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:—
| Water. | Nitrogenous substances. | Fat. | Sugar. | Extractive free from Nitrogen. | Cellulose. | Ash. | Dry Substances. | ||
| N. | Carbohydrates. | ||||||||
| per cent. | per cent. | per cent. | per cent. | per cent. | per cent. | per cent. | per cent. | per cent. | |
| Fine wheat bread | 31·51 | 7·06 | 0·46 | 4·02 | 52·56 | 0·32 | 1·09 | 1·75 | 87·79 |
| Coarse wheat bread | 40·45 | 6·15 | 0·44 | 2·08 | 49·04 | 0·62 | 1·22 | 1·65 | 85·84 |
| Rye bread | 42·27 | 6·11 | 0·43 | 2·31 | 46·94 | 0·49 | 1·46 | 1·69 | 85·31 |
| Pumpernickel | 43·42 | 7·59 | 1·51 | 3·25 | 41·87 | 0·94 | 1·42 | 2·15 | 79·74 |
Clifford Richardson gives the following results of the analysis of ordinary family loaf-bread:—
| Per cent. | |
| Water | 37·30 |
| Soluble albuminoids | 1·19 |
| Insoluble„ | 6·85 |
| Fat | 0·60 |
| Sugar | 2·16 |
| Dextrine | 2·85 |
| Starch | 47·03 |
| Fibre | 0·85 |
| Ash | 1·17 |
| 100·00 | |
| Nitrogen | 1·29 |
| Total albuminoids | 8·04 |
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.