Having devised a feasible method of estimating how large a loaf any given flour will make, the problem of the shape and texture still remains. Previous investigators had exhausted almost every possible chemical property of gluten in their search for a method of estimating strength. The author therefore determined to study its physical properties. Now gluten is what is known as a colloid substance, like albumen the chief constituent of white of egg, casein the substance which separates when milk is curdled, or clay which is a well known constituent of heavy soils. Such colloid substances can scarcely be said to possess definite physical properties of their own, for their properties vary so largely with their surroundings. The white of a fresh egg is a thick glairy liquid. On heating it becomes a white opaque solid, and the addition of certain acids produces a similar change in its properties. Casein exists in fresh milk in solution. The addition of a few drops of acid causes it to separate as finely divided curd. If, however, the milk is warmed before the acid is added the casein separates as a sticky coherent mass. Every farmer knows that lime improves the texture of soils containing much clay, because the lime causes the clay to lose its sticky cohesive nature.
Such instances show that the properties of colloid substances are profoundly modified by the presence of chemical substances. Wheat, like almost all plant substances, is slightly acid, and the degree of acidity varies in different samples. Accordingly the effect of acids on the physical properties of gluten was investigated, and it was found that by placing bits of gluten in pure water and in acid of varying concentration it could be made to assume any consistency from a state of division so fine that the separate particles could not be seen, except by noticing that their presence made the water milky, to a tough coherent mass almost like indiarubber (Fig. 11). It was found, however, that the concentration of acid in the wheat grain was never great enough to make the gluten really coherent.
Fig. 11.
But wheat contains also varying proportions of such salts as chlorides, sulphates and phosphates, which are soluble in water, and the action of such salts on gluten was next tried. It was at once found that these salts in the same concentration as they exist in the wheat grain were capable of making gluten coherent, but that the kind of coherence produced was peculiar to each salt. Phosphates produce a tough and elastic gluten such as is found in the strongest wheats. Chlorides and sulphates on the other hand make gluten hard and brittle, like the gluten of a very weak wheat (Fig. 12).
The next step was to make chemical analyses to find out the amount of soluble salts in different wheats. Strong wheats of the Fife class were found to contain not less than 1 part of soluble phosphate in 1000 parts of wheat, whilst Rivet wheat, the weakest wheat that comes on the market, contained only half that amount. Rivet, however, was found to be comparatively rich in soluble chlorides and sulphates, which are present in very small amounts in strong wheats of the Fife class. Ordinary English wheats resemble Rivet, but they contain rather more phosphate and rather less chlorides and sulphates. After making a great many analyses it was found that the amount of soluble phosphate in a wheat was a very good index of the shape and texture of the loaf which it would make. The toughness and elasticity of the gluten no doubt depend on the concentration of the soluble phosphate in the wheat grain, the more the soluble phosphate the tougher and more elastic the gluten, and a tough and elastic gluten holds the loaf in shape as it expands in the oven, and prevents the small bubbles of gas running together into large holes and spoiling the texture.
Fig. 12.
