Methods of producing oligosaccharides for use as prebiotics
Provided herein are methods for preparing prebiotic oligosaccharides by non-enzymatic methods of glycosylation of monosaccharides and/or disaccharides or by hydrolysis of polymeric sugars to monosaccharides and simultaneous glycosylation of hydrolytic products. The methods may include mixing one or more types of monosaccharides and/or disaccharides with a water-deficient system at a temperature sufficient to form one or more types of prebiotic oligosaccharides. Also provided are methods of preparing prebiotic oligosaccharides comprising mixing starch, cellulose and/or lignocellulosic biomass with a water-deficient system at a temperature sufficient to form one or more types of prebiotic oligosaccharides that collectively make up at least 25% of the products of the synthesis.
1. A method comprising mixing one or more types of monosaccharides, disaccharides, or a combination thereof with a water-deficient system at a temperature sufficient to form one or more types of prebiotic oligosaccharides, wherein
the weight ratio of monosaccharides, disaccharides, or a combination thereof to water-deficient system is 0.01 to 10; and the water-deficient system comprises a metal salt selected from an alkali metal salt and/or an alkaline earth metal salt, water, and a catalytic amount of acid wherein
the molar ratio of water to metal salt in the water-deficient system is about 2 to about 12; and
the acid has a pKa of less than 4.
2. The method of claim 1 wherein the metal salt is a lithium salt.
3. The method of claim 1 wherein the metal salt is one or more selected from the group consisting of LiBr, LiCl, NaBr, CaCl 2 , CaBr 2 , MgCl 2 , NaI, LiI, CaI 2 , MgI 2 , AlCl 3 , AlBr 3 , MgBr 2 , ZnCl 2 , ZnBr 2 , LiClO 4 , Ca(ClO 4 ) 2 , LiSCN, and Ca(SCN) 2 .
4. The method of claim 1 wherein the metal salt is lithium bromide.
5. The method of claim 1 wherein the molar ratio of water to metal salt in the water-deficient system is about 2 to about 5.
6. The method of claim 1 wherein the water deficient system is a solution.
7. The method of claim 1 wherein the acid has a pKa of about −10 to less than 4.
8. The method of claim 7 wherein the acid is one or more selected from the group consisting of HCl, H 2 SO 4 , HNO 3 , H 3 PO 4 , CH 3 SO 3 H, tosylic acid, oxalic acid, glyoxylic acid, lactic acid, citric acid, formic acid, and trifluoroacetic acid.
9. The method of claim 1 wherein the water-deficient system comprises 0.5 mM to 500 mM acid.
10. The method of claim 1 wherein the monosaccharides, disaccharides, or combination thereof comprises glucose, fructose, galactose, xylose, mannose, arabinose, sucrose, lactose, maltose, cellobiose, apiose, rhamnose, hydrolyzed starch, hydrolyzed cellulose, hydrolyzed lignocellulosic biomass, or a combination of two or more thereof.
11. The method of claim 1 , wherein the mixing comprises a combination of monosaccharides and disaccharides with the water-deficient system.
12. The method of claim 1 wherein the temperature is about 50° C. to about 160° C.
13. The method of claim 12 wherein the temperature is about 70° C. to about 120° C.
14. The method of claim 1 wherein the monosaccharides, disaccharides, or a combination thereof and the water deficient system are mixed for 1 minute to 2 days.
15. The method of claim 1 further comprising adding a diluting solvent to the mixture comprising prebiotic oligosaccharides to form a diluted mixture in which the metal salt remains substantially in solution.
16. The method of claim 15 wherein the diluting solvent is water or methanol.
17. The method of claim 15 further comprising adding a precipitating solvent to the diluted mixture to selectively precipitate the metal salt or metal salt and unreacted monosaccharides, disaccharides, or a combination thereof over the prebiotic oligosaccharides.
18. The method of claim 17 wherein the precipitating solvent is acetone, ethanol, isopropanol, methyl isobutyl ketone or a mixture of any two or more thereof.
19. The method of claim 17 further comprising recycling the precipitated metal salt to form another water deficient system.
20. The method of claim 1 further comprising purifying the prebiotic oligosaccharides.
21. The method of claim 20 wherein the purified prebiotic oligosaccharides contain less than 5% by weight metal salt.
22. The method of claim 20 wherein the purified prebiotic oligosaccharides contain less than 5% by weight HMF and furfural.