STARCH. This is perhaps the most widely distributed sub stance in the vegetable kingdom and occurs, often in great abun dance, in almost every plant. Pure starch has the appearance of a white, glistening, friable powder, and possesses a harsh feel when rubbed between the fingers; it is tasteless, has no smell, is not soluble in cold water and undergoes no change when exposed to the air. When examined under the microscope it is found to consist of granules of definite shape, size and appearance. These show very wide variations, some being minute while others attain a comparatively large size; they are, nevertheless, quite charac teristic of the plant from whence they are obtained, and the iden tification of any particular starch by comparison with those of known origin is a more or less simple matter. Granules or cells which are usually oval in shape are found to be composed of a series of concentric layers arranged around a nucleus or hilum which appears as a dark spot, the outer layers being the oldest in point of growth.
Starch belongs to the group of carbohydrates in which are included the sugars, gums and cellulose, and is composed of the elements carbon, hydrogen and oxygen. Its constitution is very complex, but it is built up in the plant from two simple compounds, water and the carbon dioxide gas present in the atmosphere, through the agency of the green colouring matter, chlorophyll, under the activating influence of sunlight. The role of starch in plant economy is that of a reserve material, and as such it must be transferred from the leaves and other green parts of the plant, where it is formed, to more permanent quarters. To facilitate this migration it is broken down into sugars which dissolve in the cell juice, in this way passing through the cell walls of the plant finally to become reconverted into starch, either in the fruit or seeds, or in the bulbs and tubers as the case may be. It is thus rendered available as a source of sustenance for the young plant until it is sufficiently advanced to carry on the process of starch formation itself. The starch granule is composed of two distinct but similar compounds, amylose and amylopectin, the former being in considerable excess, and although it is quite insoluble in cold water, when mixed with hot water the cells rupture consider ably below boiling point, and form a viscous, jelly-like liquid which is known as "starch paste," setting into a solid mass on cooling. When starch paste is heated with dilute acids the starch
is first converted into the soluble form and, by a process known as hydrolysis, is subsequently changed into a series of sugars, the principal of which are dextrin, maltose and glucose. This process is employed in the manufacture of corn or glucose syrup, used very extensively in confectionery, and of glucose sugar used in brewing. Hydrolysis is also effected by certain natural ferments or "enzymes" such as those present in saliva, pancreatic juice, malted grain (diastase) and in fact all germinated seeds, and it is the latter agent which is employed in the fermentation industries— brewing, distilling, etc.—converting the starch in the materials used into material capable of fermentation by yeast. The devel opment of starch in the growing plant already referred to, and the rebuilding of the soluble products into the cellulose which constitutes the tissues, is carried out by ferments or enzymes elaborated by the plant itself, while a similar process goes on in the human system when starchy foods are eaten and converted into soluble sugars capable of assimilation.
Dry starch heated to about 32o° F is transformed into dextrin, a pale, yellowish powder soluble in water, known as British gum.
A very sensitive reaction of starch is the production of an intense blue coloration with a solution of iodine, a characteristic much used in its identification.
The average starch content of the principal starch yielding plants, together with the temperatures at which the purified starch gelatinizes when heated with water, are, according to Lippmann, as in the following table: Average Starch Gelatinizing content % temp.° F Potato ..... . . . 18-20 Wheat ..... . . . 54-58 152 Barley ..... . . . 40-46 Oats ...... . . 35-38 Rye ..... . . . 131 Maize ..... . . . Rice ..... . • • 70-79 242 Millet ...... . • 53-55 Peas ...... . . 39-40 . .