Naked Science Forum

Non Life Sciences => Chemistry => Topic started by: GlentoranMark on 16/11/2013 11:43:29

Title: How does cornflour thicken gravy?
Post by: GlentoranMark on 16/11/2013 11:43:29
Put some cornflour or flour in water and it turns into a thick goo but reasoning makes me think that the particles should float in suspension in water and not have any effect.

Can anyone explain to me if there is a chemical process that turns water into a sort of quicksand?
Title: Re: How does cornflour thicken gravy?
Post by: distimpson on 16/11/2013 13:22:42
Cornstarch (particles of amylopectin) in water without heating is often used to demonstrate non-Newtonian fluid behavior, when forces are applied slowly it is liquid like, when applied rapidly it is solid like:
http://chemistry.about.com/od/chemistryhowtoguide/ht/oobleck.htm (http://chemistry.about.com/od/chemistryhowtoguide/ht/oobleck.htm)

When heating cornstarch, as in gravy, the polymer amylopectin dissolves. Polymers are like molecular noodles, they interact with water and entangle with each other making the solution appear thicker.

http://en.wikipedia.org/wiki/Starch_gelatinization (http://en.wikipedia.org/wiki/Starch_gelatinization)

(https://www.thenakedscientists.com/forum/proxy.php?request=http%3A%2F%2Fwww.rsc.org%2FEducation%2FTeachers%2FResources%2Fcfb%2Fimages%2Famylopectin.gif&hash=999aba26f7be6c9a6452c6a1a3c3e28f)

http://www.rsc.org/Education/Teachers/Resources/cfb/carbohydrates.htm (http://www.rsc.org/Education/Teachers/Resources/cfb/carbohydrates.htm)
Title: Re: How does cornflour thicken gravy?
Post by: GlentoranMark on 16/11/2013 15:12:42
Many thanks for the response and it's probably the right answer.... but can I have that in layman's terms  :)
Title: Re: How does cornflour thicken gravy?
Post by: distimpson on 16/11/2013 17:40:42
I'm not very good at layman's terms, guess I've been a scientist for too long, usually get in trouble for over explaining.

Here's my best for the non-Newtonian description, think of a bowl of water and add some small particles, say ball bearings. When you try to move a spoon through the mixture you have to move the water and the ball bearings.  The ball bearings have to move past each other and the water has flow around the particles, it's a bit like a log jam. The faster you go the more the particles and water have trouble pushing their way among the other particles so it acts like a solid. If you go very slowly then as you say "particles ... float in suspension in water and not have any effect", it acts like a liquid.

For the case of heating, the dissolved polymer molecules are very long and thin. They like water and kind of stick to each other in the solution with water acting a like the glue holding them together. If you get a high enough concentration it turns into a gel, like in the filling for lemon meringue pie.

Best I can do, hope someone else jumps in too.