13 May 2005: Chemistry of cinema snacks

Food chemists in the US - a nation with a term for the hard bits of popcorn that don’t go pop - have discovered a way to maximise pop-ability.

’We now have a better understanding of the science behind why unpopped kernels occur and how we can use this knowledge to go about reducing their number,’ said group leader Bruce Hamaker, director of Purdue University’s Whistler Center for Carbohydrate Research. 

Unpopped kernels are called ’old maids’ in the US; they reportedly crack teeth, destroy fillings and cause choking. 

The secret to maximizing pop-ability lies in the chemistry of the corn kernel, says Hamaker. He has identified a crystalline structure in popcorn that appears to determine pop-ability. 

His team tested a variety of popcorn brands, and found the most poppable ones share a characteristic chemical structure of the outer hull (pericarp).

Popcorn kernels contain a drop of water stored in starch, enveloped by the pericarp. With heating, the water expands, and pressure causes the pericarp to split. The starch inside the popcorn becomes inflated and bursts, turning the kernel inside out. Finally, the steam inside the kernel is released, and the popcorn is popped.

Moisture loss increases the number of unpopped kernels, so Hamaker’s team analysed pericarp properties to study moisture-loss rate during microwave heating.

’Differential scanning calorimetry profiles of ground pericarp displayed a notable exothermal event, and hybrids with superior microwave popping performance (fewer unpopped kernels) exhibited significantly higher enthalpies,’ write the authors. 

X-ray analysis showed that cellulose and arabinoxylan are the major structural components of the pericarp. Structural changes in cellulose were induced by moisture and heat, and crystallinity was greatly enhanced when the pericarp was heated in the presence of water.

The data suggest that cellulose in the pericarp is responsible for the development of exothermal events and increased crystallinity. ’The propensity of cellulose to form crystalline structures in the popcorn pericarp during microwave heating improves moisture retention and hence popping performance,’ they write.

Improving pop-ability - by breeding varieties with optimal crystalline structure; by chemical modification of corn kernels; or by genetic modification - could result in a better product in 3-5 years, predicts Hamaker. 

Bea Perks