Product description
Lettuce (Lactuca sativa) is a versatile leafy vegetable available in several types, most notably upright varieties like Romaine (Cos) with erect, elongated leaves, and loose-leaf lettuces such as Greenleaf and Redleaf with open, spread-out leaves lacking a compact head. Known for their crisp texture and mild, refreshing flavor, these lettuces are commonly used fresh in salads, wraps, and sandwiches, but can also be lightly grilled or added to other dishes. Valued for their vibrant color, high water content, and essential nutrients like vitamins A and K, Romaine and leafy lettuces are staples in both home and commercial kitchens.
The summary provided here is generally applicable to several types of lettuces, including Romaine, Butterhead, Greenleaf, and Redleaf lettuces.
Maturity and Quality
Romaine or Cos lettuce is an elongated heading lettuce type. Maturity is based on the number of leaves and head development. A very loose or easily compressible head is immature, and a very firm or hard head is overmature. For other leafy lettuces, the number of leaves and head compressibility are also indicators of maturity.
After trimming outer leaves, the leaves should be a bright to dark green color (tinged with red in some cultivars) with the inner leaves of the head being yellow or light green. The variation in leaf color corresponds to differences in the content of chlorophyll and carotenoids. The bright to dark green leaves are indicative of higher vitamin A and vitamin C contents relative to less green leaves. Leaves should be crisp and turgid, and free from insects, decay, or mechanical damage. Different lettuce types and cultivars vary considerably in sweetness (sugars) and bitterness (sesquiterpenes).
Figure 1. Lettuce leaf age from youngest (1) to oldest (5). Photo credit: Marita Cantwell, UC Davis.
| Leaf Age (1 = Youngest, 5 = Oldest) | 1 | 2 | 3 | 4 | 5 |
| Lightness | 81.0 | 72.5 | 57.8 | 48.0 | 43.6 |
| Chroma | 50.7 | 43.9 | 36.6 | 24.9 | 19.3 |
| Hue° | 106.4 | 114.4 | 121.4 | 125.8 | 127.3 |
| Chlorophyll (mg/g FW) | 0.05 | 0.11 | 0.41 | 0.72 | 1.13 |
| Carotenoids (mg/g FW) | 0.04 | 0.04 | 0.09 | 0.11 | 0.15 |
| Leaf Age or Position | Heart | Young | Midsize | Full Size |
| Total Chlorophyll (mg/100 g FW) | 4.4 | 17.3 | 37.6 | 52.2 |
| Total Carotenoids (mg/100 g FW) | 5.2 | 7.4 | 12.2 | 16.3 |
| Total Sugars (mg/g FW) | 18.2 | 12.8 | 12.8 | 8.7 |
| Vitamin C (mg/100 g FW) | 25.0 | 30.9 | 30.4 | 43.7 |
Figure 2. Quality scale (1-4) for Romaine lettuce. Photo Credit: Postharvest Team, Jiangsu Academy of Agricultural Sciences (JAAS), China.
Postharvest Handling and Storage
Storing near 0°C (32°F) is required to optimize the postharvest life of Romaine and leafy lettuce. A shelf-life of around 21 days is expected at this temperature for romaine lettuce. At 5°C (41°F), a shelf life of about 14 days can be expected if no ethylene is present in the environment. Storage life will vary by lettuce type. Vacuum cooling, hydro-vacuum cooling, and hydrocooling are all methods used to cool Romaine and leafy lettuces. Forced-air cooling may also be used, although it is a slower cooling method that usually incurs higher moisture loss.
