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
270 The nutrition handbook for food processors CHO C=N-R CH-NH-R CHOH RNH CHOH reaction CHOH 5 CHOH CH=O CHOH -H2O CHOH CHOH R R Fig. 11.5 Possible pathway for the formation of glycolaldehyde alkylamines proposed by nd Hayashi (1986 R-NH2 Ho CH2OH CH,OH A CH2OH HoH F(Ct +Cs) F(c2+c4) route O(NR) NHR CH2OH CHOH CH2OH cyclisation 3. 4-dideoxy Fig. 11.6 Transformation of hexoses and pentoses to Cs- and Ca-pyrroles and-furans. (Reproduced with permission from Tressel et al, 1998a) However, in the last two decades other mechanisms have been proposed. For example, starting from the experimental observation of free radical formation at the start of the MR, Hayashi and Namiki(1981: 1986) proposed a reducing sugar degradation pathway that produces glycolaldehyde alky limine without passin through the formation of APs(Fig. 11.5 Very recently, on the basis of extensive experiments with- H-labelled ugars,a detailed reaction scheme was proposed by Tressel et al (1995 and 1998a): the formation of various C6", Cs", and Ca-pyrroles and furans from both intact and fragmented hexoses and amines could be unambiguously attributed to distinct reaction pathways via the intermediates A-C without involving the
However, in the last two decades other mechanisms have been proposed. For example, starting from the experimental observation of free radical formation at the start of the MR, Hayashi and Namiki (1981; 1986) proposed a reducing sugar degradation pathway that produces glycolaldehyde alkylimines without passing through the formation of ARPs (Fig. 11.5). Very recently, on the basis of extensive experiments with 13C- and 2 H-labelled sugars, a detailed reaction scheme was proposed by Tressel et al (1995 and 1998a): the formation of various C6-, C5-, and C4-pyrroles and furans from both intact and fragmented hexoses and amines could be unambiguously attributed to distinct reaction pathways via the intermediates A–C without involving the 270 The nutrition handbook for food processors CHO CHOH CHOH CHOH R' C=N-R CHOH CHOH CHOH R' CH-NH-R CHOH CH=O CHOH R' + RNH2 - H2O reverse aldol reaction 5 Fig. 11.5 Possible pathway for the formation of glycolaldehyde alkylimines proposed by Namiki and Hayashi (1986). CHO OH HO OH OH CH2OH OH HO OH OH CH2OH NR O H OH OH CH2OH NR O H OH CH2OH NR O H O OH CH2OH NHR O H OH OH CH2OH O H OH CH2OH O H OH OH CH2OH O H CH2OH NR OH H O NR O H O CH2OH NHR OH H O O NHR N R OH O OH O N R O O O N R polymers polymers 2-deoxypentoses tetroses pentoses C6-pyrroles C6-furans cyclisation cyclisation beta-dicarbonyl route 3,4-dideoxy aldoketose route 3,4-dideoxy aldoketose route 3,4-dideoxy aldoketose route beta-dicarbonyl route F(c1 + c5) F(c2 + c4) F(c5 + c1) F(c5 + c1) R-NH2 3-deoxy aldoketose route (NR) A B C H H H H (NR) Fig. 11.6 Transformation of hexoses and pentoses to C5- and C4-pyrroles and -furans. (Reproduced with permission from Tressel et al, 1998a)
Thermal processing and nutritional quality 271 Table 11.1 Composition of the primary fragmentation pools Type of pool Constituents Amino acid Amines fragmentation pool Carboxylic acids Kanes and aromatics Aldehydes Amino acid specific side chain fragments: H,S(Cys), CH,SH even Sugar fragmentation CI fragments: formaldehyde, formic acid pool (S) C2 fragments: glyoxal, glycoladehyde, acetic acid 3 fragments: glyceraldehyde, methylglyoxal, hydroxyacetone dihydroxyacetone, etc. C4 fragments: tetroses, 2, 3-butanedione, 1-hydroxy-2-butanone 2-hydroxybutanal, etc. C5 fras eoxy derivati C6 fragments: pyranone, furans, glucosone, deoxyglucosone Amadori and Heyns C3-ARP/HRP derivatives: glyceraldehyde-ARP, amino fragmentation pool acid-propanone, amino acid-propanal, etc IDI C4-ARP/HRP derivatives: amino acid-tetradiuloses amino acid-butanones C5-ARP/HRP derivatives: amino acid-pentadiuloses C6-ARP/HRP derivatives: amino acid-hexadiuloses, pyrylium betaines Lipid fragmentation Propanal, al hexanal, octanal. nonanal 2-Oxoaldeh C6-9) C2 fragments: glyoxal 3 fragments: CHOCH_CHO, methylglyoxal Amadori rearrangement(Fig. 11.6). These pyrroles and furans polymerise very easily to highly coloured compounds that may be involved in the formation of melanoidins By means of experiments showing that sugars and most amino acids also undergo independent degradation(Yaylayan and Keyhani, 1996), a new concep tual approach to the Mr has been proposed recently by Yaylayan(1997). He sug- gested that in order to understand the mr better. it is more useful to define a sugar fragmentation pool [S), an amino acid fragmentation pool (A], and an interaction fragmentation pool (D, deriving from the Amadori and Heyns com- pounds( Table 11. 1). Together they constitute a primary fragmentation pool of building blocks that react to give a secondary pool of interaction intermediates and eventually a very complex final pool of stable end-products G owever. most foods contain also lipids that can degrade by autoxidation rosch, 1987) giving reactive intermediates, mainly saturated or unsaturated aldehydes or ketones and also glyoxal and methylglyoxal (in common with the
