Heres something interesting i came across today. It covers the whole salad industry and provides some pretty good reasons to grow your own lettuce.
full text at http://www.theecologist.org/archive_detail.asp?content_id=322
Bagged salads did not exist before 1992. Now two thirds of households buy
them regularly. The value of the UK salad vegetable market grew by 90 per cent
between 1992 and 2002. By 2002 it was worth £1.25 billion – more than the total
value of the sliced bread or breakfast cereal markets. This does not mean we are
eating 90 per cent more salad; volumes have grown only by 18 per cent over the
same period; just that the food industry has found ways to make much more money
out of salad.
Modified-atmosphere packaging (MAP) can extend the shelf life of prepared salad by more than 50 per cent, making it possible for supermarkets to sell washed and bagged salad from around the world. Lettuce and salad leaves are harvested from fields in the UK, southern Europe or the US one day, and reach a packing house either the same day or, if imported, a day or two later. The salad is cut or separated out into individual leaves by gangs of workers, then washed in chlorine, dried and sorted before being packaged in pillows of plastic in which the normal levels of oxygen and carbon dioxide have been altered. Typically in MAP, the oxygen is reduced from 21 per cent to 3 per cent and the CO2 levels correspondingly raised. This slows any visible deterioration or discolouring. The salad is then trucked to a supermarket’s distribution centre, where it will be dispatched for delivery to individual stores. MAP keeps it looking fresh for up to 10 days. Some lettuces imported from the US can be kept fresh for up to a month. Unfortunately, research published in 2003 in the British Journal of Nutrition suggested that MAP might actually destroy many of the vital nutrients in salad. The research detailed an experiment conducted at the Rome Institute of Food and Nutrition. Scientists took lettuce grown by a cooperative and gave it to volunteers to eat on the day it was harvested.
Lettuce from the same source was then given to volunteers to eat after it had been packed in MAP straight after harvesting and stored for three days. Blood samples from the two groups were analysed after they had eaten the salad.
The researchers noted that several antioxidant nutrients (which protect against ageing, degenerative disease and cancer) such as vitamin C, vitamin E, polyphenols and other micro-nutrients, seemed to be lost in the MAP process. The volunteers who had eaten the fresh lettuce showed an increase in antioxidant levels in their blood, but those who had eaten lettuce stored for three days in MAP showed no increase in antioxidant levels.
When the results of this trial were published, they provoked a defensive debate among packers in the UK. Jon Fielder, director of Waterwise – a company that sells ozone-based disinfecting systems to salad packers, wrote to the trade magazine The Grocer, saying that it couldn’t be the MAP that was responsible for destroying nutrients. Fielder blamed the nutrient depletion on the use of chlorine, an oxidising disinfectant, in the washing of salads. The leaves used by most UK packaged salad producers are immersed in a water-chlorine mixture. The chlorine level is usually maintained at a minimum of 50 milligrams per litre; that’s 20 times higher than in the average swimming pool.
Chlorine washes leave surface residues of chlorinated compounds on lettuce, and because of this the process is banned in organic production. Some chlorinated compounds are known to be cancer-causing, but there appears to be little research on those left on foods treated with high doses of chlorine; the process having evolved in an ad hoc way.
As well as disinfecting out bugs, they disinfect out the taste of fresh leaves, as anyone who has eaten salad straight from the garden knows,’