CHAPTER VIII.

VEGETABLE THICKENINGS

HE principal substances used as thickeners are starch, fecula, flour,

various kinds of disintegrated starch, such as roasted starch, lei'ocome,dextrine, gommeline, gomma, the gum produced by cherry and plum trees, thegenuine gums, such as arabic, Senegal, tragacanth, Bassora gum, salep, sago,linseed, and carragheea. All these substances are, in chemical parlance,carbo-hydrates ; their formula is typified by that of cellulose, with which theyhave many properties in common.

Amylaceous Matters—Starch, Fecula, C 6 H 10 0 3 , or Ci^H^oOio.

Amylum, that is, grain which needs no grinding, is a very common constituentof the entire vegetable kingdom. It exhibits an organised substance of globularshape, the size of the globules varying with the species of the plant fromwhich it is produced. Every grain is composed of concentric layers ofdifferent densities, superimposed one upon another. In the centre a smallnucleus may be seen by the aid of a microscope, which serves as a commoncentre for the different layers. The peculiar construdtion of the starchglobules is seen to greater advantage after the substance has been heated to392 0 F. (200° C.) and then swollen by immersion in water. The shape ofthese globules is polyangular. Their size varies considerably ; expressed inthousandths of a millimetre it is as follows:— Canna gigantea, ovoid shaped,175 diameter; Solatium tuberosum (potato), ovoid, 183 diameter; beans(cotyledons), oblong, 75 diameter; wheat, and similar kinds of grain, spherical,45 to 50 diameter; sago, a flattened ovoid, 35 diameter; for Zea Mais (Indiancorn) a rounded polyhedron, 24 diameter; the seed of Chenopodium quinoa,globular, 2 diameter.

The amylaceous matter is enclosed in cells while in the living plant, andis found in abundance, together with more or less nitrogenous matter, in thetrunks of many kinds of palm trees, in the perispermium and cotyledons ofseeds, more especially beans, peas, and the various kinds of grain; in theroots, tubers, and bulbs of plants, as, for instance, in carrots, rhubarb, liquorice,manioc, potatoes, florentine iris, bryonia, tulips, &c.; in some fruits, as acorns,chestnuts, buckwheat, &c. ; it is also found in many kinds of lichens.

Starch is used only in the pure state, as obtained from wheat, rice, potatoes,chestnuts, Indian corn, and the various kinds of arrow-root, with all whichvegetable substances it is intimately associated. The methods by whichstarch is prepared are based upon its insolubility in cold water, together withits comparatively high specific gravity, and its resistance to the aftion bothof weak acids and alkalies, and also to putrefaiftive fermentation at the