>95%
The heads of Romaine and leafy lettuces have moderate respiration rates, and they are higher than the rates for iceberg lettuce. Table 3 shows the expected range of respiration rates for romaine heads. The rates for leafy lettuces are about 20% higher than the rates for Romaine. Intact washed Romaine and Greenleaf lettuce leaves also have respiration rates about 10-15% higher than the rates of romaine heads. Respiration rates can vary widely, depending on the specific type and cultivar of lettuce, head maturity at harvest, harvest season, and other factors. Table 4 illustrates an example of the variation in respiration rates of Romaine leaves in relation to leaf maturity, where younger leaves exhibit higher respiration rates.
| Temperature | 0°C (32°F) | 5°C (41°F) | 10°C (50°F) | 15°C (59°F) | 20°C (68°F) | 25°C (77°F) |
| mL CO2/kg·hr | 4-6 | 8-12 | 15-20 | 19-25 | 30-38 | 45-65 |
To calculate heat production, multiply mL CO2/kg·hr by 440 to get Btu/ton/day, or by 122 to get kcal/metric ton/day.
| Leaf Age or Position | Heart | Young | Midsize | Full Size |
| Intact Leaves Respiration Rate | 7.4 ± 0.2 | 3.9 ± 0.6 | 4.0 ± 0.2 | 4.4 ± 0.4 |
| Salad-cut Pieces Respiration Rate | 8.9 ± 0.6 | 6.9 ± 0.6 | 6.7 ± 0.6 | 7.5 ± 1.1 |
Ethylene production rates are very low to low, less than 0.1 to 1 μL/kg·h at ambient temperatures. Damage and cutting cause increases in ethylene production.
Romaine lettuce is highly sensitive to ethylene, with exposure to as little as one ppm causing yellowing (only at warm temperatures), brown spots, and an increased risk of disease. Ethylene damage on Romaine typically appears as discolored spots on the midrib, which are generally larger and less defined than the ethylene-induced russet spotting seen in iceberg lettuce. Sensitivity to ethylene can vary greatly among different Romaine and leafy lettuce cultivars.
Figure 3. No damage (2 left leaves) and moderate damage (two right leaves) on romaine lettuce due to ethylene exposure. Damage is seen as discoloration on the midribs. Photo Credit: Marita Cantwell, UC Davis.
Some benefit to storage life can be obtained with low O2 atmospheres (1-3%) at temperatures of 0-5°C (32-41°F). Low O2 atmospheres will reduce respiration rates and may reduce the detrimental effects of ethylene. Intact heads are not generally benefited by atmospheres containing CO2, and injury may occur with >5% CO2 (see physiological disorders, brown stain). Cut Romaine lettuce, however, is commonly packaged in low O2 (<1%) and high CO2 (7-10%) atmospheres because these conditions are effective to control browning on the cut surfaces. On salad pieces, cut surface browning occurs more rapidly and more extensively than do symptoms of brown stain caused by the high CO2 atmosphere. Cut iceberg lettuce tolerates higher CO2 concentrations than cut romaine lettuce.
Cut or broken midribs of Romaine lettuce may discolor more rapidly than cut pieces of iceberg lettuce. This is probably due to the higher content of phenolic compounds found in romaine leaves compared to iceberg leaves. Romaine varieties can vary greatly in the rate and severity of discoloration of cut pieces.
Red and Blue LED Irradiation. Considerable research is being conducted on greenhouse hydroponic lettuces. The type and intensity of lighting, as well as nutrient and climate conditions, all affect lettuce product quality and storage life.
Physiological and Physical Disorders
Ethylene-induced Brown Spots (Russet Spotting). This disorder is caused by low concentrations of ethylene gas, which stimulates the production of phenolic compounds, leading to unsightly brown pigments (see Figure 3). Russet spots appear as dark brown spots, especially on the midribs. Under severe conditions, russet spots are also found on the green leaf tissue and throughout the head. The disorder is strictly cosmetic but makes the lettuce unmarketable. Ethylene contamination may occur from propane forklifts, transportation in mixed loads, or storage with ethylene-generating fruits such as apples and pears.
