Why are you told to use less water when cooking new rice?
― The water difference is about a teaspoon. What's really different is the rice's "age"
Every autumn, people say "use less water for new rice." The usual explanation is that new rice holds more moisture. But when you actually run the numbers, the moisture difference between new rice and rice harvested a year earlier is only about 4 grams per cup of rice. The real reason you want to adjust the water is that the rice grain itself slowly "hardens" over the course of a year.
In October, "new rice" labels start appearing in the supermarket rice aisle. Some bags even say "use less water for new rice" on the back.
If you cook it as instructed, it comes out glossy, with a faint sweet smell. Something really is different from last year's leftover rice.
But is it really true that it's "because of more moisture"? Rice is a dry grain. Does its moisture content really change that much in a year?
There are two reasons the water amount changes
Freshly harvested rice is said to hold slightly more moisture than rice that's a year old. But the difference is less than a teaspoon (5 grams) per cup.
Even after harvest, rice keeps changing little by little inside the grain. The cell walls and oils change, making it harder for stickiness to develop when cooked.
In other words, "use less water for new rice" is half about the amount of water. The other half is about the "age" of the rice grain. And when you do the math, it turns out the second factor is what actually matters.
The moisture difference really is tiny
When rice is shipped, regulations require that unpolished (brown) rice have a moisture content of 15% or less. Freshly harvested new rice is said to sit close to this upper limit, around 15%. After about a year in storage, it often drops to around 14%.
The difference is roughly 1%. A cup of rice weighs 150 grams, so 1% of that is just 1.5 grams. Even accounting for its effect on the water you add when cooking, the difference comes out to only about 4 grams — about a quarter of a tablespoon (15 grams).
Look at Figure 1. It shows the water needed to cook a cup of rice, split into "water the rice already holds" and "water added when cooking." Even when you change the rice's moisture with the slider below, the length of the added-water bar barely changes.
If you cook new rice with 10% less water, that's about 17 grams less per cup. The moisture difference (about 4 grams) only accounts for about a quarter of that. So what explains the rest?
Over a year, the rice grain "hardens"
Even dried, rice is a living seed. After harvest, slow changes keep happening inside the grain. During storage, three main things are said to occur.
First, the cell walls inside the grain harden. When cooking, water passes through these walls to reach the starch. Harder walls make it harder for water to soak in. Second, the proteins form more bonds with each other, tightening the inside of the grain.
Third, there's a change in the small amount of oil rice contains. The oil breaks down, or reacts with oxygen in the air. The breakdown products are said to cling to the starch and get in the way of swelling. The products of the oxygen reaction become the source of the smell described as "stale rice odor."
Look at Figure 2. On the left, new rice has soft walls, and water reaches deep into the grain. On the right, year-old rice has hardened walls, and water tends to stop near the surface. That's why, cooked with the same amount of water, new rice turns out soft and sticky while older rice turns out dry and crumbly.
Flip the logic around and you get a trick for cooking older rice well. Use a little more water than usual, and soak it a bit longer — that helps water reach deep into the hardened walls. Even with new rice, drying methods today are fairly standardized, so some say the standard water line is plenty. Starting with the standard line and adjusting to taste is the safest approach.
The grain's changes are said to proceed faster at higher temperatures. That's why rice warehouses are often kept at or below 15°C. At home, sealing the bag and storing it in the refrigerator's vegetable drawer is said to slow the change in odor too.
Summary
The moisture difference between new rice and older rice is only about 4 grams per cup. The real reason you want to adjust the water is that rice "hardens" over the course of a year, making it harder for water to pass through. New rice is soft and sticky because the grain is still young and readily takes in water.
The deliciousness of new rice isn't about the amount of water —
it's about the "youth" of the grain that takes the water in.
Also see "How many grains of rice are in a bowl of rice?", which counts the grains in a serving, and "Why are baked sweet potatoes so sweet when no sugar is added?", where the same kind of starch change produces sweetness.
- Put equal weights of new rice and the previous year's rice into two separate clear cups, and soak them in the same amount of water.
- Every 10 minutes, take out a few grains and hold them up to the light. Compare how long it takes for the white core to disappear and the grain to turn translucent all the way through.
- Also compare the smell of both rices dry, fresh out of the bag, by holding them up to your nose.
You should see water reach the core of the new rice faster. Results can vary by variety and storage conditions, so try comparing several times.
Want to know more? ― Terms, formulas, and textbook connectionsWe mark which level each part belongs to, from middle-school science to university specialist courses
- MSCovered in middle-school science
- HSCovered in high-school "Chemistry" or "Biology"
- HS+Advanced high-school content, or textbook sidebar material
- UnivContent not taught in high school — university specialist courses (food chemistry, culinary science)
- ResearchNot yet taught as settled fact even at university — an active research question
MSTerms: this phenomenon has a name
- Moisture content: the proportion of rice's weight made up of water. Shipping regulations cap brown rice at 15% or less.
- Starch: the component making up most of rice's weight. It's built from long chains of glucose.
- Aging (komaika): the change in rice's properties during storage, reducing stickiness and aroma when cooked.
MSHSCheck with a formula: how much does the added water differ between new and old rice?
