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Thursday, 5 July, 2001, 11:27 GMT 12:27 UK
Scientists' salad day and night
Lettuce in a supermarket
Harvesting lettuce at night could help it stay fresh
Scientists are camping overnight in a field in an attempt to cure the problem of limp lettuce.

The team from the University of Southampton is conducting the experiment to find out how to improve the quality and shelf-life of pre-packed salad.


Leaves with more rigid cell walls are able to stay fresh for longer

Dr Gail Taylor
Vitacress Salads is funding the three-year research project to improve the ability of leaves to withstand the washing and packing process.

Dr Gail Taylor, leader of the University Plants and Environment laboratory, said: "There is some good evidence that if you harvest salad at night it is of a better quality and will last longer in the supermarket."

A previous study found that by harvesting basil at 6pm, instead of 6am, its shelf life improved by 170%.

Dr Taylor said: "During the day, photosynthesis ensures there is a build up of sugars which appear to be important in producing a good quality leaf.

Samples harvested

"Other changes also occur between day and night, including reduced water loss in the evening as stomata in the leaves close to prevent transpiration."

The team investigating how these changes may affect salad quality set up camp on a Vitacress Farm near Alresford, Winchester, on Wednesday night.

Sorting salad by hand
Handling methods can affect quality
Over a 24-hour period they are harvesting a sample of baby leaf Lollo Rosso and Roquette every four hours.

The leaves will then be bagged and transported by a refrigerated vehicle to the university where they will be processed and analysed over the following 10 days.

Vitacress produces up to one million bags of pre-packed salad a week. They mix the salad "recipe" by hand, followed by two separate washes, drying and then packing.

During the process, leaves can get bruised and damaged which causes the salad to deteriorate quickly.

'Stay fresh'

It is hoped that the team from the University School of Biological Sciences will find a way to improve the ability of leaves to endure the washing and packing steps to increase quality.

Work so far suggests that the plant cell wall may be important in determining how fresh the salad leaves remain once packed.

Dr Taylor said: "The plant cell wall is a complex structure with many different types of molecule that form a network capable of shrinking and stretching, to regulate cell volume.

"Leaves with more rigid cell walls are able to stay fresh for longer."

Dr Steve Rothwell, managing director of the Vitacress's production division, said: "The research at the University of Southampton is a typical illustration of the company trying to improve the quality of a packed salad through the application of fundamental scientific research."

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