|Caesar Salad courtesy of Tracy|
Short answer: When salad is packaged oxygen is removed from the bag and replaced with other gases to slow down decomposition.
Packaged salad and other produce is treated with a process called 'modified atmosphere packaging' (MAP) or 'equilibrium modified atmosphere packaging' (EMAP OR EMA) (1). Both methods reduce the amount of oxygen present. Oxygen causes oxidation (decomposes and turns things brown) and is necessary for aerobic bacteria to survive. By changing the oxygen levels neither of these things can happen and the salad lasts a lot longer.
Modified atmosphere packaging is a process where air is replaced with a one or more gases. The packaging is not permeable (nothing can get in or out) so the gases stay the same until you open the bag. MAP can also be used to package meat, fish, fresh pasta, coffee, tea and baked goods.The gases used depend on the product. The most common are oxygen, carbon dioxide and nitrogen. The concentration varies for the product as well. For example, for vegetables 3-8% carbon dioxide, 2-5% oxygen and 87-95% nitrogen is used. For baked goods it can be 100% nitrogen.
Equilibrium modified atmosphere packaging is a process where air is replaced by other gases but the packaging is permeable so it allows certain gases to enter the packages and others to escape. This is also a common method for fruits and vegetables that need to respire. This method uses a combination of altered gases, 5% carbon dioxide, 3% oxygen and 92% nitrogen and special polymer packages.
Using these methods, the producers can cut costs by centralizing their packaging and make fewer deliveries. The consumer gets a fresh convenient product without the use of chemical preservatives. However it is expensive, the product requires refrigeration and it is not very environmentally friendly due to the large amount of packaging required. There is also some concern over product safety because the lack of spoilage may disguise dangerous levels of contamination.
1. Phillips CA. Review: modified atmosphere packaging and its effects on the microbiological quality and safety of produce. International Journal of Food Science and Technology [series on the internet]. 1996 [cited 2010 Dec 1];31:[463-76]. Available from: http://wifss.ucdavis.edu/pdf/modatmospherpkg.pdf