Wheat (Triticum vulgare) forms the principal bread-stuff of civilized nations, and is by far the most important of all cereal grasses. It has one or more slender, erect and smooth stalks, which, owing to the large proportion of siliceous matter present, possesses the strength necessary for the support of the ears. The grain is imbricated in four rows. The following are the averages of the results obtained by the analyses of 260 samples of American wheat, made by the United States Department of Agriculture, in 1883:—

Analyses of the ash of wheat by the same Department, furnished the following results:—

FLOUR.

The name flour is usually given to the product obtained by grinding wheat and removing the bran, or woody portion of the grain, by sifting or bolting. Its constituents are starch, dextrine, cellulose, and sugar (carbohydrates), the nitrogenous compounds albumen, gliadin, mucin, fibrin, and cerealin, and fat, mineral substances and water. Upon kneading flour with water, and removing the starch and soluble matters by repeated washing, an adhesive body termed gluten remains behind. This is chiefly composed of gliadin, mucin, and fibrin.

According to Wanklyn,[50] the general composition of flour is:—

The average of numerous analyses of American flour examined by the Department of Agriculture gave:—

The composition of the ash of flour from Minnesota wheat (1883), is as follows:—

Analysis of Flour.

The following are the determinations generally required in the proximate analysis of flour:—

Water.—Two or three grammes of the sample are weighed in a tared platinum dish, and heated in an air bath, until constant weight is obtained. The proportion of water should not exceed 17 per cent.

Starch.—A small amount of the flour is placed in a flask, connected with an ascending Liebig’s condenser, and boiled for several hours with water slightly acidulated with sulphuric acid. Any remaining excess of acid is then neutralised with sodium hydroxide; the solution is considerably diluted, and the glucose formed, estimated by means of Fehling’s solution (see p. 111). 100 parts of glucose represent 90 parts of starch.

Fat.—The inconsiderable proportion of fat in flour is best determined by exhausting the dried sample with ether and evaporating the solution.

Gluten (albuminoids).—As previously stated, gluten is separated by kneading the flour and repeated washing with water. After the removal of the amylaceous and soluble ingredients, the residue is carefully dried and weighed. A far more accurate method is to make a combustion of a small portion of the flour with cupric oxide, and determine the quantity of nitrogen obtained, the percentage of which, multiplied by 6·33, gives the percentage of gluten.[51] The proportion of gluten in flour ranges from about 8 to 18 per cent. From 10 to 12 per cent, is deemed necessary in order to make good bread, and, in England, any deficiency in this constituent is remedied by the addition of bean or other flour, but in the United States this practice is seldom required.

Substances soluble in cold water.—About five grammes of the flour are digested with 250 c.c. of cold water, and the solution filtered, and evaporated to dryness. Good flour is stated to yield 4·7 per cent. of extract when treated in this manner, the soluble matters consisting of sugar, gum, dextrine, vegetable albumen, and potassium phosphate. The latter salt, which constitutes about 0·4 per cent. of the extract, should form the only mineral matter present.

The Ash.—The ash of flour is determined in the usual manner, by ignition in a platinum dish. It varies in amount from 0·3 to 0·8 per cent., and should never exceed a proportion of 1·5 per cent.

When of good quality, wheaten flour is perfectly white, or has only a faint tinge of yellow. It should be free from bran, and must not show red, grey, or black specks, nor possess a disagreeable odour. It should also exhibit a neutral reaction and a decided cohesiveness, acquiring a peculiar soft and cushion-like condition when slightly compressed. Formerly, wheaten flour was mixed with various foreign meals, such as rye, corn, barley, peas, beans, rice, linseed, buckwheat, and potato starch; but at present this form of adulteration is probably but rarely resorted to, at least in the United States. The presence of mildew, darnel, ergot, and other parasites of the grain, constitutes an occasional contamination of flour. The most frequent admixture consists, however, in the addition of alum, which, although more extensively used in bread, is also employed in order to disguise the presence of damaged flour in mixtures, or to improve the appearance of an inferior grade; its addition to a damaged article serves to arrest the decomposition of the gluten, thereby preventing the flour from acquiring a dark colour, and disagreeable taste and odour.

It has recently been stated that in flour which has been kept for a long time in sacks, a transformation of the gluten sometimes occurs, resulting in the production of a poisonous alkaloid. This body may be separated by evaporating the ethereal extract of the flour to dryness, and treating the residue with water. The presence of the alkaloid in the filtered aqueous solution is recognised by means of potassium ferrocyanide. The presence of an excessive proportion of moisture is doubtless instrumental in the formation of toxic alkaloids or fungi in old flour and bread.

