1. Why Mochi Stretches Even Though Single Rice Grains Break Apart
During New Year's celebrations, when you lift a piece of mochi from your soup, it stretches out long and stringy. However, if you try to pull a single grain of everyday cooked rice, it does not stretch; it just breaks. Although both are made from rice, the grains themselves look different. Regular rice (uruchimai) grains are said to have a transparent quality, while glutinous rice (mochigome) grains appear white and opaque [2]. If the outside looks different, the inside may be different too. What is the difference?
2. Starch in Rice: Straight Amylose and Branched Amylopectin Molecules
The starch in rice is made of molecules formed by many glucose units linked together [1]. There are two ways these units connect. One is "amylose," where tens to thousands of glucose units link in a long chain that forms a spiral shape. The other is "amylopectin," where glucose chains branch off from the main amylose chain like tree branches [1].
The ratio of these two types varies by rice variety. In regular rice, starch is about 20% amylose and about 80% amylopectin. In glutinous rice, it is explained that amylopectin is nearly 100%, with no amylose included [1][3]. The 20% figure is a general guide for regular rice, but it varies by breed. This shape difference affects stickiness. Spiral-shaped amylose molecules do not tangle easily, resulting in less stickiness. Branched amylopectin molecules tangle complexly and resist breaking, making the rice sticky [1]. Regular rice is less sticky than glutinous rice because it contains this less-tangling amylose [3].
We understand that branched molecules tangle and resist breaking. However, raw glutinous rice does not stretch on its own. What must be done to make it stretchy?
3. How Steaming and Pounding Turn Rice Starch into a Stretchy Network
Mochi making begins with steaming. In raw rice, starch molecules are tightly packed and resist water. When heated with water, the bonds between molecules loosen, allowing water to enter the gaps, changing the starch into a soft, sticky state [1]. This change is called "gelatinization" (or alpha-gelatinization). In mochi making, the steaming process is thought to achieve this.
Steamed glutinous rice is then pounded with a mallet (kine) and mortar (usu). A document from the Ministry of Agriculture, Forestry and Fisheries explains that pounding steamed glutinous rice causes the branches of amylopectin to tangle complexly, forming a net-like structure that creates mochi's stickiness and stretchiness [2]. This matches the previous explanation: branched molecules tangle easily and resist breaking. Pounding spreads this tangling into a net. When pulled, parts of the net remain connected, so the mochi stretches long.
However, this document does not provide experimental evidence showing exactly what happens at the molecular level during pounding. It remains an explanation from the source material. Now we know why it stretches. But how long does the softness of freshly pounded mochi last?
4. Why Mochi Becomes Hard as Time Passes
If you leave grilled mochi on a plate while distracted, you may notice it has become hard. Foods where starch is the main ingredient, like dumplings and mochi, are said to harden as the starch "ages" over time [4]. As aging progresses, stickiness also decreases [5].
The degree of hardening is said to change depending on the type of starch, moisture content, and storage temperature [4]. It is also explained that heated starch ages faster when stored at low temperatures [5]. So, while hot mochi is sticky and stretchy, it moves toward becoming hard as it cools or is refrigerated. Stretching and hardening are both properties of mochi's starch. The detailed mechanism of aging is covered in other articles about why cooled rice hardens. Here, we simply note that stretchy mochi is not soft forever. So, are there glutinous rice varieties that resist hardening?
5. Can Shorter Starch Branches Keep Mochi from Hardening?
Research reports that the tendency for mochi to harden varies by glutinous rice variety. A report from the Crop Science Society of Japan found that hardening properties differ between varieties and are influenced by the temperature during ripening [7].
A presentation from the Aichi Prefectural Institute of Industrial Science is more specific. A glutinous rice variety called "Yawakoi-mochi," which lacks a specific starch-branching enzyme, has short branches in its amylopectin. Gyuhi (a type of mochi) made from this rice retained elasticity after 7 days of storage at 5°C (41°F), showing little change from day 1. Of the 8 varieties compared, 6 varieties other than Yawakoi-mochi showed a decrease in elasticity [6].
It is important to note that this result was for gyuhi, not general mochi, and was tested at cold temperatures, not room temperature. However, this serves as one example supporting the view that the length of amylopectin branches relates to resistance to hardening. The tangling of branches may relate to stretching, and branch length may be related to resistance to hardening. The shape of one molecule seems to determine two faces of mochi. Can you verify this yourself?
6. How to Compare Glutinous Rice and Regular Rice Grains and Check Ingredient Lists
You can try this today. Place glutinous rice and regular rice side by side on a white plate. Change the angle of the light and compare the color and transparency. Check with your own eyes if the glutinous rice looks whiter and more opaque, as the sources state.
Read the ingredient lists on packages of mochi or arare (small rice crackers/okaki). Look for "glutinous rice" (mochigome) in the ingredients. This can help you think about which rice-based foods utilize the "stretching" property.
If you want to read further, compare materials on starch and rice published by national research institutions or prefectural research institutes to see points omitted in this article.