Vegetables Produce Facts English
Recommendations for Maintaining Postharvest Quality
Maturity & 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. Heads that are immature (
After trimming outer leaves, the leaves should be a bright to dark green color (tinged with red in the red romaine cultivars) with the inner leaves of the head being yellow or light green. The bright to dark green of romaine leaves is indicative of higher vitamin A and vitamin C contents relative to iceberg lettuce types. Leaves should be crisp and turgid, and free from insects, decay or mechanical damage (U.S. Grade No. 1). Different romaine varieties may vary in sweetness and bitterness.
Temperature & Controlled Atmosphere
0°C (32°F) is required to optimize the postharvest life of romaine lettuce. A shelf-life of around 21 days is expected at this temperature. At 5°C (41°F) a shelf-life of about 14 days can be expected as long as no ethylene is in the environment. Water spray-vacuum cooling or hydrocooling are often used for romaine lettuce, but forced-air cooling may also be used.
Rates of Respiration
Romaine lettuce heads have moderate respiration rates, but they are generally higher than rates for iceberg lettuce:
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.
Ethylene production rates are very low:
Responses to Ethylene
Romaine lettuce is sensitive to ethylene. Ethylene damage appears as discolored spots on the midrib. These are generally larger and less defined than those found with ethylene-induced Russet spotting on iceberg lettuce (see physiological disorders). Varieties can vary significantly in their susceptibility to ethylene.
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 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 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. Cut iceberg lettuce tolerates higher CO2 concentrations than cut romaine lettuce.
Temperature & Controlled Atmosphere Photos
Physiological and Physical Disorders
Use of Materials
The UC Postharvest Technology Center grants users permission to download textual pages (including PDF files) from this World Wide Web site for personal use or to reproduce them for educational purposes, but credit lines and copyright notices within the pages must not be removed or modified.
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.