Product description
Crisphead lettuce (Lactuca sativa) is a compact, pale-green lettuce known for its tightly layered leaves and crunchy texture. Its flavor is mild and slightly sweet, with the inner leaves being more tender and juicier than the outer leaves. Usually sold as whole heads, it is commonly used fresh in salads, sandwiches, and wraps. Crisphead lettuce is also widely known as iceberg lettuce.
Maturity and Quality
Maturity is based on head compactness. A compact head that can be compressed with moderate hand pressure is considered ideal. A very loose head is immature, and a very firm or hard head is overmature. Immature and mature heads have better flavor (more sweetness) than overmature heads and have fewer postharvest problems. See Figures 1 and 2.
Fig.1. Maturity stages for iceberg lettuce. Photo credit: Marita Cantwell
Fig. 2. Five maturity stages for iceberg lettuce. 1 and 2 would be considered immature, 3 and 4 are mature heads, and 5 is an overmature head. Photo credit: Marita Cantwell
While less nutritious than the darker green lettuces, crispness is a very important attribute of crisphead lettuce.
After trimming the outer wrapper leaves, crisphead lettuce should have bright, uniform, light green leaves. Leaves should be crisp, turgid, and free of any signs of dehydration (wilting, shriveling) or damage (breakage).
The cut surface of the butt may discolor (due to phenolic metabolism) but this has no relationship to the quality of the head.
Postharvest Handling and Storage
The optimal storage temperature for crisphead lettuce is 0°C (32°F), and a postharvest life of 21-28 days can be achieved, depending on the cultivar. At 5°C (41°F), a storage life of about 14 days can be expected, provided there is no ethylene in the environment.
Hydro-vacuum and vacuum cooling are common methods for initially cooling crisphead lettuce. Crisphead lettuce can tolerate up to 5% weight loss with vacuum cooling without affecting the crisp texture of the heads. Forced-air cooling can also be used successfully, albeit at a slower rate and with higher moisture loss.
>95%
Respiration rates of crisphead lettuce heads vary according to the season of the year, cultivar and the head maturity. In general, crisphead lettuce heads have moderate 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 | 3-8 | 6-10 | 11-20 | 16-23 | 25-30 | 40-56 |
To calculate heat of production, multiply ml CO2/kg·hr by 440 to get Btu/ton-day or by 122 to get kcal/metric ton-day.
Ethylene production rates are very low, about 0.1 μL/(kg·h) at 5°C (41°F) to about 1 μL/(kg·h) at 25°C (77°F).
Lettuce is highly sensitive to ethylene, with just 1 ppm causing brown spots on the veins (russet spotting). In some cases, ethylene exposure may increase the incidence of disease and/or yellowing.
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 reduce the detrimental effects of ethylene. Intact heads do not benefit from atmospheres containing CO2 and injury may occur with >3% CO2 (see Physiological Disorders, brown stain). Lettuce cut for salad products, however, is commonly packaged in low O2 (<1%) and high CO2 (10%) atmospheres because these conditions control browning on the cut surfaces. On salad pieces, cut-surface browning occurs more rapidly and more extensively than the brown stain caused by CO2.
Physiological and Physical Disorders
Freezing Injury. Freeze damage can occur in the field, causing separation of the epidermis from the leaf. This weakens the leaf and leads to more rapid bacterial decay. During storage, freeze damage can occur if lettuce is stored at <-0.2°C (31.7°F). This appears as darkened, translucent, or water-soaked areas that will turn slimy and deteriorate rapidly after thawing.
Fig. 3. Freezing damage on iceberg lettuce. Photo credit: Marita Cantwell
Physical Injury. Lettuce tissue is tender and prone to cuts, compression, and collisions during harvesting, packaging, and transportation, which can cause damage. 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 heads, and results in unsightly 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.
Discoloration Related to Physical Damage. Discolored areas, usually pink or brown, on cut or damaged leaves are due to phenolic metabolism induced by wounding. Careful handling to reduce damage at harvest and low temperatures after lettuce is cut are key to reducing discoloration. In fresh-cut crisphead lettuce, modified atmospheres in combination with low temperature (0-2°C; 32-36°F) further reduce undesirable pinking and browning.
