Core course 13: Unit 3 (Part 1): Food Chemistry II

Browning reactions in food

By

Ms. Namratha Pai K

DOS in Food science and Nutrition, University of Mysore, Mysuru

Dear viewers, in today’s session under Food technology we will be learning about
Browning reactions in food.

Browning of foods is defined as the process in which there is a change in the color of the
food due to a chemical process. Browning reaction is broadly classified into two types i.e.,
Enzymatic browning and non-enzymatic browning. Non-enzymatic browning of foods includes
browning reactions without the involvement of enzymes. Although most non-enzymatic
browning in food materials is undesirable when it indicates deterioration in flavor and
appearance of the product involved, the development of brown colors in some products is
entirely acceptable. It is desirable in caramelization of sugars, browning of tea which imparts
flavor, browning of the crust in bread and roasting of meat. To understand more about non-
enzymatic browning, the topic is divided into seven major topics:

1. Types of browning reactions

2. Maillard reaction

3. Caramelization

4. Ascorbic acid browning

5. Lipid and protein browning

6. Factors influencing non-enzymatic browning reactions

7. Implications of non-enzymatic browning reactions

 1. Types of Browning reactions: Browning reactions in food are broadly classified as
enzymatic and non-enzymatic browning reactions. Enzymatic browning occurs usually in fruits
and vegetables. It is defined as a process wherein enzymes such as phenolase oxidize phenols to
orthoquinones which rapidly polymerize to form undesirable brown or black pigments such as
melanins. This process takes place in the presence of oxygen. Enzymatic browning becomes
evident when fruits and vegetables are subjected to processing or to mechanical injury. These
enzymes are present in fruits such as apples, banana, pears, gooseberry and kiwi and; in
vegetables such as lettuce, brinjal, mushrooms, asparagus, broccoli, carrot and potato. Non-
enzymatic reactions on the other hand include chemical reactions involving reducing sugars
which individually undergo caramelization or sugars along with proteins which undergo
Maillard reaction.

2. Maillard reaction: The Maillard reaction is the action of amino acids and proteins on sugars.
The reaction was first reported by Maillard in 1912. The sugar moiety is a reducing sugar with a
free carbonyl group. This reaction occurs in many foods causing browning. The end-product is
the melanoidins, which are the brown pigments. The brown color results from the following
three stages of development.

I. Initial stage with no formation of color

A. Sugar-amine condensation
B. Amadori rearrangement
II. Intermediate stage with formation of yellow pigments
C. Sugar dehydration
D. Sugar fragmentation
E. Amino acid degradation
III. Final stage with the formation of highly colored dark pigments.
F. Aldol condensation
G. Aldehyde-amine polymerization and formation of heterocyclic nitrogenous
compounds.

