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White asparagus is any asparagus that has been deprived of sunlight. It is, in essence, the exact same vegetable as green asparagus with the exception of its lack of color. Deprivation of sunlight gives it this appearance by hindering the production of chlorophyll, which would under normal circumstances gives the asparagus its green coloring.
In the United States, white asparagus is not as popular as its green counterpart. When it is available it is often sold in cans or in the produce department of certain grocery stores, however it can be difficult to find. White asparagus is more common in Europe and Asia, where it is seen as a delicacy and used in gourmet cooking. Typically it is also more expensive than other types of asparagus, often as much as three times higher in price.
To ensure that the sun does not reach the asparagus, the plant is buried beneath approximately eight to ten inches (20.32 to 25.4 cm) of mounded soil. Before the spears break through the mound, a knife is used to cut and remove them. In some cases, the tips of the asparagus are allowed to break free so that there is some color on the tips.
Another method that is used to keep the asparagus white is to cover the rows of planted asparagus with a plastic tarp, or covering. This covering is only taken away after harvesting has been completed. When using this method to grow white asparagus, the plastic should be an opaque black that does not allow sunlight to penetrate it. Plastic black drums are another method used by some to block out the sun's rays.
Harvesting white asparagus can be exceptionally tricky. This is because exposing the white spears to more than 15 minutes of sun can be enough to turn them green. To prevent this from happening, they should be put in opaque bags until they are taken out of the sun.
Prior to cooking or otherwise preparing white asparagus, it must be cleaned and the skin peeled. While it is possible to eat it raw, completely cooking the vegetable will make it more tender. The flavor of white asparagus is much milder than its green counterpart when cooked and it has a nut-like, sweet taste when it is eaten raw.
When compared to green asparagus, the nutritional value of white asparagus varies somewhat. Like green asparagus, it is a source of vitamin C, vitamin A and other nutrients. Due its lack of exposure to sunlight, however, it generally contains these same nutrients to a lesser degree than green asparagus.