Discovery of Romaine Lettuce (Lactuca sativa var. longifolia) Varieties with Reduced Pinking/Browning Discoloration for Fresh-cut Processing

Pinking and browning discoloration is a major quality problem for fresh-cut lettuce. Caused by oxidation of phenolic compounds in the presence of oxygen and enzymes, browning is often controlled by reducing oxygen levels near the cut lettuce through active modified atmosphere packaging. However, an initial nitrogen flush and the use of packaging films with low oxygen transmission rate is costly and often applied at the expense of the optimal atmospheric conditions required to maintain other quality attributes of cut lettuce. Furthermore, minor defects in the packaging, such as a pin hole in the bag or a piece of lettuce in the package seal, often lead to severe browning on the cut edges of the lettuce ribs and the loss of product quality and shelf life. Thus, browning-resistant lettuce varieties are needed by the fresh-cut produce industry.

This article reports our research findings on the selection of lettuce varieties with reduced browning. Romaine lettuce (Lactuca sativa var. longifolia) was grown in Salinas, California. The products were harvested and shipped to the Food Quality Laboratory at the US Department of Agriculture, Agricultural Research Service in Beltsville, Maryland. Six large trials were conducted from 2016 to 2018, with additional preliminary studies conducted previously. Lettuce was cut, washed and packaged according to typical commercial fresh-cut produce processing procedures. For some samples, the midrib tissue was excised, shredded, and packaged in perforated bags to simulate packages with imperfect seals or pin holes. Browning intensity and extent were evaluated visually by both trained and consumer sensory panels, and computer vision.

Overall, large variations in browning were observed among lettuce varieties. While the browning development is affected by many factors, including pre-harvest growing conditions and the quality at the time of harvest and processing, consistent browning traits were observed for certain varieties. For example, Parris Island Cos and Darkland (selection of Parris Island Cos) exhibited consistently low browning, while Tall Guzmaine and King Henry (selection of Tall Guzmaine) had the highest browning. Other varieties that are genetically related to Parris Island Cos, including Hearts Delight, Green Towers, and SM13-R2, were generally low in browning, despite variations in performance over different harvests. Conversely, Siskiyou, genetically related to Tall Guzmaine, had a relatively high degree of browning. Triple Threat, Sun Valley, and RH11-1506 also exhibited relatively high browning potential, although variations were observed between harvests.

These research findings are useful for fresh-cut processors to optimize lettuce variety selections to improve product quality and shelf life, and to reduce the operational cost by reducing the reliance on an initial nitrogen flush. The results from this study also provide important insight for breeders that can be used in new variety development.