1. Why Sweet Corn Tastes Best Right After Being Picked
Sweet corn loses its sugar rapidly after being picked from the field. The Niigata Prefecture food guide explains that this is why "freshly picked" is considered the best taste. The sugar content, or sweetness level, is typically around 14–16%, though newer varieties can reach 17–18%. However, the sugar level is an estimate that changes depending on the variety and how it is measured; it is not a fixed rule [3].
The speed at which sugar disappears also depends on temperature. In an Iwate Prefecture study using the 'Peter Corn' variety, sugar loss was greater at higher temperatures and was well preserved at temperatures below 10°C (50°F). Because the temperature of corn in the field rises and falls with the air temperature throughout the day, harvesting in the early morning when it is coolest is recommended [4]. The rate of sugar loss also varies by variety, raising the question: how does the inside of the kernel relate to its sweetness?
2. How Sweet Corn Kernels Differ From Other Types of Corn
Corn is divided into types based on kernel properties. Dent corn, which has a dent in the top of the kernel when mature, is used for cornstarch and feed. Flint corn has no dent, is resistant to pests, can be pollinated in cool temperatures, and is mainly used for food. Pop corn kernels swell and burst when heated. Sweet corn, grown as a vegetable, is characterized by having less starch and more sugar [1].
Genetic research shows that the "super-sweet" type has a significantly reduced ability to make starch inside the kernel. As a result, sugar accumulates at about six times the level of ordinary corn during the stage when the ear is still green and soft. Note that this "six times" figure compares sugar at that specific stage, not the sweetness level of ears sold in stores. This type is also said to keep better after harvest [5].
In Japan, sweet corn is classified by sugar content (sweet vs. super-sweet) and kernel color (yellow, silver, or bicolor). The current mainstream is yellow super-sweet varieties like 'Gold Rush' and 'Mirai'. Bicolor varieties like 'Yume no Corn' and 'Kan-kan Musume' are also grown [2]. According to Iwate Prefecture materials, bicolor varieties are super-sweet types that have more sugar and smaller post-harvest loss than older varieties. This means the loss is smaller, not that it stops. Even in sweet varieties, sugar levels change depending on the day of harvest. So, when is the sweetest time to pick?
3. How Farmers Use Sugar Content to Choose the Harvest Day
In the Iwate Prefecture study of 'Peter Corn', sugar content changed as days passed after the silk appeared. The best time for harvest, with high sugar and good taste, was about 20–23 days after the silk emerged. This value is specific to that variety and study; it is not a universal number for all corn [4].
Even if seeds are planted on the same day, the day the silk appears can vary from plant to plant. Counting from the day the silk appears seems easier for matching the growth of the ears in the field than counting from the planting date. The Niigata Prefecture guide mentions using accumulated temperature (the sum of daily temperatures) after pollination to decide when to harvest. Thus, both days and temperature serve as guides [3]. But what role does the silk play inside the ear?
4. How Silks Match the Number of Kernels on an Ear of Corn
Corn has a tassel at the top that produces pollen, and the ear develops from the base of the leaves. When pollen lands on the silk, pollination is complete, and the silk turns brownish [6].
The seed company Taki explains that because each silk comes from a single kernel, the number of silks equals the number of kernels. The rows of kernels are said to always be an even number. The reason given is that cells divide in two as they grow. This is an explanation from the seed company and has not been verified by detailed research [7].
To ensure pollination in home gardens, it is recommended to plant at least 10 plants in two rows so pollen falls evenly on the silks. Leaving only one ear per plant (the top one) allows nutrients to concentrate, resulting in a larger ear. This is a guideline for home gardens and differs from commercial farm planting methods [8]. How does this corn reach us during summer?
5. How Sweet Corn Shipping Moves From South to North in Summer
During summer, the source of sweet corn moves north. Shipping begins in Miyazaki around mid-April, shifting to Kyushu, then Kanto, and finally Hokkaido. Hokkaido shipping runs from early July to late October. In August, Hokkaido corn is central to the Tokyo market, based on 2015 Tokyo and Osaka market records [2].
In 2014, the national planted area was 24,400 hectares (about 60,300 acres), and the shipment volume was 201,400 tonnes (about 222,000 tons). Hokkaido accounted for 9,250 hectares (about 22,850 acres) of planting and 109,200 tonnes (about 120,400 tons) of shipment, ranking first in both. Calculating the share, Hokkaido represents about 38% of the area but about 54% of the shipment volume. The share of shipment is larger than the share of area. These are older figures, so the current situation may differ slightly [2].
Corn arrived in Japan in the 16th century, and full-scale cultivation began with the development of Hokkaido in the Meiji era. Since the 'Honey Bantam' variety was cultivated nationwide in the 1960s, high-sweetness varieties have become mainstream [2].
6. How to Count Kernel Rows and Silks on a Corn Cob
If you find an unpeeled corn cob, wash your hands and gently peel back the husk. What color is the silk? If you count the rows of kernels, do they form an even number? You can pinch one silk and follow it with your finger to the base. For further reading, the Agricultural and Food Industry Promotion Organization's "Vegetable Information" (August 2016 issue) summarizes production regions and varieties. Iwate Prefecture's agricultural research results show sugar content changes in tables. Since the data is from 2006, 2014, or 2015, check the years while reading.