Science and Cooking: From Haute Cuisine to Soft Matter Science - Viscosity and Polymers

As I posted in my October 13th blog entry, I've enrolled in an on-line course about the relationship between science and cooking.  I'm still plugging away at this, good thing I decided to do this for the knowledge and not the class credit.

Here's what went down for Week 7.

• Viscosity measures how easy something is to pour. Water has a low viscosity and cold syrup has a high viscosity.
• The molecules that make a liquid up flow past and bump into each other. To thicken it, we need to add molecules or particles that will impede the motion of the liquid.
• Want to measure the viscosity of a fluid, but you don't have a rheometer? If you have some free time on your hands (and who doesn't?), take a container and put a little hole in the bottom, and measure how long it takes for the fluid to flow through the hole.
• How thick or how thin a sauce is can make or break a recipe. Some of the ways to thicken food are:

1. Reduction: This term you've heard on your favourite cooking show means you take the material you want to thicken, heat it on a stove in an open pan, and you reduce it by simmering it until about half of the water has left, through evaporation. 
• This works because they were already enough molecules in the material you want to thicken to cause a thickening, but because of all the water molecules in the liquid, they were just too far apart to thicken. It works well if you have a stock, because it has lots of gelatin molecules. But it won't work if you just have something with only very small molecules in it. This is why you can't thicken a brine by boiling it, and why can't thicken wine by boiling it, unless you boil it all the way down till it's a glaze and there's almost nothing left.

Here is guest lecturer Carme Ruscalleda in her native Spanish to demonstrate:


2. Emulsion: Another oft-heard cooking show word, this is the process of combining two liquids (usually fat and water) that will maintain their distinct characteristics after being mixed. Common fat in water emulsifications include hollandaise sauce and mayonnaise, and common water in fat emulsifications are vinaigrettes and whole butter.
• Adding oil is a good way to thicken, but it makes what you're making taste of the oil you're using.
3. Starch-based thickener: This is a classic French technique of mixing and cooking equal parts flour and fat into something called a roux.
• The starch in the flour is heated up to the point that it hydrates and gelatinizes, turning the starch molecules into sticky polymers that make a liquid thick.
• Note that you have to cook the roux properly, otherwise it will have a floury taste, and it won't thicken well. You can use cornstarch or arrowroot or other starches, but if you put too much in, your roux will get rubbery and have an unpleasant texture.
4. Modernist thickener: These are substances used by those who worship at the altar of molecular gastronomy. Xanthan gum, a natural product, is made by fermenting a kind of bacteria. Because of the polymer molecules, very small amounts can produce a large increase in the viscosity of a liquid- in most foods, it is used at 0.5%, and can be used in lower concentrations. Xanthan gum also helps thicken commercial egg substitutes made from egg whites, to replace the fat and emulsifiers found in yolks, and is also used in gluten-free baking, as it gives the dough or batter a "stickiness" that would otherwise be achieved with gluten.
• A polymer is a very long, but very flexible molecule, made up of many, many monomers. Polymers have to get out of the way of one another in order for the fluid to flow. This is why polymer thickeners are so effective at increasing the viscosity of a fluid.

• Gels can also be used in thickening, as their long polymer molecules stick to each other, in a random way, trapping both water molecules and all other molecules, into something that effectively becomes a solid, like Jell-o. When you break them up, by pureeing it for example, the gel will reform slightly, and create a fluid gel, which acts like a thick liquid.
• Agar agar (or just agar) is a natural gelatin product made from seaweed that's been used for more than 1000 years in Asian cooking. Like xanthan gum, only a small amount of agar is needed to thicken your soups and sauces.
• Food additives like agar agar and xantha gum can be ordered online at MOLECULE-R.