Research Results

Led there be light for fresh-cut lettuce

Due to the severe wounding of plant material and subsequent storage in darkness, the shelf life of fresh-cut leafy vegetables is generally short, as determined by visual appearance. Can light during postharvest storage maintain product quality?

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Objective

After harvest, during storage, several wound-induced symptoms, such as discoloration, increased respiration, and elevated ethylene production, develop in fresh-cut lettuce. Additionally, dark storage promotes senescence processes, leading to a loss of texture and a decline in levels of micro- and macronutrients.
The question that we studied is: Can light during postharvest storage affect product quality? If so, how does it work?

Light improves quality and shelf life

The absence of photosynthesis in darkness, and as a consequence the rapid decline in carbohydrate levels, have been proposed as the main causes of dark-induced senescence. Our research showed that light levels (5-30 µmol m-2 s-1) resulted in better quality and longer shelf life of fresh-cut butterhead lettuce stored at 12°C. There was less browning on the cut edges, the leaves were more crisp, indicating delayed senescence.

Looking at the carbohydrate levels, it was seen that light results in carbohydrate accumulation in the leaves, while in dark storage the levels go down. The light also preserved the L-ascorbate levels (Vitamin C) in the fresh-cut lettuce.

Fresh-cut lettuce after 5 days storage in dark at 12C.
Fresh-cut lettuce after 5 days storage in dark at 12C.
Fresh cut lettuce after 5 days storage in light at 12C.
Fresh cut lettuce after 5 days storage in light at 12C.

Low temperature combined with low light preserves quality for a longer time

The results are an indication that it works through photosynthesis. In the following figure light response curves of lettuce show that at 10C a light level of 5 µmol m-2 s-1 PAR is slightly above the light compensation point, meaning there is net photosynthesis due to low respiration, resulting in sugar production. Moreover, it is shown that at lower storage temperature the leaves have even lower light compensation points. This supports the finding that a combination of low temperature with a low light level preserves quality of fresh-cut leafy vegetables for a longer time.

Photosynthesis light response curves of fresh cut lettuce leaves  stored at 1 and 10 degrees Celsius. Source: WUR
Photosynthesis light response curves of fresh cut lettuce leaves stored at 1 and 10 degrees Celsius. Source: WUR

Conclusion and future perspective

This research showed that adding light to green tissues in the postharvest stage stimulates photosynthesis, leading to an improvement of the sugar and Vitamin C (antioxidant) status resulting in a longer shelf life. Adding a low light level in cold storage rooms, refrigerated transport or in supermarket can be beneficial for quality of the lettuce. A combination of light (LED) treatment with post harvest management is a tool to optimize the supply chain. The current developments in LED technology can facilitate the application of light during post harvest stages.

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