Maillard reaction is called the carbonyl-amine reaction, between free amino groups from
amino acids, peptides, or proteins and the carbonyl group of a reducing sugar such as ribose,
glucose, fructose and galactose. Among the most reactive carbonyl compounds are α,β-
unsaturated aldehydes such as furaldehyde, and α-dicarbonyl compounds such as diacetal and
pyruvaldehyde. The reaction which is reversible starts between an aldose or ketose sugar and a
primary or secondary amine, the product of which is a glycosylamine. The yield of
glycosylamine is higher when the amount of water present is low. The formation of
glycosylamine is important in the browning of concentrated and dried foods. The initial product
of the reaction between glucose and ammonia is glucosylamine. This rearranges to form 1-
amino-1-deoxy-D-fructose in the presence of an acid catalyst which is called Amadori
rearrangement. The dehydration of sugar leads to the formation of Amadori Rearrangement
Products or ARPs such as furfural, reductones and fission products such as acetol,
pyruvaldehyde, diacetyl groups. The reaction takes place in two ways, in the presence of neutral
or acid solutions furfurals are formed and in the dry state or in non-aqueous solvents when
amines are present, reductones are formed. The Heyns rearrangement shows the reaction starting
with fructose instead of glucose. Both of these rearrangements bring about the same
transformation where α-D-fructopyranosylamine is converted to 2-amino-2-deoxy-α-D-
glucopyranose. The order of formation of compounds in Heyns rearrangement is ketoseamine
followed by diketoseamine and diamino sugar in the presence of aldose.
The intermediate stage leads to breakdown of Amadori compounds and the formation of
degradation products, reactive intermediates such as 3-deoxyglucosone or formation of osones
by Strecker’s reaction and volatile compounds that lead to the formation of flavor. The 3-
deoxyglucosone participates in cross-linking of proteins at much faster rates than glucose itself,
and further degradation leads to two known advanced products: 5-hydroxymethyl- 2-furaldehyde
(HMF) and pyraline.
The main pathway for the formation of brown color in foods appears to be degradation
and condensation by way of the 1,2-enol forms of the aldose or ketose amines. The aldol
condensation mechanism for the α,β-dicarbonyl compounds formed seems to be involved. The
formation of these brown pigments via the carbonyl-amine reactions has similarities to the
formation of caramels. Another step in the formation of brown pigments is the Strecker
degradation of the amino acid moiety. This degradation of the alpha amino acids results in
aldehydes containing one less carbon atom than the amino acid. The loss of the carbon atom is
accounted for by the release of carbon dioxide. In addition to CO2, it produces carbonyl
compounds and amines. It is also necessary that the amino group of the amino acid be at the
alpha position to the carbonyl group and the CO2 comes from the carboxyl group of the amino
acid moiety and not from the sugar. These dicarbonyl compounds are osones and are active
agents for Strecker degradation. Pyrazine compounds with different amounts of substitution are
formed in carbonyl-amine reactions and can cause Strecker degradation of the amino acid to
form 2,5-Dimethylpyrazine from a glucose and glycine reaction.
The final stage is characterized by the production of nitrogen-containing brown polymers
and copolymers known as melanoidins. They have been described as low-molecular weight
colored substances that are able to cross-link proteins via ε-amino groups of lysine or arginine to
produce high molecular weight colored melanoidins. Also, it has been postulated that they are
polymers consisting of repeating units of furans and/or pyrroles, formed during the advanced
stages of the Maillard reaction and linked by polycondensation reactions. Chemical structure of
melanoidins can be mainly formed by a carbohydrate skeleton with few unsaturated rings and
small nitrogen components; in other cases, they can have a protein structure linked to small
chromophores.

3. Caramelization: When sugars are treated under anhydrous conditions with heat, or at high
concentration with dilute acid, caramelization occurs, with the formation of anhydro sugars.
Glucose forms glucosan or l,2-anhydro-α-D-glucose and levoglucosan or 1,6-anhydro-β-D-
glucose with a specific rotation of +69° and -67° respectively. With similar treatment, fructose
gives rise to levulosan or 2,3-anhydro-β-D-fructofuranose. Simultaneous hydrolysis and
dehydration take place when sucrose is heated at about 200°C, and followed by a rapid
dimerization. These compounds are characterized by isosacchrosan, which is a sucrose molecule
one molecule of water lesser. It is not sweet, but mildly bitter. When dilute solutions of reducing
sugars are used, the beginning stages of caramelization involve enolization, isomerization,
dehydration, and fragmentation. Following this, polymerization reactions take place, which in
the end form pigments similar to those formed in more concentrated solutions, or at higher
temperatures. Caramels for commercial use are made from glucose syrups, but usually
caramelization is the result of reactions that take place when sucrose is heated. The reaction takes
place at 200°C. There are three stages during this process, during which water is lost and
isosacchrosan formed first followed by formation of other anhydrides. The first stage starts with
melting of the sucrose, followed by foaming which continues for 35 min. During this period one
molecule of water is lost from a molecule of sucrose. The foaming then stops. Shortly after this,
a second stage of foaming starts which lasts 55 min. During this stage about 9% of the water is
lost, and the compound formed is caramelan, a pigment with the molecular formula of C24H36018.
2 C12H22011 - 4 H20 = C24H36018
Two molecules of sucrose lose four molecules of water to form caramelan.
Caramelan melts at 138°C, is soluble in water and ethanol, and is bitter in taste. The pigment
caramelen with the molecular formula C36H50025 is formed during the third stage of foaming
which starts after about 55 min.
3 C12H22011 - 8H20 = C36H50025
Three molecules of sucrose lose eight molecules of water to form caramelen.
Caramelen melts at 154°C and is soluble in water. When the heating is continued, the result is
the formation of humin, which is an infusible, dark mass with a high molecular weight, and is
called caramelin. The molecular formula of caramelin is C125H188080. The reaction rate of
caramelization is ten times greater at pH 8 than at lower pH.

