The popularity of sushi in Japan may have been associated with Buddhism.
Buddhism was imported in the 6th century AD, and quickly became one of the major religions. One of the rules of Buddhism was to restrain from eating meat, and fish was not counted as such.
A syrup of this density is filtered to remove sugar crystals and calcium malate.
The filtered syrup is placed in the target containers while hot (usually 82 degrees C or more). To sterilize the stopper, the containers are turned upside down.