Why does popcorn pop when heated at atmospheric pressure and why do other cereal grains such as wheat, barley, rice, and dent corn fail to do so? The answer lies in the structure of the popcorn kernel itself. Popcorn kernels contain a strong pericarp or hull and a high percentage of translucent endosperm cells where densely packed starch granules are located. Most other grains contain opaque cells that have many intergranular spaces. During popping, the hull serves as a pressure vessel that allows the moisture held in the kernel to turn into superheated steam. Eventually the hull can no longer withstand the internal pressure and it fails. Rupture occurs at temperatures in the range 350-375°F (8). These temperatures correspond to saturated steam pressures inside the kernel of 135-185 psi. Microscopic studies have shown that after popping, cell walls are generally intact (8,9). It is believed that the tight packing of the translucent cells helps the escaping steam to create the foamed network. Opaque starch contains air spaces that provide channels for the escaping steam; consequently it does not foam or expand.

Popcorn quality, as perceived by the consumer, is strongly related to its expanded volume. Highly expanded popcorn is desired for its tenderness and "melt-in-the-mouth" characteristics. Kernel moisture and kernel test weight (bulk density or number of kernels per 10 g) both strongly influence the amount of expansion. Maximum expansion of popcorn typically occurs in the 12.5-15% total moisture range (8). However, each hybrid has its own optimum moisture content. The level of expansion at moistures below 10% is poor (8). Among hybrids, those with the highest test weights have the greatest expanded volumes (10). The percentage of totally popped kernels is strongly related to both popping temperature and moisture content. In one study, when the temperature of cooking oil was raised from 345 to 350°F the percentage of totally popped kernels rose from 70 to 94%. Similarly reduction in moisture content from 11.1 to 9.9% caused a reduction from 100% popping to 82% (8).

Commercial Popping

Today, most large commercial poppers use continuous dry or hot air poppers. Continuous poppers generally consist of a rotating drum with helical flights for conveying the popcorn. Hot air (410—430°F) is blown in from the feed end and heats the tumbling kernels, which are metered into the drum from above. Unpopped kernels, small pieces and hulls are separated with a screen, then the popped kernels are cooled and coated if desired. Because fresh popcorn is extremely hygroscopic, it must be immediately packaged in a moisture barrier, eg, foil, to preserve freshness. Coatings range from mixtures of oil, salt, seasonings, and coloring to candy caramel and nuts. Caramel corn is made by combining fresh popcorn with caramel, a molten mixture of sugar, glucose, butter, sodium bicarbonate, and flavor in a specialized agitated vessel. After coating, the caramel corn is cooled and separated into the appropriate cluster size before packaging.

Microwave Popcorn

Through the late 1980s microwave popcorn has been one of the fastest growing grocery product categories in the United States. By the end of 1987 annual microwave popcorn sales were estimated at $351 million, up from $124 million in 1985 and $246 million in 1986 (2). Most microwave popcorn is packed in an expandable bag that incorporates design features that focus or concentrate the microwave energy on a mixture of solid fat, popcorn and salt. Popcorn-to-fat ratios are approximately 3:1. The bags are generally designed to expand to allow room for the popped corn (11). Early commercial microwave popcorn products were refrigerated or frozen to provide fat and moisture stability. Newer packaging materials such as polyester-lined kraft paper provide about six months shelf stability for the fat-covered kernels and eliminate the need for freezing or refrigeration. The introduction of susceptors (metallized films that are good absorbers of microwave energy) to the microwave popcorn package has improved poppability by providing a localized source of intense heat (400-450°F) beneath the kernel-oil mixture.

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