Chemistry Reference
In-Depth Information
Academy in 1914. The process has ever since been referred to as the Maillard reac-
tion and its products collectively named Maillard products. During the process, so-
called reducing sugars—fructose, glucose, glyceraldehyde, lactose, arabinose, and
maltose—will bind to amino acids and nucleic acids, both DNA and RNA, pep-
tides, and proteins, and produce compounds usually called Amadori products, which
with time undergo complex changes: cyclization, dehydration, oxidation, condensa-
tion, cross-linking, and polymerization to form irreversible chemical products. In
particular, reactive carbonyls, such as glyoxal and methylglyoxal, have been found
to rapidly modify reactive side chains of proteins. Important amino acids, such as
lysine (essential amino acid) and histidine (essential for children), are often involved.
During the heating process, thousands of good-tasting and good-smelling volatile
compounds are released in addition to significant amounts of pigments (melanoids)
that often make the food or parts of the food brown or black, which is why some-
times the process is referred to as “browning.” Common browning products are bread
crusts and the roasted surface of fried meat and fish. All sorts of broths, irrespective
of vegetable or animal origin, Chinese soy, Balsamico products, smoked foods are
rich in brown/black Maillard products. But not all Maillard products are dark in
color. White Maillard products also exist; common examples are diary products,
especially cheese and powdered milk. It was suggested early on that the Maillard
process might be negative to health, at least when its products are consumed in larger
amounts, as these products will accumulate in the body, sometimes for the rest of
life, but also because the process might reduce the supply of important and essential
amino acids to the body.
7.2 hEATINg, rEDuCTIoN oF ANTIoxIDANTs, AND
ACCuMulATIoN oF MAIllArD ProDuCTs
Most of the well-known plant antioxidants are inactivated at temperatures
between 30°C and 100°C. Antioxidants in common food oils such as olive and
rapeseed oil will start disappearing at temperatures around 30°C. Heating to higher
temperatures, as almost always occurs with microwaving, eliminates almost all anti-
oxidants. The production of Maillard products occurs much in parallel to reduction
of the content of antioxidants in foods, and accelerates dramatically, almost expo-
nentially, as the temperatures are elevated above 100°C.
Maillard products based on association of carbonyl groups in sugars and proteins
have in more recent years been collectively called advanced glycation end products
(AGEs). Similar products are often formed between reactive fatty acids and proteins,
referred to as advanced lipoxidation end products (ALEs). A long list of such syn-
thetic products are identified, and two to three previously unknown such compounds
are added to the list each year. Commonly studied AGEs/ALEs are pentosidine,
N
ε
-carboxymethyl)lysin (CML) and
N
ε
-(carboxyethyl)lysin (CEL).
It is important to observe that the production of both AGEs and ALEs is not at
all dependent on enzymes. The intensity in production increases, not only with the
increase in temperature, but also with the length of storage at elevated temperatures.