Freezing Injury. Freeze damage can occur in the field, causing separation of the epidermis from the leaf. This weakens the leaf and leads to bacterial decay during storage. Freeze damage can occur during storage if the lettuce is held at <-0.2°C (31.7°F) (this temperature will vary depending on the sugar content of the lettuce). Freezing damage appears as darkened translucent or water-soaked areas that will turn slimy and deteriorate rapidly after thawing.
Figure 4. Lettuce showing water-soaked regions and peeling epidermis caused by freezing injury. Photo Credit: Postharvest Team, Jiangsu Academy of Agricultural Sciences (JAAS), China; Marita Cantwell, UC Davis.
Tipburn. This disorder is caused in the field and is related to climatic conditions, variety selection, and mineral nutrition. Leaves with tipburn are unsightly, and the damaged leaf margins are weaker and susceptible to decay.
Figure 5. Tipburn on Romaine lettuce. Photo Credit: Marita Cantwell, UC Davis.
Brown Stain. The symptoms of this disorder on romaine lettuce heads are yellowish-reddish-brown, large, depressed spots or stains. These are most noticeable on the midribs and may darken and enlarge with time. Brown stain disorder is caused by exposure to CO2-containing atmospheres, especially at concentrations above 5%. Visual symptoms of brown stain may occur less rapidly on upright varieties like Romaine than on crisphead varieties like iceberg lettuce.
Yellowing. Romaine and dark green leafy lettuces begin to yellow within 1-2 days when stored at ambient temperature, and yellowing occurs more easily in low-humidity environments. Generally, yellowing does not occur if the product is held at 10°C (50°F) or lower.
Figure 6. Lettuce displaying yellowing near the tips of the leaves. Photo Credit: Postharvest Team, Jiangsu Academy of Agricultural Sciences (JAAS), China.
Water Loss and Wilting. Lettuce is prone to water loss after harvest, especially in environments with high temperatures and low humidity. As water is lost, the edges of the leaves become dry and the leaves soften, losing crispness. A barrier to the environment, such as a thin plastic bag, can greatly reduce the rate of water loss and maintain appearance.
Figure 7. Romaine lettuce displaying water loss and wilting. Photo Credit: Postharvest Team, Jiangsu Academy of Agricultural Sciences (JAAS), China.
Pink Rib. Pink rib is a postharvest physiological disorder in lettuce, where the leaf midribs and base may develop a pink or red discoloration. This disorder is often seen in overmature heads, heads stored too long, and/or in heads stored at higher than recommended storage temperatures. The midribs usually take on a generalized pinkish coloration (see the photos for iceberg lettuce), and this is not associated with physical damage. Ethylene exposure does not appear to affect pink rib disorder, and low O2atmospheres do not control it.
Physical Injury. Lettuce tissue is tender and prone to cuts, compression, and other damage during harvesting, packaging, and transportation. Mechanical injury can accelerate respiration rates, increase the risk of pathogen infection, and shorten the storage period. Breakage of the midribs often occurs during field packing, especially on overmature romaine heads, and results in unsightly pinking or browning and increased susceptibility to decay. Product harvested early in the morning, when pulp temperatures are lower, is more susceptible to midrib cracking and breakage.
Figure 8. An example of physical damage that may occur during harvesting, packaging, and transportation of lettuce. Photo Credit: Postharvest Team, Jiangsu Academy of Agricultural Sciences (JAAS), China.
Discoloration due to Physical Injury. Most ‘pinking’ and browning discoloration is related to mechanical damage at harvest and subsequent injury-induced production of phenolic compounds, leading to unsightly pink and brown areas. Reducing cut-to-cool time and storing romaine lettuce below 2.5°C (36°F) are useful strategies to minimize discoloration in damaged areas.
Figure 9. Lettuce with discoloration due to mechanical damage. Photo Credit: Postharvest Team, Jiangsu Academy of Agricultural Sciences (JAAS), China.
