Introduction

The taming of fire, permitting the thermal processing of vegetable foodstuffs in particular, extended enormously the number of natural products that could be used as foods by humans and gave a tremendous impulse to the extraordinary diffusion and development of the human population in almost every region of the world (De Bry, 1994). Foodstuffs can be roughly divided in two classes, those that are or are not edible in their raw form. The most important naturally edible foods are meat and milk, which are heated mainly for eliminating dangerous microorganisms, and some fruits, used by plants to attract animals for diffusing their seeds in the environment. In contrast, many plants protect themselves and especially their seeds and tubers from the consumption of insects and superior animals with several antinutritional components that may be deactivated only by thermal treatments. For this reason cereals, grain legumes, and vegetables, such as potatoes, although they are considered the base of a balanced diet in view of the most up-to-date dietary recommendations, are never consumed raw.

With the exception of milk, fruit juices, and some other foods, in which a fresh and natural appearance is required, thermal treatments have also relevant hedonistic consequences, as they confer the desired sensory and texture features to foods. Bread and baked products, or chocolate, coffee, and malt are well known products that are consumed world-wide; here thermal treatments produce the characteristic aroma, taste, and colour (Arnoldi, 2001). Such sensory characteristics have positive psychological effects that facilitate digestion and therefore contribute to an individual's well-being.

During thermal treatment many reactions take place at a molecular level:

  • Denaturation of proteins, with the important consequence of the deactivation of enzymes that destabilise foods or decrease their digestibility, such as lipases, lipoxygenases, hydrolases, and trypsin inhibitors.
  • Lipid autoxidation.
  • Transformations of minor compounds, for example vitamins.
  • Reactions involving free or protein-bound amino acids.

The last reactions belong essentially to four categories:

  • breaking and/or recombination of intramolecular or intermolecular disulfide bridges;
  • reactions of the basic and acidic side chains of amino acids to give isopep-tides (for example Lys + Asp);
  • reactions involving the side chains of amino acids and reducing sugars in a very complex process generally named as 'Maillard reaction' (MR);
  • reactions involving the side chains of amino acids through leaving group elimination to give reactive dehydro intermediates, which can produce cross-linked amino acids.

The Maillard reaction is described in this chapter and some information given on those reactions involving the side chains of amino acids. The Maillard reaction, or non-enzymatic browning, is one of the most important processes involving on one hand amino acids, peptides and proteins, and on the other reducing sugars (Ledl and Schleicher, 1990; Friedman, 1996). The MR is a complex mixture of competitive organic reactions, such as tautomerisations, eliminations, aldol condensations, retroaldol fragmentations, oxidations and reductions. Their interpretation and control is difficult because they occur simultaneously and give rise to many reactive intermediates.

Soon after the discovery of the MR it became clear that it influences the nutritive value of foods. The loss in nutritional quality and, potentially, in safety is attributed to the destruction of essential amino acids, interaction with metal ions, decrease in digestibility, inhibition of enzymes, deactivation of vitamins and formation of anti-nutritional or toxic compounds. However, while the reaction has its negative effects, the positive effects are considerably greater.

11.2 The Maillard reaction

About 90 years ago Maillard (1912) observed a rapid browning and CO2 development while reacting amino acids and sugars: he had discovered a new reaction that became known as the 'Maillard reaction' or non-enzymatic browning. Nineteen years later Amadori (1931) detected the formation of rearranged stable products from aldoses and amino acids that became known as the Amadori rearrangement products (ARPs). The development of industrial food processing, especially after World War II, gave a large impulse to research in this field and after some years Hodge (1953) was able to propose an overall picture of the reactions of non-enzymatic browning in a review that, after almost 50 years, remains one of the most cited in food chemistry.

The mechanism of non-enzymatic browning is generally studied in simple model systems in order to control all the parameters and the results are extrapolated to foods quite efficiently.

The reactants include reducing sugars. Pentoses, such as ribose, arabinose or xylose are very effective in non-enzymatic browning, hexoses, such as glucose or fructose, are less reactive, and reducing disaccharides, such as maltose or lactose, react rather slowly. Sucrose as well as bound sugars (for example glycoproteins, glycolipids, and flavonoids) may give reducing sugars through hydrolysis, induced by heating or very often by yeast fermentation, as in cocoa bean preparation before roasting or dough leavening.

The other reactants are proteins or free amino acids; these may already be present in the raw material or they may be produced by fermentation. In some cases (e.g. cheese) biogenic amines can react as amino compounds. Small amounts of ammonia may be produced from amino acids during the Maillard reaction or large amounts added for the preparation of a particular kind of caramel colouring.

A very simplified general picture of the MR may be found in Fig. 11.1. Following the classical interpretation by Hodge (1953), the initial step is the condensation of the carbonyl group of an aldose with an amino group to give an unstable glycosylamine 1 which undergoes a reversible rearrangement to the ARP (Amadori, 1931), i.e. a 1-amino-1-deoxy-2-ketose 2 (Fig. 11.2). Fructose reacts in a similar way to give the corresponding rearranged product, 2-amino-2-deoxy-

Early stage

Intermediate stage

Advanced stage

Early stage

Intermediate stage

Polymerisations

Fig. 11.1 Simplified scheme of the Maillard reaction.

R-NH2

protein or amino acid

H HO

OH CH2OH

1-amino-1-desoxyaldose 1

1-amino-1-desoxyaldose 1

OH OH CH2OH

h2c-nhr

1-amino-1-desoxyketose 2 Amadori rearranged product

Fig. 11.2 Mechanism of the Amadori rearrangement.

1-amino-1-desoxyketose 2 Amadori rearranged product

Fig. 11.2 Mechanism of the Amadori rearrangement.

2-aldose 3 (Fig. 11.3, Heyns, 1962). The formation of these compounds, that have been separated from model systems as well as from foods, takes place easily even at room temperature and is very well documented also in physiological conditions. Here long-lived body proteins and enzymes can be modified by reducing sugars such as glucose through the formation of ARPs (a process known as gly-cation) with subsequent impairment of many physiological functions. This takes place especially in diabetic patients and during aging (Baynes, 2000; Furth, 1997; James and Crabbe 1998; Singh et al, 2001; Sullivan, 1996). A detailed description of the synthetic procedures, physico-chemical characterisation, properties and reactivity of the ARPs may be found in an excellent review by Yaylayan and Huyggues-Despointes (1994).

Where the water content is low and pH values are in the range 3-6, ARPs are considered the main precursors of reactive intermediates in model systems.

2-amino-2-desoxy-D-glucose 3 Heyns rearranged product

2-amino-2-desoxy-D-glucose 3 Heyns rearranged product

Fig. 11.3 Heyns products.

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