A: Food safety expert, Caroline Gunn says:
“Yes, the bagged salad leaves are washed in a chlorine rinse, which some industry sources confirm can reduce nutrient levels. But washing any salad vegetable leaves, even in water at home, will tend to rinse out some water-soluble nutrients. Unfortunately because of food safety scares in the US, UK and Japan with E.coli and Salmonella causing serious illness in people eating salad vegetables, processors must use this chlorine rinse for safety. The rinsed and dried leaves are then bagged in a modified atmospheric pressure (MAP) bag that has had the oxygen, nitrogen and carbon dioxide levels changed to reduce the rate of deterioration of the product and enhance shelf life.
The lettuce is sold in supermarkets 3-4 days after packing, though this can be extended to 10 days. A study in the British Journal of Nutrition in 2003 noted antioxidant nutrients such as vitamin C and E, polyphenols and other micronutrients seemed to be lost in the MAP process. However, this has always been a concern in the industry and there has been considerable research on how to minimise any loss of nutrients and develop strains of lettuce delivering high levels of antioxidants and other phytonutrients with selective breeding. Overall, for that one study, which showed a loss of antioxidant nutrients (only possibly due to MAP), you could find several to show otherwise.
It is accepted that the nutritional quality of a product is optimal straight after harvest and the longer it is stored, the greater the decline in nutrient quality. But it has not in any way been conclusively proved that this packaging technology accelerates the nutrient loss that happens normally during storage. Any product that has been processed will never be as nutritious as the freshly picked whole fruit or vegetable, but most of us can’t access this level of nutrition on a daily basis. Serve-yourself salad leaves will be much the same in nutritional quality as the bagged ones, but they won’t have the same shelf life. For maximum nutrient benefit (if you’re not growing your own), aim to buy smaller quantities and turn them over fast.”