Everyone has, at some time, been served a sauce so overthickened with starch that it turned as gluey as wallpaper paste. The flavor is usually even worse than the texture because the gluey starch inhibits flavor release, which is how the flavor chemicals get to your taste buds.
Cooks today have much better alternatives: modern hydrocolloids, which are powders that set or thicken when mixed with water. Traditional starches and gelatin are hydrocolloids, but now stores have begun to carry many other hydrocolloids that are even more useful, such as agar, carrageenan, locust bean gum, and xanthan gum.
Both agar and carrageenan are extracts made from seaweed. If you have ever played with seaweed on the beach, you’ve noted its rubbery and gel-like consistency. Agar has been used in Japanese cooking for a thousand years, but has only just become popular in Western cuisines. Carrageenan is named for a small Irish fishing village, where they have traditionally made a pudding by boiling seaweed in sweetened milk. Locust bean gum is made from the seedpods of carob, which you can find sold as a chocolate substitute in most any health food store. Perhaps the most flexible modern hydrocolloid is xanthan gum, which is made by fermenting a natural bacteria, in much the same way that vinegar and yeast are made.
These modern hydrocolloids, and others like them, have many advantages over gelatin and traditional starches. They work at a wider range of pH and temperature. They perform better when reheating. They can make gels that don’t weep. And they work at very small concentrations. When thickening with xanthan gum, for example, we typically add just 0.1-0.2 g for every 100 g of liquid, and when making a solid gel, we usually use about 0.5 g for every 100 g of liquid. The trickiest part of using these ingredients is often measuring out such small weights precisely! But because the amounts are so small, they don’t interfere with the taste of the final dish nearly as much as conventional starches do.
In this recipe for gelato, we exploit yet another advantage of hydrocolloids: the way they affect the size of the ice crystals in ice cream or sorbet as it freezes. The size of the crystals is the biggest factor in the texture and consistency of an ice cream or sorbet; generally speaking, the smaller, the better.
Finally, hydrocolloids are often called stabilizers because they help foods stabilize at a good consistency without too much added fat or sugar (we’ve assembled a useful table of emulsion stabilizers that summarizes the properties and uses of each one–you can find it below). In our pistachio gelato, the nuts themselves are the only source of fat. But to get the texture just right, a little bit of carrageenan goes a long way.
Nathan Myhrvold, coauthor of Modernist Cuisine and Modernist Cuisine at Home