The Chemistry of Cookery

by W. Mattieu Williams

The difference between new and stale bread is familiar enough, but the nature of the difference is by no means so commonly understood. It is generally supposed to be a simple result of mere drying. That this is not a true explanation may be easily proved by repeating the experiments of Boussingault, who placed a very stale loaf (six days old) in an oven for an hour, during which time it was, of course, being further dried; but, nevertheless, it came out as a new loaf. He found that during the six days, while becoming stale, it only lost 1 per cent. of its weight by drying, and that during the one hour in the oven it lost 3½ per cent. in becoming new, and apparently more moist. By using an air-tight case instead of an ordinary oven, he repeated the experiment several times in succession on the same piece of bread, making it alternately stale and new each time.

For this experiment the oven should be but moderately heated—260° to 300° Fahr. is sufficient. I am fond of hot rolls for breakfast, and frequently have them à la Boussingault, by treating stale bread-crusts in this manner. My wife tells me that when the crusts have been long neglected, and are thin, the Boussingault hot rolls are improved by dipping the crust in water before putting it into the oven. This is not necessary in experimenting with a whole loaf or a thick piece of stale bread.

The crumb of bread, whether new or stale, contains about 45 per cent. of water. Miller says ‘the difference in properties between the two depends simply upon difference in molecular arrangement.’

This ‘molecular arrangement’ is the customary modern method of explaining a multitude of similar physical and chemical problems, or, as I would rather say, of evading explanation under the cover of a vague conventional phrase.

I have made some simple experiments which supply a visible explanation of the facts without invoking the aid of any invisible atoms or molecules, or any imaginary arrangements or rearrangements of these imaginary entities.

I find that, as bread becomes stale, its porosity appears to increase, and that when renewed by reheating, it returns to its original apparently smaller degree of porosity. That this change can be only apparent is evident from the facts that the total quantity of solid material in the loaf remains the same, and its total dimensions are retained more or less completely by the rigidity of the crust. I say ‘more or less,’ because this depends upon the thickness and hardness of the crust, and also upon the completeness of its surrounding. Lightly-baked loaves shrink a little in dimensions in becoming stale, and partly regain the loss on reheating, but this difference only exaggerates the apparent paradox of varying porosity, as the diminished bulk of a given quantity of material displays increased porosity, and the increase of total dimensions accompanies the diminished porosity.

I have obtained a reconciliation of this paradox by careful examination of the structure of the crumb. This shows that the larger or decidedly visible pores are cells having walls of somewhat silky appearance. The silky lustre and structure is, I have no doubt, due to a varnish of dextrin, the gummy nature of which I have already described. On looking a little more closely at this inner surface of the big blow-holes with the aid of a hand-lens of moderate power, I find that it is not a continuous varnish of gum, but a net-work or agglomeration of gummy fibres and particles, barely touching each other.

My theory of the change that takes place as the bread becomes stale is, that these fibres and particles gradually approach each other either by shrinkage or adhesive attraction, and thus consolidate and harden the walls of each of the millions of easily visible pores, these walls forming the solid material of which the loaf is made up. In doing so they naturally increase the dimensions of the visible pores, while the microscopic interstices or spaces between the minute fibres of the cell walls are diminished by the approximation or adhesion of the fibres to each other.

This adhesion is probably aided by an oozing out or efflorescence of the vapour held by the fibres, and its condensation on their surfaces. This point, be it understood, is merely hypothetical, as the efflorescence is not visible. All the other phenomena I have just described are visible either with the naked eye or by the aid of a lens.

When the stale bread is again heated, a general expansion occurs by the conversion of liquid water into aqueous vapour, every grain of water thus converted expanding to 1,700 times its former bulk. As this happens throughout, i.e. upon the surface of every one of the countless fibres or particles, there must be a general elbowing in the crowd, breaking up the recent adhesion between these fibres and thrusting them all apart in the directions of least resistance; i.e. towards the open spaces of the larger and visible pores, producing that apparent diminution of porosity that I have observed as the easily visible characteristic of the change.