Although small quantities can be popped in a stove-top kettle, or pot in a home kitchen, commercial sale of freshly popped popcorn employs specially designed popcorn machines, which were invented in Chicago, Illinois, by Charles Cretors in 1885. Cretors successfully introduced his invention at the Columbian Exposition in 1893. At this same world's fair, F.W. Rueckheim introduced a molasses-flavored "Candied Popcorn", the first caramel corn; his brother, Louis, slightly altered the recipe and introduced it as Cracker Jack popcorn in 1896.
Cretors's invention introduced the first
patented steam-driven popcorn machine that popped corn in oil.
Previously, vendors popped corn by holding a wire basket over an open
flame. At best, the result was a hot, dry, unevenly cooked snack.
Cretors's machine popped corn in a mixture of one-third clarified
butter, two-thirds lard, and salt. This mixture could withstand the 450
°F (232 °C) temperature needed to pop corn and it did without producing
much smoke. A fire under a boiler created steam that drove a small
engine; that engine drove the gears, shaft, and agitator that stirred
the corn and powered a small automated clown puppet-like figure, "the
Toasty Roasty Man", an attention attracting amusement intended to drum
up business. A wire connected to the top of the cooking pan allowed the
operator to disengage the drive mechanism, lift the cover, and dump
popped corn into the storage bin beneath. Exhaust from the steam engine
was piped to a hollow pan below the corn storage bin and kept freshly
popped corn uniformly warm for the first time ever.
An in-home hot-air popcorn maker
A very different method of popcorn-making can still be seen on the
streets of some Chinese cities today. The un-popped corn kernels are
poured into a large cast-iron canister — sometimes called a 'popcorn
hammer' — that is then sealed with a heavy lid and slowly turned over a
curbside fire in rotisserie fashion. When a pressure gauge on the
canister reaches a certain level, the canister is removed from the fire,
a large canvas sack is put over the lid, and the seal is released. With
a huge boom, all of the popcorn explodes at once and is poured into the
sack. This method is believed to have been developed during the Song
dynasty originally for puffing rice.
Individual consumers can
also buy and use specialized popping appliances that typically generate
no more than a gallon of popped corn per batch. Some of these appliances
also accept a small volume of oil or melted butter to assist thermal
transfer from a stationary heating element, but others are "air poppers"
which rapidly circulate heated air up through the interior, keeping the
un-popped kernels in motion to avoid burning and then blowing the
popped kernels out through the chute. The majority of popcorn sold for
home consumption is now packaged in a microwave popcorn bag for use in a
Expansion and yield
Popping results are
sensitive to the rate at which the kernels are heated. If heated too
quickly, the steam in the outer layers of the kernel can reach high
pressures and rupture the hull before the starch in the center of the
kernel can fully gelatinize, leading to partially popped kernels with
hard centers. Heating too slowly leads to entirely unpopped kernels: the
tip of the kernel, where it attached to the cob, is not entirely
moisture-proof, and when heated slowly, the steam can leak out of the
tip fast enough to keep the pressure from rising sufficiently to break
the hull and cause the pop.
Producers and sellers of popcorn
consider two major factors in evaluating the quality of popcorn: what
percentage of the kernels will pop, and how much each popped kernel
expands. Expansion is an important factor to both the consumer and
vendor. For the consumer, larger pieces of popcorn tend to be more
tender and are associated with higher quality. For the grower,
distributor, and vendor, expansion is closely correlated with profit:
vendors such as theaters buy popcorn by weight and sell it by volume.
For both these reasons, higher-expansion popcorn fetches a higher profit
per unit weight.
Popcorn will pop when freshly harvested, but
not well: its high moisture content leads to poor expansion and chewy
pieces of popcorn. Kernels with a high moisture content are also
susceptible to mold when stored. For these reasons, popcorn growers and
distributors dry the kernels until they reach the moisture level at
which they expand the most. This differs by variety and conditions, but
is generally in the range of 14–15% moisture by weight. If the kernels
are over-dried, the expansion rate will suffer and the percentage of
kernels that pop at all will decline.
Two explanations exist
for kernels which do not pop at proper temperatures, known in the
popcorn industry as "old maids". The first is that unpopped kernels do
not have enough moisture to create enough steam for an explosion. The
second explanation, according to research led by Dr. Bruce Hamaker of
Purdue University, is that the unpopped kernel may have a leaky hull.
Popcorn varieties are broadly categorized by the shape of the kernels,
the color of the kernels, or the shape of the popped corn. While the
kernels may come in a variety of colors, the popped corn is always
off-yellow or white as it is only the hull (or pericarp) that is
colored. "Rice" type popcorns have a long kernel pointed at both ends;
"pearl" type kernels are rounded at the top. Commercial popcorn
production has moved mostly to pearl types. Historically, pearl popcorns
were usually yellow and rice popcorns usually white. Today both shapes
are available in both colors, as well as others including black, red,
and variegated. Commercial production is dominated by white and yellow.
"Mushroom"-shaped popcorn, left, is less fragile and less tender than "butterfly"-shaped, right
In popcorn jargon, a popped kernel of corn is known as a "flake". Two
shapes of flakes are commercially important. "Butterfly" flakes are
irregular in shape and have a number of protruding "wings". "Mushroom"
flakes are largely ball-shaped, with few wings. Butterfly flakes are
regarded as having better mouthfeel, with greater tenderness and less
noticeable hulls. Mushroom flakes are less fragile than butterfly flakes
and are therefore often used for packaged popcorn or confectionery,
such as caramel corn. The kernels from a single cob of popcorn may form
both butterfly and mushroom flakes; hybrids that produce 100% butterfly
flakes or 100% mushroom flakes exist, the latter developed only as
recently as 1998. Growing conditions and popping environment can also
affect the butterfly-to-mushroom ratio.