REVIEWS
Food Browning and Its Prevention: An Overview
†
Mendel Friedman
Western Regional Research Center, Agricultural Research Service, U.S. Department of Agriculture,
800 Buchanan Street, Albany, California 94710
Enzymatic and nonenzymatic browning reactions of amino acids and proteins with carbohydrates,
oxidized lipids, and oxidized phenols cause deterioration of food during storage and processing. The
loss in nutritional quality and potentially in safety is attributed to destruction of essential amino
acids, decrease in digestibility, inhibition of proteolytic and glycolytic enzymes, interaction with
metal ions, and formation of antinutritional and toxic compounds. Studies in this area include
influence of damage to essential amino acids on nutrition and food safety, nutritional damage as a
function of processing conditions, and simultaneous formation of deleterious and beneficial
compounds. These compounds include kidney-damaging Maillard reaction products, mutagens,
carcinogens, antimutagens, antioxidants, antibiotics, and antiallergens. This overview covers the
formation, nutrition, and safety of glycated proteins, characterized browning products, and
heterocyclic amines. Possible approaches to inhibiting browning reactions and preventing adverse
effects of browning during food processing and food consumption, including protection against adverse
effects of heterocyclic amines by N-acetylcysteine, caffeine, chlorophyll, conjugated linoleic acid,
lignin, and tea extracts, are also described. This research subject covers a complex relationship of
the chemistry, biology, and pathology of browning products and the impact on human nutrition
and health. Future study should differentiate antinutritional and toxicological relationships, define
individual and combined potencies of browning products, and develop means to prevent the formation
and to minimize the adverse manifestations of the most antinutritional and toxic compounds. Such
studies should lead to better and safer foods and improved human health.
Keywords: Browning prevention; food browning; food safety; glycated proteins; glycosylation;
heterocyclic amines; human health; Maillard products; nutrition
INTRODUCTION
Amino-carbonyl and related interactions of food
constituents encompass those changes commonly termed
browning reactions. Specifically, reactions of amines,
amino acids, peptides, and proteins with reducing
sugars and vitamin C (nonenzymatic browning, often
called Maillard reactions) and quinones (enzymatic
browning) cause deterioration of food during storage and
commercial or domestic processing. The loss of nutri-
tional quality is attributed to the destruction of essential
amino acids and a decrease in digestibility and inhibi-
tion of proteolytic and glycolytic enzymes. The produc-
tion of antinutritional and toxic compounds may further
†
This paper is based on the Wise and Helen Bur-
roughs Lecture, Department of Food Science and Hu-
man Nutrition, Iowa State University, Ames, IA, March
29, 1995.
MARCH 1996
VOLUME 44, NUMBER 3
© Copyright 1996 by the American Chemical Society
This article not subject to U.S. Copyright. Published 1996 by the American Chemical Society

reduce the nutritional value and possibly the safety of
foods (Carpenter, 1991; Finley and Friedman, 1973;
Friedman, 1977a,b, 1982a; Friedman et al., 1989; Hur-
rell, 1990; Mauron, 1990; Percival and Schneeman,
1979; Schumacher and Kroh, 1994; Smith and Fried-
man, 1984). A special class of browning reactions
results in the formation of heterocyclic amines in meat
and fish. These are derived from the interaction
between amino acids, carbohydrates, and creatinine,
which is present only in animal food. Such reactions
occur widely in foods subjected to heat processing and
storage.
To develop rational approaches to minimize adverse
consequences of browning reactions and optimize ben-
eficial ones, studies are needed to relate compositional
changes to nutritional and toxicological consequences.
To catalyze progress, a need exists to define known
chemical, biochemical, nutritional, and toxicological
indices of browning and its prevention. This limited
review addresses the nutritional and safety conse-
quences of the browning reaction from selected studies
in the widely scattered literature. Specifically covered
are (a) formation, nutritional values, and safety of
specific browning products; (b) formation and nutritional
value of glycated proteins; (c) antimutagenic, antioxi-
dant, antibiotic, and antiallergenic browning products;
(d) formation and risk to human health of heterocyclic
amines; and (e) possible approaches to minimizing and
preventing food browning and the resulting adverse
consequences to food quality and safety.
