Popping of Corn 151 
cell wall is a comparatively fragile structure, incapable of holding any 
great pressure, and playing no significant part in the process. In the 
softer varieties of corn the steam generated during the application of 
the heat tends to leak out through the more porous matrix of col- 
loidal material so that the explosion, when it finally occurs, is much 
less violent; and it comes at a lower pressure than in good popping 
varieties because the confining structure is not strong enough to hold 
a greater pressure. 
All kinds of corn pop more or less, and the process is also char- 
acteristic of the seeds of many other species of grasses. But it is only 
in the small-seeded flint varieties known as popcorns, and in some of 
the sorghums, that the grain undergoes so great a change as is gen- 
erally indicated in the popular term popping. The necessary structure 
for successful popping is a flinty endosperm. The range of moisture 
permissible is much wider than is generally supposed. 
Popping and Protein Content.—Since the hardness of the endo- 
sperm is determined by the degree to which the interstices between 
the starch grains are filled with the colloid rich in protein, a close 
correlation might be expected between the popping quality and the 
protein content of the grain. A general correlation does occur, but 
analyses such as those given by Carr and Ripley (p. 262) show that 
it is not so close as might be expected. 
It may be remarked in passing that the ordinary analysis of the 
whole grain of corn is scarcely more than useless in the solution of a 
problem like the one here at hand. A grain of corn consists of three 
separate and distinct parts—pericarp, endosperm, and embryo—mem- 
bers of three different morphological generations of the plant, and pos- 
sessed of three distinct genetic possibilities, and capable of having three 
uncorrelated chemical compositions. 
The protein concerned in the popping process is located in the 
interstices between the starch grains of the endosperm, that in other 
parts of the grain playing only a passive part. An analysis of the 
whole grain will indicate feeding qualities and many other things, but 
to take such an analysis as an index to popping qualities, or, as some 
have done, to the hereditary constitution of the embryo, is exactly as 
scientific as carrying out a nitrogen determination after spilling an 
unknown quantity of a substance of unknown composition into the 
digestion flask. 
Analyses of endosperms carefully separated from the rest of the 
grains show a much closer correlation between protein content and 
popping qualities. But even here too much must not be expected. Pro- 
tein content is only a matter of relativity after all. Starch grains vary 
much in size and in shape, and there is consequent variation in the 
amount of protein-bearing colloid necessary to fill all the spaces suffi- 
ciently to produce flinty translucency. The endosperm of the flintiest 
type the author has ever analyzed had only a little more than 6% 
protein, while that of a relatively soft, floury variety had more than 
12%. Although the former had large enough a grain to be classified 
among the flints, it popped well; the latter merely split open when 
heated. But microscopic examination showed the one to have large, 
