Non enzymic browning

Mr. OMAR ALAJIL (‫أ‬.‫العجيل‬ ‫عمر‬)
M.Sc Food Technology

TABLE OF CONTENT
 INTRODUCTION
 Changes during non-enzymic browning
 Types of Non-enzymatic Browning
 Maillard Reaction mechanisms and products
 How to measure browning
 Kinetic of non enzymatic browning
 Controlling Factors of the Maillard Reaction
Products
 Prevention of non enzymatic browning
 CONCLUSION

Introduction
 Foods may develop a variety of
brown colors, from yellow-brown to
red-brown to black-brown, during
handling, processing, and storage.

 These colors are desirable in certain foods
(e.g., coffee, beer, bread, maple syrup).
 In other foods, such as most dehydrated
fruits and vegetables, dried eggs, and
canned or dried milk, browning is
detrimental.
 Even when desirable, browning should not
be excessive, as in potato chips, French
fries, and apple juice.
 Numerous reactions lead to browning in
foods. Some of these may also generate
flavors and/or alter the nutritional
properties of foods.

enzymatic and nonenzymatic
browning
 Enzymatic browning
occurs when fresh food or beverage
products are exposed to air and there are
active enzymes present within the food or
beverage product. When an apple slice or
banana turns brown, it is usually due to
polyphenol oxidase enzyme activity.
 Non-enzymatic browning
can produce brown, fluorescent, highly
cross-linked pigments, such as melanoidin
chromophores.

Types of Nonenzymatic
Browning
A number of chemical processes not
involving enzymes may result in food
browning.
1. Maillard reaction
2. Caramelization
3. ascorbic acid browning
4. metalpolyphenol browning.

Changes during non-enzymic
browning
 this changes some times are
desirable and sometimes undesirable
 Produces flavor
 Produces color
 Produces antioxidant products
 Produces toxic products
 loss and Destroys nutrients (lysine)
 formation of undesirable products such
as HMF (5-hydroxymethylfurfural)

1. Caramelization
 This reaction leads to brown products
when sugars are heated dry or in
solution.
 The large quantities of industrial caramel
color that are added to beverages (cola
drinks), baked goods, and confections
are made by heating high-conversion
corn syrups in the presence of catalysts
(acids, alkalis, salts).

 The chemical transformations involved in
caramelization are complex and poorly
understood.
 They include dehydration, fragmentation,
and polymerization.
 On the heating of pentoses, furfural is
formed which polymerizes to brown
products.
 Heating hexoses results in
hydroxymethylfur-fural, which polymerizes
similarly.

2. Ascorbic Acid Browning
 When ascorbic acid is heated in the
presence of acids, furfural is formed.
 The latter, either by itself or after
reacting with amino compounds,
polymerizes to brown products.
 Citrus juices, especially their
concentrates, develop browning, which
has been attributed to ascorbic acid
degradation.

3. Metal-Polyphenol
Browning
 Polyphenolic compounds form
complexes with certain metals. The
polyphenols of fruits and vegetables most
commonly chelate iron. The resulting iron
complexes are bluish black pigments.
 Cutting apples with a non-stainless-steel
knife results in darkening of both the blade
and the surface of the apple.
 This darkening is independent of the
enzymatic browning that might develop as
a result of cutting.

 The iron of the tissue must first be
oxidized to the ferric state for the
blackish complex to appear.
 Canned or pickled cauliflower may turn
dark due to the interaction of
polyphenols in the tissue with iron from
external sources

4. The Maillard Browning
 Maillard reaction is caused by the condensation of
an amino group and a reducing compound,
resulting complex changes in biological and food
system.
 This reaction was described for the first time by
Louis Maillard in 1912.

John deMan States that
Maillard reaction is:
 “The sequence of events that begins
with reaction of the amino group of
amino acids with a glycosidic hydroxyl
group of sugars; the sequence
terminates with the formation of brown
nitrogenous polymers or melanoidins”

Maillard Reaction mechanisms
and products
 This reaction is actually a series of
reactions occurring from the first
encounter of a carbonyl compound with
an amine compound to the formation of
brown pigments.
 It is also known as the carbonyl-amine
reaction, and its brown products are
often called melanoidins, indicating their
visual similarity to the melanins of
enzymatic browning.

 The most common carbonyl compounds
of foods involved in the Maillard reaction
are reducing sugars, and the most
common amine compounds are amino
acids.

 Among sugars, pentoses are more
reactive than hexoses, and hexoses are
more reactive than reducing
disaccharides.
 When free amino acids react with
sugars, lysine appears to be the most
active among them.

 Maillard reaction occurs when
virtually all foods are heated, and
also occurs during storage.
 Maillard reaction form products that
are desirable or undesirable


desirable
- caramel aromas
- and golden brown colors,
undesirable
- foods darkness
- and off-flavor development
- loss of nutrition component (Lysine)

 the Maillard reaction can seriously lower
the nutritive value of the food.
 Toasting, for example, may reduce to
one-half the protein efficiency ratio of
bread.