The following outline of categories and examples gives
an indication of the complex dynamics of browning in
food and in vivo.
1. nonenzymatic browning
(a) heat catalyzed protein- and amino acid-carbo-
hydrate reactions, e.g., wheat gluten plus glucose (bread
crust); lactalbumin plus lactose (stored milk powder);
free amino acids plus glucose (fried potatoes and foods
for parenteral nutrition)
(b) in vivo protein-carbohydrate reactions, e.g., he-
moglobin plus glucose in diabetics or eye lens protein
plus glucose (cataracts)
(c) protein-oxidized fatty acid reactions, e.g., casein
plus oxidized linoleic acid
(d) heterocyclic amine formation, e.g., heat-catalyzed
reactions of amino acids, glucose, and creatinine to form
polycyclic amines in cooked meat and fish
2. enzymatic browning
(a) polyphenol-oxidase-catalyzed oxidation of polyphe-
nol compounds in fruits and vegetables to quinones,
which then polymerize to dark melanin pigments of
unknown structure, e.g., formation of brown or black
spots from chlorogenic acid in bananas, pears, lettuce,
and potatoes and the browning of fruit juices
(b) reaction of polyphenol-derived quinones with free
amino acids and proteins to form dark polymers, e.g.,
reactions of casein with oxidized chlorogenic acid in
mixed foods containing both casein and potatoes
Since the discovery of browning reactions over 80
years ago (Maillard, 1912), food scientists have been
studying the mechanisms of browning and its effects on
organoleptic properties, appearance, nutritional quality,
and safety. In contrast, medical scientists have only
been exploring relationships between in vivo browning
and disease and aging for about 15 years. When the
medical researchers determined that the in vivo reac-
tions were quite similar to those occurring in food, they
utilized the available knowledge discovered by food
scientists. From such broad-based cross-fertilization of
ideas, we can expect further progress that will benefit
both food science and medicine.
Since more than one type of browning can occur
simultaneously in food, this integrated overview at-
tempts to develop a better understanding of the complex
overlapping aspects of browning to help stimulate
further progress on the pervasive impact of browning
on human nutrition and health.
Figures 1-7 illustrate the formation and structures
of nonenzymatic browning products. Figure 8 illus-
trates enzymatic browning and its prevention and
Figures 9 and 10 show the formation and structures of
heterocyclic amines.
REACTIONS
Protein Glycation. Nonenzymatic glycosylation, or
glycation, is the covalent attachment of sugars to R-or
²-NH
2
groups of amino acids and proteins to form
glycated proteins (Figures 1-5). This should be dif-
ferentiated from enzyme-catalyzed glycosylation, during
which oligosaccharide chains are attached to asparagine
or serine side chains through glycosidic linkages to form
glycoproteins. The first glycation product or Schiff base
rearranges to a more stable ketoamine or Amadori
product. The Amadori products can then form cross-
links between adjacent proteins or with other amino
groups. The resulting polymeric aggregates are called
advanced glycation end products.
Furth (1988) and Ahmed and Furth (1990, 1991)
summarize several assays for glycated proteins. These
include the thiobarbituric method, periodate oxidation,
borohydride reduction, serum fructosamine assay, and
other separation methods based on the use of boronate
complexes of sugars, fluorescence spectroscopy, and
immunoassays for specific derivatives. Hydrolysis of
glycated proteins results in transformation of about 30%
of the Amadori compound fructosyllysine to furosine. A
minor cyclization product called pyridosine is also
formed. About 50% of the fructoselysine reverts to
lysine. Furosine can be separated by HPLC and de-
tected at 280 nm. An improved ninhydrin assay was
developed by Friedman et al. (1984a) to measure NH
2
groups in proteins and in enzymatic digests (Pearce et
al., 1988).
Browning Reaction Products. The availability of
sophisticated chromatographic and analytical tech-
niques made it possible to isolate, characterize, and
analyze several specific compounds formed in vitro and
in vivo in the early and advanced stages of the Maillard
Figure 1. Molecular events in the initial stages of the
Maillard reaction (Finot et al., 1977; Friedman, 1982).
632 J. Agric. Food Chem., Vol. 44, No. 3, 1996 Reviews