Starch is a carbohydrate having the composition (C6H10O5)n, which occurs in granules in most. green plants. It is generally formed by plastids; in leaves and other parts exposed to light, within chloroplastids; in rhizomes, tubers, roots, or other parts not exposed to light, in contact with leucoplastlds. A watery central mass, the hilum, is first formed, and then layers are added by apposition, the grains within plastids being commonly conceentric, whilst those merely in contact with a plastid at one end grow most at that end and become excentric. The form and size of the grain is characteristic of different species, so that the adulteration of food-starches can be readily detected by the microscope. The smallest known grains are those of rice, less than one five-thousandth of an inch across; the largest, those of tous-les-mois, one three-hundredth. The grains are doubly refracting and appear stratified. They contain from 2 to 5-1/2 per cent. of a less soluble substance known as farinose or starch-cellulose, forming a skeleton to the granule, the remainder being formed of the more soluble granulose. Starch, or rather granulose, turns blue with iodine and is very inert. When heated with water the grains absorb the water, swell, and ultimately burst, forming starch-paste, but not until heated to a very high temperature does it become truly soluble. When treated with malt extract (diastase), saliva or the pancreatic secretion, starch passes into dextrin, which is isomerous with starch, and maltose (C12H22O11). The ultimate product of its hydrolysis by dilute sulphuric acid is dextrose or grape-sugar (C6H12O6), thus:-
(C6H10O5)n + nH2O = n(C6H12O6).
In the process of assimilation in green plants starch is the first visible product; but it originates from sugar (q.v.) in solution, and is itself always of the nature of a reserve substance. After the sugar has passed from the leaves to stems, roots, or seeds, starch is re-formed in them by leucoplastids. In germinating seeds or at the rise of the sap, diastatic action once more turns the starch into sugar. Starch is the most important heat-giving or force-producing ingredient in human food. Potatoes contain 15, wheaten bread 48, haricots 49, peas 51, oatmeal 63, maize 64, rice 76, ana sago, tapioca, arrowroot, and cornflour abont 83 per cent. of starch. Besides its use under these various forms as food, enormous quantities of starch, which in England are made chiefly from rice and maize, are employed for laundry purposes and for stiffening textiles.