A nitrogen flush is the process of using nitrogen gas to remove oxygen from food packaging, thus increasing its shelf life. Because nitrogen is the heavier gas, it sinks to the bottom, pushing out the oxygen. It's similar to placing a stone in a cup of water, causing the water to spill out.The nitrogen packaging system essentially involves replacing oxygen with nitrogen in food packing. To begin with, the food item to be preserved is placed in an unsealed wrapping or pack. Once this is done, food-grade nitrogen is introduced into the packing, driving out oxygen-rich air as well as any moisture present.