1. Cooked Rice Weighs More Than Twice as Much as Raw Rice
Place 100g of rice on a scale, cook it, and weigh the finished rice. A kitchen scale lets you compare your own result with the calculation below. The moisture content of non-glutinous rice (uruchimai) is about 15%, while cooked rice is about 60–65% moisture.[1] In 100g of rice, 15g is water, and the remaining 85g is starch and other solids. If we assume this 85g does not change during cooking, the weight of the cooked rice would be about 212g (85 ÷ 0.40) or 243g (85 ÷ 0.35). This means the rice becomes about 2.1 to 2.4 times its original weight. This is a calculation based on data, not a direct measurement.
The standard amount of water added is about 1.5 times the weight of the rice.[1] For 100g of rice, you add 150g of water. However, the final rice does not weigh 250g because some water turns into steam and escapes during heating. Calculations suggest the rice absorbs about 110 to 140g of water. Rice takes in water, but it does not become cooked rice by soaking alone. Heating is required. What happens inside the rice grains after they absorb water?
2. Why Rice Needs Both Heat and Time, Not Just Soaking, to Soften
In raw rice, starch molecules are packed in a dense structure. When water is added and heat is applied, water enters the gaps between molecules, loosening the structure and making it pliable.[2] This process is called gelatinization, and it is the central change that turns hard rice into cooked rice.
According to Ministry of Agriculture, Forestry and Fisheries data, gelatinization begins around 60°C. At the same time, enzymes that break down starch produce glucose, which is the source of sweetness.[1] However, 60°C is a rough guide; the exact value varies depending on the rice variety and how it is measured. One research group measured the electrical properties of rice during cooking. The temperature at which the electrical capacitance began to rise nearly matched the starch gelatinization start temperature, about 5 minutes after heating began. In differential scanning calorimetry measurements, gelatinization progressed quickly from about the 7-minute mark, just before boiling, and 50% of the starch had gelatinized 7 minutes after heating began.[3] These values come from laboratory equipment and amounts, so they are not the times in a home pot. The difficult part is the center of the grain. One source gives about 20 minutes at 98°C or higher as the guideline for fully gelatinizing the center.[1] Even if the surface gelatinizes first, if water and heat do not reach the center, the core will remain hard.
This is why rice is soaked in water before cooking. Soaking for at least 30 minutes, and preferably 60 minutes, is the recommended guideline.[1] If soaked too long, the texture suffers, and in summer, there is a risk of spoilage. By ensuring water reaches the inside of the grains during soaking, gelatinization during heating can proceed more easily to the center. So, what is the order of events after you turn on the heat?
3. What Happens During the About 40 Minutes of Cooking in the Pot
Ministry of Agriculture data divides the time in the pot into four stages.[1] The first is the temperature rise period, lasting about 10 minutes. It is considered appropriate to reach about 100°C within 10 minutes of ignition. If this stage is too short, water does not reach the center, leaving a hard core. If it is too long, the rice absorbs all the water before boiling, causing uneven hardness. The next stage is the boiling period (about 5 minutes) and the simmering period (10–15 minutes). Together, these two periods, totaling about 20 minutes, are especially important for gelatinizing the center. The final stage is the steaming period, about 10 minutes after turning off the heat. The total time is about 35–40 minutes.
Steaming is not just about waiting. During the steaming period, water on the surface of the grains is absorbed into the grains, evening out the moisture distribution.[1] Even after the heat is off, the rice-making process is not finished.
In traditional hearth cooking (kamado-taki), there is a saying that describes the heat control: "Hajime chorochoro, naka pappā." This suggests starting with low heat, then using high heat to bring it to a boil, maintaining a boil with medium heat, and finally using strong heat before turning off the heat to steam. The details of this saying vary slightly by region.[6] The timing for reducing the heat is judged by how the bubbles rise and by the sound of boiling, and in one example the heat is adjusted so that the water boils 7 to 8 minutes after ignition.[7] This method assumes that the rice absorbs all of a set amount of water. Were there other ways of handling the water?
4. How the Absorption Method Differs from Discarding Water or Steaming Rice
Everyday rice cooking in Japan today uses the "absorption method" (takidashi-ho). This method adds the necessary amount of water before heating, and no excess water remains at the end.[1] You determine the water amount, and everything except what evaporates is absorbed by the rice. While there is plenty of water, the rice is being boiled; as water decreases, it enters a steaming state.[4]
In contrast, the "draining method" (yudori-ho) involves boiling the rice in a large amount of water, removing the thick rice broth (omoyu) after boiling, and finishing by steaming. Until the Edo period, the absorption method and the draining method were used side by side. Later, the absorption method became the mainstream.[4] Some general explanations suggest that for hearth pots, the draining method required less effort in heat control.
In a hands-on workshop in Tottori Prefecture, Japanese and Thai rice were cooked in earthenware pots using the draining method. Japanese rice was also cooked by the absorption method without draining. After boiling over, draining the excess water reportedly made the rice less likely to stick to the inside of the pot.[5] This is the result of an experience workshop, not evidence that the draining method was used in the Yayoi period.
Furthermore, it is said that before the Heian period, steaming was the common method for non-glutinous rice as well.[4] However, the transition of cooking methods varies depending on the source, and it seems different methods were used depending on the era and purpose. Today in Japan, steaming is used for glutinous rice dishes like okowa, ceremonial rice, and steaming rice for sake brewing. Absorbing all the water, boiling and draining water, or steaming—all have the same goal: to deliver water and heat to the rice to cause gelatinization. The difference lies in how the water is handled. The absorption method is one of the three, and it became the everyday standard in Japan, though the available sources are not enough to settle on one reason why.
5. How to Check the Change in Rice Weight with a Kitchen Scale
To verify this at home, one kitchen scale is enough. Weigh the rice, then weigh the rice after soaking and draining the water, and finally weigh the cooked rice. Be careful handling the container as the freshly cooked rice is hot. In the calculations for this article, the cooked rice should be about 2.1 to 2.4 times the weight of the raw rice. The water amount in a rice cooker is said to be about 1.3 to 1.4 times the weight of the rice,[1] and the ratio is thought to vary slightly depending on the cooking tool and water amount.
Your measured result may differ from the calculation, because the moisture of the cooked rice changes with the amount of water and the cooking method. You can keep the result as a record of how your own pot and rice behave. Many of the numbers in this article are based on the Ministry of Agriculture, Forestry and Fisheries page on "Cooking Characteristics of Rice."[1] Guidelines for each stage of soaking and heating can also be read there.