Identification of romaine lettuce (Lactuca sativa var. longifolia) varieties with reduced browning discoloration for fresh-cut processing

Authors: Luo, Yaguang - Sunny; BORNHORST, ELLEN - Orise Fellow; TENG, ZI - University Of Maryland; Zhou, Bin; Park, Eunhee; Turner, Ellen; Simko, Ivan

Submitted to: Postharvest Biology and Technology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/7/2019
Publication Date: 6/17/2019
Citation: Luo, Y., Bornhorst, E., Teng, Z., Zhou, B., Park, E., Turner, E.R., Simko, I. 2019. Identification of romaine lettuce (Lactuca sativa var. longifolia) varieties with reduced browning discoloration for fresh-cut processing. Postharvest Biology and Technology. 156:110931. https://doi.org/10.1016/j.postharvbio.2019.110931.
DOI: https://doi.org/10.1016/j.postharvbio.2019.110931

Interpretive Summary: Lettuce is one of the top ten most valuable crops in the US, with an annual farm-gate value of over $2.3 billion. Fresh-cut lettuce in packaged, ready-to-eat salads is becoming increasingly popular. Discoloration or browning is one of the main reasons that the quality of packaged, fresh-cut lettuce can only be maintained for a short amount of time. There is a need to identify lettuce varieties that have less browning and would last longer in fresh-cut packaging. To address this issue, scientists at the USDA ARS selected fourteen different varieties of Romaine lettuce and compared the amount of browning and enzyme activity over time for each variety. A computer-assisted image analysis program was used to objectively measure the amount of browning on the lettuce pieces. Results showed that the lettuce varieties that were genetically related had similar amounts of browning. For example, varieties related to Tall Guzmaine had the greatest amount of browning, while varieties related to Parris Island Cos had the least amount of browning. Biochemical assays indicated that enzymatic activities that may influence browning were also dependent on the lettuce genetics and storage time. We used statistics to determine that the amount of browning is correlated to activity of two of the four enzymes that we evaluated. This research provides lettuce growers and breeders with information on the role of enzymes in lettuce browning and data that they can use to select better cultivars with less browning for fresh-cut salad applications.

Technical Abstract: Discoloration (browning) represents a major challenge that limits the quality and shelf life of fresh-cut lettuce. In this study, we aimed at finding Romaine lettuce (Lactuca sativa var. longifolia) accessions with low browning potential. Fourteen accessions genetically associated with common cultivars were shredded and packaged in perforated bags. Images were captured daily for 5 days and analyzed with computer vision technology to quantify browning intensity via L*a*b* color values, hue angle (h°), and browning index (BI). Enzymatic activities, including phenylalanine lyase (PAL), peroxidase (POD), polyphenol oxidase (PPO), and total phenolic content (TPC) were determined daily for 5 days. After five days of storage, the accessions in the Tall Guzmaine and Parris Island Cos pedigree groups exhibited the greatest and least browning, respectively. In addition, while the PAL, POD, and TPC increased substantially over time, the PPO of twelve accessions fluctuated with minor increase. Pearson correlation showed that the increase in PAL, POD, and TPC explained at least 78% of the change in browning index over time, indicating that the underlying enzymes and phenolics play a significant role in browning. Comparing across the accessions, those with higher PAL and lower POD accumulation during storage tended to show a greater browning index at the end of storage, although the correlation was weak. The accumulation of TPC and PPO, on the other hand, showed no correlation to the final BI among the lettuce accessions. This systematic study provides lettuce growers and breeders with guidance on cultivar selection, and it also supplies researchers in plant physiology and genetics with more information on the roles of enzymes in lettuce browning.