Lettuce (Romaine or Cos)
Maturity Indices
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(<30 leaves before trimming) and mature (about 35 leaves) have much better flavor than overmature heads (less bitterness, more sweetness) and also have fewer postharvest problems. Extra trimming to leave only the inner, most tender light green leaves results in “Romaine hearts”.
Quality Indices
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.
Optimum Temperature and Relative Humidity
0°C (32°F) with >95% RH 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.
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 bacterial decay during storage. Freeze damage can occur during storage if the lettuce is held 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.
Rates of Respiration
Romaine lettuce heads have moderate respiration rates, but they are generally higher than rates for iceberg lettuce:
Temperature | 5°C (41°F) | 10°C (50°F) | 15°C (59°F) | 20°C (68°F) |
ml CO2/kg·hr | 9 – 12 | 15 – 20 | 19 – 25 | 30 – 38 |
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.
Rates of Ethylene Production
Ethylene production rates are very low: <0.2 µL/kg·hr at 0-5°C (32-41°F).
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.
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 O2atmospheres will reduce respiration rates and reduce the detrimental effects of ethylene. Intact heads are not generally benefitted 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 CO2concentrations than cut romaine lettuce.
در تکه های سالادی، قهوه ای شدن سطح برش کاهو سریع تر و شدیدتر از علائم لکه قهوهای ناشی از دیاکسید کربن خودش را نشان می دهد. کاهوی آیسبرگ برش خورده به غلظت های بالای دی اکسید کربن مقاوم تر از کاهوهای معمولی برشخورده است.