Thermal processing and nutritional quality A. Arnoldi, University of Milan 11.1 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 dif fusion 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 he 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 charac- teristics 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:
11 Thermal processing and nutritional quality A. Arnoldi, University of Milan 11.1 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:
266 The nutrition handbook for food processors 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 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 con- densations, retroaldol fragmentations, oxidations and reductions. Their interpre- tation 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 eater 11.2 The maillard reaction About 90 years ago Maillard(1912)observed a rapid browning and CO2 devel opment while reacting amino acids and sugars: he had discovered a new reaction that became known as the"Maillard reaction or non-enzymatic browning. Nine teen 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 Il, gave a large impulse to research in this field and
• 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 isopeptides (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 266 The nutrition handbook for food processors
Thermal processing and nutritional quality 267 after some years Hodge(1953)was able to propose an overall picture of the reac tions 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 extrapo- lated to foods quite efficiently xylose are very effective in non-enzymatic brof.s, such as ribose, arabinose or The reactants include reducing sugars pentose 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 6 A very simplified general picture of the MR may be found in Fig. 11.1.Fol- lowing the classical interpretation by Hodge (1953), the initial step is the con- densation of the carbonyl group of an aldose with an amino group to give an unstable glycosylamine I which undergoes a reversible rearrangement to the ARP (Amadori, 1931), i.e. a l-amino-l-deoxy-2-ketose 2(Fig. 11.2). Fructose reacts in a similar way to give the corresponding rearranged product, 2-amino-2-deoxy First interactions between Early stage sugars and amino groups. rearrangements Intermediate stage Fissions, cyclisations, 9 dehydrations, condensations oligomerizations Advanced stage Polymerisations Fig. ll1 Simplified scheme of the Maillard reaction
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-deoxyThermal processing and nutritional quality 267 Early stage First interactions between sugars and amino groups, rearrangements Fissions, cyclisations, dehydrations, condensations, oligomerisations Polymerisations Intermediate stage Advanced stage Fig. 11.1 Simplified scheme of the Maillard reaction
268 The nutrition handbook for food processors HOH H R-NH2 HO HO-H H CH2OH 1-amino-1-desoxyaldose 1 HC-NHR H CH2OH H2C-NHR OF HO-H 1-amino-1-des ketose 2 CH2OH 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 condi tions. 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 descrip- tion 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-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 glycation) 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. 268 The nutrition handbook for food processors HO O H H HO H HO H OH H OH HC H OH HO H H OH H OH CH2OH NR HC OH HO H H OH H OH CH2OH NHR H2C O HO H H OH H OH CH2OH NHR O H HO H HO H H H OH NHR OH O OH H H HO H OH H OH H NHR R NH2 1-amino-1-desoxyaldose 1 1-amino-1-desoxyketose 2 Amadori rearranged product protein or amino acid Fig. 11.2 Mechanism of the Amadori rearrangement
Thermal proc and nutritional quality 269 HOH 2-amino-2-desoxy-D-glucose 3 Heyns rearranged product Fig 11.3 Heyns products. C H2N COOH HC R Strecker hyde R R OH A NH3 OH Fig. 11.4 Mechanism of the Strecker degradation of amino acids Below pH 3 and above pH 8 or at temperatures above 130C(caramelisation) sugars will degrade in the absence of amines (Ledl and Schleicher, 1990). Ring opening followed by 1, 2 or 2, 3-enolisation are crucial steps in ARP transfor- mation and are followed by dehydration and fragmentation with the formation of many very reactive dicarbonyl fragments. This complex of reactions is con- sidered the intermediate stage of the mr Maillard observed also the production of CO2, which is explained by a process named the Strecker degradation(Fig. 11. 4). The mechanism involves the reac tion of an amino acid with an a-dicarbonyl compound to produce an azovinylo- gous B-ketoacid 4, that undergoes decarboxylation. In this way amino acids are converted to aldehydes containing one less carbon atom per molecule. These are very reactive and often have very peculiar sensory properties. The aldehydes that derive from cysteine and methionine degrade further to give hydrogen sulfide, 2 methylthio-propanal, and methanethiol: that means that the Strecker degradation is responsible for the incorporation of sulfur in some Maillard reaction products MRPs). Another important consequence of the Strecker reaction is the incorpo- ration of nitrogen in very reactive fragments deriving from sugars, such as 5
Below pH 3 and above pH 8 or at temperatures above 130°C (caramelisation), sugars will degrade in the absence of amines (Ledl and Schleicher, 1990). Ring opening followed by 1, 2 or 2, 3-enolisation are crucial steps in ARP transformation and are followed by dehydration and fragmentation with the formation of many very reactive dicarbonyl fragments. This complex of reactions is considered the intermediate stage of the MR. Maillard observed also the production of CO2, which is explained by a process named the Strecker degradation (Fig. 11.4). The mechanism involves the reaction of an amino acid with an a-dicarbonyl compound to produce an azovinylogous b-ketoacid 4, that undergoes decarboxylation. In this way amino acids are converted to aldehydes containing one less carbon atom per molecule. These are very reactive and often have very peculiar sensory properties. The aldehydes that derive from cysteine and methionine degrade further to give hydrogen sulfide, 2- methylthio-propanal, and methanethiol: that means that the Strecker degradation is responsible for the incorporation of sulfur in some Maillard reaction products (MRPs). Another important consequence of the Strecker reaction is the incorporation of nitrogen in very reactive fragments deriving from sugars, such as 5. Thermal processing and nutritional quality 269 HO O H H HO H H H NHR OH OH 2-amino-2-desoxy-D-glucose 3 Heyns rearranged product Fig. 11.3 Heyns products. C C O O H2NHC COOH R C C O N HC COOH R C C OH N CH R C C OH NH2 O CH R NH3 C HC OH O + + 4 Strecker aldehyde 5 Fig. 11.4 Mechanism of the Strecker degradation of amino acids