Food gases generally consist of CO2, N2, O2, etc. CO2 can inhibit the development of molds and aerobes, N2 has antioxidant and bacteriological effects, and O2 oxidizes vitamins and fats.
Tissue of fresh meat fish and shellfish is active. It's constantly consuming oxygen all the time.

In anaerobic condition, myoglobin, a muscle pigment, is reduced to a dark color.

Beef and fish cannot keep fresh without oxygen.

A small amount of ethylene oxide can also be added to the preservative mixture to increase their ability to kill bacteria.

There are many types of fresh-keeping mixture. Different ingredients and proportions of fresh-keeping mixture can be selected according to the fresh-keeping items.