4. Ascorbic Acid Browning

Browning of ascorbic acid can be briefly defined as the thermal decomposition of ascorbic acid
under both aerobic and anaerobic conditions, by oxidative or non-oxidative mechanisms, either
in the presence or absence of amino compounds. Non-enzymatic browning is one of the main
reasons for the loss of commercial value in citrus products. These damages, degradation of
ascorbic acid followed by browning is also a concern in non-citrus foods such as asparagus,
broccoli, cauliflower, peas, potatoes, spinach, apples, green beans, apricots, melons, strawberries,
corn, and dehydrated fruits. In citrus juices, non-enzymatic browning is from reactions of sugars,
amino acids, and ascorbic acid. In freshly produced commercial juice packed in Tetrapak, is
mainly due to non-enzymatic browning involving carbonyl compounds formed from L-ascorbic
acid degradation. The presence of amino acids and possibly other amino compounds enhance
browning.
When oxygen is present in the system, ascorbic acid is degraded primarily to dehydro ascorbic
acid (DHAA). DHAA is not stable and spontaneously converts to 2,3-diketo-l-gulonic acid
Under anaerobic conditions, ascorbic acid undergoes the generation of diketogulonic acid via its
keto tautomer, followed by β elimination at C-4 from this compound and decarboxylation to give
rise to 3-deoxypentosone, which is further degraded to furfural. Under aerobic conditions,
xylosone is produced by simple decarboxylation of diketogulonic acid and that is later converted
to reductones. In the presence of amino acids, ascorbic acid, DHAA, and their oxidation products
furfural, reductones, and 3-deoxypentosone may contribute to the browning of foods by means of
a Maillard-type reaction.
5. Lipid and protein browning: Lipid oxidation occurs in oils and lard, and also in foods with
low amounts of lipids, such as products of vegetable origin. The reaction is both desirable and
undesirable. It is desirable in the production of cheese or fried food aromas and undesirable when
odor, appearance and flavor are affected. Moreover, toxic compound formation and loss of
nutritional quality can also be observed. Although the lipids can be oxidized by both enzymatic
and non-enzymatic reactions, the latter is the main involved reaction. This reaction arises from
free radical or reactive oxygen species generated during food processing and storage,
hydroperoxides being the initial products. These compounds are quite unstable further leading to
many reactions and pathways. The enzymatic oxidation of lipids occurs sequentially. Lipolytic
enzymes can act on lipids to produce polyunsaturated fatty acids that are then oxidized by either
lipoxygenase or cyclooxygenase to form hydroperoxides or endoperoxides, respectively. Later,
these compounds suffer a series of reactions to produce, among other compounds, longchain
fatty acids responsible for important characteristics and functions. Via polymerization, brown-
colored oxypolymers are produced subsequently from the lipid oxidation derivatives. Radical
transfer occurs early in lipid oxidation, and this process underlies the antioxidant effect for lipids.
In addition, protein radicals can also transfer radicals to other proteins, lipids, carbohydrates,
vitamins, and other molecules, especially in the presence of metal ions such as iron and copper
The interaction between oxidized fatty acids and amino groups has been related to the browning
detected during the progressive accumulation of lipofuscin an age-related yellow-brown
pigments in lysosomes of men and animals. According to the mechanism proposed for the
protein browning caused by acetaldehyde, the carbonyl compounds derived from unsaturated
lipids readily react with protein-free amino groups to produce, by repeated aldol condensations,
the formation of brown pigments. More recently, another mechanism based on the
polymerization of the intermediate products 2-(1-hydroxyalkyl) pyrroles has been proposed. 2-
(1-Hydroxyalkyl) pyrroles (I) have been found to be originated from the reaction of 4,5-epoxy-2-
alkenals formed during lipid peroxidation with the amino group of amino acids and/or proteins,
and their formation is always accompanied by the production of N-substituted pyrroles (II).
Compounds derived from reaction of 4,5-epoxy-2-alkenals and phenylalanine have been found to
be flavor compounds analogous to those of Maillard reaction. Therefore, flavors traditionally
connected to Maillard reaction may also be produced as a result of lipid oxidation. However, the
N-substituted 2-(1-hydroxyalkyl) pyrroles are unstable and polymerize rapidly and
spontaneously to produce brown macromolecules with fluorescent melanoidin-like
characteristics.
6. Factors influencing non-enzymatic browning reactions:

