Why is soy milk white but soy sauce black?

Question:

My friend was eating spam and asked me that.

Answer:

SOY MILK:
There is a simple yet profound difference between traditional Chinese and Japanese soy milk processing: the Chinese method boils the filtrate (soy milk) after a cold filtration, while the Japanese method boils the slurry first, followed by hot filtration of the slurry. The latter method results in a higher yield of soy milk but requires the use of an anti-foaming agent or natural defoamer during the boiling step. Bringing filtered soy milk to a boil avoids the problem of foaming. It is generally opaque, white or off-white in color, and approximately the same consistency as cow's milk.


SOY SAUCE:
Brewing, the traditional method of making soy sauce, consists of three steps: koji -making, brine fermentation, and refinement.

Koji-making
•1 Carefully selected soybeans and wheat are crushed and blended together under controlled conditions. Water is added to the mixture, which is boiled until the grains are thoroughly cooked and softened. The mash, as it is known, is allowed to cool to about 80°F (27°C) before a proprietary seed mold (Aspergillus) is added. The mixture is allowed to mature for three days in large perforated vats through which air is circulated. This resulting culture of soy, wheat, and mold is known as koji.
Brine fermentation
•2 The koji is transferred to fermentation tanks, where it is mixed with water and salt to produce a mash called moromi. Lactic acid bacteria and yeasts are then added to promote further fermentation. The moromi must ferment for several months, during which time the soy and wheat paste turns into a semi-liquid, reddish-brown "mature mash." This fermentation process creates over 200 different flavor compounds.

Refinement
•3 After approximately six months of moromi fermentation, the raw soy sauce is separated from the cake of wheat and soy residue by pressing it through layers of filtration cloth. The liquid that emerges is then pasteurized. The pasteurization process serves two purposes. It helps prolong the shelf life of the finished product, and it forms additional aromatic and flavor compounds. Finally, the liquid is bottled as soy sauce.