Lettuce
Lettuce is a leafy vegetable belonging to the Asteraceae family, scientifically known as Lactuca sativa. This versatile plant is primarily grown for its leaves, which serve as a key ingredient in salads and various dishes. Lettuce comes in two main categories: head lettuce, which includes varieties like iceberg, known for its compact shape and crisp texture, and leaf lettuce, which encompasses types such as romaine and butterhead, characterized by their loose leaves and diverse colors. Historically, romaine lettuce is believed to be the first domesticated type, with evidence of cultivation dating back over 5,000 years, particularly in ancient Egypt where it was valued for both its leaves and oil-rich seeds.
In terms of cultivation, lettuce thrives in cooler temperatures, with optimal growing conditions for head varieties ranging from 60 to 65 degrees Fahrenheit. Its nutritional profile varies, with romaine lettuce being notably higher in vitamins and nutrients compared to head lettuce. While lettuce is recognized for its low calorie content and high water concentration, it also serves various culinary purposes, from salads to garnishes, and even as a low-carb alternative for wraps. Additionally, lettuce seed oil has been explored for its potential health benefits, including its use as a natural sleep aid and treatment for anxiety. This makes lettuce not only a staple in diets worldwide but also a subject of interest in herbal medicine.
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Lettuce
Lettuce is a leafy vegetable that belongs to the Asteraceae family of seed-bearing flowering plants. The common form of the plant is known by the scientific name Lactuca sativa. Lettuce is typically grown for its leaves, which are most often used as a base ingredient in salads. The plant's oil-bearing seeds also have culinary and non-culinary uses.
The many varieties of lettuce can be grouped into two broad categories: head lettuce and leaf lettuce. Head lettuce, also known as crisphead or iceberg lettuce, grows in a tight, compact spherical shape and is light green in color with a crisp texture. Leaf varieties include romaine lettuce, butterhead lettuce, and a wide range of loose-leaf subtypes that are usually named for the color or appearance of their leaves. Romaine lettuce, also known as cos lettuce, has looser, elongated dark green leaves supported by pale ribs. Butterhead types include Boston lettuce and Bibb lettuce, both of which have loose heads and soft, delicate green leaves. The most widely consumed types of loose-leaf lettuce include green leaf, red leaf, and oak leaf lettuce. The leaves found in these varieties flower out from a compacted stalk, with the ends of red lettuce leaves displaying a burgundy hue and the ends of oak lettuce leaves taking on a brownish-red tint. Beyond these basic varieties, lettuce is also available in hundreds of different hybrids.
Brief History
Romaine is believed to be the first type of domesticated lettuce. It has been part of the human diet for at least five thousand years, with ancient Egyptian bas-relief carvings dating to the third millennium BCE confirming its cultural prominence. However, these early types of domesticated romaine lettuce bear little resemblance to the carefully cultivated varieties available today. They were developed from a wild form of the plant, which had a far more bitter taste than modern lettuce. It is thought that the ancient Egyptians grew lettuce not so much for its edible leaves, but rather for its oil-rich seeds and the milk-like latex secretion it produced. This secretion was believed to be an aphrodisiac and had associations with Min, an ancient Egyptian fertility deity.
Beyond its purported aphrodisiac properties, lettuce secretions were also used as a sleep aid. During the era of the Roman Empire, it was common to serve lettuce-based dishes at the end of a meal to help encourage sleep. As different varieties continued to be cultivated, lettuce types that lacked strong sleep-inducing properties were developed. This development marked a general shift from lettuce being served at the end of a meal to its use as an appetizer.
Overview
When grown outdoors, head lettuce is best suited to cooler conditions. Horticultural guidelines identify an optimum temperature range of about 60 to 65 degrees Fahrenheit (15 to 18 degrees Celsius). Prolonged exposure to hot temperatures will adversely affect the growth and development of head lettuce plants, although they can withstand conditions as warm as 80 to 85 degrees Fahrenheit (26 to 29 degrees Celsius) for a few days. Mature head lettuce plants can also survive brief periods of exposure to temperatures below the freezing mark.
Head lettuce seeds can germinate in temperatures slightly above the freezing mark, but a range of 70 to 75 degrees Fahrenheit (21 to 24 degrees Celsius) is considered ideal for germination. Soil conditions should be slightly toward the acidic side of neutral, with pH balances between 6.0 and 6.8.
