Soy sauce is easily oxidized. When exposed to air, it becomes blackish and its taste also deteriorates.
How to keep soy sauce fresh has been a long-standing challenge for soy sauce manufacturers. These days, new type of soy sauce bottle has been developed and sold in the market.
The Fig. below is related to patent of a bottle.
It is a double bag structure, with a check valve attached at its spout so that air only enters the space between the bags. When you grasped by hand, double bags deflate and soy sauce within the inner bag will be pushed out. However, when you release the hand, the shape of the inner bag will remain intact and air flows between the inner bag and the outer bag. As a result, the outer bag will return to its original shape.
The bottle does not allow air to enter the inner bag containing soy sauce. Consequently, oxidation does not take place and soy sauce remains fresh.