Somen vs. Udon noodles: What is the Difference?
When it comes to the noodles commonly eaten during the summer season in Japan, Somen (そうめん) is a long-time favorite, widely enjoyed in many households,
where the boiled noodles are chilled and typically served in a glass bowl with iced water, separately from broth, like traditional Soba noodles.
We often make the broth of Somen and Soba by watering down the Mentsuyu soup base, which also applies to the dish of Udon.
Somen vs. Udon
Although Soba mainly consists of buckwheat flour, Somen/Udon noodles use the same ingredients; wheat flour, salt, and water. Then, what is the difference between them?
Based on this article on Kanmen.com, as for machine-made dry noodles, the difference between Somen and Udon noodles only comes from the difference in long diameter.
Specifically, the JAS (Japanese Agricultural Standard) definition states the Somen noodle has a diameter of less than 1.3 mm, while that of Udon is 1.7 mm or more.
Incidentally, the variety whose long diameter ranges from 1.3 mm to less than 1.7 mm is called Hiyamugi (ひやむぎ).
As mentioned above, in the summertime, we usually eat Somen cold, but the noodles also come in a hot soup like Udon, and the hot version of Somen is generally called Nyumen (にゅうめん).
(Reference Page: Wikipedia 素麺 )