The cultivation of romaine lettuce follows similar guidelines, but it also grows well in slightly warmer conditions. Butterhead and loose-leaf varieties have even higher heat tolerances. Head lettuce crops usually mature in about 75 to 100 days, while romaine is ready for harvest after 75 to 85 days. Butterhead and loose-leaf lettuce plants have a more flexible window and can typically be harvested after 45 to 55 days. Lettuce can also be grown in indoor greenhouses, which offer more easily controlled growing conditions.
In 2014, worldwide lettuce and chicory production reached a total of 24.976 million tons. In order, the world's top ten producers of lettuce in 2014 were China, the United States, India, Spain, Italy, Japan, Turkey, Iran, Mexico, and Germany.
In modern times, lettuce is most often used as the primary ingredient in salads, in which its leaves are mixed with other vegetables and served with or without dressing. Lettuce is also a popular garnish in sandwiches and wraps. Less commonly, chopped or shredded lettuce is added to soups, and lettuce heads or whole leaves can also be marinated for braising, grilling, sautéing, or searing. Butterhead lettuce leaves can be substituted as a low-carbohydrate alternative to flour-based breads, wraps, or tortillas. Because of its high water concentration, lettuce is sometimes combined with other vegetables or fruits to make juice.
Lettuce is very low in calories, providing only about 15 calories per 100 grams. It also delivers about one gram of fiber per cup. However, vitamin and nutrient content varies considerably among the different varieties of lettuce. Head lettuce offers the lowest nutritional value of all commonly consumed types, while romaine lettuce is the most nutritious.
The main vitamins and nutrients in lettuce include folate, vitamin A, and vitamin K. It also contains smaller concentrations of vitamin C, calcium, phosphorus, and potassium. Romaine lettuce contains about three times as much folate and vitamin K as head lettuce while providing ten times as much vitamin A.
Lettuce seed oil has long been used as a folk remedy for insomnia and sleep disturbances, and recent scientific studies have confirmed its efficacy as a natural sleep aid. The results of a study published in the International Journal of General Medicine in 2011 found that lettuce seed oil was useful for inducing sleep, particularly in elderly patients, without producing any hazardous or undesirable side effects. Lettuce seed oil also appears to have anxiolytic properties, with potential as a natural treatment for mild to moderate anxiety.
Bibliography
"A Guide to Lettuce." Good Housekeeping, 6 June 2012, www.goodhousekeeping.com/food-recipes/cooking/tips/g532/types-of-lettuce/?slide=1. Accessed 1 Mar. 2017.
Hart, Leah. "Nutritional Value of Romaine vs. Iceberg Lettuce." LiveSTRONG, 26 Jan. 2014, www.livestrong.com/article/408131-nutritional-value-of-romaine-vs-iceberg-lettuce/. Accessed 1 Mar. 2017.
"Lettuce." Agricultural Marketing Resource Center, May 2015, www.agmrc.org/commodities-products/vegetables/lettuce/. Accessed 1 Mar. 2017.
"Lettuce." Vegetable Research and Information Center, University of California–Davis, vric.ucdavis.edu/pdf/lettuce.pdf. Accessed 1 Mar. 2017.
"Production Quantities of Lettuce and Chicory by Country." Food and Agriculture Organization of the United Nations, 2015, www.fao.org/faostat/en/#data/QC/visualize. Accessed 1 Mar. 2017.
Rupp, Rebecca. "From Ancient Egypt to Outer Space: The Delicious History of Lettuce." National Geographic, 13 Aug. 2015, theplate.nationalgeographic.com/2015/08/13/from-ancient-egypt-to-outer-space-the-delicious-history-of-lettuce/. Accessed 1 Mar. 2017.
Sanders, Douglas. "Lettuce: Horticulture Information Leaflet." North Carolina State University, 1 Jan. 2001, content.ces.ncsu.edu/lettuce. Accessed 1 Mar. 2017.
Yakoot, Mostafa, et al. "Pilot Study on the Efficacy and Safety of Lettuce Seed Oil in Patients with Sleep Disorders." International Journal of General Medicine, vol. 4, 2011, pp. 451–56.