∼ 44 % (boron: sugar = 1.5). With temperature increased to 70 °C, the conversion yield of d-tagatose was further improved to ∼ 61 % (boron: sugar = 1.5), with the minimized formation of byproducts. Moreover, high purity (∼83 %) and concentrated d-tagatose solution (∼40 g/L) was obtained after sequential desorption. The proposed extraction-assisted isomerization strategy achieved improving the yield and purity of d-tagatose, proving its feasibility in industrial applications.

Isomerization of Galactose to Tagatose: Recent Advances in Non-enzymatic Isomerization

The valorization of galactose derived from acid whey to low-calorie tagatose has gained increasing attention. Enzymatic isomerization is of great interest but faces several challenges, such as poor thermal stability of enzymes and a long processing time. In this work, non-enzymatic (supercritical fluids, triethylamine, arginine, boronate affinity, hydrotalcite, Sn-β zeolite, and calcium hydroxide) pathways for galactose to tagatose isomerization were critically discussed. Unfortunately, most of these chemicals showed poor tagatose yields (<30%), except for calcium hydroxide (>70%). The latter is able to form a tagatose-calcium hydroxide-water complex, which stimulates the equilibrium toward tagatose and prevents sugar degradation. Nevertheless, the excessive use of calcium hydroxide may pose challenges in terms of economic and environmental feasibility. Moreover, the proposed mechanisms for the base (enediol intermediate) and Lewis acid (hydride shift between C-2 and C-1) catalysis of galactose were elucidated. Overall, it is crucial to explore novel and effective catalysts as well as integrated systems for isomerizing of galactose to tagatose.

Physicochemical Characterization of Texture-Modified Pumpkin by Vacuum Enzyme Impregnation: Textural, Chemical, and Image Analysis

Texture-modified pumpkin was developed by using vacuum enzyme impregnation to soften texture to tolerable limits for the elderly population with swallowing and chewing difficulties. The impregnation process and macrostructural and microstructural enzyme action were explored by the laser light backscattering imaging technique and a microscopic study by digital image analysis. Texture was analyzed by a compression assay. The effect of enzyme treatment on antioxidant capacity and sugar content was evaluated and compared to the traditional cooking effect. Image analysis data demonstrated the effectiveness of the impregnation process and enzyme action on plant cell walls. Enzyme-treated samples at the end of the process had lower stiffness values with no fracture point, significantly greater antioxidant capacity and significantly lower total and reducing sugars contents than traditionally cooked pumpkins. The results herein obtained demonstrate the capability of using vacuum impregnation treatment with enzymes to soften pumpkins and their positive effects on antioxidant capacity and sugar content to develop safe and sensory-accepted texture-modified products for specific elderly populations.

Preparation of combined cross-linked enzyme aggregates containing galactitol dehydrogenase and NADH oxidase for L-tagatose synthesis via in situ cofactor regeneration

The combined cross-linked enzyme aggregates (combi-CLEAs) containing galactitol dehydrogenase (Gdh) and NADH oxidase (Nox) were prepared for L-tagatose synthesis. To prevent the excess consumption of cofactor, Nox in the combi-CLEAs was used to in situ regenerate NAD⁺. In the immobilization process, ammonia sulfate and glutaraldehyde were used as the precipitant and cross-linking reagent, respectively. The preparation conditions were optimized as follows: 60% ammonium sulfate, 1:1 (molar ratio) of Gdh to Nox, 20:1 (molar ratio) of protein to glutaraldehyde, and 6 h of cross-linking time at 35 °C. Under these conditions, the activity of the combi-CLEAs was 210 U g^-1. The combi-CLEAs exhibited higher thermostability and preserved 51.5% of the original activity after eight cycles of reuses at 45 °C. The combi-CLEAs were utilized for the preparation of L-tagatose without by-products. Therefore, the combi-CLEAs have the industrial potential for the bioconversion of galactitol to L-tagatose.

An overview of D-galactose utilization through microbial fermentation and enzyme-catalyzed conversion

D-Galactose is an abundant carbohydrate monomer in nature and widely exists in macroalgae, plants, and dairy wastes. D-Galactose is useful as a raw material for biomass fuel production or low-calorie sweetener production, attracting increased attention. This article summarizes the studies on biotechnological processes for galactose utilization. Two main research directions of microbial fermentation and enzyme-catalyzed conversion from galactose-rich biomass are extensively reviewed. The review provides the recent discoveries for biofuel production from macroalgae, including the innovative methods in the pretreatment process and technological development in the fermentation process. As modern people pay more attention to health, enzyme technologies for low-calorie sweetener production are more urgently needed. D-Tagatose is a promising low-calorie alternative to sugar. We discuss the recent studies on characterization and genetic modification of L-arabinose isomerase to improve the bioconversion of D-galactose to D-tagatose. In addition, the trends and critical challenges in both research directions are outlined at the end. KEY POINTS: • The value and significance of galactose utilization are highlighted. • Biofuel production from galactose-rich biomass is accomplished by fermentation. • L-arabinose isomerase is a tool for bioconversion of D-galactose to D-tagatose.

