Week 7: Viscosity and Polymers, Harvard Style

Classic crepes are an example of a liquid batter that thickens during cooking to the point where there is no fluid flow, unless you count the Grand Marnier sauce.

Viscosity is a word that sounds like a bitter subject - but it introduces an engaging week of food science that's all about thickening power. Hello sauces!

Viscosity is the resistance of a fluid to flow. The larger the food molecules, the slower the flow. The more resistance to flow, the higher the viscosity.

Where this truly matters is in sauce-making, right? There are four basic ways to change sauce thickness: 1) reduction 2) adding starch 3) making an emulsion 4) modernist thickeners

Diving into this rich pool of intrigue, we begin with reductions. So in a reduction, you have a sauce on the stove and you keep heating it to reduce the available liquid via evaporation. As liquid goes down,  the food molecules become more concentrated and bump into each other, which slows the sauce's ability to flow. In a classic reduction, you reduce by half - and it only works because there were already enough molecules in the pan to cause thickening. So by increasing the concentration as you drove off the water, resistance to flow increased. Voila!

Now when you add a starch to your sauce, like cornstarch or flour, you're adding larger molecules into the food, and this also impedes flow. Think of a roux - butter and flour. You heat the starch in the flour so it hydrates and gelatinizes (and by doing so, those molecules swell and get bigger). The starch molecules become sticky polymers and rub against each other, creating the sensation of thickening. The problem is, you need a lot of flour to get this thickening, and if you use too much, your sauce gets rubbery - and those starch molecules can dilute desirable flavors, too. Pity the poor bechamel sauce if this happens.

Up next is the emulsion technique, like when you make mayonnaise. When you add oil to the eggs, you suspend tiny droplets of one phase of a liquid into another. The droplets become like people on a subway platform, running and bumping into each other and trying to get out of each others' way. The more you add, the stiffer everything becomes. The problem here is, your mayo can become too oily, too rich. (And the emulsion can break, but we're not floundering in that issue just now.)

With modernist thickeners such as xanthan and guar gum, these polymer molecules are way better at thickening than starch. Xanthan gum is a fantastic thickener when you use only a small amount, and it doesn't impede flavor or add unwanted carbs. Can you thicken gravy for chicken-fried steak using xanthan gum? Of course you can! These thickeners allow better control of viscosity, have greater thermal stability and don't contribute their own flavor, ureka!

A-ha Moment of the Week: Ketchup is an example of a product that is very thick at rest but liquifies with force (and you know this if you've banged on the bottle, only to have the contents shoot out). Via the process known as "shear thinning," at rest it's a fluid-gel hybrid where the gel is broken and some water can flow, but it's still thickened by polymers. It behaves like a solid but flows when force is applied. The ketchup actually undergoes a change in viscosity, wow.

What is a polymer, since I mention it? It's a long, flexible molecule made up of monomers, and can bend in any direction. OK, that's done.

Equations of the Week: n = nof(0) (where this o has a line through it and represents "phi.")
N is the viscosity of the solution, no is the viscosity of the pure continuous phase, f(phi) is a function that increases with phi.

(phi) = V dispersed, divided by V total where V dispersed is the volume occupied by non-solvent molecules and V total is the total volume of the solution. Just typing this in, I feel like the Scarecrow in the Wizard of Oz after he got his brain.

Next Week: Emulsions and Foams