1. D. C. Marmion, Handbook of U.S. Colorants for Foods, Drugs, and Cosmetics, 2nd ed., John Wiley and Sons, New York, 1984.
2. D. C. Marmion, Handbook of U.S. Colorants for Foods, Drugs, and Cosmetics, 3rd ed., John Wiley and Sons, New York, 1991.
3. F. J. Francis, Handbook of Food Colorants, Eagen Press, St. Paul, Minn., 1999.
Haemoglobin from blood is the only commercially important colorant in the haem group. The term haem, or heme, is used to describe iron derivatives of the cyclic tetrapyr-role protoporphin with the structure shown in Figure 1. The numbering system is similar to that for chlorophyll (see Colorants: chlorophylls, Fig. 2). This structure is basic to a number of compounds such as chlorophyll, important in photosynthesis; haemoglobin, in oxygen transport; cytochrome, in energy transport; and a number of other enzymes. Haemoglobin is probably the best known, and it is composed of a haem group associated with a protein. Haemoglobin forms a loose association with oxygen as the basis for oxygen transport and carbon dioxide removal and is remarkably stable in vivo but not in vitro. This introduces some difficulty in using haems as colorants.
Free haem can be easily extracted from blood by treatment with acidified organic solvents. Preparation of haem from blood has been known for centuries and can be accomplished by the addition of whole blood to glacial acetic acid saturated with salt at 100°C. Crystalline haem precipitates out, but it is not stable enough to function as a food colorant. More stable compounds can be produced by allowing other ligands such as carbon monoxide, nitrous oxide, hydroxides, and cyanide to displace the oxygen from the central iron atom. A number of patents exist (1), and they fall into three broad areas: (1) treatment of whole blood with a variety of reagents, (2) stabilization of the color with a variety of ligands, and (3) purification of haem derivatives. Treatment of whole blood to remove the haemoglobin has considerable appeal because the iron is in a very nutritionally available form as a dietary supplement and the protein has a high biological value. It would be still another advantage if the preparation could be made stable enough to act as a food colorant. A number of the patents claim
Figure 1. Structure of haem.
Figure 1. Structure of haem.
that the preparations are suitable for coloring products that normally contain meat, meat analogues, sausages, and so on.
The use of whole blood as a food ingredient is a very old custom. The Europeans are famous for "blood pudding," which may be a delectable food item but is black in color not red. Haemoglobin is denatured in the cooking process. Houghton reported an interesting application of naturally occurring haem pigments, namely, the bile pigments bilirubin and biliverdin (2). They occur in gallstones and hair balls and are in demand in Chinese medicine as an aphrodisiac.
Blood is permitted as a food ingredient if it has been collected and processed in an appropriate sanitary manner, but colorants derived from blood are not permitted in the United States.
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