Light Shortens The Life Of Supermarket Vegetables

Their latest work has been published in the Journal of the Science of Food and Agriculture, and shows that the useful life of chard is reduced by 11 days if it is exposed to light instead of keeping it in the dark, even though it has been covered in the best protective wrap.

“We have established that there is a negative effect on the sensory quality of minimally processed vegetables (MPV) if they are exposed to light. These are foods that are ready for immediate consumption after being washed, peeled, cut and wrapped in protective polymeric wraps ”, Susan Sanz explains to SINC and who is from the Department of Food Technology at the University of La Rioja. Her team has shown that the useful life of leeks is reduced from approximately 26 to 18 days if they are exposed to light, broccoli from 14 to 11 days, and cauliflower from 11 to 3 days.

According to Sanz, “the colour is affected in particular in non-pigmented green vegetables (cauliflower, asparagus, leeks and the white part of the chard), whereas in green vegetables (broccoli, leeks and the green part of the chard) the sensory attribute which suffers the most deterioration is texture”.

Light activates the opening of the stomata (pores in which gas exchange occurs in plants), causing an increase in respiratory and photosynthetic levels. In the case of chard, for example, the green parts have a high stomatal density, for which reason a greater interchange of oxygen and carbon dioxide occurs, in addition to a loss of water which, similarly, promotes dehydration. The white parts, on the other hand, “respire much less”, but the light affects their coloration more.

The scientists have illuminated various containers storing non pigmented vegetables and have shown that they have atmospheres with higher CO2 levels and less O2 content than those kept in the dark, using the same type of wrap. As regards green vegetables stored in illuminated conditions, they have an increased respiratory rate, although this is compensated for by the photosynthetic activity of the plant itself. In this case, the composition of the atmosphere on the inside of the container and the lifetime of the product depend on the permeability of the wrap used.

Scientists have shown that fresh or near fresh vegetables ”do not usually exceed the two weeks’ useful shelf life”, and that light promotes their deterioration since this accelerates the transpiration and respiration of these plants, and changes the way they behave. Sanz does point out that not only is it important not to break the cold chain, but it is also important to control the atmosphere inside the container so that it suits the type of vegetable”.

The response to light exposure is different depending on whether a leaf, an inflorescence (such as the cauliflower or broccoli), a root or a stem is involved. “These factors should be taken into account when establishing the optimum conditions of process, storage and commercialisation”, Sanz states.

The researcher recognises that the ideal situation would be to keep the vegetables both in the cold and in the dark, which is what actually happens in distributors’ coolers and sales points, but at the end of the day they must be on view to the consumer. Her team is already carrying out tests to see which is the ideal colour for lights at the point of sale so that the vegetables are not affected so much. In any case, they recommend the shelves are refilled frequently so that the products are exposed to light for the least time possible, as well as using “attractive containers, but adapted to each type of vegetable”.

“We are used to seeing clear wraps and it is nice to be able to see the product, but we need to progress towards dyed wraps, which could have a small window in them in order to see inside the product”, Sanz emphasises. She quotes the consumers’ willingness to accept crisp bags as an example. As light affects the oxidation of fatty products, previously clear plastic bags for this product are currently protected by brown paper, or have been replaced directly with bags made from aluminium with a photo of the crisps on the outside.