Harvesting and culinary use

Green leafy vegetables are considered an essential part of the diet to meet the daily nutrient requirements and can be used fresh as a salad or cooked or processed as per the consumer's interest.

Green leafy vegetables are essential for a balanced diet with rich nutrients, high dietary fibre, low lipids, rich folate,ascorbic acid and vitamin k potassium and magnesium.  They are also high in phytochemicals such as Beta carotene and flavanoids.

Their low caloric value makes them ideal for weight management and is beneficial in lowering the risk of cardiovascular diseases and cancer.

Green leafy vegetables are also valued for individuals with type 2 diabetes due to their high Mg content, high fibre content, and low glycemic index.

Reference:

Muhammad Atif Randhawa BSc, MSc, PhD, Muhammad Wasim Sajid BSc, MSc, in N\Handbook of Fertility, 2015

Harvesting

Lettuce should be harvested when full size but just before maturity. The leaves taste best when they're still young and tender.

Depending on the variety and time of year, lettuce generally lives 65–130 days from planting to harvesting. However, lettuces left too long in the soil produce flowers (bolt) and become bitter.

Leaf lettuce should be harvested before maturity, simply by removing outer leaves so that the centre leaves can continue to grow.

Butterhead, romaine, and looseleaf types are harvested by removing the outer leaves, digging up the whole plant, or cutting the plant about an inch above the soil surface.

A second harvest is often possible when using the first or third methods.

Crisphead lettuce should be harvested when the centre is firm.

Mature lettuce gets bitter and woody and will go bad quickly. Gardens should be checked every day for ready-to-harvest leaves.

Lettuce can be kept in the refrigerator for up to 10 days in a loose plastic bag.

Wilted lettuce leaves can be revigorated by placing them in a bowl of cold water with ice cubes and soaking for about 15 minutes.

However, the high water content of lettuce (94.9 %) creates problems when attempting to preserve the plant – it cannot be successfully frozen, canned or dried and must be eaten fresh.

Culinary use

Lettuce is most often used for salads, either alone or with other greens, vegetables, meats and cheeses, although it is also seen in other kinds of food, such as soups, sandwiches and wraps; it can also be grilled, while the stems are eaten both raw and cooked..

Romaine lettuce is often used for Caesar salads.

The consumption of lettuce in China developed differently from Western countries due to health risks and cultural aversion to eating raw leaves.

In that country, "salads" were created from cooked vegetables and served hot or cold.

In China, lettuce is eaten raw or cooked, primarily in soups and stir-frys and used as a primary ingredient in the preparation of lettuce soup.

Depending on the variety, lettuce is an excellent source (20% of the Daily Value) (DV), or higher) of vitamin K (97% DV) and vitamin A (21% DV) (table), with higher concentrations of the provitamin A compound, beta-carotene, found in darker green lettuces, such as romaine.

Except for the iceberg variety, lettuce is also a good source (10–19% DV) of folate and iron.

Consumption and use of lettuce, historical and current 

Europe and North America initially dominated the market for lettuce, but by the late 20th century, the consumption of lettuce had spread throughout the world.

World production of lettuce and chicory for 2017 was 27 million tonnes, 56% of which came from China.