Thais use a number of different seasoning sauces in cooking, especially in stir-fried dishes. One of the most basic and universally used sauces is soy sauce. In their development of the humble fermented soy beans, Thais have gone well beyond their Asian neighbours. Soy sauce - see-eew in Thai - in Thailand is like vinegar in European cooking, in its variety.
A selection of soy and other sauces on sale in an old market.
Basically, there are two types of soy sauce used in Thai cooking, which the Thai refer to as “white” soy sauce and “black” soy sauce. Unfortunately, the difference is not as obvious as the difference between white and brown vinegars. Both the white and black soy sauces are in fact a dark brownish-black in color and almost indistinguishable to the uninitiated. The white, or 'light' sauce is close to its Chinese and Japanese forebears, while the black sauce is much thicker and sweet. It has an aroma that reminds me a lot of molasses. In Indonesia, where it's much more commonly used, the black soy sauce is known as kecap manis. A third type, coloured red, is used in only a few dishes.
To further complicate things, another kind of sauce has gained popularity. Simply called “seasoning sauce” in Thai, the sauce is quite similar in taste to Japanese styled soy sauce.
But wait, there's more. Light soy sauce made from soy mixed with shiitake mushrooms is commonly available in most Thai supermarkets. The mushrooms give it an earthier taste.
Finally, on the subject of sauces made from the humble soy bean, we have to include fermented soy beans (tao jeeo). While it's true that soy sauce is itself fermented, or brewed to be more precise, fermented soy beans is something different altogether. The sauce is light brown in color and thick, making it appear almost like gravy. It will also have a large amount of split soy beans floating in it (they look like peanuts, but if you're allergic don't worry, there are no peanuts in fermented soy beans). While soy sauce is one of the ingredients of fermented soy beans, it is mainly just beans and salt water.