[thetada]

I've been thinking about this for ages. I think it has legs as a practical investigation for sixth form / senior highschool chemistry students.

If you cook baked beans in a saucepan and then leave a few in the bottom to dry out, they stick fast like concrete. They only need to be soaked in water for about 2 minutes before the pan can be easily wiped clean. Other starchy foods, like Weetabix, do the same thing. Obviously hydrogen bonds between glucose monomers in the starch is going to be a factor, but why do they stick to the bottom of the bowl / pan? If you're using a stainless steel pan, what kind of intermolecular forces will form between the pan and the beans? I feel like stainless steel pans couldn't form hydrogen bonds. It also seems to make sense that stainless steel should not form an oxide layer on its surface, but it also feels like metals will generally form oxide layers. Would an oxide layer enable the formation of hydrogen bonds / dipole-dipole interactions? Weetabix (don't know how far beyond UK borders this cereal has penetrated - it's a wheat-based cereal served in densely packed tablets that are crispy until you add milk, upon which they rapidly turn soggy) is traditionally served in ceramic bowls. Would ceramic surfaces form hydrogen bonds with glucose monomers? If so, a water droplet ought to roll more slowly across ceramic than across a hydrophobic material. Perhaps what makes more sense is that at the sub-microscopic scale, the surface is nowhere near so smooth, but instead has ridges, channels, craters and canyons. Is it simply that when the beans / weetabix are mixed with water, they can flow easily into these cracks, but once they have set hard, they are fixed in place?

I would love to hear anyone's thoughts on this, whether it concerns the science or the use of this point for a practical investigation.

[thetada]

Just been tweeting about this and someone else has suggested involvement of proteins. Since they have hydrophobic and hydrophilic regions, could they somehow bind the starchy environment to the ceramic / metal surface?

[Corribus]

Sticking of food to metal cookery is partly chemical but largely physical. The metal surface is not a perfect surface. There are pores, uneven spots, notches, etc. Perfect for food to fit into and stick. Any cook knows, for example, that you have to heat a stainless steel pan thoroughly with oil before putting meat into it; otherwise the meat will stick like crazy. A significant contributor to this sticking is that when it is cold, meat placed in the pan will conform to the topology of the surface. This increased contact surface area helps to create a great bond between the meat and the metal. On the other hand, if the pan is blazing hot first, the surface of meat will be flash seared, limiting its ability to fill those pores and gaps in the metal surface. I imagine the fast production of steam from the meat also helps prevent interactions between the protein rich meat surface and the oily metal surface. (The metal surface also swells to some degree when it is heated, which may help to close those pores and microscopic cracks.)

There is a chemical aspect, too. Proteins seem to stick much worse than starches. Proteins, particularly when denatured, can bind to metals. Metals also usually contain oxides on the surface that are quite polar and can react with proteins to form both covalent and strong intermolecular bonds.

Nonstick cookery works by reducing the chemical adhesion between the food surface and the pan surface. Properly seasoned cast-iron is almost perfectly nonstick because a thin layer of polymerized oil separates the metal pan from the food surface, preventing sticking by both limiting chemical reactions between the metal and the food, and also creating a smoother cooking surface. And we've discussed how Teflon pans work before, here: http://www.chemicalforums.com/index.php?topic=72456.msg276198

[thetada]

Thanks Corribus!

Glad to see I had some good hunches. Apart from the forum post on teflon (which I look forward to reading), can you recommend any other materials? Come to think of it, I can't believe I haven't checked my copy of McGee yet. I'll bet there's something in there. Anyway, I reckon there's definitely plenty of scope for a school investigation.

Thanks again for your input

[Corribus]

You may find this thread (and the link in the opening post) useful reading on cast iron seasoning.

[thetada]

That was useful, thanks.

[Corribus]

Did I forget to include a link in my last post?
This is the thread I was referring to. Sorry about that!

http://www.chemicalforums.com/index.php?topic=72038.msg260615#msg260615