THE CHEMISTRY OF COOKERY, 213 



per is added ; no such effect is produced by gum-arabic, and thus we 

 have an easy test for distinguishing between true and fictitious gum- 

 arabic. 



The technical name for describing this persistence of composition 

 with changes of properties is isomerism^ and bodies thus related are 

 said to be isomeric with each other. Another distinguishing charac- 

 teristic of dextrin is that it produces a right-handed rotation on a ray 

 of polarized light — hence its name, from dexter, the right. 



The conversion of starch into dextrin is a very important element 

 of the subject of vegetable cooking, inasmuch as starch-food can not 

 be assimilated until this conversion has taken place, either before or 

 after we eat it. I will therefore describe other methods by which this 

 change may be effected. 



If starch be boiled in a dilute solution of almost any acid, it is con- 

 verted into dextrin. A solution containing less than one per cent of 

 sulphuric or nitric acid is sufficiently strong for this purpose. One 

 method of commercial manufacture (Payen's) is to moisten ten parts 

 of starch with three of water, containing y^ of its weight of nitric 

 acid, spreading the paste upon shelves, allowing it to dry in the air, 

 and then heating it for an hour and a half at about 240° Fahr. 



But the most remarkable and interesting agent in effecting this 

 conversion is diastase. It is one of those mysterious compounds which 

 have received the general name of " ferments." They are disturbers 

 of chemical peace, molecular agitators that initiate chemical revolutions, 

 which may be beneficent or very mischievous. The morbific matter 

 of contagious diseases, the venom of snake-bite, and a multitude of 

 other poisons, are ferments. Yeast is a familiar example of a ferment, 

 and one that is the best understood. I must not be tempted into a 

 dissertation on this subject, but may merely remark that modern re- 

 search indicates that many of these ferments are microscopic creatures, 

 linking the vegetable with the animal world ; they may be described 

 as living things, seeing that they grow from germs and generate other 

 germs that produce their like. Where this is proved, we can under- 

 stand how a minute germ may, by falling upon suitable nourishment, 

 increase and multiply, and thus effect upon large quantities of matter 

 the chemical revolution above named. 



I have already described the action of rennet upon milk, and the 

 very small quantity which produces coagulation. There appears to be 

 no intercession of living microbia in this case, nor have any been yet 

 demonstrated to constitute the ferment of diastase, though they may 

 be suspected. Be this as it may, diastase is a most beneficent ferment. 

 It communicates to the infant plant its first breath of active life, and 

 operates in the very first stage of animal digestion. 



In a grain of wheat, for example, the embryo is surrounded with 

 its first food. While the seed remains dry above-ground there is no 

 assimilation of the insoluble starch or gluten, no growth, nor other 



