Despite being made, in its most basic form, from just rice and water, mochi—and its Korean and Chinese cousins, tteok and nian gao—manages to have a pillowy, chewy, even slightly stretchy texture that is missing from kneaded cakes and breads made from other grains.
Their secret is in the starch.
Plants make starch as a storage system for sugar, linking small sugar molecules into long chains, a stable “save-it-for-later” packet of energy, produced either in tubers or roots for the plant’s own use (think cassava or potatoes) or inside a seed as kick-start for baby plants to use until they get their photosynthesis act together (rye, corn, rice, wheat, barley, beans, etc.). This process involves building up many layers of starch molecules into granules between 1 and 100 microns in size, or roughly the width of a human hair.
Inside of these granules are the two molecular forms of starch. One, amylose, is a single long chain of glucose. The other, amylopectin, has highly branched chains of glucose, which split off in different directions like a tree canopy. Most plants produce starch granules that are a mix of amylose and amylopectin, in different ratios. When cooked in the presence of moisture, the granules of starch absorb water molecules and swell up as water works its way in between the starch chains. This is called gelatinization (even though there is no gelatin, which is protein-based, is involved). When it is completed, the water molecules have irreversibly disrupted the tidy arrangements of the starch molecules, leaving you with a thickened mixture of hydrated, swollen granules.
Though both types of starch are made from the same basic subunit—glucose—the differences in shape cause amylose and amylopectin to behave very differently once cooked. These differences mean that starchy foods with different ratios of the two molecules can have very different textures and properties. Puddings made with cornstarch will have a less stringy texture than tapioca starch (which comes from cassava), rice noodles and rice paper need the power of amylose to gel and then set, and potato starch will thicken a sauce much more quickly than wheat starch. These properties are exploited to great effect by the snack food industry and are also responsible for the textural diversity in the rices eaten by different cultures around the world.
You may have heard different varieties of rice described by their grain length (long-grained basmati rice, short-grained sushi rice, etc). Long-grain rices are, yes, longer than short-grain rices, and have a lower width-to-length ratio (they’re skinnier, and short-grained rices are chubbier). Grain length also implies a change in the balance between amylose and amylopectin: the shorter the grain, the less amylose is present. In especially short-grained sticky and “sweet” rices (which aren’t particularly sugary, but are often used in desserts) there is essentially no string-like and orderly amylose—the starch is almost entirely branched amylopectin. Longer-grained rices, with more amylose, require more water to cook, have distinct and separated grains after cooking, and become quite hard when they cool, caused by the orderly amylose molecules lining up and crystallizing outside the starch granules. Shorter-grained rices have a stickier texture as a result of their high levels of amylopectin, allowing a perfect clump of sushi rice to cling together to form the base for nigiri. Long-grain rices make for terrible sushi, just as pilafs made with sticky rice inevitably end in gooey tragedy.
Mochi relies on the all-amylopectin starch of sticky rice to do its thing. The mochi-making process begins with smashing cooked sticky rice repeatedly with a mallet (if you are celebrating the New Year and are particularly intrepid) or stirring and kneading dough made with milled rice flour—mochiko—in between bursts of heat from a stove or microwave. The swollen, gelatinized granules of starch will burst, and the pounding/kneading will help the floppy aymlopectin molecules start to line up into a network, especially as the dough begins to cool. In the absence of amylose, which would form a tough gel as the rice dough gets colder and the straight chains begin aligning with each other, the cooled amylopectin-based gel remains relatively disorganized. The texture stays fairly soft and supple, and because amylopectin is so floppy and slow to move around, mochi can even be frozen and thawed without losing much of these characteristics, making ice cream-stuffed daifuku possible.
So the next time you’re digging into a plate of spicy sausage and rice cakes, or chewing through a skewer of dango, thank the weird, floppy, inefficiently crystalizing behavior of amylopectin for your treat, and the foresight of early rice breeders thousands of years ago for recognizing the potential of short-grained mutant varieties that stuck together when cooked.
Arielle Johnson, the former head of research for MAD, in Copenhagen, has a Ph.D. in flavor chemistry from UC Davis. She is a director’s fellow at the MIT Media Lab. Tweet her your food-science questions @ariellejjohnson.