Vegetables respire. They obviously do not have lungs and breathe like we do, but they use oxygen and dissipate heat and CO₂ when they produce their own energy. The respiration of greens continues after harvest, and the higher the respiration rate the faster they deteriorate and become limp and dull.

 - To avoid the vegetables becoming dull and uninspiring before they reach the dinner table, every step of the process from harvesting in the field or greenhouse to storage at the consumer must be optimal, says PhD Mette Marie Løkke from Aarhus University.

To better understand what is going on inside the vegetables, scientists from Aarhus University studied changes in the quality of green leafy vegetables, and how these changes are influenced by the time of harvest, packaging and storage temperature.

 The studies were performed on wild rocket and broccoli using a new wireless system, where the concentration of oxygen was measured continuously in order to determine the respiration rate of the greens and its dependence on variety, time of harvest, storage temperature and length of storage.

Oxygen measured with wireless sensors

When using the new measuring method the scientists used a new type of oxygen sensor. The sensor enabled the scientists to measure the consumption of oxygen in sealed chambers containing individual varieties of either rocket or broccoli. The temperature inside the chamber was measured at the same time so it was possible to monitor the dynamic effect of the temperature on respiration rate without taking air samples.

The new analytical method gave a unique insight into the strong correlation between temperature and vegetable respiration rate. Although the variety, time of harvest and type of packaging all affected the respiration rate and thus the shelf life of the products, temperature had the largest influence. Respiration rate increases as soon as the temperature rises.

Crisp and green appears fresher

Perception of quality is best measured by sensory analysis. In the study the scientists looked at how temperature, length of storage at the different temperatures, and type of packaging affected the quality by using a panel of people who rated the appearance, smell and texture of packaged wild rocket. Not surprisingly, it turned out that the leaves kept fresher at the low temperatures.

Leaves packaged in material that was not completely airtight lost freshness due to ageing processes, whereas the leaves in the more airtight packages lost freshness because of fermentation processes.

The test panel’s immediate assessment of the wild rocket in the packaging before it was opened was highly dependent on whether the salad had a fresh green colour. Once the lettuce had been removed from the packaging, the panel’s rating of its freshness also depended on smell and texture. This means that a fresh green colour, crisp leaves and a neutral smell is judged to be fresh, while yellow and limp leaves smelling a little of silage signal the opposite.

The study confirms how important it is to handle and store vegetables like wild rocket and broccoli at a low temperature throughout the processing chain.

- It is important to maintain the keeping qualities and freshness of vegetables as this reduces food waste, increases the amount of available foods and ensures a high food quality, says PhD Mette Marie Løkke from Aarhus University.

vurderer produkternes kvalitet ude i butikkerne, siger Merete Edelenbos.

Innovation project for new solutions

It is often necessary to package fresh fruit and vegetables to preserve their quality and avoid damage to them. Scientists from the Department of Food Science at Aarhus University are working on this in collaboration with the Danish Technological Institute so that even better ways of packaging fresh fruit and vegetables can be found

Read more about the innovation project (in Danish) at www.friskpak.dk

Further information: PhD Mette Marie Løkke, Department of Engineering and Department of Food Science, e-mail: mettem.loekke@agrsci.dk, telephone: +45 8715 7874