Many of the foods that you buy wrapped in plastic use what's 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 also can extend the seasons of certain fruits and vegetables.
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In the case of lettuce in a bag, the modified atmosphere does two things. First, 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. Second, it discourages bacteria.
According to the magazine Practical Hydroponics & Greenhouses, atmospheres with high levels of carbon dioxide and low levels of oxygen can:
- Block the mode of action and biosynthesis of ethylene, a ubiquitous plant hormone that promotes aging and senescence.
- Reduce rot by directly inhibiting the growth of pathogens and by maintaining the health and integrity of the plant tissue, which reduces its susceptibility to infection.
- Slow yellowing of green tissues by preventing chlorophyll degradation.
- Maintain the food and nutritional value and flavor of produce by slowing the loss of food reserves, particularly sugars, inhibiting the loss of labile vitamins such as vitamins C and A, and by slowing the accumulation of undesirable secondary metabolites in the plant's tissues, such as free ammonia.
- Slow cell membrane degradation and loss of cellular compartmentation and function.
- Inhibit discoloration of cut surfaces.
That collection of features is what keeps the lettuce fresh longer. There are no chemicals involved!
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