Pure wheaten flour is coloured yellow when treated with ammonium hydroxide, whereas corn meal assumes a pale brown colour, and the meals prepared from peas, beans, etc., become dark brown in colour when tested in this way. Nitric acid imparts an orange-yellow colour to wheaten flour, but fails to change the colour of potato-starch, with which it forms a stiff and tenacious paste.

Potato-starch is readily detected by examining a thin layer of the sample on a slide under the microscope, and adding a dilute solution of potassium hydroxide, which, while not affecting the wheaten starch, causes the potato-starch granules to swell up very considerably. Leguminous starches, such as peas, etc., contain approximately 2·5 per cent. of mineral matter; in pure flour, the average proportion of ash is only about 0·7 per cent., and this difference is sometimes useful in the detection of an admixture of the former.

The external envelope of the granules of potato-starch offers far less resistance when triturated in a mortar than that of wheat, and upon this fact a simple test for their detection is founded. It is executed by rubbing up a mixture consisting of equal parts of the sample and sand with water, diluting and filtering the paste formed, and then adding to it a solution of 1 part of iodine in 20 parts of water. In the absence of potato-starch, an evanescent pink colour is produced; in case it is present, the colour obtained is dark purple, which in time also disappears.

Among the methods which have been suggested for the detection of such accidental impurities as darnel, ergot, and mildew, are the following:—If pure flour is digested for some time with dilute alcohol, the latter either remains quite clear or it acquires a very light straw-colour; with flour contaminated with darnel, the alcohol shows a decided greenish tint, and possesses an acrid and disagreeable taste. In case the alcohol used is acidulated with about 5 per cent. of hydrochloric acid, the extract obtained exhibits a purple-red colour with flour containing mildew, and a blood-red colour with flour containing ergot. When flour contaminated with ergot or other moulds, is treated with a dilute solution of aniline violet, the dye is almost wholly absorbed by the damaged granules, which are thus rendered more noticeable in the microscopic examination.

The following test is often used for the detection of alum in flour:—A small quantity of the suspected sample is made into a paste with a little water and mixed with a few drops of an alcoholic tincture of logwood; a little ammonium carbonate solution is then added. In the presence of alum, a lavender-blue coloured lake is formed, which often becomes more apparent upon allowing the mixture to remain at rest for a few hours. The production of a brown or pink coloration is an indication of the absence of alum. A modification of this test, proposed by Blyth, consists in immersing for several hours in the cold aqueous extract of the flour a strip of gelatine, with which the alum combines; the gelatine is subsequently submitted to the action of the logwood tincture and ammonium carbonate as above.

For the quantitative estimation of alum in flour, the following processes are usually employed:—A considerable quantity of the sample is incinerated in a platinum dish, the ash is boiled with dilute hydrochloric acid and the solution filtered. The filtrate is next boiled and added to a concentrated solution of pure sodium hydroxide, the mixture being again boiled and afterwards filtered hot. A little sodium diphosphate is now added to the filtrate which is then slightly acidulated with hydrochloric acid, and finally made barely alkaline by addition of ammonium hydroxide. The resulting precipitate, which, in the presence of alum, consists of aluminium phosphate, is brought upon a filter, well washed, and then weighed.

Another method, which is a modification of that of DuprÉ, is as follows:—The ash obtained by the calcination of the flour (or bread), is fused, together with four times its weight of pure mixed sodium and potassium carbonates, the fused mass treated with hydrochloric acid, the solution evaporated to dryness and the separated silica collected and weighed. A few drops of sodium phosphate solution are added to the filtrate from the silica, then ammonium hydroxide in excess, by which the calcium, magnesium, ferric and aluminium phosphates are precipitated. The two latter are next separated by boiling the liquid with an excess of acetic acid (in which they are insoluble), and brought upon a filter, washed, dried, and weighed. The iron sometimes accompanying the precipitate of aluminium phosphate, can be determined by reduction with zinc and titration with potassium permanganate. If the presence of alum is indicated by the logwood test, and it is quantitatively determined by either of the preceding methods, it has been suggested that an allowance be made for the small proportion of aluminium silicate occasionally found in unadulterated flour or bread, and a deduction from the total alum present of one part of alum for every part of silica obtained is considered proper. The weight of aluminium phosphate found, multiplied by 3·873, or by 3·702, gives respectively the corresponding amounts of potash-alum or ammonia-alum contained in the sample examined.

BREAD.

