Lettuce is one of the most important salad vegetables grown in the United States:
Four types of lettuce dominate U.S. production:
- Crisphead, head or iceberg
- Cos or romaine
- Leaf
- Butterhead (Boston or Bibb)
Crisphead, head or iceberg is the predominant lettuce grown in the U.S.A. and is best adapted for long distance shipment. Per-capita consumption of head lettuce exceeds 25 lb annually. Leaf types are grown for local and regional markets, primarily, although, with proper cooling, packaging and refrigerated truck transportation, these lettuces may also be distributed nationally.
Most crisphead or iceberg lettuce is grown in the cool, coastal valleys of California. It requires a long, cool growing season. Leaf lettuce matures quickly and is easy to grow and a good type of lettuce for home gardens.
Because lettuce is so fragile, it is handled as little as possible. Lettuce is extremely perishable and needs to be handled delicately, and marketed rapidly. A stronger, bitter taste and toughness develops if harvest is delayed or if crop is over-mature, and then the product becomes unmarketable.
Head lettuce is harvested when the heads are of good size (about 2 lbs), well formed and solid. Three-to-four undamaged wrapper leaves are left on each head. Heads are put into rigid cardboard containers in the field and are graded according to size and pack. Leaf, butterhead and cos types are cut, trimmed and tied into compact bundles before placing in cartons.
The mechanical “lettuce harvestor” is used in most commercial Head lettuce harvest. This allows harvested heads of lettuce to be conveyed upward into the machine with their wrapper leaves attached . By removing them mechanically, the machine paves the way for graders and packers riding the machine to complete their tasks efficiently and comfortably. Some fresh market lettuce is hand cut and trimmed, and placed in cardboard cartons in the field. No lettuce is washed before it gets to the store. Lettuce and other leafy items must be kept clean and free of soil and mud.
All packaging is done before cooling. Iceberg or head lettuce may be closely trimmed and wrapped in film. Polyethylene films are most common but new PVC films maintain freshness longer and are easier to wrap resulting in a neater wrapping. Packing is done in the field. A wire-bound or waxed film carton designed to hold 20-24 heads is used. Lettuce should be “flat packed” (non bulge to avoid crushing the heads. There are two layers of heads; the bottom layer is packed stem-down and the top layer, stems up. This keeps the milky latex from the stems from dripping on the heads.
Whole plants of leaf lettuce are put in a plastic sleeve and sold 24-36 per fiber-board carton. Iceberg lettuce is commonly packaged in 43 to 48-lb, 24 count, cartons.
The lettuce is then trucked to a central area for vacuum cooling. Lettuce must be cooled before shipment. Vacuum cooling is the primary method. (In a few areas it is not vacuum cooled, but placed in a cooler for temporary holding until trucked to a nearby market.)
Lettuce should be precooled to 32-34 F soon after harvest and stored at 32F and 90-95% humidity for retention of quality and shelf life. For vacuum cooling, containers and film wraps should be perforated or readily permeable to water vapor. To aid vacuum cooling, clean water is sprinkled on the heads of lettuce prior to carton closure if they are dry and warmer than 78F. Thorough precooling is essential because mechanically refrigerated rail cars or trucks do not have enough capacity to cool warm lettuce during transit.
Lettuce is highly perishable and deteriorates rapidly with increasing temperature. At 32 F, head lettuce can be held in good condition for 2 to 3 weeks- the time period depending on maturity, quality, and handling condition of the lettuce at harvest. The storage life at 38 F is only about half at that at 32 F.
Lettuce is easily damaged by freezing, so all parts of the storage room must be kept above the highest freezing point of lettuce (31.6F).
If lettuce needs to be in transit over-seas for a month, an atmosphere of 2% carbon dioxide and 3% oxygen is recommended, because the reduction in decay achieved by 2% carbon dioxide outweighs the danger of injury.
More lettuce of all types is being processed into fresh-cut salad mixes for commercial and home ease. Supermarkets are devoting increased space to cut lettuce and lettuce mixes of many types, some complete with salad dressings. The supply of this profitable and popular product is dominated by a handful of producers because of the large capital investment that must be made to produce this product and meet the exacting demands of the produce industry. Some differences to conventionally marketed lettuces exist in production, harvest timing and handling and the way the product is processed and marketed.
Cut lettuce, which is found in grocery stores in plastic bags “ready-to-eat” may be hand cut and cored. However harvesters for this purpose are now also available.
After harvest the lettuce is then placed in bulk containers which are transported to a processing facility. There it is cut and washed in suitably cold water. It is then centrifuged to remove excess water and is often mixed with other types of lettuce or greens, shredded carrot and/or red cabbage. It may be treated with a chlorine-containing compound and/or an antioxidant or preservative during washing or before packaging. It is then bagged in special plastic films that maintain, internally, a certain ratio of atmospheric gases (N2, O2, and CO2) that is lower in O2 and higher in CO2. The bags are then placed in cartons for temporary cold storage or for immediate shipment to market.
With proper vacuum cooling and packaging, refrigerated transportation, and water controlled atmosphere, the industry can supply whole and packaged lettuces to national and international markets. As we shop in our local grocery stores and select lettuce, we can only be amazed at the amount of labor, planning, engineering and science, and sales that have been combined to produce a beautiful head of lettuce or other lettuce product. *
* Commercial Vegetable Production Guides, Oregon State University.
* North Carolina State University Horticulture Information Leaflets.
* www.southernvacuumcooling.com
