1. Why Rice Cooked in the Morning Gets Hard by Noon, and Why Rice Type Matters
When you pack rice cooked in the morning into a lunchbox, it is often hard by noon. This happens because starch that became gelatinous during cooking tries to return to its original regular state as time passes. This change is called aging.[4]
A research institute in Hiroshima Prefecture explains that the speed of this aging changes depending on the type of starch, the amount of water, and the temperature at which it is kept.[4] This means the speed at which rice hardens may also differ depending on the variety. Researchers have focused on amylose, a component of starch.
Explanations from the Agriculture & Livestock Industries Corporation (ALIC) state that aging happens more easily with amylose than with amylopectin.[4] Therefore, varieties with less amylose have been bred for bento rice. However, choosing rice is not just about this one factor. There are rankings for taste and standards used by companies making bento.
2. How Much Amylose Do Different Rice Varieties Have?
In ordinary non-glutinous rice, starch is about 20% amylose and about 80% amylopectin. "Silky Pearl," bred in 2001, has about 10% amylose. The National Agriculture and Food Research Organization states it is sticky and does not harden easily even when cold, making it ideal for bento and onigiri.[4] Note that this is the breeder's description, not experimental data comparing hardness of different varieties after cooling.
In "Pika Maru," amylose is 10.1%, while Hinohikari grown in the same city (Chikugo, Fukuoka Prefecture) was reported at 17.5%.[4] Dividing 10.1 by 17.5 gives about 0.58. So Pika Maru has about 60% of the amylose of Hinohikari. Its overall eating quality is said to be slightly better than Koshihikari and Hinohikari, and it is suited for general table rice, blending, and frozen cooked rice.
"Milky Queen" lowers amylose content to about 3/5 that of Koshihikari to increase stickiness. It also has a characteristic where the brown rice looks cloudy white.[4] This document mentions stickiness and overall quality; the claim that it does not harden when cold appears on sales pages, not in this specific research summary.
3. Why Temperature During Ripening Changes a Variety's Amylose Amount
Amylose amount is not a single fixed value for each variety. If temperatures are low during the ripening period, especially the 20 days after the ear emerges, amylose increases. If temperatures are high, it decreases.[4]
If amylose drops too much, the rice becomes cloudy white and very sticky, sometimes producing an odor called "mochi smell." This means that even for the same variety, the properties of the cooked rice can change between a hot summer and a cool one.
This variability differs by variety. The breeder's materials state that Pika Maru is stable at about 10% amylose regardless of ripening temperature.[4] In areas where temperatures change every year, this stability itself can be a reason to choose the variety.
4. Does the Rice Taste Ranking Test Rice Packed in Lunchboxes?
The widely known ranking for rice is the Eating Quality Ranking by the Japan Grain Inspection Association. It started with the 1971 harvest. It compares white cooked rice against a standard mix of Koshihikari from several regions across six items: appearance, aroma, taste, stickiness, hardness, and overall evaluation.[4] Results are divided into five levels, with "Special A" being the highest.
The Association itself explains the purpose is to promote high-quality rice production and expand consumption.[4] It is a measuring stick to find rice that many people feel is delicious, serving as encouragement for producing regions.
If you look at the six items, there is no "hardness after cooling" or "ease of cooking." This does not mean the ranking is wrong; it just looks at a different scope. Do companies making large amounts of bento and onigiri have their own measuring stick?
5. How Do Bento Makers Choose Rice by Their Process Steps?
The Hokkaido Research Organization's Central Agricultural Experiment Station interviewed 13 companies using commercial rice (wholesale, food service, mid-meal, processed rice). Almost all products go through four steps: milling, cooking, processing/shaping, and storage/distribution. At each step, they check if the raw rice fits based on three views: quality/eating quality, economics, and workability.[4]
This material states that traditional inspections focused on eating quality and quality, but there was no evaluation method for workability or economics. So, they created new numerical measures for how much rice increases when cooked, stickiness, color change (yellowing) after keeping warm, and the degree of aging in cooked rice. The aging degree is an evaluation value for the storage/distribution step.
The goal is to choose raw rice that has good workability and small changes in taste over time.[4] When comparing varieties, there were differences in the aging degree values. However, this study focused on commercial rice for food service and processing, not standards for choosing home lunchbox rice.
6. How to Find Your Home Rice Variety in Breeding Records
Try this with an adult: Use your home bag of rice as a clue. If the variety name is on the bag, write it in a notebook. If there is rice stored safely by an adult from the same morning cooking, eat a small amount of freshly cooked rice and rice that has cooled by noon, noting hardness and stickiness. If you cannot store it safely, do not taste it; just look up the variety name.
Next, search for pages from organizations like the National Agriculture and Food Research Organization using the variety name. If you find materials mentioning amylose, compare them with the numbers in this article. If you cannot find your exact variety, comparing similar varieties is also useful.
Your one-time comparison at home mixes factors like water amount and time left out. Do not decide it is just a difference in variety; use it as material to record by changing conditions one by one.