A number of factors can affect the formation of these pigments. Among these are pH,
temperature, moisture content, time, concentration, and nature of the reactants. Also, one of these
factors may affect another.
Temperature: The rate of browning increases with rising temperature. In model systems the
development increases 2 to 3 times for each 10°C rise in temperature. In natural systems,
particularly those high in sugar content, the increase may be faster. Two methods have been used
to measure these changes: (1) measure of color development and (2) measure the evolution of
CO2.
pH: Although browning reactions usually slow down as the pH decreases until the optimum
stability pH for reducing sugars is passed, this is not so important for food products. Maillard
reaction and ascorbic acid degradation is faster at an alkaline pH or a nearly dry state.
Moisture: The moisture content seems to have an important effect on the rate of browning. It is
quite likely that for moisture contents above 30% a decrease in reaction is caused by dilution.
Acids: The development of brown color in dried fruits is largely caused by the reaction between
amino acids and glucose, a reaction which is speeded by the presence of organic acids.
Fermentation: The formation of ketoseamines in dried whole egg or egg white is avoided by
fermentation of the glucose before drying.
Oxygen: Removal of oxygen decreases the rate of browning reactions especially in ascorbic acid
oxidation.
Technologies: Modified-atmosphere packages, microwave heating, ultrasound-assisted thermal
processing, pulsed electric field processing and carbon dioxide-assisted high-pressure processing
are some examples of technological processes that allow ascorbic acid retention and
consequently prevent undesirable browning.

7. Implications of non-enzymatic browning reactions

Advantages: Processing such as baking, frying, and roasting are based on the Maillard reaction
for flavor, aroma, and color formation. Maillard browning may be desirable during manufacture
of meat, coffee, tea, chocolate, nuts, potato chips, crackers, and beer and in toasting and baking
bread. In foods, antioxidant properties of Maillard reaction products have been found in honey
and in tomato purees. The antimicrobial activity of coffee melanoidins against different
pathogenic bacteria has been reported.

Disadvantages: In processes such as pasteurization, sterilization, drying, and storage, the

Maillard reaction often causes detrimental nutritional and organoleptic changes such as lysine
damage. Other types of undesirable effects produced in processed foods by Maillard reaction
may include the formation of mutagenic and carcinogenic compounds. Frying or grilling of
meat and fish may generate low (ppb) levels of mutagenic/carcinogenic heterocyclic amines via
Maillard reaction. The formation of these compounds depends on cooking temperature and
time, cooking technique and equipment, heat mass transport, and/or chemical parameters. The
formation of carcinogen acrylamide is observed in a range of cooked foods. Moderate levels of
acrylamide (5–50 μg/kg) is found in heated protein-rich foods, and higher levels (150–4000
μg/kg) in carbohydrate rich food, such as potato, beet root, certain heated commercial potato
products, and crisp bread. It is absent in raw or boiled foods, but it is present at significant
levels in fried, grilled, baked, and toasted foods. On the basis of the large number of existing
studies, the International Agency for Research on Cancer has classified acrylamide as “probably
carcinogenic” to humans. The Maillard reaction is one of the main reactions causing
deterioration of proteins during processing and storage of foods. This reaction can promote
nutritional changes such as loss of nutritional quality, attributed to the destruction of essential
amino acids or reduction of protein digestibility and amino acid availability.

Conclusion:

To sum up, the Maillard reaction results in the formation of brown nitrogenous polymers
and copolymers. The reaction involves the sugar-amine condensation, the Amadori
rearrangement resulting in the keto form, and the Strecker degradation which results in action on
the alpha amino acids with the loss of one molecule of CO2 and the formation of an aldehyde
followed by formation of colored pigments. When sugars are treated under anhydrous conditions
with heat, or at high concentration with dilute acid, caramelization occurs, with the formation of
anhydro sugars. Ascorbic acid browning occurs due to thermal decomposition of ascorbic acid
under both aerobic and anaerobic conditions, by oxidative or non-oxidative mechanisms, either
in the presence or absence of amino compounds. These reactions can be controlled with factors
such as pH, temperature, moisture content, time, concentration, and nature of the reactants.
Browning reactions in food can therefore be advantageous and disadvantageous depending on its
use in food system.