Starch Chemistry

Starches are polysaccharides, or strings of sugar molecules. Starch consists of two types of molecules:

• Amylose, which is linear
• Amylopectin, which is branched

In a starch granule, amylose and amylopectin strands arrange themselves in a starburst around a central point called a hilum. Hydrogen bonds between the strands give the granule its shape. Granules come in a range of sizes, and different starches have different proportions of amylose and amylopectin.

If you add cold water to a starch, the granules absorb a little bit of it, but they remain pretty much unchanged. But if you add right amount of warm water, the starch granules swell, break down and release some of their contents into the water. In other words, they gelatinize. You can get a similar affect by mixing starch with cool water and heating it to its gelatinization temperature. Or, you can stir the mixture vigorously -- the mechanical action of stirring will help break the granules down. Check out the animation below to see just how this happens.

Lots of factors can affect the consistency of this gelatinous mix:

• Large starch granules swell more easily than small granules
• The more amylose a starch contains, the more swelling it takes to gelatinize it. Starches with high amylose content also make a stronger, firmer gel because more amylose can move out of the granule and into the water.

So that's the first clue to the chemistry of Play-Doh compound. The compound needs to be firm, but pliable. The starch it uses needs to have enough amylose to create sturdy, moldable dough. For these reasons, Play-Doh compound contains wheat starch, which contains around 25 percent amylose and 75 percent amylopectin.

We'll look at how other ingredients affect how Play-Doh compound works in the next section.

Back to Water and Starch The interaction between starch and water also explains why Play-Doh compound gets dry and grainy if you leave it out overnight or play with it for extended periods. When water evaporates out of the dough and into the air, it leaves the dry, starchy ingredients behind. The remaining dough is dry and flaky to the touch.