This section is from the book "Experimental Cookery From The Chemical And Physical Standpoint", by Belle Lowe. Also available from Amazon: Experimental cookery.
Egg. In thin batters, to improve the baking quality or to give structure to the finished product, eggs are added. Without their addition only a thickened mass with no increased volume is obtained. The egg protein coagulates when heated and with the gluten retains the gas formed in the batter.
Fat. The addition of fat gives a shorter dough or a more tender one. By short, bakers refer to the gluten breaking quickly and not adhering tenaciously. Fat causes the gluten threads to pull or break apart more easily. In other words, it might be stated that when the gluten particles are "greased" or lubricated they adhere less strongly.
Starch. Bailey and Le Vesconte have found that the addition of starch to a bread flour results in a decreased extensibility. The addition of 10 per cent of starch decreases the extensibility only a little, but the addition of larger quantities decreases it to a greater extent. The gas-retaining capacity of the dough and the quality of the bread are also impaired. However, since the added starch is not embedded in gluten and the diastase can act on such starch quickly, Alsberg states that carbon dioxide will be formed more rapidly, with which statement Johnson and Bailey agreed.
Cooked mashed potatoes are sometimes added to bread doughs. Since the starch is cooked, judging from Kuhlmann and Golossowa's results, the water-binding capacity of the dough and bread should be increased; but too large a quantity of mashed potatoes because of the dilution of the gluten may impair the quality of the bread.
Sugar. Jago reports that sugar causes a diminution of the amount of gluten that can be washed from a dough. When the sugar was added to the dough and the gluten extracted with alcohol much more gluten was dissolved, from which he concluded that sugar has a solvent action on gluten.
Baking powder. Baking powder is added to dough as a leavening agent, that is, to produce carbon dioxide gas to stretch the gluten and produce a porous texture. The ingredients of the baking powder and the residue of the salts formed from the reaction of the baking powder may have either an inhibiting or accelerating effect upon the hydration capacity of the gluten, depending upon the particular cations and anions of the baking powders or their salts.