A disaccharide or biose is the carbohydrate formed when two monosaccharides undergo a condensation reaction which involves the elimination of a small molecule, such as water, from the functional groups only. Like monosaccharides, disaccharides form an aqueous solution when dissolved in water. One of the more known examples is sucrose, the other lactose.
There are two different types of disaccharides: reducing disaccharides, in which one monosaccharide, the reducing sugar, still has a free hemiacetal unit; and non-reducing disaccharides, in which the components bond through an acetal linkage between their anomeric centers and neither monosaccharide has a free hemiacetal unit. Cellobiose and maltose are examples of reducing disaccharides. Sucrose and trehalose are examples of non-reducing disaccharides.  
Disaccharides are formed when two monosaccharides are joined together and a molecule of water is removed. For example; milk sugar (lactose) is made from glucose and galactose whereas the sugar from sugar cane and sugar beets (sucrose) is made from glucose and fructose.
The two monosaccharides are bonded via a dehydration reaction (also called a condensation reaction or dehydration synthesis) that leads to the loss of a molecule of water and formation of a glycosidic bond.
The glycosidic bond can be formed between any hydroxyl group on the component monosaccharide. So, even if both component sugars are the same (e.g., glucose), different bond combinations (regiochemistry) and stereochemistry (alpha- or beta-) result in disaccharides that are diastereoisomers with different chemical and physical properties.
Depending on the monosaccharide constituents, disaccharides are sometimes crystalline, sometimes water-soluble, and sometimes sweet-tasting and sticky-feeling.
Disaccharide Unit 1 Unit 2 Bond Sucrose (table sugar, cane sugar, beet sugar, or saccharose) glucose fructose α(1→2)β Lactulose galactose fructose β(1→4) Lactose (milk sugar) galactose glucose β(1→4) Maltose glucose glucose α(1→4) Trehalose glucose glucose α(1→1)α Cellobiose glucose glucose β(1→4)
Less common disaccharides include:
Disaccharide Units Bond Kojibiose two glucose monomers α(1→2)  Nigerose two glucose monomers α(1→3) Isomaltose two glucose monomers α(1→6) β,β-Trehalose two glucose monomers β(1→1)β α,β-Trehalose two glucose monomers α(1→1)β Sophorose two glucose monomers β(1→2) Laminaribiose two glucose monomers β(1→3) Gentiobiose two glucose monomers β(1→6) Turanose a glucose monomer and a fructose monomer α(1→3) Maltulose a glucose monomer and a fructose monomer α(1→4) Palatinose a glucose monomer and a fructose monomer α(1→6) Gentiobiulose a glucose monomer and a fructose monomer β(1→6) Mannobiose[disambiguation needed ] two mannose monomers either α(1→2), α(1→3), α(1→4), or α(1→6) Melibiose a galactose monomer and a glucose monomer α(1→6) Melibiulose a galactose monomer and a fructose monomer α(1→6) Rutinose a rhamnose monomer and a glucose monomer α(1→6) Rutinulose a rhamnose monomer and a fructose monomer β(1→6) Xylobiose two xylopyranose monomers β(1→4)
- ^ Biose on www.merriam-webster.com
- ^ IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version: (2006–) "disaccharides".
- ^ "Nomenclature of Carbohydrates (Recommendations 1996)2-Carb-36 Disaccharides.". http://www.chem.qmul.ac.uk/iupac/2carb/36.html.
- ^ "Disaccharides and Oligiosaccharides". http://faculty.virginia.edu/mcgarveylab/Carbsyn/Carblist/html/disacch.html. Retrieved 2008-01-29.
- ^ F.W.Parrish; W.B.Hahn,G.R.Mandels (July 1968). "Crypticity of Myrothecium verrucaria Spores to Maltose and Induction of Transport by Maltulose, a Common Maltose Contaminant". J. Bacteriol. (American Society for Microbiology) 96 (1): 227–233. PMC 252277. PMID 5690932. http://jb.asm.org/cgi/reprint/96/1/227.pdf. Retrieved 2008-11-21.
- ^ Matsuda, K.; Abe, Y; Fujioka, K (November 1957). "Kojibiose (2-O-alpha-D-Glucopyranosyl-D-Glucose): Isolation and Structure: Chemical Synthesis". Nature 180 (4593): 985–6. doi:10.1038/180985a0. PMID 13483573.
- ^ T. Taga; Y. Miwa, Z. Min (1997). "α,β-Trehalose Monohydrate". Acta Cryst. C53 (2): 234–236. doi:10.1107/S0108270196012693. http://scripts.iucr.org/cgi-bin/paper?S0108270196012693.
Types of carbohydrates General: Geometry MonosaccharidesAldodiose (Glycolaldehyde)>7 MultipleDisaccharidesOther oligosaccharidesFructooligosaccharide (FOS) · Galactooligosaccharides (GOS) · Mannan-oligosaccharides (MOS)
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