Recent advances in the synthesis and applications of mordenite zeolite - review

Among the many industrially important zeolites, mordenite is found to be interesting because of its unique and exceptional physical and chemical properties. Mordenite (high silica zeolite) is generally prepared by the hydrothermal method using TEA+ cations. TEA+ cations are the best templating agent, though they can create a number of issues, for instance, generating poison and high manufacturing cost, wastewater contamination, and environmental pollution. Hence, it is necessary to find a mordenite synthesis method without using an organic template or low-cost template. In this review, a number of unique sources were used in the preparation of mordenite zeolite, for instance, silica sources (rice husk ash, silica gel, silica fumes), alumina sources (metakaolin, faujasite zeolite) and sources containing both silica and alumina (waste coal fly ash). These synthesis approaches are also based on the absence of a template or low-cost mixed organic templates (for instance, glycerol (GL), ethylene glycol (EG), and polyethylene glycol 200 (PEG)) or pyrrolidine-based mesoporogen (N-cetyl-N-methylpyrrolidinium) modifying the mordenite framework which can create unique properties. The framework properties and optical properties (indium-exchanged mordenite zeolite) have been discussed. Mordenite is generally used in alkylation, dewaxing, reforming, hydrocracking, catalysis, separation, and purification reactions because of its large pore size, strong acidity, and high thermal and chemical stability, although the applications are not limited for mordenite zeolite. Recently, several applications such as electrochemical detection, isomerization, carbonylation, hydrodeoxygenation, adsorption, biomass conversion, biological applications (antibacterial activity), photocatalysis, fuel cells and polymerization reactions using mordenite zeolite were explored which have been described in detail in this review.

A Three-Step Process for the Bioconversion of Whey Permeate into a Glucose-Free D-Tagatose Syrup

We have developed a sustainable three-stage process for the revaluation of cheese whey permeate into D-tagatose, a rare sugar with functional properties used as sweetener. The experimental conditions (pH, temperature, cofactors, etc.) for each step were independently optimized. In the first step, concentrated whey containing 180–200 g/L of lactose was fully hydrolyzed by β-galactosidase from Bifidobacterium bifidum (Saphera®) in 3 h at 45 °C. Secondly, glucose was selectively removed by treatment with Pichia pastoris cells for 3 h at 30 °C. The best results were obtained with 350 mg of cells (previously grown for 16 h) per mL of solution. Finally, L-arabinose isomerase US100 from Bacillus stearothermophilus was employed to isomerize D-galactose into D-tagatose at pH 7.5 and 65 °C, in presence of 0.5 mM MnSO4. After 7 h, the concentration of D-tagatose was approximately 30 g/L (33.3% yield, referred to the initial D-galactose present in whey). The proposed integrated process takes place under mild conditions (neutral pH, moderate temperatures) in a short time (13 h), yielding a glucose-free syrup containing D-tagatose and galactose in a ratio 1:2 (w/w).

One-pot synthesis of sweetening syrup from lactose

Lactose has become the main byproduct of many dairy products and ingredients. Current applications of lactose are insufficient to use the recovered lactose from manufacturing operations. Here we exemplified a new process for converting aqueous lactose into a sweeting syrup via one-pot synthesis. The synthesis consisted of two-steps: (1) enzymatic hydrolysis of lactose and (2) catalytic isomerization over MgO/SiO2. The hydrolysis of lactose over β-galactosidase converted 95.77 ± 0.67% of lactose into glucose and galactose. The catalytic isomerization was performed over MgO/SiO2 with different MgO loadings (10-40 wt.%). A battery of tests was conducted to characterize the different catalysts, including surface properties, basicity, and microstructure. The one-pot synthesis, enzymatic hydrolysis and catalytic isomerization over 20%-MgO/SiO2, converted 99.3% of lactose into a sweetening syrup made of glucose (30.48%), galactose (33.51%), fructose (16.92%), D-tagatose (10.54%), and lactulose (3.62%). The outcomes of this research present an opportunity for expanding the utilization of lactose.

Food Industrial Production of Monosaccharides Using Microbial, Enzymatic, and Chemical Methods

Most monosaccharides in nature are hexoses, which have six carbon atoms; the most well-known hexose is d-glucose. Various hexoses with distinct characteristics can be produced from inexpensive polysaccharides for applications in the food industry. Therefore, identification of the health-related functions of hexose will facilitate the consumption of hexoses in food products to improve quality of life. The hexoses available in foods include N-acetyl glucosamine, d-glucosamine, d-fructose, d-mannose, d-galactose, other d-hexoses, and l-hexoses. Here, an updated overview of food industrial production methods for natural hexoses by microbial, enzymatic, and chemical methods is provided.