Best-before dates have expired, at least for fruit and vegetables. This week Tesco revealed it is to stop using the date stamps on almost 70 of its own-brand products in an attempt to tackle food waste. But is older produce as good for you as fresher stuff?
Many nutrients, including certain vitamins in fresh produce, are unstable and levels drop after items are harvested. “Micronutrients start to reduce from the moment [produce] is cut, which is why frozen peas are actually better than canned or fresh, as they are frozen soon after picking,” said Adam Hardgrave, a food safety expert from the Food Standards Agency’s foodborne disease control team.
According to one study, spinach kept at refrigerator temperatures for eight days lost almost half of its folate, a B vitamin, with the loss accelerated at higher temperatures. Meanwhile a study by Unilever scientists found broccoli stored at room temperature retained only 44% of its vitamin C after seven days and just 28% after 14 days. However, when kept at 4C, 80% of the vitamin was retained even after three weeks. Carrots, on the other hand, lost vitamin C at a far lower rate.
Luke LaBorde, professor of food science at Penn State University who led the research into spinach, said the rate at which levels of such substances fall varies for different plants and nutrients, with environmental factors like temperature and oxygen levels also important. A key factor, he says, is the rate of respiration of produce after harvest, which is linked to the onset of a process of deterioration called senescence.
Once fruit or vegetables reach this point, unstable substances like vitamin C can no longer be replenished by cells. “Some of these high-respiring, rapid-senescing products might be things like spinach, lettuce or peas,” he said. “On the other hand, some have very low respiring [rates] like root crops, and some are intermediate like peppers.”
But the situation is complex. “Vitamin A and iron are stable, but vitamin C is lost over time,” said Dr Christine Bruhn from the University of California, Davis. “The important thing is to eat fruits and veggies. The best advice is to buy and eat soon. Remember, frozen, canned and dried forms all contribute to health.”
Dr Kirsten Brandt of the human nutrition research centre at Newcastle University agrees there is no one-size-fits-all explanation for how quickly substances like vitamins break down in produce. “For every fruit and vegetable it is more or less different because it depends on the life cycle of that plant. If you take onions, they can be stored for many months without losing any nutrients,” she said, pointing out that in nature they sit dormant in the ground for long periods.
Apples can also retain certain nutrients for lengthy periods, she says, if kept at the right temperature. “Their lifestyle means that once they are ripe they sit there and wait for the animal [to take them],” she says. “It is totally unnatural for broccoli to be cut off and therefore you have quite a short shelf life, and the broccoli will start dying quite soon”.
In one study, researchers found that average levels of vitamin C across 31 types of apples fell by 30% after 10 days at room temperature, but only by 19% after three months at 1C. However, the speed at which such nutrients break down varies from cultivar to cultivar.
Storing produce in the fridge to slow the breakdown of vitamins can have downsides, though. For example, chilling potatoes increases their glucose content. Then, when the potatoes are fried or roasted, this sugar reacts with other substances to form a chemical called acrylamide which has been linked to cancer.
“Most fruits and vegetables benefit from refrigeration. There [are] some exceptions,” said LaBorde. “For instance tomatoes generally should not be refrigerated because they are sensitive to cold temperatures and will lose flavour overnight,” he said. “Basil leaves and bananas will also suffer rapid quality loss in the refrigerator, in this case as harmless, though unattractive, darkening.”
And there is more. “For vegetables generally, once they have been cut off their plant there is only one way [for nutrients] and that is down,” said Brandt. “For fruit that has been harvested when it is still not quite ripe, then during storage it might ripen more,” she said. “So you can actually have a situation where the nutrient content increases.”
There is also good news for some nutrients like iron and calcium. “Minerals aren’t going to degrade; they will always be there,” said LaBorde.
When it comes to how nutritious produce is, Brandt says it is worth trusting your eyes. “As long as the spinach and the broccoli looks fresh, it probably is [fresh] from a nutritional point of view. [There] can be small differences, but you don’t see a big loss until the point where you also see a change in the way it looks,” she says, noting that consumers should look out for spots or produce turning pale green or yellow. “That is because the chlorophyll, the green compound in the cells, is one of the first things to be broken down when senescence starts.”
LaBorde added that even if fruit and vegetables are not at peak freshness, they should not be shunned. “Just because it may not be super optimum, there are other [beneficial substances] that may still be there such as polyphenols or fibre,” he says. “The fruit or the vegetable that is not eaten has no nutritional value.”
