Light Treatment Extended Shelf-Life and Postharvest Quality of Chinese Kale (Brassica oleracea var. alboglabra)

Background and Objectives : Chinese kale (Brassica oleracea var. alboglabra) is a leafy vegetable in the Brassicaceae family. It contains phytochemicals such as glucosinolates, vitamin C, polyphenols, carotenoids, and anthocyanins. Chinese kale has a short shelf life of about three days, resulting in a rapid decline in postharvest quality, such as color alteration and chlorophyll degradation. The deterioration leads to income loss, waste, and the production of poor-quality products for consumers. Effective strategies to reduce postharvest losses are, therefore, essential. Light treatment has been developed and shown to be effective in prolonging postharvest quality in vegetables, including broccoli, spinach, and basil. The effectiveness of light treatment depends on the light source, intensity, and quality, as well as the type of treatment (continuous or pulsed). Several internal and external factors influence the efficiency of light treatment, including the genotypes and cultivation conditions, which can range from open fields to greenhouses and plant factories. Within the highly regulated environment of a plant factory, vegetables may exhibit differing physiological and textural characteristics. The aim of this study was to determine the effectiveness of white, red and blue light treatment in prolonging key postharvest quality characteristics of Chinese kale, including chlorophyll content, color, and firmness, cultivated specifically under plant factory conditions.  

Methodology : Chinese kale cv. Inthanon F1 plants were cultivated in a deep flow technique (DFT) hydroponic system. The plants were grown under white LED light at 100 µmol m^-2s^-1 PPFD, following a 16-hour light/8-hour dark cycle. After 28 days of growth, the plants that consisting of 5 true leaves were harvested for light treatment. Three Chinese kale plants were packed into Active Pak^TM bags which facilitate gas and moisture transfer creating equilibrium modified atmospheric packaging. The experiments were conducted in nine replications and the bags were placed horizontally in a plexiglass box measuring 55 cm x 55 cm x 54 cm. Inside the box, LED panels emitted red light (660 ± 10 nm), blue light (460 ± 10 nm), or white light (400–780 nm) at intensities of 20, 50, and 100 µmol m^-2s^-1 PPFD for 180 min. After the light treatment, the Chinese kale plants were kept in the dark at 20 °C for 14 days. The 2^nd, 3^rd and 4^th leaves from the top from each plant were taken on days 0, 3, 7, 10, and 14 days of storage and used immediately for measurement. Their firmness, chlorophyll index and colour measurement were measured in the middle portion of the leaves, avoiding the veins. Firmness was assessed using a DS/DF Series Digital Force Gauge. Lower values indicated a less rigid leaf texture with a crisper feel, while higher values suggested a tougher texture. The chlorophyll index was measured with a Portable Chlorophyll Meter CM-B and the results were expressed as SPAD values. Color measurements were conducted using a Professional Digital Electronic Colorimeter WR-18. Senescence symptoms including yellowing and microbial infection were also measured. The effects of light type and intensity on postharvest quality were analysed using R software. Density plots were created using the ggplot2 package. All statistical analyses were conducted at a significance threshold of α = 0.05. The data were analysed using one-way analysis of variance (ANOVA) followed by Tukey's multiple comparison tests.

Main Results : The effects of white, red, and blue light at 20, 50, and 100 µmol m^-2s^-1 PPFD on Chinese kale under plant factory conditions were investigated. Red light at 20 µmol m^-2s^-1 PPFD (R20) and white light at 50 µmol m^-2s^-1 PPFD (W50) maintained chlorophyll content and firmness with less variability compared to untreated kale. Blue light at 20 µmol m^-2s^-1 PPFD (B20) maintained firmness better than blue light at higher intensities, with no significant differences in other parameters. Based on these results, R20, B20, and W50 were selected and combined with equilibrium modified atmospheric packaging during cold storage. During storage, light treatment strongly maintained chlorophyll content. By day 7, untreated kale exhibited a significant decrease in chlorophyll, while light-treated kale particularly W50 and R20 better preserved chlorophyll than B20. This trend continued through day 10. On day 14, only W50- and R20-treated kale retained measurable chlorophyll, whereas B20 and untreated kale showed severe senescence and were unmeasurable. B20-treated kale showed no senescence symptoms until day 7, while R20- and W50-treated kale remained symptom-free until day 10. By day 14, only 4% of W50-treated kale and 7% of R20-treated kale showed microbial infection, with 11% and 30% showing yellowing, respectively. Based on cost-effectiveness, white light treatment can be applied at the commercial scale to improve long-term storage of Chinese kale and other leafy vegetables.

Conclusions : Red and white light emissions at 20 and 50 µmol m^-2s^-1 PPFD, combined with equilibrium modified atmospheric packaging, extended the shelf life of Chinese kale to 14 days and drastically reduced microbial infection. From a cost-effectiveness perspective, white light treatment is suitable for commercial-scale application to enhance long-term storage of Chinese kale and other leafy vegetables.