Over the centuries people like the Mongolian warriors used animal blood as a source of food, often ingesting it fresh. Today, some Masai of Tanzania still follow this practice, ingesting blood for nutrition as they travel with their herds. Elsewhere, blood (primarily from pigs, cows, chickens and geese) is still used as a thickening agent in some dishes, such as blood sausage (also known as black pudding because of the dark color of cooked blood). Blood should never be boiled, or it will clot. A little vinegar keeps blood from clotting during storage. In winemaking, blood is used as a fining agent to help clear suspended particles and clarify the wine. Blood is usually available by special order through some butcher shops.
From The Food Lover's Companion, Fourth edition by Sharon Tyler Herbst and Ron Herbst. Copyright © 2007, 2001, 1995, 1990 by Barron's Educational Series, Inc.