Amadori rearrangement (Fig. 11.6). These pyrroles and furans polymerise very easily to highly coloured compounds that may be involved in the formation of melanoidins. By means of experiments showing that sugars and most amino acids also undergo independent degradation (Yaylayan and Keyhani, 1996), a new conceptual approach to the MR has been proposed recently by Yaylayan (1997). He suggested that in order to understand the MR better, it is more useful to define a sugar fragmentation pool {S}, an amino acid fragmentation pool {A}, and an interaction fragmentation pool {D}, deriving from the Amadori and Heyns compounds (Table 11.1). Together they constitute a primary fragmentation pool of building blocks that react to give a secondary pool of interaction intermediates and eventually a very complex final pool of stable end-products. However, most foods contain also lipids that can degrade by autoxidation (Grosch, 1987) giving reactive intermediates, mainly saturated or unsaturated aldehydes or ketones and also glyoxal and methylglyoxal (in common with the Thermal processing and nutritional quality 271 Table 11.1 Composition of the primary fragmentation pools Type of pool Constituents Amino acid Amines fragmentation pool Carboxylic acids {A} Alkanes and aromatics Aldehydes Amino acid specific side chain fragments: H2S (Cys), CH3SH (Met), styrene (Phe) Sugar fragmentation C1 fragments: formaldehyde, formic acid pool {S} C2 fragments: glyoxal, glycoladehyde, acetic acid C3 fragments: glyceraldehyde, methylglyoxal, hydroxyacetone, dihydroxyacetone, etc. C4 fragments: tetroses, 2, 3-butanedione, 1-hydroxy-2-butanone, 2-hydroxybutanal, etc. C5 fragments: pentoses, pentuloses, deoxy derivatives, furanones, furans C6 fragments: pyranones, furans, glucosones, deoxyglucosones Amadori and Heyns C3-ARP/HRP derivatives: glyceraldehyde-ARP, amino fragmentation pool acid-propanone, amino acid-propanal, etc. {D} C4-ARP/HRP derivatives: amino acid-tetradiuloses, amino acid-butanones C5-ARP/HRP derivatives: amino acid-pentadiuloses C6-ARP/HRP derivatives: amino acid-hexadiuloses, pyrylium betaines Lipid fragmentation Propanal, pentanal, hexanal, octanal, nonanal pool {L} 2-Oxoaldehydes (C6–9) C2 fragments: glyoxal C3 fragments: CHOCH2CHO, methylglyoxal Formic acid, acids
272 The nutrition handbook for food processors Interaction pool A S heterocycles D}{} Pri fragmentation pool dimers Fig. 11.7 Conceptual representation of the Maillard reaction: genealogy of primary ragmentation pools, interaction pools (containing self-interaction pools as well as mix interaction pools) and end-products Maillard reaction) and malondialdehyde(Table 11.1). These belong to a fourth pool, the lipid fragmentation pool (L)(' Agostina et al, 1998)and in this way the scheme proposed by Yaylayan(1997)was revised to include it(Fig. 11.7) Clear interconnections between the MR and lipid autoxidation have been exten sively studied in the case of food aromas, where many end-products deriving from lipids and amino acids or sugars are very well documented(whitfield, 1992), but certainly they may be relevant also for other sensory aspects, such as colour or taste, or for nutrition, although these research areas have been almost completely neglected until the present. Depending on food composition and heating intensity applied, thousands of fferent end products may be formed in the advanced stage of the MR: they are classified here according to their functions in foods(Fig. 11.8). Very volatile com- pounds, such as pyrazines, pyridines, furans, thiophenes, thiazoles, thiazolines, and dithiazine are of interest when considering aroma: some low molecular weight compounds relate to taste(Frank et al, 2001: Ottiger et al, 2001), others behave as antioxidants and a few are mutagenic. Polymers(melanoidins) that in sugar/amino acid model systems and some foods such as coffee, roasted malt, or chocolate are the major MRPs, and determine the colour of the food. This review will discuss only the mechanism of formation of MRPs that have some nutritional significance or may be used as molecular markers for quantify- ing the MR in foods. A very detailed description of the pathways leading to most Maillard reaction icts may be found in an excellent review by ledl and Schleicher(1990)
