Supermarket salad healthier than the homegrown variety
'Superfood' vegetables grown in back gardens and allotments may actually be less healthy than those that have been sitting on a supermarket shelf for a week.
In the first study of its kind, the University of Reading has found that levels of cancer-fighting nutrients in supermarket rocket leaves peak between five and seven days after being processed.
The tests found initial levels plummet during the time between picking and washing.
But after a week of being left in the fridge at 4C (40F), levels of the nutrients had risen threefold, suggesting that as the leaves slowly die, some of their nutritional benefits are enhanced.
Rocket leaves contain sulforaphane, which has properties that protect against cancer and gastrointestinal. The compound is also found in green vegetables such as watercress and broccoli, which helps give the foods "superfood" status. High levels of the compound cause rocket's hot, peppery flavour. It is the plant's natural defence against being eaten by bugs.
"Violent" methods of harvesting, such as mechanical farming, trigger a sudden drop in levels of the compound. But the study established that sulforaphane levels show "surprising" recovery, peaking around five days to a week after being processed.
It means freshly picked leaves may contain fewer anti-cancer properties than those that have been sitting in a fridge for days.
However, while levels of sulforaphane in rocket rise over days, other nutrients will decline, affecting their overall health benefits. Dr Luke Bell, from the University of Reading, said: "The discovery is really surprising, going against the assumption that nutrients found in rocket will dissipate following harvest.
"Our study has shown that processing actually has a potentially beneficial effect, and that rocket lovers can have confidence in the health boost a bag of rocket will give them.
"The biggest boost in these cancer-fighting compounds came seven days after processing, but begin to tail off after that.
"With regular consumption of rocket and sulforaphane, consumers could potentially improve their long-term health and reduce the risk of developing other chronic diseases, such as cardiovascular disease."