Sept. 7, 1883.] 



♦ KNOWLEDGE ♦ 



151 



CHEMISTRY OF THE CEREALS, 



By William Jago, F.C.S. 



No. v.— BREAD-MAKmG (Continued). 



IN our last paper an explanation was given of how by 

 the action of fermentation the dough becomes charged 

 with carbon dioxide gas. Let us be quite sure that this 

 action is understood ; certain minute organisms possess the 

 power, under proper conditions, of effecting the decomposi- 

 tion of sugar into alcohol and carbon dioxide. When moist 

 flour is exposed to air, seeds of the particular organism, which 

 is a very low form of plant life, fall on the mass and fructify 

 there. The flour is now changed into " leaven," and if a 

 little be added to a further quantity of flour and water, the 

 whole mass begins to ferment — that is to say, the growth of 

 this plant proceedo through the whole, and sots up an active 

 conversion of the sugar. 



From the earliest times this method of causing dough to 

 " rise " has been in common use, a portion of the one 

 parcel of dough being set aside as leaven for the next. The 

 coarser varieties of bread, particularly the German black 

 bread, are still made in this fashion. For the finer varieties 

 of bread beer-yeast is now largely used. There are two 

 forms of this substance known in England — V)rewers' yeast 

 and "German" or "dry yeast." This latter is simply 

 brewers' yeast from which most of the water has been 

 squeezed out, leaving a slightly moist greyish mass. Dry 

 yeast consists almost entirely of the cells of this particular 

 alcohol-producing plant whose action we have been study- 

 ing. It has the advantage over leaven of being more 

 concentrated and of greater purity. 



In making bread with yeast, a small quantity of flour, 

 yeast, and warm water are mixed together, and set aside 

 in a warm place to undergo fermentation. This constitutes 

 the " sponge " ; with yeast in a healthy condition the 

 dough is seen to rise, and bubbles of carbon dioxide from 

 time to time escape. The sponge is next mixed with the 

 main quantity of flour and water, and the whole mass 

 thoroughly kneaded with the hands, and sometimes with 

 the feet. The dough is now allowed to remain for some 

 hours, the fermentation going on all the time : it is next 

 cut into loaves ; these go on fermenting until their size is 

 nearly doubled ; they are then baked, and still further in- 

 crease in volume through the expansion of the gases under 

 the influence of heat. The heat of the oven rapidly arrests 

 the fermentation by killing the yeast plant. 



On the continent the making of bread has attained 

 greater perfection than in England ; Paris and Vienna are 

 justly renowned for the excellence of the bread there made. 

 Their method of proceeding differs considerably from that 

 in use with us. A portion of tlie dough from one baking 

 is reserved for the next, and after remaining for about ten 

 hours, is mixed with an equal quantity of freshly-made 

 dough ; this, after standing some hours, is again kneaded 

 with a larger (juantity of fresh dough, and after another 

 interval is mixed with more dough and also some beer 

 yeast. About half of the fermented dough is made into 

 loaves, which, after a time are baked. Tlit> remainder is 

 mixed witli more fresh dough and yeast, allowed to ferment, 

 again divided, the half being baked, and the rest mixed 

 with more dough and yeast. This division and re-fermen- 

 tation is altogether repeated some Ave or si.x times ; the 

 bread last produced is of the finest and whitest quality. 



It will be seen that by the method of fermentation the 



carbon dioxide; is produced at the expense of the sugar, 



and indirectly of the starch of the grain ; the loss, however, 



ut a very small one, for 1 oz. of sugar yields when fer- 



mented about twelve pints, or 400 cubic inches of carbon 

 dioxide. 



With good flour the operation of baking produces but 

 little change in the composition of the interior of the loaf : 

 the starch will be found to have undergone but little altera- 

 tion, a small proportion will have been rendered soluble, 

 but the greater number of granules have not even had 

 their walls broken, being simply swollen by the absorption 

 of water. As may be imagined, the quantity of this com- 

 pound present in Ijread is considerable ; from the results 

 of analyses of twenty-five diflerent loaves Dr. Odling 

 arrived at a mean of 43-43 per cent, of water. But although 

 the " crumb " of the loaf is but little changed, there is a 

 decided alteration in the crust. In the first place, it has 

 a much sweeter taste, and, secondly, by proper treatment, 

 yields a sticky substance. These characters point re- 

 spectively to the conversion of starch into sugar and 

 dextrin. These bodies may be extracted from the crust 

 by soaking it in hot water, and then gently evaporating 

 the clear solution. Under the influence of the greater 

 heat to which the exterior of the loaf is exposed, the starch 

 has suffered decomposition. When bread is " burnt," the 

 decomposition has gone a step or steps further; pro- 

 vided the heat has not been too intense, the sugar 

 is converted into "caramel." This is the material 

 used so largely for browning gravies and other similar pur- 

 poses. In composition caramel consists of a number of 

 closely-allied substances produced by the evolution of water 

 from the sugar in various proportions. As the intensity of 

 the heat is increased, the sugar gets more and more changed, 

 until at last only charcoal remains. It will be of interest 

 here to mention the composition of a sample of good bread : 

 After first removing as much of the water as possible, the 

 following results were obtained: — sugar, 3 6; altered 

 starch, 18 0; unaltered starch, 53 5; gluten, with a little 

 starch, 20-7 per cent. 



Bread when new is very different to the same article 

 when a few days old ; instead of being soft and spongy, a 

 loaf will have become hard and almost brittle. The taste 

 too will have altered, and that, with most peoples' palates, 

 for the worse. The bread is usually supposed to have 

 become dry ; and certainly the cracks which score a cut 

 surface are suggestive of a loss of moisture. On weighing 

 the bread it will be found, though, not to have lost much ; and 

 what is far more striking, if the bread Ije re-baked, it again 

 acquires the pleasant qualities of " new " bread. An experi- 

 ment of re-baking stale bread showed it to have actually 

 lost over 3 per cent, in the oven, so that, with less water, 

 the bread had lost the peculiar dryness of taste. It is 

 necessary, therefore, to go a step further in order to explain 

 the change the bread has undergone in becoming stale : 

 the water is still there, but instead of being in the free 

 state, has gradually combined with certain of the com- 

 pounds present, probably the starch. A gentle lieat 

 again decomposes these compound.s, and the liberated water 

 gives newness to the loaf. 



We have so far been supposing that the flour used in 

 bread-making is perfectly sound and of the best quality. 

 Failing these conditions, other chemical changes occur by 

 wliicli the bread is more or less deteriorated. We have 

 from time to time referred to the starch-converting power 

 of the nitrogenous matters of grain ; the bran, owing to 

 the presence of ccrealin, is particularly active in this re- 

 spect. So, too, is gluten, after the flour has been ex- 

 posed for some time to warmtli and moisture. The 

 change of starch to sugar by these bodies is not well 

 understood ; there, is, however, no reason to believe that 

 it resembles fermentation by being the work of some 

 livinf organism. There is some confusion on this point, 



