Chemistry of Gluten Free Starches

Two of the most commonly used starches in gluten-free cooking are rice and tapioca, which illustrate the different properties of grain starches versus root starches.  A starch is many molecules of glucose hooked together, and plants use starches to store sugar.  Starches are actually granules and are semi crystalline.  Some plants contain starches in their grain, as with rice and sorghum, and other plants store starches in their tubers or roots, as with tapioca and potatoes.  Either way, starches are stored by plants in one of two forms—either amylose or amylopectin.  In amylose, starch is stored in long, straight chains of molecules.

In amylopectin, starches are stored as smaller branched shapes, as shown below.

A plant’s starch can contain from 1% to 28% amylose, and the amount of amylose that a starch contains affects how it behaves in recipes.  Grain starches are typically high in amylose whereas root starches are lower. When starch is heated in water, the water enters the starch granule from the outside-in until the grain is hydrated.  Once hydrated the bonding of amylose and amylopectin holds the shape of the granule together, and it begins to swell from the center.  In general, grain starches tend to swell less easily than root starches during the heating/hydration process. One of the greatest problems with gluten-free baking is that higher amylose grains tend to make baked goods dry and crumbly.  This is because cooling makes higher amylose starches harden.  Cooling makes long starch chains link tightly together forming hard crystals (like day-old takeout Chinese rice.)  In order to make high amylose content breads palatable, they need to be reheated so that the crystals melt and the bread becomes soft again.  For this reason, gluten-free bakers most often work with a mixture of flours, balancing high amylose starches such as rice with lower amylose starches such as tapioca.