This is a copy of a sign we post over the Japanese Usukuchi soy sauce,explaining the difference between Usukuchi and low sodium soy sauces, and why "light soy sauce" is an ambiguous term in Asian cooking:
*Because a lot of people pick this item up by mistake, we felt some clarification was in order:
Japanese cuisine sometimes calls for USUKUCHI SOY SAUCE, as opposed to regular Japanese soy sauce. USUKUCHI SOY SAUCE is LIGHT-COLORED soy sauce, but it contains MORE SALT per serving than regular soy sauce; it's used in dishes where you don't want the color to get too dark (chicken and white-meat fish, for example). If you want a lower sodium soy sauce, look for the ones labeled "less salt," "milder," or "lite" soy sauce (GREEN LABELS by Kikkoman and Yamasa brands). Some Japanese cookbooks call for "light soy sauce," and they usually mean USUKUCHI SOY SAUCE. However, you should check your particular cook book or recipe to make sure they don't mean low sodium soy sauce.
Compounding the confusion is the fact that Chinese cuisine involves the use of what is called "light" (or "thin") soy sauce and "dark" (or "black") soy sauce. Chinese light or thin soy sauce is similar to Japanese regular soy sauce (but again, it is NOT a low sodium soy sauce). Chinese dark soy sauce is thicker and sweeter (than regular Japanese soy sauce or Chinese light soy sauce), and there is no substitute for it.
If a recipe calls for "light" soy sauce, try to determine first if it's a Japanese or Chinese recipe. If it's Chinese, "light soy sauce" means Chinese light (also called thin) soy sauce (oftentimes, you can substitute with a Japanese regular soy sauce like Kikkoman or Yamasa). If the recipe is Japanese, it probably means Usukuchi soy sauce, but it's good to double check to see if the author is calling for low sodium soy sauce. (It's unlikely that a recipe would actually call for low sodium soy sauce, since all you'd need to do is reduce the amount of regular soy sauce.)
One more thing about low sodium soy sauce. The Japanese Kikkoman and Yamasa brands of reduced sodium soy sauces are relatively expensive, as compared with the regular soy sauces by those same brands. This is because the process of producing soy sauce initially requires the same amount of salt, whether you're producing regular or low sodium soy sauce. In the case of low sodium soy sauce, the same step-by-step procedures are used as in regular soy sauce, with the additional step at the end of removing excess sodium. Since low sodium soy sauces are more expensive than their regular, "high octane" counterparts, they are meant to be used mainly at the table for dipping (with sashimi, for example). If you need to reduce the amount of sodium in a recipe, just use less regular soy sauce, instead of substituting the more costly low sodium soy sauce. Also note that low sodium soy sauces should be kept refrigerated...they will spoil more easily since they have less salt (which is, after all, a preservative).
This is something we've probably explained hundreds of times over the last 20 years, so here is it in print. Hope it helps someone out there from accidentally getting a sodium overdose!