Bread is usually prepared by mixing flour with water, kneading it into a uniform dough, submitting it to a process of “raising,” either by means of a ferment or by the direct incorporation of carbonic acid gas, and finally baking the resulting mass.

Unleavened bread, however, is made by simply kneading flour with water, with the addition of a little salt, and baking. The oatcake of the Scotch, the passover bread of the Israelites, and the corncakes of the Southern States are the best known varieties of unleavened bread.

The porosity peculiar to raised bread is caused by the generation of a gas, either previous to, or during the process of baking. In former times (and to some extent at present, notably in Paris), fermented bread was made by the use of leaven, which is dough in a state of incipient decomposition; but in this country, the common agent employed in raising bread is yeast, which consists of minute vegetable cells (Torula cerevisiÆ) forming either the froth or deposit of fermenting worts.

By the action of these ferments, the gluten of the flour first undergoes a modification and enters into a peculiar combination with the starch-granules, which become more or less ruptured; the soluble albumen is rendered insoluble, and the starch is transformed, first into sugar, then into carbonic acid and alcohol. These changes are perfectly analogous to those which occur in the fermentation of the wort in the preparation of fermented liquors.

Other and minor decompositions likewise occur, such as the partial conversion of the starch into dextrine, the sugar into lactic acid, and the alcohol into acetic acid, but the most essential change is the production of alcohol and carbonic acid. The alcohol formed is mainly volatilised, although an average proportion of 0·3 per cent. of this compound has been found in samples of fresh bread. The escape of the carbonic acid is retarded by the gluten, and to its expansion is due the porous or spongy appearance of well-made bread.

Of late years, artificial substitutes for the fermentation process in the production of porous bread have been extensively employed. By the use of these agents, the liberation of carbonic acid in the dough is accomplished and a slight gain of weight is effected, as none of the original ingredients of the flour are lost by fermentation.

“AËrated bread” is made by kneading the flour under pressure with water highly charged with carbonic acid gas, which, upon the removal of the pressure, expands, and gives porosity to the bread. The use of “baking powders” effects the same result in a more convenient manner, and is largely practised in families. These compounds generally consist of sodium bicarbonate (sometimes partially replaced by the corresponding ammonia salt), and tartaric acid, or potassium bitartrate, together with rice or other flour. A more commendable preparation is a mixture of sodium bicarbonate with potassium or calcium acid phosphates, the use of which is claimed to restore to the bread the phosphates lost by the removal of the bran from the flour. Baking powders are often mixed in the dry state with flour, and the produce, which is known under the name of “self-raising flour,” only requires to be kneaded with water and baked to form porous bread. However great the convenience attending the use of these compounds, they are often open to the objection that their decomposition gives rise to the formation of aperient salts, e.g. sodium tartrate, and that they are very frequently contaminated with alum.

As a result of the chemical changes which take place in the fermentation of the flour and the subsequent application of heat, the composition of bread differs materially from that of the grain from which it is prepared. As already mentioned, the soluble albuminoids are rendered insoluble, and the starch is partially transformed into sugar (maltose). The unconverted starch is modified in its physical condition, the ruptured granules being far more readily acted upon by the digestive fluids than before. The proportion of soluble carbohydrates is naturally augmented in bread. The amount of ash is also somewhat increased, chiefly owing to the addition of salt, but it should not exceed a proportion of 2 per cent. The quantity of water in bread varies considerably. Wanklyn fixes 34 per cent. as the standard; greater proportions have, however, been frequently found. In ten samples of apparently normal bread, examined by E. S. Wood, Analyst to the Massachusetts State Board of Health, the amounts of moisture contained varied from 34 to 44 per cent. The quantity of water decreases very rapidly upon exposure to the air. Thus, Clifford Richardson[52] found that bread which showed 36 per cent. of moisture when freshly baked, contained but 5·86 per cent. after drying for two weeks. Stale bread would seem to contain water in a peculiar molecular condition, and, as is well known, upon heating (“toasting”), it reassumes the porous state.

According to analyses collected by KÖnig,[53] the mean composition of bread is as follows:—

Clifford Richardson gives the following results of the analysis of ordinary family loaf-bread:—

The analysis of bread is conducted essentially in the same manner as that of flour. Under ordinary circumstances, the determinations required are limited to an estimation of the moisture contained in the crumb, the amount of the ash, and special tests for the presence of alum and copper salts. Owing to the broken condition of the starch granules in bread, their identification by the microscope is usually rendered exceedingly difficult. The logwood test for alum in bread is applied by Bell as follows:—About 10 grammes of the crumb are immersed in a little water containing 5 c.c. each of the freshly prepared logwood tincture and solution of ammonium carbonate for about five minutes, after which the liquid is decanted, and the bread dried at a gentle heat. In the presence of alum the bread will acquire the characteristic lavender tint mentioned under Flour. It should be added, that salts of magnesia also produce a lavender lake with alum; but this fact does not affect the usefulness of the process as a preliminary test to the quantitative determination of the mineral impurities present in the sample under examination. The quantitative examination of alum in bread is made by one of the methods described on p. 93. Bread, free from alum, will sometimes yield 0·013 per cent. of aluminium phosphate, and this amount should therefore be deducted from the weight of the precipitate obtained.

