STRUCTURE OF STARCH: It is the reserve carbohydrate of plant kingdom. The most common sources of starch include but not limited to; Potatoes, tapioca, cereals including rice and wheat, and other food grains. Starch is composed of amylose and amylopectin. When starch is treated with boiling water, 10-20% is solubilized; this part is called amylose. Amylose is made up of glucose units with alpha-1,4 glycosidic linkages to form an unbranched long chain with a molecular weight 400,000 D or more. The insoluble part absorbs water and forms paste like gel and this is called amylopectin. Amylopectin is also made up of glucose units, but is highly branched with molecular weight more than 1 million. The branching points are made by alpha-1,6 linkage similar to isomaltose.
HYDROLYSIS OF STARCH: Starch will form a blue colored complex with iodine; this color disappears on heating and reappears when cooled. This is a sensitive test for starch. Starch is non reducing because the free sugar groups are negligible in number. When starch is hydrolysed by mild acid, smaller and smaller fragments are produced. Thus hydrolysis for a short time produces amylodextrin which gives violet color with iodine and is nonreducing. Further hydrolysis produces erythrodextrin which gives red color with iodine and mild reduction of Benedict's solution. Later achrodextrins (no color with iodine, but reducing) and further on, maltose (no color with iodine, but powerfully reducing) are formed on continued hydrolysis.
ACTION OF AMYLASES ON STARCH: Salivary amylase and pancreatic amylase are alpha-amylases, which act at random on alpha1,4 glycosidic bonds to split starch into smaller units (dextrins), and finally to alpha-maltose. Beta-amylases are of plant origin (almond, germinating seeds, etc) which split starch to form beta-maltose. They act on amylose to split maltose units consecutively. Thus the enzyme starts its action from one end. When beta-amylase acts on amylopectin, maltose units are liberated from the ends of the branches of amylopectin, until the action of enzyme is blocked at the 1,6-glycosidic linkage. The action of beta-amylase stops at branching points, leaving a large molecule, called limit dextrin or residual dextrin.
RELATED;
No comments:
Post a Comment