Eng. Verzija

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Chemical and Biochemical Reactions

Each reaction class can involve different reactants or substrates depending on the

specific food and the particular conditions for handling, processing, or storage.

They are treated as reaction classes because the general nature of the substratesor reactants is similar for all foods. Thus, nonenzymic browning involves reaction

of carbonyl compounds, which can arise from existing reducing sugars or from

diverse reactions, such as oxidation of ascorbic acid, hydrolysis of starch, or

oxidation of lipids.

Oxidation may involve lipids, proteins, vitamins, or pigments, and more

specifically, oxidation of lipids may involve triacylglycerols in one food or

phospholipids in another. Discussion of these reactions in detail will occur in

subsequent chapters of this book.

4.1.4.6 Nonenzymic Browning [10,12,30,59]

Under some conditions, reducing sugars produce brown colors that are desirable

and important in some foods. Other brown colors obtained upon heating or

during long-term storage of foods containing reducing sugars are undesirable.

Common browning of foods on heating or on storage is usually due to a chemical

reaction between reducing sugars, mainly D-glucose, and a free amino acid or afree amino group of an amino acid that is part of a protein chain. This reaction is

called the Maillard reaction. It is also called nonenzymic browning to differentiate

it from the often rapid, enzyme-catalyzed browning commonly observed in freshly

cut fruits and vegetables, such as apples and potatoes.

When aldoses or ketoses are heated in solution with amines, a variety of

reactions ensue, producing numerous compounds, some of which are flavors,

aromas, and dark-colored polymeric materials, but both reactants, disappear only

slowly. The flavors, aromas, and colors may be either desirable or undersirable.

They may be produced by frying, roasting, baking, or storage.

The reducing sugar reacts reversibly with the amine to produce a glycosylamine,

as illustrates with D-glucose (Fig. 22). This undergoes a reaction called the

Amadori rearrangement to give, in the case of D-glucose, a derivative of 1-amino-



1-deoxy-D- fructose. Reaction continues, espectially at pH 5 or lower, to give an

intermediate that dehydrates. Eventually a furan derivative is formed; that from a

hexose is 5-hydroxymethyl-2-furaldehyde (HMF) (Fig. 23). Under less acidic

conditions (higher than pH5), the reactive cyclic compounds (HMF and others)

polymerize quickly to a dark-colored, insoluble material containing nitrogen.

Maillard browning products, including soluble and insoluble polymers, are found

where reducing sugars and amino acids, proteins, and/or other nitrogen-

containing compounds are heated together, such as in soy sauce and bread

crusts. Maillard reaction products are important contributors to the flavor of milk

chocolate. The Maillard reaction is also important in the production of caramels,

toffees, and fudges, during which reducing sugars also react with milk



proteins. D-Glucose undergoes the browning reaction faster than does D-fructose.

Application of heat is generally required for nonenzymic browning. While Maillard

reactions are useful, they also have a negative side. Reaction of reducing sugars

with amino acids destroys the amino acid. This is of particular importance with L-

lysine, an essential amino acid whose e-amino group can react when the amino

acid is part of a protein molecule. Also, a relationship has been found between

formation of mutagenic compounds and cooking of protein-rich foods. Mutagenic

heterocyclic amines have been isolated from broiled and fried meat and fish, and

from beef extracts.

Heating of carbohydrates, in particular sucrose (see Sec. 4.2.3) and reducing

sugars, without nitrogen-containing compounds effects a complex group of



reactions termed caramelization. Reaction is facilitated by small amounts of acids

and certain salts.

Mostly thermolysis causes dehydration of the sugar molecule with introduction of

double bonds or formation of anhydro rings.

Introduction of double bonds leads to unsaturated rings such as furans.

Conjugated double bonds absorb light and produce color. Often unsaturated rings

will condense to polymers yielding useful colors. Catalysts increase the reaction

rate and are often used to direct the reaction to specific types of caramel colors,

solubilities, and acidities.

Brown caramel color made by heating a sucrose (see Sec. 4.2.3) solution with

ammonium bisulfite is used in cola soft drinks, other acidic beverages, baked

goods, syrups, candies, pet



foods, and dry seasonings. Its solutions are acidic (pH 24.5) and contain colloidal

particles with negative charges. The acidic salt catalyzes cleavage of the glycosidic

bond of sucrose; the ammonium ion participates in the Amadori rearrangement.

Another caramel color, also made by heating sugar with ammonium salts, is

reddish brown, imparts pH values of 4.24.8 to water, contains colloidal particleswith positive charges, and is used in baked goods, syrups, and puddings. Caramel

color made by heating sugar without an ammonium salt is also reddish brown,

but contains colloidal particles with slightly negative charges and has a solution

pH of 34. It is used in beer and other alcoholic beverages. The nonenzymic

browning caramel pigments are large polymeric molecules with complex, variable,



and unknown structures. It is these polymers that form the colloidal particles.

Their rate of formation increases with increasing temperature and pH.

Certain pyrolytic reactions of sugars (Fig. 24) produce unsaturated ring systems

that have unique flavors and fragrances in addition to the coloring materials.Maltol (3-hydroxy-2-methylpyran-4-one) and isomaltol (3-hydroxy-2-acetylfuran)

contribute to the flavor of bread. 2H-4-Hydroxy-5-methylfuran-3-one can be used

to enhance various flavors and sweeteners.

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