Originally posted February 11, 2011
The most careful designs are easily blown by enough pressure
February 11, 2011
I was prepared. For once. My class notes were finished the night before. They had been double-checked. The topic was engaging. The demo was money. This class was going to be good.
You see, things usually don’t happen this way. I’m usually scrambling to get all of my slides put together in time. I’m usually trying to make sure that I’ve got enough interesting things to say to fill the hour and fifteen minute time slot. This is entirely nerve-wracking. Especially for someone who is normally under the opinion that he is really not all that interesting.
But, for once, I was ready. I was confident. I had all of my loose ends locked up.
Or so I thought.
Popcorn
The plan was pretty good. I had just lectured on the Gas Laws. (Everybody remembers those, right? PV = nRT and all that) I was going to use the gas laws in class to figure out how much pressure it takes to cause a popcorn kernel to pop.
Popcorn is really interesting stuff. Most other types of corn (sweet corn, dent corn, etc) don’t pop when they’re cooked … at least not with the same intensity. Popcorn has a couple things going for it in this respect. First, and foremost, the shell surrounding the kernel is more dense than in other types of corn. This facilitates the water vapor, trapped inside the kernel, to build up pressure. In the other kinds of corn, the water vapor just passes through the shell. Popcorn, in general, also contains a large percentage starch by weight (roughly 65%). With the starchy part ultimately fluffing up to become the popcorny goodness, this is quite an important fact. In fact, the transition that the starch undergoes – from inside the shell and tough to puffy tastiness – is really rather amazing.
Here is a picture of some starch granules taken with a scanning electron microscope:
Here is another picture of what the starch granules look like in an unpopped kernel of corn:
Image taken from “Mechanism of Popcorn Popping” Journal of Cereal Science 1983 1 p43-52 – yes, you can publish real research on this sort of thing
Starch, while being ubiquitous in the foods that we eat and seemingly very boring, is an amazing composition brought about by fairly simple chemistry. On the molecular scale of things corn starch is made up of the polymers amylose and amylopectin, very chemical sounding names that really just mean long chains of sugar molecules. Plants can do some really interesting things with sugar. One of which is taking individual sugar molecules and stringing them together into polymers. Corn does this (primarily) with glucose to make amylose (a linear string of sugar molecules) and amylopectin (a branched chain of sugar molecules).
Chemical structure of glucose, water, amylose – a linear chain of glucose molecules, and amylopectin – a branched chain of glucose molecules. Make note of the recurrence of OH groups (red) in sugar, water, and the polymers.
The red OH groups in these molecules are what help them to become soluble in water (remember – like attracts like or chemicals are homosexual). Granted, glucose on its own is much more soluble in water than is amylose. (For the amylose polymer you are effectively removing 2 OH groups from each glucose molecule that would otherwise be strongly interacting with water.) This polymerization does not remove the attractive power of each OH group for another; it just removes the overall attractiveness of one glucose unit to water molecules. These strong OH-OH interactions are what eventually lead amylose and amylopectin to form starch granules. The starch granules are so tight (the attraction between the polymers so nearly complete) that they can almost keep any water from meaningful interactions with the polymer OH groups.
A rendering of the interactions among the amylose OH groups as they may be found in a starch granule. This drawing is purposefully misleading to show how compact this sort of relationship is and to illustrate how starch granules protect themselves from being dissolved by water.
Back to the popcorn
As you are heating up the popcorn, the water trapped inside of each kernel is able to start penetrating and weakening the starch granule. The hot water eventually “gelatinizes” the starch (weakens its structure). This same thing happens when you use corn starch to thicken gravy or when you make mashed potatoes. The starch granules loosen and the individual polymers start to stretch out and interact with the hot water.
And, when the popcorn pops, most of that hot water leaves the kernel as steam while the polymer strands, with their plentiful OH groups, reconnect in a new form, which appears to look like millions of little balloons. Of course we’re all familiar with what popped popcorn looks like to our human eyes. But, when we look a little closer we can see what has actually happened to the structure of the starch granules and understand a little bit of how the amylose polymers are formed by steam and pressure into their white, fluffy, new shape.
from “Mechanism of Popcorn Popping” Journal of Cereal Science 1983 1 p43-52. Popped popcorn is really just a foam with solid amylose fibers (white) surrounding little pockets of air (black).
Back to my class
So, I had this class on popcorn all ready to go. We were going to weigh the popcorn before and after popping, measure the volume of an unpopped kernel of corn, and use the knowledge that popcorn pops most efficiently at 177oC to determine the pressure inside the kernel of corn. (For PV=nRT … we now know Volume, number of moles, R – the gas constant, and T).
But, for whatever reason, the scale that I brought to class stopped working. It wouldn’t register any mass reading at all. I was pretty livid and beyond frustrated. My perfect plan was destroyed in an instant. I may have actually popped myself. But, my students, being really quick and very engaged, pulled me through. Watching me fiddle and curse under my breath for several minutes trying to fix the stupid balance, they started surfing the net with their smart-phones trying to find out what the mass of a popped and unpopped kernel. And, they did all of this without any prompting from me. The students were actually all pretty involved in the lecture now, and I felt that, in the end, it was a success. By the way, popcorn pops when its internal pressure reaches about 9 times the pressure at the Earth’s surface (about 4 times the amount of pressure inside of your car’s tires) … which is pretty amazing!
Ahhh, the best laid plans of mice and men …
I am very grateful to my students for getting me through my minor meltdown and making the class a success!! I should be so lucky with every class that I teach.
Cheers
-mrh
By the way, I am also very grateful to the kind soul who supplied me with a copy of the paper, “Mechanism of Popcorn Popping”. Having this made the class a resounding success. Nudge Nudge Wink Wink!