Why does lettuce in a bag stay fresh longer than a head?

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:

    * Block the mode of action and biosynthesis of ethylene, a ubiquitous plant hormone which promotes aging and senescence.

    * Reduce rots 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 flavour 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 discolouration of cut surfaces.