Starch is the most important carbohydrate of our diet. It is found exclusively in plants as stored food. The chief sources are cereals, potatoes, corn and rice.
Properties
1. Starch is a colourless, amorphous, tasteless powder. It is insoluble in water.
Soluble starch can be obtained by heating ordinary starch with 10% HCI for 24 hours followed by reprecipitation by adding alcohol. Starch so obtained is water soluble. Alternatively it can also be prepared by heating starch with water and glycerol under pressure.
When starch solution is heated to about 200°C, a lower polysaccharide (dextrin) is produced which is used as adhesive under the name of british gum. Actually, synthetic gum is polymer of more than one type of monosaccharides.
2. Hydrolysis : When boiled with dilute acid, it yields dextrin, maltose and ultimately glucose. When hydrolysed with the enzyme diastase, it gives maltose.
3. Action of iodine : With iodine, starch gives a blue colour which disappears on heating and reappears on cooling. In fact it is the amylose that gives a blue colour with iodine, amylopectin gives a red brown colour with iodine.
