quarta-feira, 2 de janeiro de 2019


CHEMISTRY AND PERFECT POPCORN. Heather Hudson, Tim Harrison and Natalie Fey, at The School of Chemistry, University of Bristol, UK

A popular snack across the globe; sold often overpriced but ever-so tempting, in many cinemas and enjoyed at home on the sofa or in the office as a gloomy afternoon pick-me-up; popcorn is an all-time favourite. With so many possible flavour, colour and shape combinations and a few minutes of cooking time it’s not surprising that, in 2018 alone, a whopping 232.34 million Americans consumed popcorni. What perhaps is unexpected is that popcorn has been eaten for thousands of years, with the oldest known popped kernel found in a cave in New Mexico and dating back 5600 years!ii

The challenge is to engineer “The Perfect Popcorn”. Many factors influence this with the nation’s favourite flavour dominating. There are many intriguing flavours on offer. These flavours include cheese on toast; mince pie; gin and tonic; ketchup; and even “pregnancy mix” – ice cream and dill pickle flavoured popcorn! Feel free to indulge on that last one here http://poparazzispopcorn.com/pregnancy-mix/. I personally, will be sticking to simple toffee flavour.

Flavours aside, one thing everyone can agree on is that popcorn is no good if it doesn’t pop. How does soft delicious popcorn form from such a tough seed?

Popcorn kernels are usually of the flint corn variety, with Zea Mays Everta being the most widely grown cultivation.iii Popcorn kernels have an outer shell called a pericarp, which holds the starchy endosperm and water. When it is cooked, this water is superheated into steam. This steam is trapped. The hot, gaseous water molecules have a lot of kinetic energy (transferred from heat energy) and move around rapidly, bouncing off each other and the pericarp. This exerts a pressure onto the resisting shell which acts like a pressure cooker, and ‘cooks’ the starch inside into a gel. At 180°C a pressure of 930 KPa is reached which even the tough kernel can no longer withstand, and so it breaks open-it pops.

Once the pericarp has ruptured the pressure is immediately and dramatically reduced and this (according to the Ideal Gas Equation, PV = nRTiv) causes rapid expansion of the steam in the gelatinous starch into thin, jelly-like bubbles 30-50 times the size of the kernel. These cool quickly and solidify into those well-known butterfly and mushroom shaped clouds.v In traditional microwave popcorn, the inside of the bag is lined with flavourings so when the popcorn explodes it bounces into the flavourings and gets coated in them.

Water content is important. Fresh popcorn seeds are generally no good as popcorn as they have high water content. This leads to the popcorn treat being chewy and under expanded. In contrast, over dried kernels have insufficient water to generate the necessary pressures or cause the starch to gelatinise, and very few will expand and pop. It has been calculated that 14-15% moisture relative to mass is optimal.

For the perfect popcorn, the second thing to consider is how you cook it. If heated too quickly, the pressures inside the kernel reach bursting point before the starch can fully gelatinise, resulting in partially popped popcorn with a hard centre. If heated too slowly, the steam escapes through pores formed where the corn was originally attached to the cob, and the pressure fails to exert enough force to rupture the kernel. The area over which the pressure acts is constant, so the force relies entirely on pressure (p= f/a).

Fortunately, for those not in-the-know, microwavable packs have clear, tried and tested instructions. Even then, you still end up with those few, annoying unpopped kernels (historically called “old-maids”) rattling around in the bag and getting stuck in your teeth. To what does the ability of kernels to pop then come down? Researchers in America took it upon themselves to find out and analysed the structure of 14 different genetic varieties (all yellow) before testing them under the same microwave conditions. Shockingly, in one of the batches 47% of the kernels failed to perform! They discovered that the structure of the pericarp had a big influence into the results. This is because of the essential requirement for a build-up of pressure inside the tough exterior and the need to be able to withstand it for just long enough. The shell is made of highly ordered crystalline cellulose molecules. The more highly structured and densely packed the molecules are, the stronger the material and the better it withstands the pressure from the gaseous water molecules. This leads to a more complete gelatinisation of the starch and a more successful explosion.

From this discovery, selective farming of the best grains can occur, so who knows, perhaps in a few years’ time, unpopped kernels will be a thing of the past!vi

So, what happens when you ‘cook’ the starchy interior of the kernel?

Scheme 1: the structure of amylopectin (left), a highly branched polysaccharide of α – glucose, and the structure of amylose (right), an unbranched polysaccharide. Together, these 2 polymers make up starch.

Starch is an example of the semi-crystalline structure possessed by many polymers in the solid state, made up of amorphous domains and crystalline, chain-folded regions. It consists of 2 different polymer chain types, as shown above in scheme 1, linked by the stronger intermolecular bonds- hydrogen bonding. The endothermic process of gelatinisation of starch in water reduces the crystalline character and density of the granules as they swell with water absorption. Under standard conditions water molecules cannot fit into the densely packed, entangled amylopectin chains, however heating breaks down the intermolecular hydrogen bonds, the crystalline regions become less closely packed, and the system becomes less ordered, which coincides with a favourable increase in entropy. Consequently, water can enter the amorphous space with ease. It forms new hydrogen bonds with the now free hydroxyl and oxygen groups, dissolving the chains and eventually breaking down the granule structure. The starch now has a viscous ‘gel-like’ texture.vii The temperature at which this transition occurs (180°C for popcorn starch) is known as the glass transition temperature (Tg). Above Tg, polymers are viscous, rubbery and soft compared to below Tgwhere polymeric material is hard and brittle; the transition between the two states is a pseudo 2nd order transition.viii

Next time you enjoy a simple bucket of popcorn, know that you’re enjoying the product of reactions of starch molecules, popping their way to satisfy those cravings.

i https://www.statista.com/statistics/285906/consumption-of-popcorn-products-in-the-us-trend/ (last accessed November 2018)
ii https://www.thespruceeats.com/the-history-of-popcorn-1328768 (last accessed November 2018)
iii https://en.wikipedia.org/wiki/Popcorn (last accessed November 2018)
iv Laugier, Alexander; Garai, Jozef. "Derivation of the Ideal Gas Law." Journal of Chemical Education,. 2007, Vol. 84, Iss. 11, pgs. 1832 -1833
v http://www.chemistryislife.com/t-3 (last accessed November 2018)
vi https://www.sciencedaily.com/releases/2005/04/050415112829.htm (last accessed November 2018)
vii https://www.revolvy.com/page/Starch-gelatinization (last accessed November 2018)
viii http://polymerdatabase.com/polymer%20physics/GlassTransition.html (last accessed November 2018)

The Authors:

Heather Hudson, at the time of writing, is a final year undergraduate student in the School of Chemistry, University of Bristol, UK.

Tim Harrison is the Bristol ChemLabS Director of Outreach, School of Chemistry, University of Bristol, UK.

Natalie Fey is a Lecturer based in the Centre for Computational Chemistry, School of Chemistry, University of Bristol, UK.

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