Vegetables Produce Facts English

Lettuce, Crisphead

Recommendations for Maintaining Postharvest Quality

Marita Cantwell and Trevor Suslow

Department of Plant Sciences, University of California, Davis

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Maturity is based on head compactness. A compact head which can be compressed with moderate hand pressure is considered ideal maturity. A very loose head is immature and a very firm or hard head is overmature. Heads that are immature and mature have much better flavor than overmature heads and also have fewer postharvest problems.

Quality Indices

After trimming outer wrapper leaves, the leaves should be a bright light green color. Leaves should be crisp and turgid.

Maturity & Quality Photos

Title: Lettuce Maturity (1)

Photo Credit: Adel Kader, UC Davis

Title: Lettuce Maturity (2)

Photo Credit: Adel Kader, UC Davis

Temperature & Controlled Atmosphere

Optimum Temperature

0°C (32°F) is required to optimize lettuce storage life. A shelf-life of 21-28 days can be expected at this temperature and RH. At 5°C (41°F) a shelf-life of 14 days can be expected as long as no ethylene is in the environment. Vacuum cooling is usually used for iceberg lettuce, but forced-air cooling may also be used successfully.

Freezing Injury. Freeze damage can occur in the field and cause 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 the lettuce is stored at 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.

Relative Humidity

>95%

Rates of Respiration

Iceberg 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)
ml CO₂/kg·hr	3-8	6-10	11-20	16-23	25-30

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.

Rates of Ethylene Production

Very low,

Responses to Ethylene

Iceberg lettuce is extremely sensitive to ethylene. Russet spotting (see physiological disorders) is the most common symptom of ethylene exposure.

Responses to Controlled Atmospheres (CA)

Some benefit to shelf-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 are not benefited by atmospheres containing CO₂ and injury may occur with >2% 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 do symptoms of brown stain caused by CO₂.

Temperature & Controlled Atmosphere Photos

Title: Brown Stain

Photo Credit: Adel Kader, UC Davis

Title: Brown Stain Under Modified Atmospheres &Ethylene

Photo Credit: Adel Kader, UC Davis

Title: Ethylene-induced Russet Spotting

Photo Credit: Adel Kader, UC Davis

Disorders

Physiological and Physical Disorders

Many disorders have been identified for iceberg lettuce. Some very common and important disorders are the following:

Tipburn. A disorder caused in the field and is related to climactic conditions, cultivar selection and mineral nutrition. Leaves with tipburn are unsightly and the damaged leaf margins are weaker and susceptible to decay.

Russet Spotting. A common disorder due to exposure to low concentrations of ethylene which stimulates the production of phenolic compounds which lead to brown pigments. Russet spots appear as dark brown spots especially on the midribs. Under severe conditions, russet spots are 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 fork lifts, transport in mixed loads, or storage with ethylene-generating fruits such as apples, pears and peaches.

Brown Stain. The symptoms of this disorder are yellowish-reddish-brown large, depressed spots on the midribs mostly. These may darken or enlarge with time. Brown stain also appears as reddish-brown streaks in some cases. Brown stain is caused by exposure to above 3% CO₂ atmospheres, especially at low temperatures.

Pink rib. A disorder in which the midribs take on a pinkish coloration. Overmature heads and high storage temperatures increase the disorder. Ethylene exposure does not increase the disorder and low O₂ atmospheres do not control it.

Breakage of the midribs often occurs during field packing and causes increased browning and increased susceptibility to decay.

Pathological Disorders

Bacterial soft-rots are caused by numerous bacteria species and result in a slimy breakdown of the infected tissue. Soft-rots may follow fungal infections. Trimming outer leaves, rapid cooling and low temperature storage reduce development of bacterial soft-rots.

Fungal pathogens. May also lead to a watery breakdown of lettuce (watery soft-rot caused by Sclerotinia or gray mold rot caused by Botrytis cinerea) but 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.

Disorders Photos

Title: Grey Mold

Photo Credit: Adel Kader, UC Davis

Title: Mechanical Damage

Photo Credit: Adel Kader, UC Davis

Title: Pink Rib

Photo Credit: Adel Kader, UC Davis

Title: Russet Spotting

Photo Credit: Adel Kader, UC Davis

Title: Rusty Brown Discoloration

Photo Credit: Adel Kader, UC Davis

Title: Severe Symptoms of Russet Spotting

Photo Credit: Adel Kader, UC Davis

Date

June 2002

Use of Materials

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Except for these specified uses, no part of the textual materials available on the UC Postharvest Technology Center Web site may be copied, downloaded, stored in a retrieval system, further transmitted or otherwise reproduced, stored, disseminated, transferred or used, in any form or by any means, except as permitted herein or with the University of California's prior written agreement. Request permission from UC Postharvest Technology Center. Distribution for commercial purposes is prohibited.

The information in this fact sheet represents our best understanding of the current state of knowledge at the time of the latest update, and does not represent an exhaustive review of all research results. Links to any of these UC Postharvest Technology Center pages are permitted, but no endorsement of the linking site or products mentioned in the linking page is intended or implied by such a link.

How to Cite

Author(s) names. Initial publication or update date (located at the top). Title. Link to the specific Produce Fact Sheet webpage (Accessed date)

Example: Cantwell, M. and T. Suslow. 2002. Lettuce, Crisphead: Recommendations for Maintaining Postharvest Quality.

http://ucanr.edu/sites/Postharvest_Technology_Center_/Commodity_Resources/Fact_Sheets/Datastores/Vegetables_English/?uid=19&ds=799 (Accessed January 18, 2014).

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Vegetables Produce Facts English

Lettuce, Crisphead

Ask the Produce Docs for this Commodity

Publications for this Commodity

Lettuce, Crisphead PDF

Maturity & Quality Photos

Temperature & Controlled Atmosphere Photos

Disorders Photos

Use of Materials

How to Cite