Many of the foods that you buy wrapped in plastic today are using what is called modified atmosphere packaging, or MAP. The package is filled with a mix of gases that are beneficial to the food. Most foods do better if they are stored away from oxygen. Also, many microorganisms have a hard time with high concentrations of CO2 (just like people -- see How does dry ice work?). So eliminating oxygen and adding C02 is very common in MAP.
The amount of time MAP can add to shelf life can be amazing. The shelf life of meat can go from three to 21 days, cheese from seven to 180 days, and fresh pasta from three to 60 days [ref]! That's not bad when you consider that the cost of adding the gas is practically nothing. Shelf-life extension often lowers the cost of a product by reducing spoilage, and also opens up long-distance import/export options. It can also extend the seasons of certain fruits and vegetables.
In the case of lettuce in a bag, the modified atmosphere does two things:
- It changes the way the green leaves behave. The green leaves are still alive even after you pick the lettuce, and you want to keep them alive as long as possible by changing their behavior.
- It discourages bacteria.
According to Practical Hydroponics & Greenhouses, High CO2 and low O2 atmospheres can:
That collection of features is what keeps the lettuce fresh longer. There are no chemicals involved!