Lactulose, alias 4-O-β-D-galactopyranosyl-D-fructose, is a disaccharide composed of galactose and fructose, which is not found in nature. Generally, lactulose is present in a syrupy product. Lactulose has important physiological and pharmacological functions and is widely used in many fields such as clinical medicine, health care products and food additives.
Lactulose chemical name 4-O-β-D-galactopyranosyl-D-fructose, molecular weight 342, is an isomer with lactose. The finished lactulose is a light yellow clear viscous body with a sweet taste. The crystal is a white irregular powder with a relative density of 1.5, a melting point of 169 ° C, and is easily soluble in water with a solubility of 76.4 ± 1.4%. Its sweetness is equivalent to less than sucrose, about 48% to 60% sucrose, with a feeling of cool alcohol, low viscosity, low calorific value, high safety, good stability, no Maillard reaction.
1. Chemical method
Alkali single catalysis: The system mainly uses sodium hydroxide, potassium hydroxide, potassium carbonate, and a tertiary amine as catalysts, and the conversion rate of lactulose is about 20%. Such an alkaline agent acts on lactose to isomerize lactose to form lactulose, but at the same time produces a considerable amount of degradation products such as galactose, fructose, etc., and the product is not only difficult to separate, but also has a deep color. The production of by-products and pigments not only reduces the yield of lactulose but also makes it difficult to further purify the syrup and to prepare crystals.
Acid-base synergistic catalysis: The system is based on the addition of boric acid to the reaction system. Under alkaline conditions, boric acid can form lactulose to form a lactulose-borate complex, so that the equilibrium of the reaction is favorable to the formation of lactulose. Move in the direction. The formation of the complex and the shift of the equilibrium of the reaction minimize the formation of degradation products. Only a small part of lactulose is converted into by-products during the whole reaction process. The conversion rate of lactulose is about 70% and the highest is 75%. After the completion of the conversion, the pH of the reaction system is converted to acidity, the complex is decomposed to form lactulose and borate, and the borate is removed to obtain pure lactulose. Since borate is a weak acid salt, the general anion exchange resin can hardly be removed below the safety standard, and an ion exchange resin that specifically removes boric acid is required, which is expensive. Synergistic preparation of lactulose by acid-base reduces the formation of by-products, improves the yield of lactulose, and facilitates the purification and purification of lactulose, which is the main method for the industrial production of lactulose.
Sodium aluminate catalysis: The mechanism of action of this system is similar to that of boric acid. Sodium aluminate can be complexed with lactulose produced by isomerization reaction, and the conversion rate of lactulose is about 60%; but in general, aluminate The formation ratio of by-products in the system is higher than that of the borate system, and the removal of aluminum ions is difficult. In recent years, some scholars have used non-uniform catalysts including zeolite, sepiolite, eggshell powder, oyster shell powder to prepare lactulose, and found that 20% lactulose conversion can be obtained at 15g/L and 90°C. Rate; using eggshell powder as the catalyst, 18%-21% lactulose conversion rate was obtained under the reaction of 12g/L and 96°C for 120min. Although zeolite, sepiolite, eggshell powder, and oyster shell powder are not high in the conversion rate of lactulose, they remain in a solid state and are easily removed by filtration. Therefore, the preparation of lactulose by a heterogeneous catalyst has strong development potential.
2. Enzymatic method
Enzymatic method is the main method for the biological production of lactulose. The enzyme used is mainly β-galactosidase. The principle is to hydrolyze lactose into galactose and glucose through the hydrolysis activity of β-galactosidase, through β-half The transglycosylation activity of lactosidase transfers galactose to the fructose receptor to form lactulose. Β-galactosidase, the full name of β-D-galactoside galactose hydrolase, in addition to higher hydrolyzed lactose activity, the enzyme also has a higher transfer of galactose to the fructose receptor Glycoside activity. Early studies have shown that the active site on β-galactosidase has two functional groups: the thiol group of Cys and the imidazolyl group of His, which play an important role in the hydrolysis of lactose by β-galactosidase. It is speculated that the thio group can be used as a generalized acid to protonate the oxygen atom of galactoside, and the imidazolyl can act as a nucleophile to attack the nucleophilic center on the first carbon atom of the galactose molecule to form a carbon-containing bond. Covalent intermediates. After the imidazole group is cleaved, the sulfhydryl anion extracts one proton from the water molecule, thereby forming an -OH attack C. Β-galactosidase is mainly derived from animals, plants, and microorganisms. Such as bacteria in the lactic acid bacteria, Bacillus circulans, Escherichia coli, Streptococcus thermophilus, gastrointestinal bacteria, etc.; Aspergillus oryzae, Aspergillus niger, Aspergillus niger, P. chrysogenum, Aspergillus charcoal, etc. in the mold; Kluyveromyces cerevisiae, Kluyveromyces cerevisiae, Candida tropicalis, etc.; Streptomyces coelicolor in actinomycetes produce β-galactosidase. Due to the rapid growth of microorganisms and the biological characteristics of efficient metabolism, β-galactosidase derived from microorganisms has industrial application value; commercial enzyme sources generally consider yeast to be the safest, followed by Aspergillus niger. At present, due to the low activity of β-galactosidase transglycoside, the conversion rate of lactulose is low, which limits the application of biological methods in the preparation of lactulose. Many scholars have done some research on this, the purpose is to screen out the microbial strains with high β-galactosidase transglycoside activity. Using a genetically modified heat-resistant lactase, lactose and fructose as substrates, reacted at 80 ° C for 6 h to produce 50 mg/ml of lactulose. The enzymatic preparation of lactulose can overcome the disadvantages of a large number of colored by-products produced by chemical methods, the high cost of separating these by-products and the degradation of lactulose during the separation process, which has strong theoretical and practical significance. This is an important development direction for the preparation of lactulose in the future.