Page 42 - Academic Press Encyclopedia of Physical Science and Technology 3rd BioChemistry

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Encyclopedia of Physical Science and Technology EN006C-254 June 28, 2001 19:52

114 Food Colors

Betalains are stable in the pH range 3.5–7.0, which is ize or react with amino compounds and form brown prod-
the pH range of most foods, but they are sensitive to heat, ucts. Phenolase (or phenol oxidase), however, is the princi-
oxidation, and light. pal browning enzyme. This enzyme oxidizes o-diphenols
to o-quinones, which, by nonenzymatic processes, are ul-
timately converted to brown polymers known as melanins.
F. Miscellaneous Natural Food Colors
Melanins are formed in both animal and plant tissues. A
There are several hundred additional natural pigments typical substrate of phenolase in animals is tyrosine. This
that are not as widely represented in foods as the pre- amino acid is converted to melanin by a series of reactions,
viously discussed coloring substances. Among them are some of which are shown in Fig. 12.
the quinones and xanthones, which are yellow pigments. In dark hair, skin, eyes, and other animal tissues,
An example of a quinone is juglone, which is present in melanin is attached to proteins. Tyrosine is also a phe-
walnuts and pecans. Mangiferin, a representative of xan- nolase substrate in plant tissues (e.g., potatoes), but o-
thones, is found in mangoes. Tannins include two types diphenols and polyphenols are by far the most common
of pale yellow to light brown compounds, characterized substrates of enzymatic browning in foods of plant origin.
by their property to convert animal hides to leather. One The following phenolic compounds have been associated
type consists of condensed tannins, to which reference was with enzymatic browning in some foods: chlorogenic acid,
made in relation to the leucoanthocyanidins, and the other caffeic acid, and catechin in apples, apricots, peaches, and
type comprises hydrolyzable tannins, which are esters of pears; 3,4-dihydroxyphenylethylamine in bananas; (−)-
a sugar, usually glucose, with gallic acid, ellagic acid, or epicatechin, (+)-catechin, (+)-gallocatechin, and (−)-
both. Corilagin is an example of a gallotannin, in which epigallocatechin galate in tea leaves and cocoa beans;
glucose is esterified with three gallic acid molecules. A catechins in grapes; and tyrosine and chlorogenic acid
yellow pigment that has attracted much attention because in potatoes. The structures of four of these phenolics are
of its toxicity to humans and nonruminant animals is shown in Fig. 13.
gossypol. It is present in cottonseeds, which are used as Many fresh fruits and vegetables brown slowly as they
animal feed and have been considered a potential source of senesce. The enzymatic browning of these commodities
protein for human use. Several biologically very important is more rapid when they are subjected to processing, such
food constituents are colored, such as phytochrome (yel- as the pressing of apples in making cider or the peeling
low), vitamin B 2 (riboflavin, orange-yellow), and vitamin and cutting of potatoes in preparing potato products. Since
B 12 (red), although their contribution to food coloration is enzyme, substrate, and oxygen must all be present for the
negligible. development of this type of browning, elimination of any
one of the three agents will prevent the browning. Heat
inactivation of the enzyme, the exclusion of oxygen (by
III. FOOD BROWNING keeping the commodity under water or packaging it under
vacuum or inert gas), and the selection of varieties poor in
Foodsmaydevelopavarietyofbrowncolors,fromyellow- substrate content or enzyme activity are ways of prevent-
brown to red-brown to black-brown, during handling, pro- ingthisdiscoloration.Also,storageatlowtemperatureand
cessing, and storage. These colors are desirable in certain the addition of sulfur dioxide, ascorbic acid, citric acid,
foods (e.g., coffee, beer, bread, maple syrup). In other sodium chloride,or combinations of these compounds will
foods, such as most dehydrated fruits and vegetables, dried inhibit browning.
eggs, and canned or dried milk, browning is detrimental.
Even when desirable, browning should not be excessive,
B. Nonenzymatic Browning
as in potato chips, french fries, and apple juice. Numerous
reactions lead to browning in foods. Some of these may A number of chemical processes not involving enzymes
also generate flavors and/or alter the nutritional proper- may result in food browning. Briefly discussed here are the
ties of foods. Conventionally, browning is discussed as Maillard reaction, caramelization, ascorbic acid brown-
enzymatic and nonenzymatic browning. ing, and metalpolyphenol browning.

A. Enzymatic Browning 1. The Maillard Reaction (Maillard Browning)

Several enzymes may initiate reactions that eventually This reaction is actually a series of reactions occurring
produce brown colors in foods. For example, the action from the first encounter of a carbonyl compound with an
of ascorbate oxidase on ascorbic acid or of lipoxidase on amine compound to the formation of brown pigments. It is
lipids leads to carbonyl products that may either polymer- also known as the carbonyl–amine reaction, and its brown