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Friday, November 20, 2009

NMR and Food Chemistry - Popcorn

Both liquid state and solid state NMR have become very important tools in the food industry for both research and quality control. Since starch is a very important biopolymer and a major constituent in many foods, a great deal of work has been done on its chemical and physical characterization. Starches are 1 -4 linked polymers of glucose. Native starches are a mixture of a linear polymer (amylose) and a branched polymer (amylopectin). Furthermore, the starches are of two main types differing in their crystal structures and water content: the A type (cereal starches) and the B type (tuber starches). In addition to A or B starches, the starch granules in plants can have some amorphous starch as well. Corn starch is of the A type.

Popcorn is a snack enjoyed by millions around the world. As a child, I remember being fascinated at watching it pop and wondering what was going on. Much study has been devoted to the physics of popping corn. Essentially, every kernel of corn is a pressure vessel. When cooked, the moisture trapped within the starchy endosperm of the kernel is superheated. When the steam pressure inside the kernel becomes high enough the hull (pericarp) of the kernel explodes and the superheated water in the starch granules suddenly vaporizes, expands and rapidly cools making a solid foam out of the starchy endosperm. Each starch granule is a bubble in the solid foam.
Although much work has been done to understand starch and much work has been done to understand the popping of corn, I was unable to find any efforts directed to the chemical changes in corn starch before and after popping. This prompted me to collect a few spectra to address the issue. The first figure below shows the starch region of the 13C CPMAS spectra of unpopped corn (bottom trace), popped corn (middle trace) and cooked but unpopped corn (upper trace). The spectrum of unpopped corn is a mixture of A type starch (with a characteristic three line pattern in the C1 region) and amorphous starch (with a broad distribution of overlapping lines in the C1 region). The spectra of popped corn and cooked but unpopped corn are essentially identical and characteristic of amorphous starch. The data indicate that the heating and dehydration of the corn transform the crystalline A starch into amorphous starch. The observation is consistent with published studies on the hydration of starches. Aside from this change, there is no evidence for any other chemical transformation in the starch.

The second figure shows an expansion of the C1 region highlighting the conversion of crystalline A type starch into amorphous starch.

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