Blast in the Past makes Lettuce Less Limp

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After harvest, products like lettuce can quickly lose their looks and crunch. Now, research shows that a short spell of high light before the harvest can help greens stay fresher and more nutritious.

As most of us probably already know, leafy vegetables are some of the quickest fruits and veg to turn bad. Previous research has shown the part of the problem is in the way we ship and store our food – namely, in the dark. It also showed that keeping the veggies in the light could limit decay processes like sugar breakdown, increase the accumulation of antioxidants, prevent chlorophyll breakdown and lower browning-related enzymes. Unfortunately, lighting also encourages the rootless lettuce heads to open the stomatal pores on their leaves. Which, without functional roots and a ready water source, can quickly lead to water loss and subsequent limpness.

Wondering whether the ‘benefits of lighting’ could be brought into play while the plants still had their roots firmly attached, Qianxixi Min and colleagues at Wageningen University in the Netherlands, designed an experiment involving pre-harvest light-blasting as a possible path to promote lettuce longevity.

The scientists grew lettuce plants until nearly ready to pick – about 5 weeks – and then put them in a special chamber where they could blast them with different levels of (high) light for about a week.

After harvest, the lettuces were given scores, ranging from 1 (very bad) to 9 (excellent), over time, as a way of understanding how rapidly lettuce quality degraded depending on treatment.

As an aside – while ‘Quality 9’ lettuce includes leafy greens with a bright and natural colour, no browning, firm and crispy texture and a fresh grass smell, ‘Quality 1’ products  are completely discoloured and come with both ‘leaking liquid’ and a ‘fermented smell’.

Pick a lettuce

As it turns out, most people don’t buy the ‘Quality 1’ lettuce. In fact, they tend to stop wanting to pay money for the product at about Quality 6.

So, using ‘Quality 6’ as a cutoff, the scientists looked at how their light-blasted lettuce fared… and found that high light treatment could increase shelf life by up to 6 days.

To understand the ‘Causes behind the Crisp’ at the molecular level, the scientists measured levels of carbohydrates, and of vitamin C (ascorbic acid). Higher levels of carbohydrates had previously been linked to maintenance of shape, crispness and discolouration, as well as being linked to more favourable flavour profiles (if lettuce can be said to have such a thing). At the end of the light blasting treatments, the levels of carbohydrates were found to positively correlate with the amount of light applied. Vitamin C levels were similarly linked to light, in line with both previous studies that demonstrated light-stimulated accumulation of Vit C, and – given its role in preventing browning – also in line with the light blasted lettuce’s extended freshness. 

The light treatment method described here has the advantage of only being applied at the end of the plant’s life, both limiting the influence on normal crop growth, and lowering the amount of energy required (compared to whole-life blasting). This might be particularly relevant for lettuce grown indoors, for example in vertical farms and the like. The study shows that a simple light treatment might be helpful in increasing both the shelf-life and nutritional quality of lettuce. And while high-nutrition isn’t really our goal when buying lettuce (let’s be honest, the thing is 96% water: if I want vitamin C and carbohydrates I’ll buy an orange or a potato), we’re happy to hear that the end product is a lettuce that can keep its distance from ‘fermented and leaking’.

Reference:

Min Q, Marcelis LFM, Nicole CCS and Woltering EJ (2021) High Light Intensity Applied Shortly Before Harvest Improves Lettuce Nutritional Quality and Extends the Shelf Life. Front. Plant Sci. 12:615355. doi: 10.3389/fpls.2021.615355

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