Why does the color of soy sauce darken over time?

The change in the color of soy sauce is a natural phenomenon, similar to aging, that occurs as fermentation and aging progress.
The browning of soy sauce occurs due to the formation of brown melanoidins, which result from a reaction between amino acids (derived from proteins) and sugars (derived from carbohydrates).

This natural process causes the soy sauce to transition from a pale yellow to a slightly darker yellow that stimulates the palate, and eventually to a rich brown color.
To delay this browning, it is best to minimize air exposure and store the soy sauce in the refrigerator.

Doing so not only helps suppress color change but also helps preserve various nutrients and beneficial components that contribute to health by maintaining a lower temperature over time.