Maillard reaction is a non-enzymatic browning reaction, 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. Maillard reaction occurs when
virtually all foods are heated, and also occurs during storage. Most of
the effect of Maillard reaction, including the caramel aromas and golden
brown colors, are desirable. Nevertheless, some of the effect of Maillard
reaction, including foods darkness and off-flavor development, are undesirable.
Maillard Reaction Products
* Aroma and Flavor
Maillard-derived aromas are extremely complex and many components are
formed in trace amounts by side-reactions and obscure pathways. The deoxyosones
are considered to be primary source of aroma volatile compounds. Deoxyosones
undergo cyclization/dehydration to produce flavor important furan derivatives
and different types of furans are formed depending in osone structure.
The aroma profile also varies with the temperature and time of heating.
At any given temperature-time combination, a unique aroma, which is not
likely to be reproduced at any other combination of heating conditions,
The formation of a specific flavor may require the simultaneous generation
of 100 or 200 individual chemicals in the proper concentration and delicate
balance. A large number and wide variety of flavor and aroma compounds
are formed via the Maillard reaction. Moreover, reactant composition, environment
and processing could influence the reaction.
The development of color is an extremely important feature of the Maillard
reaction but relatively little is known about the chemical nature of the
compounds responsible. There are both good and bad sides of the Maillard
reaction for color development. Color development in meats and bread baking
is desirable while the browning of dry milk or dehydrated products is undesirable.
Controlling Factors of the Maillard
* 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, in practice 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.
However, aw values for maximum
browning are affected by other components of the system: humectants, such
as glycerol, can lower the aw
value for maximum browning.
Since the reaction itself has a strong influence on pH it is hard to
evaluate the pH influence. However, reactions take place by all pathways,
with the pH of the system influencing the ratio of products formed and
the rate of color formation can be reduced by decreasing the pH. The most
desirable meaty and pot-roasted aroma was obtained at pH 4.7, but pH had
a less dramatic effect on aroma than did temperature, time or water content.
The left 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.
Why the absorbance of the product produced from
L-lysine is the highest one?
The temperature dependence of chemical reaction is often expressed
as the activation energy (Ea). Activation temperature data for the Maillard
reaction have been reported within a wide range 10-160KJ/mole, depending
on, what effect of the reaction has been measured. The activation energy
is also 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.
There are four types of browning reactions in foods: Maillard, caramelization,
ascorbic acid oxidation and phenolase browning. The former three are non
enzymatic in nature, and the phenolase or enzymatic-catalytic oxidation
browning is commercial significance, particularly in fruit and vegetables
where phenolase are very common. Enzymatic browning is rare at intact tissue,
since phenolic substrates and phenolase are separated. Enzymatic browning
is very common at the cut surface of light-colored fruits and vegetables.
The cut surface may rapidly change to brown color due to the oxidation
of phenols to orthoquines, which in turn polymerize to form brown pigment
or melanins quickly. Enzymes that catalyze the phenols oxidation may be
classified as phenolase, which are oligomers in food and contain one copper
prosthetic group per subunit.
Two types of reaction are involved in the phenolase catalytic reaction,
e.g.. hyroxylation and oxidation (following figure). Hydroxylation of monophenols
is slow and rate-determining step in reaction. Besides tyrosine, major
substrates of phenolases in plant include caffeic acid, chlorogenic acid
and protocatechuic acid which are abundant in plants. But tyrosine and
chlorogenic acid are the two most prevalent substrates for phenolase due
to their relatively high reaction rates. Phenolase is active in pH5-7,
and may be irreversibly inactivated at pH lower than pH 3.
How to control undesirable enzymatic browning
reaction during the fruit and vegetable processing?