Tomato paste is a thick paste that is made by cooking tomatoes for several hours to reduce moisture, straining them to remove the seeds and skin, and cooking them again to reduce them to a thick, rich concentrate. In contrast, tomato purée is a liquid with a consistency between crushed tomatoes and tomato paste, and consists of tomatoes that have been boiled briefly and strained. Tomato paste became commercially available in the early 20th Century. In the UK, paste is referred to as purée or concentrate.