Cooked rice is said to be about 60% water. Since the "dry matter" of rice doesn't change when cooked, that's what determines how much water is needed. Take 1 cup of rice (150 grams), comparing new rice at 15% moisture with old rice at 14%. Water lost as steam during cooking is the same for both, so we leave it out of the calculation.
| In symbols | W = M × (1 − w) × 60/40 − M × w |
| In words | Added water = dry matter × 1.5 − water the rice already holds |
| Where it comes from | The weight of "dry matter" is conserved when cooking. If cooked rice is 60% water, the water is 60/40 = 1.5 times the dry matter. |
| Symbol | Meaning and unit |
| W | Water added when cooking (grams) |
| M | Weight of rice (grams). About 150 grams per cup |
| w | Rice's moisture fraction (15% = 0.15) |
| Weight of 1 cup of rice | About 150 grams |
| New rice's moisture | Said to be around 15% |
| Year-old rice's moisture | Said to be around 14% |
| Cooked rice's moisture | About 60% |
| 1 tablespoon of water | 15 grams |
| New rice's dry matter | 150 × 0.85 = 127.5 grams |
| Water needed to cook new rice | 127.5 × 1.5 = 191.25 grams |
| Water new rice already holds | 150 × 0.15 = 22.5 grams |
| Water added to new rice | 191.25 − 22.5 = 168.75 grams |
| Old rice's dry matter | 150 × 0.86 = 129 grams |
| Water needed to cook old rice | 129 × 1.5 = 193.5 grams |
| Water old rice already holds | 150 × 0.14 = 21 grams |
| Water added to old rice | 193.5 − 21 = 172.5 grams |
| Difference in added water | 172.5 − 168.75 = 3.75 grams |
| In tablespoons | 3.75 ÷ 15 = 0.25 tablespoons |
| New rice's water reduced by 10% | 168.75 × 0.1 ≒ 16.9 grams |
The moisture difference only accounts for a quarter tablespoon (about 4 grams). That doesn't even reach a quarter of the traditional "cut it by 10%" rule (about 17 grams). The rest is likely a "preference" adjustment that compensates for the difference in grain properties.
HSHS+What happens inside the grain while cooking
HSRaw starch is in a crystal-like state with its molecules packed tightly together — hard to digest and hard as-is. Heated together with water, water seeps into the gaps, the packed structure unravels, and it turns soft and sticky. This is called gelatinization.
HS+Gelatinization requires enough water to reach close to the starch. In older rice, the cell walls and protein cross-links narrow the channels water travels through. As a result, with the same time and the same amount of water, gelatinization is said to have more trouble reaching the core of the grain.
UnivAging, in the language of chemistry
During storage, the enzyme lipase is said to break down oils, increasing free fatty acids. Free fatty acids are thought to enter the helical structure of amylose, a type of starch, forming an amylose–lipid complex that suppresses gelatinization and swelling. Meanwhile, lipid peroxidation of the oil produces aldehydes such as hexanal, which are considered the main components of stale-rice odor. Among proteins, disulfide bonds are said to increase, tightening the grain's structure.
📖 For the derivation of the formula and further reading: Gelatinization (Japanese Wikipedia) / Lipid peroxidation (Japanese Wikipedia)
ResearchWhat's still not fully understood
- How much does each of the three changes contribute? Among the cell wall, protein, and oil changes, how much each contributes to the decline in texture is said to vary by variety and storage temperature, and hasn't been fully sorted out.
- The true identity of new rice's "aroma." Which components, and how much of each, contribute to the pleasant smell unique to new rice is still under analysis.
- Storage methods that slow aging. Low-temperature, low-oxygen storage, and breeding efforts to reduce the enzymes that break down oil, are among the approaches being researched.
In short, even this article's content is "the explanation as currently understood." The figures are approximate; actual rice moisture varies by growing region, drying method, and storage conditions.
Textbook connections (by level)
| Level | Subject/Unit | Where in this article |
|---|---|---|
| MS | Science (plant structure, starch) · Math (percentages) | Moisture percentage and the added-water calculation |
| HS | Chemistry (sugars/starch) · Biology (cell walls) | Gelatinization and the role of cell walls |
| HS+ | Chemistry (fat oxidation) | The source of stale-rice odor |
| Univ | Food chemistry · culinary science | Amylose–lipid complex, lipid peroxidation |
| Research | Cereal science · food engineering | Separating out the causes of aging, storage technology |
| ― | Everyday connections | Water amounts for new vs. old rice, where to store rice |
- Ministry of Education, Culture, Sports, Science and Technology, Council for Science and Technology, Subdivision on Resources, "Standard Tables of Food Composition in Japan (Eighth Revised Edition)" (moisture content of rice and cooked rice) — 文部科学省 科学技術・学術審議会 資源調査分科会『日本食品標準成分表（八訂）』
- Ministry of Agriculture, Forestry and Fisheries, "Agricultural Products Grading Standards" (upper limit on moisture content of unpolished non-glutinous rice) — 農林水産省『農産物規格規程』
- Koyu Ishitani & Kenichi Otsubo (eds.), "The Science of Rice" (Food Science Series), Asakura Publishing — 石谷孝佑・大坪研一 編『米の科学』（シリーズ食品の科学）朝倉書店
- Wikipedia (Japanese): "Gelatinization" (糊化)
- Wikipedia (Japanese): "Lipid peroxidation" (脂質過酸化反応)
※ This article is a general-audience science explainer. The figures given are approximations meant to illustrate the mechanism. The right amount of water for rice depends on variety, rice cooker, and personal preference. Follow your rice cooker's manual.