How Do I Thicken My Soup?
We rarely think when it comes to thickening soups and sauces. We usually just reach for that box of cornstarch are whip up a quick batch of roux, never really thinking of what we are doing. But do you really understand when to use one thickening agent over another? Why do most gravies use flour or a roux and why do many fruit desserts use cornstarch, arrowroot or tapioca?
In part I of this series we will examine the myriad of varieties of thickeners out there and when you should use each. In part II we will focus completely on the use of flour as a thickener and more specifically we will focus on roux and how to properly make one from a white roux all the way to that darling of Cajun cooking-dark roux.
There are an almost infinite number of ways to thicken foods. Many of these though will contribute their own texture and flavor to the dish they are thickening. Examples of some of these types of thickeners are grated starchy vegetables, blood, tomato puree or paste, cereal grains (such as oatmeal, rice, couscous, etc.) egg yolks, cream, yogurt and bread. All of these items will adequately thicken a sauce or a soup. Unfortunately they will also leave a very distinct taste to whatever it is that they are thickening. Many times that is perfectly fine. Thickening a chili with tomato paste is fine as that flavor is desirable to chili. Pureeing some of the beans in a bean soup to thicken it adds body while reinforcing the bean flavor. Using yogurt to give body to a curry works because the yogurt adds a distinct sourness to the curry and helps to mellow some of the hotter spices. All these items work well, but what if you want to thicken something but not add another flavor dimension? Not to worry, there are a whole range of items that can be used to thicken foods without adding unwanted flavors.
Most of these thickeners fall into 3 categories. The first category is proteins and it includes things such as egg whites, gelatin, agar and carrageenan. The second category is vegetable gums and includes such items at guar gum, locust bean gum and xanthan gum. These first 2 categories are rarely seen outside of manufacturing, with a few notable exceptions, and don't really concern most cooks. As such we won't be focusing on them. The third and final category is the starches. This is what most home cooks and restaurants chefs deal with most of the time. While this large group of thickeners contain many obscure items such as kudzu starch, sago, and katakuri starch, it also contains a number items that most home cooks will be very familiar with; cornstarch, arrowroot and tapioca. It is these final three items that concern us today.
All starches, including flour, work in relatively the same way, chemically speaking. The starch molecules trap water molecules thus decreasing the amount of "free-flowing" water in a sauce or a soup. As the starches heat up these starch molecules untangle and stretch out allowing them to trap more water molecules, decreasing the amount of "free-flowing" water even more and thus thickening the liquid. This is a simplification of the process and because each of these starches is slightly different they thicken in different ways and produce slightly different results. At this point we will leave flour behind, as that will be discussed in part II.
Cornstarch, arrowroot and tapioca all thicken in very similar ways. Sauces thickened with these starches are all translucent and have a nice shiny sheen to them. This is wonderful when working with fruit sauces, pie filling, meat glazes or broths that you want to give a slight body to, but they tend to make dairy based soups look artificial and make gravies and other sauces look just plain strange as most people expect these things to be rather opaque. The other advantages these starches have, over flour, is that it takes a lot less of them to thicken an item, and they thicken rather quickly, without leaving a starchy taste to the product, unlike flour with which the starchy taste needs to be cooked out.
Usually about 1 tablespoon of these starches to 1 cup of liquid is adequate for most thickening purposes. Cornstarch, arrowroot and tapioca all have a few disadvantages also so I usually try to keep at least 2 of these starches on hand at all times.
Cornstarch needs to be made into a slurry with a cold liquid before adding to a hot liquid or the starch will gelatinize on immediate contact, causing lumps that you will never be able to whisk out and will need to be strained out. The same goes for arrowroot. Instant tapioca makes a great thickener for items that will receive prolonged cooking or freezing, but will always form little gelatinous balls. This really isn't an issue though in fruit pie fillings, especially those topped with a top crust. This can be avoided by pulverizing the tapioca in a coffee grinder. Cornstarch will break down in the presence of highly acidic foods or if cooked longer than a few minutes, but has a distinct advantage over arrowroot in dairy products where arrowroot has the tendency to get slimy.
Now you can see why I always keep a couple of options stocked in my pantry. Each one has its advantages and disadvantages, so next time give it a little thought and reach for the item that will best get the job done. And don't forget to look beyond the traditional thickening agents. As you can see, there are many options for thickening a soup or sauce, just think about the end product you want.