Aug. 10, 1883.] 



K^JOWLEDGE ♦ 



93 



CHEMISTRY OF THE CEREALS. 



By William J ago, F.C.S. 

 No. III. 



IT has been the object of the former two papers to give 

 some idea of the nature and properties of that interest- 

 ing group of compounds, starch, cellulose, and dextrin. 

 Viewing these bodies in their relation to plant-life, there is 

 every reason to believe that starch is the first-formed of the 

 three ; and that the plant, in the act of growing, draws on 

 its store of starch for the production of cellulose, as that 

 material is required for the building up of its structure. 

 So far, the process is one of development : from one or- 

 ganised substance, another, to be used for a more special 

 purpose, is formed. Coming to the dextrin, we find evi- 

 dence, not of life, but of death ; the starch-cells which, de- 

 spite their minuteness, are so delicately fashioned, have lost 

 all trace of structure, and are transformed into a perfectly 

 homogeneous body. Notwithstanding this (as already 

 known to the reader), the chemical composition is as yet 

 unaltered, so far as the number of atoms composing the 

 molecule is concerned. The starch undergoes yet another 

 stage of degeneration within the grain, by which it is con- 

 verted into sugar ; the gum and sugar of the cereals having 

 thus a common origin, are classed together in the analyses 

 given at the commencement of these papers. 



Starch, dextrin, and a number of other compounds are 

 frequently termed " carbo-hydrates." This name is applied 

 because they contain the element carbon combined with hydro- 

 gen and oxygen in the proportions of those elements neces- 

 sary to form water. Thus CsHioO., may be viewed as six atoms 

 of carbon and five molecules of H.,0, although the actual 

 groupings of the atoms within the molecule is undoubtedly 

 more complicated than this idea represents. Reference 

 has been made to this classification as carbo-hydrates, 

 because we thus have a connecting link between the starch 

 group and sugar, for this latter body is also a carbo-hydrate, 

 having the formula CiiHjoOg. The number of carbon atoms 

 is again the same ; but instead of five, there are here six 

 molecules of H^O. Starch passing through the modifica- 

 tions of soluble starch and dextrin is changed into sugar by 

 a very simple chemical operation : — 



CeH, A + H,,0 = C„Hi,0,, 



To changes of this kind, which consist of the assimila- 

 tion of the elements of water by a compound, the name 

 " hydrolysis " has been applied. Starch and water if 

 left to themselves do not readily combine to produce 

 sugar, but their combination may be eflected by a number 

 of bodies which do not necessarily themselves undergo 

 change during the hydrolysis. Several of the dilute acids 

 may bo used for this purpose ; the saliva also possesses the 

 property in a remarkable degree. This latter fact admits 

 of easy experimental proof in the following manner. Make 

 a tolerably stiff solution of arrowroot or cornllour by boil- 

 ing with water only ; let it cool, and when lukewarm, place 

 a spoonful in the mouth, and mix it thoroughly with the 

 saliva. Retain the mixture in the mouth, and very shortly the 

 solution becomes thin and watery,and acquires a sweet taste; 

 the starch has by that time become almost wholly converted 

 into sugar. This change is also, under certain circum- 

 stances, eflected by another group of bodies, and these are 

 of special interest, because they are constituents of the 

 grain itself. 



Sugar is so familiar to all, that it becomes scarcely 

 necessary to mention that it is a soluble body, and that 

 its great and leading character is its sweetness. Its 

 further properties must bo dealt with somewhat later, and, 



as its production from starch is the foundation of some 

 most important industries, we shall then describe these 

 properties in detail. [The foot-note appended by the Editor 

 to the last article of this series, giving a second version of 

 the discovery of dextrin, afibrds a clue to one industry, at 

 least, to which reference is here made.] 



All grains contain a certain small proportion of fat. 

 This is not, however, a very important constituent. 

 Chemical analysis shows the fats to be compounds con- 

 sisting of carbon, hydrogen, and a small proportion of 

 oxygen. 



We must in the next place turn our attention to re- 

 maining group of constituents of the cereals. These will 

 be found in the table of analyses, given at the head of the 

 first paper, classed under the term "albuminoids." The 

 percentage varies from 7 '2 in rice to IG'O in oats : wheat 

 again occupies an intermediate position, with a percentage 

 of 10'9. The albuminoids differ essentially in chemical 

 composition from the bodies we have heretofore studied, in 

 that they contain in addition to carbon, hydrogen, and 

 oxygen, the elements, nitrogen and sulphur. On analysis 

 they are found, on the average, to consist of carbon .5.3 '.3, 

 hydrogen 7-1, nitrogen, 15-7, oxygen 22-1, and sulphur 1-8 

 per cent. From these proportions it is impossible to deduce 

 a simple formula. The sulphur is only present in small 

 quantity, but as in a molecule we must have at least one 

 atom, we can get no formula simpler than that calculated 

 by Lieberkuhn, which is written C7.,Hii.2Ni,0.j.,S. 



The albuminoids are so-called because, both in chemical 

 composition and properties, they are strikingly like albu- 

 men ; this latter substance is well known to everybody, 

 occurring, as it does, in an almost pure form in white 

 of egg. 



When wheat-flour is made into a paste, and then washed 

 with a large quantity of water, a separation into three 

 distinct substances occurs. As already described, starch 

 is one of these substances, and renders the washing water 

 milky in appearance ; gluten is another, and remains in 

 the hand. It is a very sticky, elastic body, of a light-grey 

 colour. On setting the washing water aside to rest, the 

 starch falls to the bottom ; remaining in solution there is, 

 however, a little dextrin and sugar, and also another highly 

 important substance, viz., vegetable albumen. The gluten 

 on the one hand, and the vegetable albumen on the other, 

 are two of the most important of the group of albuminoids ; 

 they represent respectively the soluble and the insoluble 

 albuminoids ; this of itself is a most important subdivision 

 of the group. The soluble albuminoids consist of two 

 separate bodies — vegetable albumen, and legumin or vege- 

 table casein. On l)oiling the solution, the albumen coagu- 

 lates, and is deposited in white flakes ; the albumen thus 

 obtained is to all intents and purposes identical with that 

 of the white of egg and of blood. 



Another albuminoid of importance is found in the husk 

 or bran of wheat. This body is soluble in water, and 

 possesses in a remarkable degree the property of converting 

 starch into sugar ; hence if an infusion of bran be added 

 to starch solution, the latter speedily becomes thin and 

 watery. As a result of this action, bread made of whole 

 meal has always a specially sweet taste ; this is often so 

 pronounced that the bread after very short use becomes 

 distasteful to many. Corealiii shares with some of the 

 other albuminoids tliis peculiar property. 



C.luten is quite insoluble in water, and has scarcely any 

 taste ; when dry, it closely resembles glue in appearance ; 

 in the grain, it exists in a pul\-erulent form, and acquires 

 its toughness and elasticity on the addition of water. The 

 toughn(>ss of gluten gives dough made from wheat-flour its 

 peculiar elasticity, hence it becomes so deliciously light by 



