Leafy vegetables generally have a short post-harvest life due to physical damage and lack of light during storage and transportation. How long a leafy vegetable or any other vegetable lasts on the shelf and how long it looks and smells fresh are determined by the growing conditions of that vegetable. In academic research, scientists are establishing a link between nutrient content and shelf life. Key nutritional elements and quality markers include the levels of carbohydrates (glucose, fructose, sucrose, and starch) and vitamin C defined as the total ascorbic acid (TAsA). Both carbohydrates and TAsA levels are affected by light conditions.
In this study, our objective was to investigate the effect of light treatments applied before harvest on the post-harvest performance of lettuce, as a representative crop of leafy vegetables. Most of the previous research on the effects of light intensity on product quality has been done by applying different light levels during the entire cultivation period. In our experiments, we applied different light levels only at the End of Production – 6 or 7 days before harvest. The advantage is that the light treatments will only have limited influence on other aspects of the crop, such as growth, yield and morphology. And it also minimizes energy usage.
Experiments with high-pressure sodium lighting and white LED grow lights
We did two experiments, one with high-pressure sodium (HPS) lighting and one with white LED grow lights. In both experiments, high light intensities applied at the End of Production increased the percentage of dry matter, total ascorbic acid (TAsA) and carbohydrates (glucose, fructose, sucrose, and starch) at harvest. These higher levels were maintained during the shelf life, which was extended as a result of the End of Production lighting. The levels of AsA and carbohydrates at harvest correlated positively with the prolonged shelf life. This means that improved energy (more carbohydrates) and antioxidants (vitamin C) in the crop at harvest are drivers for a longer shelf life. In addition, TAsA may play a role in limiting tissue browning.
Why is this research important?
This research is important for consumers and retailers who might assume that lettuce grown in a vertical farm is not as nutrient rich or fresh tasting as those grown in a greenhouse or outdoors. Our results show that an End of Production LED lighting application to lettuce crops can boost their carbohydrate content and more than double their vitamin C content to make them look and taste great. This strategy can also be used to extend shelf life and nutritional content for lettuce grown in a greenhouse.