When you just say "sauce" (pronounced so-su) in Japan, it usually means a thicker version of Worcestershire sauce. We recently discussed it here! The major brand is Bulldog, and, according to their website, it's made from these ingredients: tomatoes, prunes, apples, lemon juice, carrots, onions, vinegar, sugar, HFCS (haha), salt, starch, hydrolyzed protein(?), spices, and yeast extract.
It's an essential condiment in Japan. Without it, korokke (above) and tonkatsu will suffer. You can't make yakisoba and okonomiyaki. Some people even use it on sunny-side up egg or a bowl of rice.
They come in thin(usutah), medium-thick (chuuno), and extra thick (tokuno or tonkatsu) forms. Medium-thick is the most popular kind in the Eastern part of Japan (the Kanto region). While the Kanto people like me tend to use the medium-thick for most so-su applications, the Kansai (Western Japan) people prefer the thin variety, and additionally have specialized sauces for okonomiyaki and takoyaki (octopus balls).
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