This section is from the book "Experimental Cookery From The Chemical And Physical Standpoint", by Belle Lowe. Also available from Amazon: Experimental cookery.
The hydration of a gluten and hence its tenacity or cohesiveness, and its elasticity or extensibility, may be altered, that is increased or decreased, by: (1) over-grinding, (2) bringing the dough to a certain acidity or alkalinity and subsequent neutralization, (3) treating with alcohol, (4) salts, (5) alkalies, (6) acids, (7) proportion of water used, (8) added substances, (9) temperature, (10) time of standing, (11) mechanical treatment of the dough, and (12) fermentation.
Over-grinding has been considered. The effect of mechanical treatment of the dough and fermentation upon gluten will be considered with the discussion of bread making. The effect of proportion of water used has been considered and that of added substances will be considered under dough structure.
Bringing the dough to a certain acidity or alkalinity and subsequent neutralization. The quality of the gluten can be altered by increasing the acidity or alkalinity. Gortner and Johnson have found that "doughs brought to pH 3.0 or 11.0 by the addition of acid or alkali and in which the acid or alkali has been subsequently neutralized, have lost their baking strength."
Treating with alcohol. Gortner and Johnson have found that flour treated with 70 to 85 per cent alcohol, from which the alcohol is evaporated, and the flour dried and remilled, when used in bread has lost its baking quality. They attribute this to a loss in colloidal properties brought about by action of alcohol on the glutenin of the flour.
Salts. The imbibition of water by gluten, and thus the swelling, may be depressed considerably by salts. This may be brought about by the salts found in the flour, by the addition of salts like baking powders to flour mixtures, or by the addition of the so-called flour improvers to weak flours. The inhibiting effect may be due to the cation or the anion of the salt. Different salts depress the hydration capacity of the gluten in different proportions, that is, in the order of a lyotropic series. See also the section on proteins.
Bailey and Le Vesconte have reported that the addition of monocalcium phosphate to a dough effected a slight increase in the extensibility of gluten. They find no increase in extensibility of the dough, as measured in the Chopin extensimeter, when calcium sulfate and magnesium sulfate are added in the proportions used in commercial practise.
Alkalies. The hydration capacity of gluten on the alkaline side of the isoelectric point reaches its maximum capacity at a pH 11.0. Beyond a pH 11.0 the gluten dissolves very rapidly with increasing alkalinity. Since gluten proteins tend to imbibe water and disperse in a weakly alkaline medium, doughs with a slight alkaline reaction are sticky. With excess alkali they become yellow and develop a soapy taste. Since the proteins are peptized readily with alkali, a product with a pH higher than 7 is quite tender. Thus chocolate cakes and gingerbread with excess alkali (soda) are very tender, though the cell walls are very thick.
Acids. Acids as well as salts and alkalies markedly influence the hydration capacity of gluten. The hydration capacity of gluten is increased with increased hydrogen-ion concentration until a pH of 3.0 is reached. With a pH lower than 3.0 the hydration capacity is decreased. Gortner and Sharp have shown that the various acids produce a maximum hydration at pH 3.0. On the acid side of the isoelectric point the gluten is not dispersed as rapidly as on the alkaline side, so that doughs with an acid reaction are more tenacious, and not so sticky as ones with an alkaline reaction. During fermentation of bread the development of acids gives an acid reaction to the dough, increases the hydration and solubility of the proteins and thus the tenderness of the resulting bread.
Time of standing. Bailey and Le Vesconte have reported that time of standing of the dough after mixing influences the extensibility of the dough. The greater part of this increase in elasticity or ease of handling is quite noticeable in many dough products. Pastry and other products are easier to handle if allowed to stand a short time after mixing, before they are rolled or baked.
Temperature. Bailey and Le Vesconte have reported that temperature markedly influences the extensibility of a flour and water dough. With increasing temperature the dough becomes more elastic and less tenacious. Kent-Jones in reporting the experiments of Luers* comments on the influence of temperature on hydration capacity. At a temperature of 8° to 9°C. the imbibition of water by gliadin was quite low and he speaks of it as almost "an anti-swelling effect." At higher temperatures the gluten becomes more elastic. This is of great importance in baking, for without this quality the cell walls of the dough would break and coalesce before the temperature was reached at which the protein is coagulated. As a result the volume of the product would either not increase or would be lessened during baking.
