Bibliography

1. O. Fennema, "Educational Programs in Food Science: A Continuing Struggle for Legitimacy, Respect, and Recognition," Food Technol. 43, 170-182 (1989).

2. E. Garfield, SCI Journal Citation Reports, Institute for Scientific Information, Philadelphia, Penn., 1996, p. 83.

3. M. P. Vitek, et al., "Advanced Glycation End Products Contribute to Amyloidosis in Alzheimer's Disease," Proc. Natl. Acad. Sci. U.S.A. 91, 4766-4770 (1994).

4. B. Halliwell, "Antioxidants in Human Health and Disease," Annu. Rev. Nutr. 16, 33-50 (1996).

Peter Sporns University of Alberta Edmonton, Alberta Canada

FOOD CHEMISTRY: MECHANISM AND THEORY

Food chemistry deals with the chemical identity of food components and chemical reactions governing the changes and performance of individual or interacting food components during handling, processing, and storage. Although the individual constituents may often be readily identified, the interactions of food components are extremely complex.

Food chemistry is a branch of chemistry with its foundation built on chemical principles and reaction mechanisms, and a comprehension of the subject often requires thorough understanding and application of knowledge from various chemistry and chemistry-related disciplines. In this regard, food chemistry is quite similar to biochemistry, except that the former relates chemistry to food systems and the latter to biological systems. For example, a biochemist may be interested in elucidating the biosyn-thetic pathways of wheat storage proteins, but for a food chemist, it is more relevant to relate the chemical structures of these proteins to functional properties, such as changes and effects in dough quality. Another example is found in the Maillard reaction. An organic chemist may investigate the chemical reactions and their mechanism in the formation of melanoidin compounds. A biochemist is likely more interested in the reactions because they are related to the aging process of certain vital proteins, such as lens crystalline, collagen, and elastin. A food chemist is also interested in the Maillard reaction, but more in linking the reaction mechanism to physical and chemical changes in food systems such as flavor development, browning, and nutritional loss.

In this article reaction mechanisms of sufficient importance in food systems and their current developments are presented. A number of good background references have been published (1-5).

CARBOHYDRATES The Maillard Reaction

In 1912, the French chemist Louis-Camille Maillard first observed that yellow-brown pigments formed in the reaction among sugars and amino acids, peptides, and proteins in a heated solution. Food chemists have recognized the practical relevance of this reaction to many chemical and physical changes during processing and storage of food. The first review (in English) on the Maillard chemistry in food systems was published in 1951 (6). Since then, numerous reviews on this subject have appeared (eg, 7-9,15). The biological importance of this reaction has been recognized only in the last 20 years. It is now well established that the reaction is linked to glycosylated hemoglobin (HbAlc) in diabetes, hardened lens crystallins in cataract disease, and a number of other aging proteins (10,11).

The Maillard reaction comprises a series of reactions: (1) formation of glycosylamine via a Schiff-base formation between a reducing sugar and the amino group of an amino acid, (2) Amadori rearrangement in which glycosylamine is converted to ketosamine, (3) enolization (C1-C2 or C2-

C3), followed by cyclodehydration. In general, under acidic conditions, the nitrogen is protonated, and 1,2-enolization is assisted by the positively charged nitrogen acting as an electron sink. Alkali and strong basic amine favor 2,3-enolization (Fig. 1).

The actual reactions are far more complicated than those outlined here, and there are many variations in the pathway. The initial step in the Amadori rearrangement is suggested as N-protonation. However, addition of the proton to the ring oxygen has also been proposed. Most discussions on the Maillard reaction concern the monosub-stituted amines, but the ketosamine formed in the reaction can also react with another molecule of an aldose resulting in disubstitution. Another variation is the Heynes rearrangement in the conversion of D-fructosylamine (a ketosamine) to 2-amino-2-deoxy-D-glucose (an aldosylamine).

A pathway that involves sugar fragmentation and free-radical formation prior to the Amadori rearrangement has been suggested (12,13) (Fig. 2). The radical has been structurally identified to be N,N'-disubstituted pyrazine cation radical and it is formed by the dimerization of a two-carbon enaminol product from the cleavage of glycosylamine.

Direct dehydration of the Amadori compound has been proposed as an alternative to enolization (14,15) (Fig. 3). In this mechanism, the Amadori compound undergoes a trans-elimination at C2-C3, followed by a second dehydration at C3-C4, to form a hydroxypyran and finally a pyr-ylium ion. The highly electrophilic pyrylium ion can undergo various nucleophilic additions, ring opening, and recyclization.

Maillard reaction products have very diverse structures and are involved in various secondary reactions. A compilation of 450 volatile Maillard reaction products and related compounds (16) and reviews on this subject are available (7-9,15). Dicarbonyl compounds, such as 3-deoxyglycosulose generated by the 1,2-enolization pathway and the glycosulos-3-ene formed via the 2,3-enoliza-tion are the key intermediates for subsequent degradative reactions relating to color and flavor production. One of the well-known pathways is the Strecker degradation in which the carbonyl forms a Schiffbase with the a-amino group of an amino acid. Enolization, decarboxylation, and hydrolysis yield an aldehyde corresponding to the original amino acid with one fewer carbon. The aldehydes derived from this degradative pathway constitute many important flavor compounds in food systems. Compounds generated by degradation of the dicarbonyl compounds include pyrroles, pyrazines, oxazoles and derivatives, pyrrolines, pyrrolidines, pyrones, thiazole, and thiazoline.

Sulfite inhibition of nonenzymatic browning also involves the dicarbonyl intermediates reacting with sulfur oxoanion to form stable sulfonate product (17). The sulfur oxoanion may replace the C4 hydroxy group of the deoxy-glycosulose or undergoes 1,4-addition to the double bond of the a,^-unsaturated glycosulos-3-ene.

Formation of melanoidins is caused by polymerization of unsaturated carbonyl compounds. Condensation between 3-deoxyglycosulose and its enamine, which is a re-ductone, has been suggested to be the structural unit of the polymer (18,19) (Fig. 4), although polymers of repeating units of furan or pyrrole have also been proposed.

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