A processing technique used to prolong the shelf life of pre-packed salad leaves may remove vital nutrients, according to new research.

Research published in the British Journal of Nutrition suggested that bags of salad leaves such as lettuce and rocket did not boost vitamin C levels in the body like their fresh counterparts, reported BBC Online.

To maintain the freshness of pre-packaged salad leaves they undergo a process known as modified-atmosphere packaging, which according to scientists in Italy may remove antioxidant nutrients.

The scientists, from the human nutrition unit at the National Research Institute for Food in Rome, took blood samples from 11 volunteers who had been fed pre-packaged lettuce. The volunteers showed no increase in levels of vitamin C or certain other nutrients. When fresh lettuce was consumed, however, the researchers found that levels of vitamin C and certain antioxidants in the blood increased significantly.

According to the British Nutrition Foundation, however, there are still nutritional reasons for consuming pre-packaged salads.

"It does contribute to your five a day portions of fruit and vegetables," spokesperson Sarah Stanner was quoted by BBC Online as saying.