Fig. 4. Rating scale for salad-cut iceberg lettuce discoloration, where 1 = no discoloration, 2 = slight, 3=moderate, 4=moderately severe and 5 = severe discoloration. Photo credit: Marita Cantwell
Tipburn. A disorder that occurs in the field and is related to weather conditions, lettuce cultivar, and mineral nutrition. Leaves with tipburn are unsightly, and the damaged leaf margins are weaker and more susceptible to decay.
Ethylene-induced Brown Spots (Russet Spotting). Russet spotting is caused by exposure to low concentrations of ethylene gas, which stimulates the production of phenolic compounds that, in turn, lead to brown pigmented areas. Russet spots appear as dark brown spots, especially on the midribs. Under severe conditions, russet spots are also found on green leaf tissue and throughout the head. The disorder is strictly cosmetic but makes the lettuce unmarketable. Ethylene contamination during distribution may occur from propane forklifts, transport in mixed loads, or storage with ethylene-producing fruits.
Fig. 5. Russet spotting on iceberg lettuce midribs due to ethylene exposure. Photo credit: Marita Cantwell
Fig. 6. Symptoms of russet spotting on iceberg leaves and midribs. Left to right: no symptoms, slight, moderate and severe symptoms. Photo credit: Marita Cantwell.
Fig. 7. Rating scale for russet spotting on iceberg lettuce, in which 1= none, 2= slight, 3= moderate, 4= moderately severe, and 5= severe. Photo credit: Marita Cantwell
Pink Rib. Pink rib is a physiological disorder in postharvest lettuce, in which the leaf veins, base, and sometimes the flesh may develop a pink or reddish discoloration, typically localized to the midribs. This disorder is most often seen in overmature heads. Higher-than-recommended storage temperatures or prolonged storage can increase its incidence. Ethylene exposure does not affect pink rib disorder, and low O2 atmospheres do not control it.
Fig. 8. Examples of pink rib disorder of iceberg lettuce. Photo credit: Adel Kader.
Brown Stain. The symptoms of this disorder on lettuce heads are large, yellowish-reddish-brown, depressed spots or stains. These are most noticeable on the midribs and may darken and enlarge over time. Brown stain is caused by exposure to CO2-containing atmospheres, especially at concentrations above 3%. Visual symptoms of brown stain may appear more rapidly on crisphead lettuce cultivars than on upright cultivars such as Romaine. Brown stain should not be confused with Rusty Brown Discoloration, a disorder of unknown cause.
Fig. 9. Brown stain due to high CO2 on stored iceberg lettuce. Photo credit: Adel Kader.
Fig. 10. Brown stain on iceberg lettuce pieces caused by high CO2 modified atmospheres. The translucent and brown appearance Is similar on salad pieces and whole heads. Photo credit: Marita Cantwell
Fig 11. Rating scale for brown stain in iceberg lettuce, where 1 =none, 2 = slight, 3=moderate, 4=moderately severe and 5 = severe. Photo Credit: Marita Cantwell
Fig. 12. Rusty Brown Discoloration of undetermined cause on stored iceberg lettuce. Photo credit: Adel Kader.
Water Loss and Wilting. Crisphead Lettuce is prone to water loss after harvest, especially in high-temperature, low-humidity environments. As water is lost, the outer leaves soften, losing crispness. Symptoms of dehydration may occur with a weight loss of more than 5%.
Pathological Disorders
Bacterial soft rot in lettuce is usually caused by Erwinia species, but may be caused by multiple species of bacteria, and is characterized by wet, rotten areas with a strong, unpleasant odor. As the disease progresses, the lesions enlarge and can spread throughout the lettuce. The disease is more severe in high-temperature, high-humidity environments. Bacterial soft rot may follow fungal infections. Trimming outer leaves, rapid cooling, and low-temperature storage reduce the development of bacterial soft rot.
Fungal Pathogens. Fungal pathogens may also cause a watery breakdown of lettuce, such as the watery soft rot caused by Sclerotinia or gray mold rot caused by Botrytis cinerea. These are distinguished from bacterial soft rots by the development of black and gray spores. Trimming and low temperatures also reduce the severity of these rots.
Fig. 13. Symptoms of russet spotting and gray mold (Botrytis cinerea) on stored iceberg lettuce. Photo credit: Adel Kader.
References
References from scientifically validated sources will be added in the future.