The Light of the Final Week Before Harvest

When we pick out vegetables, the first things that come to mind are what happened after harvest. When they were picked, how cold they were kept in transit, how many days they have been sitting on the shelf. All of that is true. But how many days a vegetable will last after harvest is, to a certain extent, already decided before it is picked. The light it received during its final week is one of those factors.

Only the Final Week's Light Was Changed

In 2021, researchers at Wageningen University in the Netherlands published a paper on this question in the journal Frontiers in Plant Science. The research laboratory of the lighting company Signify also took part as a co-author.

The researchers grew leaf lettuce for 5 weeks, then varied the light intensity only during the final 6–7 days before harvest. One experiment was conducted in a greenhouse, the other in an indoor growing room like a vertical farm. They then harvested the leaves, kept them in the dark at 10℃, and had people score appearance, crispness and smell to see how many days it took for the produce to fall below the threshold of consumer acceptability.

The Stronger the Light, the Longer It Lasted

The results pointed in the same direction in both experiments. In the indoor cultivation experiment, lettuce that received the weakest light during the final six days lasted about 3.6 days, the medium intensity about 6.6 days, and the strongest light about 9.5 days. Same lettuce, same 5 weeks; the only difference was the light of the final six days. In the greenhouse experiment as well, lettuce given light during the final week lasted 3–4 days longer than lettuce kept in darkness.

What mainly determined shelf life was yellowing discoloration of the leaves and browning due to senescence. In the plants that received stronger light, these changes came later.

What Is Stored Up at Harvest Is What Sustains It

The reason the researchers identified lies in what accumulates inside the leaf. Lettuce that received strong light in its final week had more sugars, starch and vitamin C (ascorbic acid) at harvest, and a higher dry weight ratio. These differences were maintained throughout storage in the dark.

Harvested leaves can no longer produce nutrients from light. They hold out by using what remains. The paper shows a correlation in which the more sugars and vitamin C present at harvest, the longer the shelf life, and interprets this as the leaf's energy and antioxidant status underpinning that shelf life.

The researchers write that changing the light only in the final few days has little effect on the crop's growth and appearance, and can also reduce the energy required compared with providing strong light throughout the entire cultivation period. They do note, however, that further research is needed on the daily duration of lighting, the color of the light, and how many days the final treatment should last. This study dealt with a single cultivar of leaf lettuce.

Being Able to Control the Light

In open-field farming, you cannot choose the weather of the week before harvest. In indoor cultivation, when and how much light to provide is a setpoint.

N.THING creates an environment in which light, temperature, humidity, carbon dioxide and nutrients are all controlled, and uses the data from one growing cycle to determine the setpoints for the next. N.FARM.AI controls the LEDs, air conditioning and irrigation equipment according to those setpoints. Each crop has its own cultivation recipe, and harvested vegetables are shipped the same day.

Post-harvest quality is not a matter of refrigeration and transport alone. It is also a matter of the final few days of growing.

You can learn more about N.THING's cultivation technology on the Technology page.