28 BULLETIN 1187, U. S. DEPARTMENT OF AGRICULTURE. 
dough which becomes entrapped in the particles of gluten that causes 
the dough to rise in the process preliminary to baking. Like all 
other proteins, gluten becomes dry and hard upon exposure to heat, 
so that the shape of the loaf becomes set soon after it is placed in 
the oven. 
It is plain that wheats with a high, medium, or low protein con- 
tent will, in milling, yield a flour having a correspondingly high, 
medium, or low protein or gluten content. Except those wheats 
which have an excessively high protein content, and which as a rule 
yield flours of less strength than those having medium protein con- 
tent, the average of a large number of samples within the same class 
of wheat shows that the baking strength of a high-grade flour is 
usually high if the percentage of protein is high, and low if the per- 
centage is low. Greater variations are found in wheats with a low 
or high percentage of protein than in those with a medium percentage. 
These variations, moreover, fully illustrate the fact that the results 
of total crude protein determinations are not always indicative of 
baking strength. This lack of relationship is due apparently to the 
physical properties of the gluten proteins, as the same kind of pro- 
tein from different classes and grades of wheat is chemically identical. 
The percentage of crude protein in both wheat and wheat products 
is obtained by multiplying the percentage of total nitrogen as deter- 
mined by the Kjeldahl method (p. 36) by the factor 5.7. The term 
11 crude protein" is used because wheat and wheat products all con- 
tain nitrogen-bearing substances which are not protein or glutinous 
in nature. By use of the factor 5.7 the approximate percentage of 
true gluten present in the sample is determined, as it has been found 
that true gluten contains approximately 17.6 per cent nitrogen. 
The apparatus for making crude protein tests are pictured in figures 
22, 23, and 24. 
The methods used to arrive at the percentage of crude gluten 
vary. The most common method (described in full on page 40) 
is to mill out the flour from the wheat and make a dough ball of the 
flour by adding water. The dough ball is then washed under a stream 
of water until everything but the gluten is gone. This remains as a 
tough, elastic mass known as wet gluten. When dried in an oven 
at 100° C. or 212° F. for 24 hours, or until it ceases to lose weight, 
it is called dry gluten. When carried out by the same person under 
the same conditions this method is fairly accurate as a means of 
determining the relative percentage of crude gluten present in the 
sample of flour under examination. An indication of the quality of 
the crude gluten may also be had by observing the color, consistency, 
and the general condition of the washed gluten. 
It does not show, however, the total amount of gluten in the wheat 
itself. The amount of flour extracted from any lot of wheat varies 
according to the practice of the miller. Because of the distribution 
of nitrogen compounds within the kernel, the amount of gluten in 
any given sample of flour varies directly with the percentage extrac- 
tion, being higher as a rule in the straight flours. It will be seen, 
therefore, that results obtained by making gluten determinations on 
flour are of questionable value as an index to the quantity of the 
gluten of the original wheat kernel unless the determination is made 
on a straight 100 per cent flour. As was pointed out above, at the 
present time there is no absolute method for determining directly 