The average of the results obtained by Dr. Edward G. Love, New York State Board of Health, from the examination of the crumb of ten samples of the cheaper varieties of wheaten bread were as follows:—

That the addition of alum to bread is prevalent seems to admit of little doubt. The British Public Analysts, in 1879, tested 1287 samples of bread, of which 95 (or 7·3 per cent.) contained alum. Of 18 samples examined, in 1880, in the city of Washington, 8 were adulterated with the salt. The question of the sanitary effects produced by the use of alumed bread is one which has given rise to very extended discussion. According to some authorities, the conversion of alum into an insoluble salt by the fermentation process, which takes place in bread-making, is regarded as a proof that it remains inert, and is consequently harmless in its effects. Others contend that its action as a preventive of excessive fermentation is at the expense of valuable nutritious constituents of the flour, and that its combination with the phosphates present in the grain results in the formation of an insoluble salt which tends to retard digestion. Experiments have been made by J. West Knights, on the comparative action of artificial gastric juice upon pure and alumed bread, which apparently support this latter view.

Another objection to the use of alum is that it is frequently employed for the purpose of disguising the bad quality of damaged and inferior grades of flour. The presence of copper salts in bread is of rare occurrence. Their detection is accomplished by treating a portion of the crumb with a dilute solution of potassium ferrocyanide acidulated with acetic acid, which, in presence of copper, will impart a reddish-brown colour to the bread. If contained in any appreciable proportion, it can be extracted from the ash obtained by the incineration of the bread, and deposited upon the interior of a weighed platinum capsule by the electrolytic method.

Starch (C6H10O5).—Starch, which enters so largely into the composition of cereals, is a carbo-hydrate, i. e. hydrogen and oxygen are contained in the proportions necessary to form water. In this respect, it is identical with woody fibre, cellulose, and dextrine.

The well-known dark-blue colour produced upon the addition of a solution of iodine to starch-paste forms the usual qualitative test for its presence. This coloration is discharged by alkalies and by a solution of sulphurous acid. The quantitative estimation of starch in mixtures is best effected by heating the dry substance in a closed tube for 24 hours, together with a dilute hot alcoholic solution of potassium hydroxide. The hot liquid is next filtered, the residue washed with alcohol, and the filtrate heated with 2 per cent. solution of hydrochloric acid until it ceases to show the blue coloration when tested with iodine. It is then rendered alkaline, and the proportion of starch originally present, calculated from the amount of sugar formed, as determined by Fehling’s solution. Although identical in chemical composition, the various forms of starch met with in the vegetable kingdom vary in size and exhibit characteristic differences in the appearance of the granules. The following are measurements of several varieties of starch granules:—

The larger granules of potato starch, when suspended in water, subside more rapidly than those of wheat starch; they are also far more readily ruptured.

The identification of the various starches is accomplished by means of the microscope. Starch possesses an organised structure which, fortunately, differs in different plants. Besides varying in size, the granules develope in a different manner and form from centres of growth, and therefore exhibit characteristic conditions and positions. These distinctions, together with their effect upon polarised light, are of great utility in the determination of the source of any particular starch. For this purpose, it is necessary to become familiar with the distinctive microscopical appearance of each individual starch. A collection of those most usually met with should be made, and, after careful study, preserved in a dried state for comparative purposes. Polarised light is a very useful adjunct in the examination of starch granules. In the microscopical investigation, a minute portion of the sample is placed upon the glass slide and well moistened with a solution of 1 part glycerine in 2 parts of water; it is then protected by a thin glass cover, which is put on with gentle pressure. The appearance of various starches, under polarised light, is seen in Plate IX., where the cross lies at the hilum or nucleus of the granule and the form and relative size is visible in outline. This plate, and Plates VII. and XII. are copied, with permission, from Bulletin No. 11 of the Chemical Division of the U.S. Department of Agriculture. The original negatives (made by Clifford Richardson) were used, but the auto-types are presented in a somewhat modified form.