With higher temperatures of mixing the gluten swells more rapidly and this affects the rapidity with which it develops or becomes attenuated with mixing.
Swanson in his article, "The Theory of Colloidal Behavior in Dough," gives a good description of dough structure. The following account is based upon his article, upon other reading, and upon observation.
Bread dough has two continuous phases: water constitutes one, and gluten the other.
* Luers, H., Koll. Zeitschrift, 25: Nov., 1919.
Starch in dough. When flour is mixed with water to form a dough the starch granules retain their shape. They also have a certain rigidity and little adhesive power, so that they are not easily pulled or squeezed out of shape and do not adhere to each other.
Gluten particles in dough. In contrast to the starch particles in dough, the gluten particles behave as if their shape is very irregular. In the flour the shape may be regular, but it is the shape in the dough that gives the dough part of its distinctive baking properties. On account of this irregular shape the particles have a very large surface area. When swollen with water they can be easily pulled, squeezed, or packed into different shapes.
The protein particles in the flour may be compared to chewing gum, for many of the properties are similar. Anyone who has chewed gum is familiar with its manipulation, so that it makes an excellent illustration. The size of the piece of gum is very much larger than that of the protein particle. The stick of chewing gum when first placed in the mouth may be brittle and crumbly. As it becomes moist it sticks together and develops tenacity, adhesiveness, and cohesiveness. If two sticks of gum that have been chewed are brought in contact with each other they adhere or stick together; if pulled, they stretch. In the literature on dough, this change in shape through hydration, surface contact, and particles adhering to each other is called attenuation. The more the pieces of gum are pulled the more attenuated they become, but if the elastic strands touch each other in large areas they adhere, and when worked more and more, they become matted together in one piece.
When the protein particles in the dough become moist they swell and form gluten. The behavior of the protein particles when moistened is similar to that of the gum. When first moistened they are crumbly and do not adhere together. After the particles become hydrated they become sticky and tenacious. When the dough is stirred the surfaces of the particles come in contact and adhere to each other. The stirring of the dough stretches and pulls the surface contacts so that the gluten becomes attenuated and filaments formed. Attentuation of the gluten is often referred to as developing the gluten or dough.
Gluten forms a continuous phase in dough. The protein in the dough forms a continuous network or mesh structure throughout the dough. In this meshwork are the starch granules, the sugar, salts, and water.
The protein particles compose only a portion of the flour content, although after they are hydrated the relative volume that they occupy in the dough is greater than their volume in the flour. If to the chewing gum we add some small round particles that do not change their shape and are about the size of small tapioca a better picture of dough structure is obtained. The small round particles will prevent the chewing gum from forming one solid mass and will tend to keep the pieces of gum apart, so that manipulation and sliding over the round particles is required to bring different surfaces of the gum in contact with each other. In the dough the whole starch granules do not swell appreciably until a temperature of 60° to 65 °C. is reached. Thus in a dough mixed at ordinary temperatures they do not change their volume or shape.
A good quality of gluten adheres strongly to other particles of gluten and forms many strands of network around the starch granules. A good quality of gluten must be elastic enough to stretch around the starch granules, and in addition stretch and retain gas bubbles formed in the dough that make the structure porous. No dough will retain all the gas formed within it, but a gluten of good quality will retain a very large portion of the gas bubbles. A poor quality of gluten will not stretch when the gas is formed but will break in places, and too large a proportion of the gas formed is lost.
There are many different qualities of gluten in different wheats. Some may be very soft and not very tenacious; some may be like the ones described above; and others may be so strong that they are tenacious enough not to stretch without excess pressure. Such glutens are improved by blending with weaker glutens. Flour from the large mills is blended to produce a working quality of gluten that gives similar results when baked in different batches. Strong glutens may be improved by the addition of malt extract, which contains a larger proportion of the proteolytic enzyme proteinase than is found in the flour. Since part of the protein is hydrolyzed by the proteinase there is not such a compact mass left and the remaining gluten is softer and more elastic.