Introduction

1 Categorisation

There are two main types of mechanism by which browning of food occurs, depending on whether the process is mediated by enzymes or not. The division is not precise and, in a specific case, it is usually difficult to rule out one or the other mechanism, unless conditions are such, for instance, during heat processing, that enzymes would have been inactivated.

Under such conditions, only nonenzymic browning can occur. Nonenzymic browning itself is subdivided roughly (again because there is an overlap) into three types of reactions.

The first, called the Maillard reaction, occurs between a carbonyl compound, which here is usually a reducing sugar, and an amine, which here is usually an amino acid, a peptide, or a protein. The second is caramelisation, a reaction where the sugars react on their own, but normally requires more drastic conditions. (Some discuss this under the heading of 'active' aldehydes.) The third is ascorbic acid oxidation. The last, although it need not involve any enzyme at all, is nearest to enzymic browning, since it often does involve ascorbic acid oxidase, which, however, does not affect the phenols, which are the normal substrate in enzymic browning, but may involve other enzymes, e.g., laccase or peroxidase.

Here, much attention will be given to the Maillard reaction, since one can consider caramelisation and ascorbic acid oxidation as special cases of it. Also, the Maillard reaction is the one of physiological significance.

2 Historical: Louis-Camille Maillard

Maillard (1878–1936) qualified in medicine from the University of Nancy in 1903, where he joined the Chemical Division of the School of Medicine. In 1914, he became the head of a biological group in the Chemical Laboratory, University of Paris, and, in 1919, he was appointed as the Professor of Biological and Medical Chemistry at the University of Algiers.

Maillard was interested in Emil Fischer's synthesis of peptides, which he thought, correctly as it turned out, could be achieved under milder conditions by the use of glycerol. This logically led on to the use of sugars as another type of polyhydroxy compound to bring about the formation of peptides and to the discovery that reducing sugars showed extra reactivity. He published seven more papers on the sugar–amino acid reaction (see ref. 2). Further light has recently been shone on Maillard's career.

Robert Ling (1861–1937), Lecturer in Brewing and Malting at the Sir John Cass Institute, London, had earlier noted that kilning led to amino compounds being produced from proteins, and that these reacted at 120–140 °C with sugars, such as glucose and maltose, produced simultaneously, giving what he thought were probably glucosamine-like compounds.

3 The Maillard Reaction

The Maillard reaction is incredibly complex. For instance, a simple example such as the reaction of glucose with ammonia gives evidence, using simple methods, of the formation of more than 15 compounds and the reaction of glucose with glycine gives more than 24. Using HPLC and TLC on solvent-soluble material only [0.1% (w/w) of reactants], about 100 components are detectable as reaction products of xylose and glycine.

In order to understand something so complex, it is necessary to draw up a simplified scheme of the reactions involved. This has been done most successfully by Hodge (see Scheme 1.1). The discussion here is based on this.

Hodge subdivides the Maillard reaction as follows:

I Initial stage: products colourless, without absorption in the ultraviolet (about 280 nm).

Reaction A: Sugar–amine condensation

Reaction B: Amadori rearrangement

II Intermediate stage: products colourless or yellow, with strong absorption in the ultraviolet.

Reaction C: Sugar dehydration

Reaction D: Sugar fragmentation

Reaction E: Amino acid degradation (Strecker degradation)

III Final stage: products highly coloured.

Reaction F: Aldol condensation

Reaction G: Aldehyde–amine condensation and formation of heterocyclic nitrogen compounds

It is worth noting that Mauron calls the three stages Early, Advanced, and Final Maillard reactions, respectively. The way these reactions fit together is outlined in Scheme 1.1. The final products of nonenzymic browning are called melanoidins to distinguish them from the melanins produced by enzymic browning. Theoretically, the distinction is clear; however, in practice, it is very difficult to classify the dark-brown products formed in foods, since they tend to be very complex mixtures and are chemically relatively intractable.

Reaction H has been inserted into Scheme 1.1. It represents the much more recently discovered free-radical breakdown of Maillard intermediates (see Chapter 2).

Oxygen plays an essential role in enzymic browning, but is not essential for nonenzymic browning. It may help in fact, for example, in the formation of reductones, such as dehydroascorbic acid, but it may also hinder the progress of the reaction, for example, in oxidising 2-oxopropanal to 2-oxopropanoic acid.

Table 1.1 lists 12 symptoms of nonenzymic browning and shows how these develop in relation to the three stages of it. Note in particular that, as far as browning itself is concerned and also off-flavour production, there is an induction period.

The Maillard reaction is exceptionally widespread. It occurs virtually ubiquitously in foodstuffs, particularly during processing at elevated temperatures (roasting, baking, extruding) or during storage for prolonged periods. It is important in the manufacture of reaction flavours, coffee, and chocolate. It occurs also in textiles. It plays a complex role in humic substances in the soil and the sea. The changes it brings about in food have both nutritional and toxicological effects. It has important medical implications, since it occurs in the body wherever there is contact between amino compounds and reducing sugars, particularly for prolonged periods (e.g., aging, cataract, diabetes, and dialysis fluid).

The reactions contributing to the Maillard reaction will be discussed further individually in the next chapter.

4 The Literature

The most important set of volumes on the Maillard reaction are the books recording the papers presented at the international Maillard symposia, held about every four years, since the...