I worked In Morrison’s twenty years ago. Fish can be kept out on display for days. If you the cut it up and package it, the use by date goes up.Nitrogen flushing is common in lots of chilled, sealed perishables.
Some of the cheaper end of the market sandwiches can be 12 to 14 days old by the time you get them off the supermarket shelf.
Nitrogen works wonders.....
The machine for doing the packaging was at the back of the counter. No Nitrogen or CO2 was used in this process.
Wonderful devices, supermarkets.
Wonderful for keeping the owners rich.