How to measure browning
 As the thermal treatment resulted in the
increase in colored substances content,
which is measured as an increase in
absorbance at 420 and 560 nm by
reactance colorimetry A420.

Kinetic of non enzymatic
browning
 First-order and zero-order kinetic
models have been used to evaluate the
development of non-enzymaticbrowning.
These kinetic models are expressed by
the equations

 It is not always possible to apply kinetics
as simple as first-order or zero-order to
describe the colour changes produced in
fruit purees,
 since these changes can be due not
only to the Maillard reaction but also to
the thermal destruction of pigments
present in the samples.

 First-order kinetics has been suggested for
the destruction of natural fruit pigments.
 Based on data for the deterioration
produced by thermal treatments, a two
stage mechanism is proposed.
 The first stage, color formation, is zero-
order and the second stage, pigment
destruction, is first-order.

 This can be written as
- sugars. Amino acids k0 colored
polymers
- pigments k1 uncolored products
 This two-stage kinetic mechanism can
be expressed mathematically.

 If C is the variable used to measure the
color changes caused by non-enzymatic
browning reactions, then Where the left-
hand side represents the fractional
conversion at time t.
 Eqn (5) is the final expression for the kinetic
model of the two-stage mechanism
proposed for the evolution of color changes
caused by non-enzymatic browning.
 The value of K expresses the maximum
color difference (DE?) obtained for long
treatment times.
 It was observed that K increased with
treatment temperature.

 Consequently, an increase in temperature
implies a greater increase in colour.
 According to the proposed kinetic model, K
represents the ratio of kinetic constants k0
(colour appearance) and k1 (pigment
destruction).
 It was observed that the kinetic constant
values for the colour formation stage
increased with treatment temperature, while
the kinetic constant values for the pigment
destruction stage showed no definite
tendency.

 However, for higher temperatures the
values of this constant were similar.
 The kinetic constants for the colour
formation stage increased in value with
increasing temperature.

 An analogous behavior was observed to
describe the increase in color.
 The value of the Kinetic constant
increased with treatment temperature,
which indicates that the yellowish hues
decreased more rapidly with increasing
treatment temperature.

 The zero order model (eqn [1])
was the best model to describe
this variation.

Controlling Factors of the
Maillard Reaction Products
 factors that influence the browning
reaction are temperature, pH, moisture
level (aw), oxygen, metals, phosphates,
sulfur dioxide, and other inhibitors
 Raising the temperature and/or the pH
accelerates the Maillard reaction.
Intermediate water activity appears to
maximize this reaction.

1- Water activity (aw)
 Water is produced during Maillard
reaction, thus the reaction occurs less
readily in foods with a high aw values
while, at low aw, the mobility of
reactants is limited, despite their
presence at increased concentrations.

 As the figure shown the Maillard
reaction occurs most rapidly at
intermediate aw values (0.5-0.8),
 and aw is of most significance to the
reaction in dried and intermediate-
moisture foods (IMFs), which have aw
values in this range.
 humectants, such as glycerol, can lower
the aw value for maximum browning.

2- pH
 Since the reaction itself has a strong
influence on pH it is hard to evaluate the
pH influence.
 Ph can have a little affect on final
products and aroma
 pH had a less dramatic effect on aroma
than did temperature, time or water
content.

 The figure is "effect of pH on Maillard
browning of L-lysine, L-alanine, and L-
arginine heated with D-glucose at 121C
for 10 min".
 Under weakly alkaline conditions, and
with a strongly basic secondary amino
components, the 2,3-enolization
pathway is favored.

3- Temperature
 The temperature dependence of chemical
reaction is often expressed as the
activation energy (Ea).
 The activation energy is highly dependent
on pH.
 The temperature dependence of the
Maillard reaction is also influenced by the
participating reactants.
 So it is difficult to isolate the effect of
temperature as a single variable.

Prevention of non enzymatic
browning
 As already indicated, nonenzymatic
browning is desirable in certain
instances and undesirable in others.
 The availability of reactants and the type
of conditions (temperature, pH,
moisture) will determine the extent of
browning.

 A chemical preservative often used to
inhibit nonen-zymatic (and enzymatic)
browning is sulfur dioxide.
 An obvious way to prevent metal-
polyphenol browning is to eliminate
contact between susceptible tissues and
reactive metals and use inoffensive
equipment (stainless steel, glass-lined
tanks, etc.)

Conclusion
 Non enzymatic browning is desirable in some products like
bread crust and meat and undesirable in other products
like apple puri, there is many types of browning like maillard
browning, Caramelization, ascorbic acid browning and
metalpolyphenol browning.
 The maillard reaction is actually known as the carbonyl-
amine reaction, and its brown products are often called
melanoidins. The most common carbonyl compounds of
foods involved in the Maillard reaction are reducing sugars,
and the most common amine compounds are amino acids.
 factors that influence the browning reaction are
temperature, pH, moisture level (aw), oxygen, metals,
phosphates, sulfur dioxide, and other inhibitors.

Non enzymic browning

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