1. Why Is Freshly Cooked Rice Fluffy and Sweet?
Freshly cooked rice is fluffy and tastes sweet when you chew it. How does dry, hard raw rice change so much? The main component of rice is starch. In raw rice, starch exists as hard grains. When you cook it, these grains absorb water, swell up, and become soft and sticky. [1]
This process is called "gelatinization" (or alpha-gelatinization). Starch that has been gelatinized is easier for the body to digest. [1] Fluffy rice is starch holding a lot of water. But how long does this state last?
2. Does Cooled Rice Get Hard Just Because It Dries Out?
Rice in a lunchbox or rice left overnight becomes hard and crumbly. You might think the water evaporated and it dried out. However, a university professor’s column explains that the cause is not dryness. [2]
Instead, water separates from the structure of the gelatinized starch. Some of it returns to a dense structure similar to raw starch. The crumbly texture is not the result of water escaping into the air, but the result of the starch arrangement changing. This phenomenon is called "aging" (or beta-gelatinization). [2] Another source also states that water is released from between the molecules of gelatinized starch, causing it to become dense again. [3]
Bread hardening is also related to this same aging phenomenon. So, at what temperature does this aging happen most easily? What about the refrigerator?
3. Does the Refrigerator Make Rice Harder Than a Warm Room?
It is often said that cooled rice becomes harder in the refrigerator than if left in a warm room. Is this true? Experiments using starch paste compared the speed of aging at different temperatures. The results showed that aging progressed faster at lower temperatures. The fastest aging occurred at 0°C (32°F). [4] Note that these results are for starch paste, not rice itself.
Similar patterns have been reported for rice. A study that stored cooked rice at 4°C (39°F) for up to 4 days found that starch aggregation increased as time passed. [5] This paper notes that it is well known that refrigeration speeds up starch aging. Another study used X-rays to examine rice and found that aging progressed in rice stored in the fridge. [6]
Numbers directly comparing room temperature and the fridge for rice itself were not confirmed here. Still, these experiments suggest that the refrigerator is a low temperature that encourages aging. Is there a temperature that stops aging?
4. Which Storage Temperatures Keep Rice From Getting Hard?
If aging happens easily at low temperatures, you can consider avoiding the temperature range where it occurs easily. Actually, aging is said to be unlikely at temperatures above 60°C (140°F) or below 0°C (32°F). [1] If you keep rice hot or freeze it, the starch has a harder time changing its arrangement.
A column from Tokyo University of Agriculture also states that aging can be prevented by drying gelatinized starch at high temperatures or by freezing it rapidly. [2] Instant noodles and frozen rice products use this idea. An explanation from Mitsubishi Electric says that refrigerating rice makes it lose flavor easily, but rapid cooling and freezing slows down aging and keeps the starch well gelatinized. [7]
This suggests that portioning freshly cooked rice and freezing it seems more suitable for slowing aging than storing it in the fridge. Since this explanation comes from an appliance manufacturer, it is good to read university materials as well to confirm the principle. By the way, even under the same storage conditions, some rice gets hard easily and some does not. What is the difference?
5. What Is Rice That Stays Soft Even When Cool?
Rice for convenience store onigiri and bento boxes sits for a long time before being eaten. Therefore, companies that handle prepared foods (called "chuushoku" in Japan) look for rice that does not get hard when cool. [9]
Rice starch is made of amylose, which has little stickiness, and amylopectin, which is very sticky. In regular short-grain rice (Japonica), about 80% is amylopectin and about 20% is amylose. Glutinous rice is almost all amylopectin. [8] The less amylose, the stickier the rice.
It is suggested that amylose leads the aging process. In starch paste experiments, glutinous starch aged the least, and no aging was seen even at 0°C (32°F). [4] This is different from real onigiri or mochi, but it shows that less amylose means less aging.
Documents from the Hokkaido Prefectural Kamikawa Agricultural Experiment Station say that Hokkaido rice is soft and tasty, but it becomes hard and crumbly when cool. This property is called "aging tendency" and is deeply related to the molecular structure of the starch. [9] Even tasty rice has different properties when cool. So, how do you choose rice that does not age easily?
6. Is Choosing Rice That Hardens Slowly the Best Solution?
To find rice that does not age easily, researchers looked for numerical indicators. Research from the Hokkaido Prefectural Kamikawa Agricultural Experiment Station found that a larger ratio of amylopectin branch lengths (Fr.I/II ratio) means less aging. [9] However, this relationship holds only when the Fr.I/II ratio does not exceed 1.5. It also means that the amount of amylose alone does not decide everything. Examples of rice that ages less include "Haenuki" and "Koshihikari".
Another study compared cooked rice from different varieties stored for a set time. The degree of physical change differed by variety. The larger the change, the more obvious the X-ray diffraction peaks indicating starch recrystallization. [10]
Is rice that ages the least the best? It is not that simple. The same source states that low-amylose rice is used in rice dishes to reduce quality loss from aging. However, it also points out that rice with a strong effect against aging might be too chewy for some people's taste. [10]
Whether rice is tasty is not decided by hardness alone. It is chosen by considering both how it looks when cool and how it tastes when freshly cooked.
7. How Can You Test Rice Storage Methods Yourself?
You can compare storage methods with freshly cooked rice. Divide the same amount of fresh rice into portions and wrap them in plastic wrap. Eat one immediately. Put the others in the refrigerator and the freezer, then take them out the next day. Write down how they feel when you press them with your finger while cold. Then heat them in a microwave and taste them. Be careful of burns, and follow the instructions for your home refrigerator and freezer.
Read the label on the rice bag. Check the variety name and origin of the rice at home. Look for descriptions at bento shops or onigiri stores that talk about the texture when the rice is cool.
SAFETY NOTE: For safety, do not leave food at room temperature for a long time.