Editing Reducing sugar (section)

{{Short description|Sugars that contain free OH group at the anomeric carbon atom}}
[[File:D-glucose lineal.svg|thumb|class=skin-invert-image|Reducing form of [[glucose]] (the [[aldehyde|aldehyde group]] is on the far right)]]

A '''reducing sugar''' is any [[sugar]] that is capable of acting as a [[reducing agent]].<ref>{{cite book |last1=Pratt |first1=Charlotte W. |last2=Cornely |first2=Kathleen  |date=2013 |title=Essential Biochemistry |publisher=Wiley |edition=Third |page=626 |isbn= 978-1118083505}}</ref> In an [[alkaline]] solution, a reducing sugar forms some [[aldehyde]] or [[ketone]], which allows it to act as a reducing agent, for example in [[Benedict's reagent]]. In such a reaction, the sugar becomes a [[carboxylic acid]].

All [[monosaccharide]]s are reducing sugars, along with some [[disaccharide]]s, some [[oligosaccharide]]s, and some [[polysaccharide]]s. The monosaccharides can be divided into two groups: the [[aldose]]s, which have an aldehyde group, and the [[ketose]]s, which have a ketone group. Ketoses must first [[tautomerization|tautomerize]] to aldoses before they can act as reducing sugars. The common dietary monosaccharides [[galactose]], [[glucose]] and [[fructose]] are all reducing sugars.

Disaccharides are formed from two monosaccharides and can be classified as either reducing or nonreducing. Nonreducing disaccharides like [[sucrose]] and [[trehalose]] have [[glycosidic bond]]s between their [[anomeric carbon]]s and thus cannot convert to an open-chain form with an aldehyde group; they are stuck in the cyclic form. Reducing disaccharides like [[lactose]] and [[maltose]] have only one of their two anomeric carbons involved in the glycosidic bond, while the other is free and can convert to an open-chain form with an aldehyde group.

The aldehyde functional group allows the sugar to act as a reducing agent, for example, in the [[Tollens' reagent|Tollens' test]] or [[Benedict's test]]. The cyclic [[hemiacetal]] forms of [[aldose]]s can open to reveal an aldehyde, and certain ketoses can undergo tautomerization to become aldoses. However, [[acetal]]s, including those found in polysaccharide linkages, cannot easily become free aldehydes.

Reducing sugars react with amino acids in the [[Maillard reaction]], a series of reactions that occurs while cooking food at high temperatures and that is important in determining the flavor of food. Also, the levels of reducing sugars in wine, juice, and sugarcane are indicative of the quality of these food products.