Elementary plant Biochemistry and Biotechnology (2+1)

Reaction with alkali

Dilute alkali

 

• Sugars in weak alkaline solutions undergo isomerization to form 1,2-enediolfollowed by the formation of a mixture of sugars.


Strong alkali


• Under strong alkaline conditions sugar undergo caramelization reactions.


Reducing property of sugars.


• Sugars are classified as either reducing or non-reducing depending upon the presence of potentially free aldehyde or keto groups. The reducing property is mainly due to the ability of these sugars to reduce metal ions such as copper or silver to form insoluble cuprous oxide, under alkaline condition. The aldehyde group of aldoses is oxidized to carboxylic acid.




• This reducingproperty is the basis for qualitative (Fehling's, Benedict's, Barfoed's and Nylander's tests) and quantitative reactions. All monosaccharides are reducing. In the case of oligosaccharides, if the molecule possesses a free aldehyde or ketone group it belongs to reducing sugar (maltose and lactose). If the reducing groups are involved in the formation of glycosodic linkage., the sugar belongs to the non- reducing group (trehalose, sucrose, raffinose and stachyose).