Irish moss, or carrageen moss (Irish carraigín, "moss of the rock") is a species of red algae (Chondrus crispus) which grows abundantly along the rocky parts of the Atlantic coast of Europe and North America. In its fresh condition the plant is soft and cartilaginous, varying in colour from a greenish-yellow to a dark purple or purplish-brown; but when washed and sun-dried for preservation it has a yellowish translucent horn-like aspect and consistency. The principal constituent of Irish moss is a mucilaginous body, of which it contains about 55%; the plant also has nearly 10% of albuminoids and about 15% of mineral matter, and is rich in iodine and sulphur. When softened in water it has a sea-like odour, and because of the abundant mucilage it will form a jelly when boiled, containing from 20 to 30 times its weight of water.
Irish moss is a major source of carrageenan, which is commonly used as a thickener and stabilizer in processed foods, including ice cream and luncheon meat. It may also be used as a thickener in calico-printing and for fining beer or wine. Irish moss is frequently mixed with Gigarlina mammillosa, G. acicularis and other seaweeds with which it is associated in growth.