Pathological Disorders
Bacterial Soft-rots. Bacterial soft rots in lettuce are usually caused by Erwinia species, but can also result from multiple bacterial species. They are characterized by wet, rotten areas that may emit an unpleasant odor. As the disease advances, the lesions enlarge and can spread throughout the entire lettuce. The disease tends to be more severe in environments with high temperature and humidity. Bacterial soft rot may occur after fungal infections. Trimming outer leaves, rapid cooling, and storing at low temperatures help reduce the development of bacterial soft rots.
Figure 10. Lettuce with severe bacterial rot (Erwinia spp.) Photo Credit: Postharvest Team, Jiangsu Academy of Agricultural Sciences (JAAS), China.
Bacterial Leaf Spot. This disorder, caused by Pseudomonas spp., begins with water-soaked spots that gradually grow into irregular, dark brown to black lesions. The affected areas become dry, papery in texture, and do not emit an unpleasant odor.
Figure 11. Lettuce with severe bacterial leaf spot caused by Pseudomonas spp. Photo Credit: Postharvest Team, Jiangsu Academy of Agricultural Sciences (JAAS), China.
Fungal Pathogens. Fungal pathogens can also cause a watery breakdown of lettuce, such as watery soft-rot caused by Sclerotinia or gray mold rot caused by Botrytis cinerea. These are distinguished from bacterial soft-rots by the presence of black and gray spores. Trimming leaves and maintaining low temperatures also help reduce the severity of these rots.
Figure 12. Gray mold (Botrytis cinerea) decay on lettuce. Photo Credit: Postharvest Team, Jiangsu Academy of Agricultural Sciences (JAAS), China.
References
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[2] California Leafy Greens Marketing Agreement (LGMA). 2024. Commodity Specific Food Safety Guidelines for the Production and Harvest of Lettuce and Leafy Greens. https://assets.lgma.ca.gov/downloads/CA-LGMA-Metrics_2024.08.21-FINAL.pdf.
[3] Cantwell, M.I. 2019. Postharvest management of injury-induced pinking in romaine hearts. Acta Horticulturae 1256: 179-185. https://doi.org/10.17660/ActaHortic.2019.1256.25
[4] Cantwell, M.I. 2024. Postharvest handling systems: Flower, leafy, and stem vegetables. In Postharvest Technology of Horticultural Crops 4th ed: Vegetable Crops, Herbs, and Flowers, Volume 7, Chapter 6, UC ANR Publ. 21661. https://anrpublications.org/Details.aspx?itemNo=21661.
[5] Gil, M.I. and J.A. Tudela. 2020. Leafy vegetables: Lettuce, escarole, and radicchio. Ch. 21.2. Controlled and Modified Atmospheres for Fresh and Fresh-Cut Produce, M.I. Gil and R.A. Beaudry, eds. Elsevier. https://doi.org/10.1016/B978-0-12-804599-2.00043-0.pp. 537-542.
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[8] Saltveit, M. E. 2016. Lettuce. Pp. 386-389. In: Gross, K.C., C. Y. Wang, and M. Saltveit, eds. The Commercial Storage of Fruits, Vegetables, and Florist and Nursery Stocks. Agriculture Handbook 66, USDA. https://www.ars.usda.gov/is/np/CommercialStorage/CommercialStorage.pdf.
[9] USDA. 2016. United States standards for grades of greenhouse leaf lettuce. https://www.ams.usda.gov/sites/default/files/media/GreenhouseLeafLettuceStandard.pdf
[10] USDA. 2016. United States standards for grades of romaine. https://www.ams.usda.gov/sites/default/files/media/RomaineStandard.pdf.
[11] Yang, X., J. Hu, Z. Wang, T. Huang, Y. Xiang, L. Zhang, J. Peng, F.A. Thomas-Barberan, and Q. Yang. 2023. Pre-harvest nitrogen limitation and continuous lighting improve the quality and flavor of lettuce (Lactuca sativa L.) under hydroponic conditions in greenhouse. Journal of Agricultural and Food Chemistry 71: 710−720. https://doi.org/10.1021/acs.jafc.2c07420.