Maillard reaction) and malondialdehyde (Table 11.1). These belong to a fourth pool, the lipid fragmentation pool {L} (D’Agostina et al, 1998) and in this way the scheme proposed by Yaylayan (1997) was revised to include it (Fig. 11.7). Clear interconnections between the MR and lipid autoxidation have been extensively studied in the case of food aromas, where many end-products deriving from lipids and amino acids or sugars are very well documented (Whitfield, 1992), but certainly they may be relevant also for other sensory aspects, such as colour or taste, or for nutrition, although these research areas have been almost completely neglected until the present. Depending on food composition and heating intensity applied, thousands of different end products may be formed in the advanced stage of the MR: they are classified here according to their functions in foods (Fig. 11.8). Very volatile compounds, such as pyrazines, pyridines, furans, thiophenes, thiazoles, thiazolines, and dithiazines are of interest, when considering aroma; some low molecular weight compounds relate to taste (Frank et al, 2001; Ottiger et al, 2001), others behave as antioxidants and a few are mutagenic. Polymers (melanoidins) that in sugar/amino acid model systems and some foods such as coffee, roasted malt, or chocolate are the major MRPs, and determine the colour of the food. This review will discuss only the mechanism of formation of MRPs that have some nutritional significance or may be used as molecular markers for quantifying the MR in foods. A very detailed description of the pathways leading to most Maillard reaction products may be found in an excellent review by Ledl and Schleicher (1990). 272 The nutrition handbook for food processors {A} {S} {D} {L} polymers Interaction pool Primary fragmentation pool oligomers dimers heterocycles Fig. 11.7 Conceptual representation of the Maillard reaction: generalogy of primary fragmentation pools, interaction pools (containing self-interaction pools as well as mixinteraction pools) and end-products
Thermal processing and nutritional quality 273 Volatile compounds Antioxidants Maillard reaction Toxic compounds Tasty compounds Brown compound Fig. 11.8 Functional classification of Maillard reaction products 11.3 Nutritional consequences and molecular markers of the mailllard reaction in food As the Mr involves some of the most important food nutrients, its nutritional consequences must be carefully considered. Researcher attention has previously been focused mainly on milk and milk products, where thermal treatments are necessary for obtaining microbial stabilisation and the preservation of high nutritional quality. Vegetable products, which become edible only after thermal treatments, have been relatively neglected so far The degradation of sugars per se is never considered a problem because it is only rarely they are lacking in diet. However, free or protein-bound essential amino acids may be damaged irreversibly; the amount of free amino acids in food is always very low and they are important as constituents of proteins. This means that the most relevant nutritional effect of the Maillard reaction is non-enzymatic glycosylation of proteins which involves mostly lysine, whose bioavailability may be drastically impaired. This should be distinguished very clearly from enzy matic glycosylation, a normal step in the biosynthesis of glycoproteins, in which oligosaccharides are bound to serine or asparagine through a glycosidic bond. The first glycation products are then converted to the Amadori product, fruc tosyllysine, that eventually can cross-link with other amino groups intramolecu larly or intermolecularly. The resulting polymeric aggregates are called advanced glycation end products(AGEs) Lysine availability is an important nutritional parameter especially in foods for particular classes of consumers, such as infant formulas(Ferrer et al, 2000) Statistically significant losses of available lysine(about 20%)with respect to raw milk have been reported as a consequence of the thermal treatment applied in the preparation of these foods Because the reactions of lysine are so relevant in nutrition, over a period of time different MRPs have been proposed as markers of protein glycosylation
11.3 Nutritional consequences and molecular markers of the Maillard reaction in food As the MR involves some of the most important food nutrients, its nutritional consequences must be carefully considered. Researcher attention has previously been focused mainly on milk and milk products, where thermal treatments are necessary for obtaining microbial stabilisation and the preservation of high nutritional quality. Vegetable products, which become edible only after thermal treatments, have been relatively neglected so far. The degradation of sugars per se is never considered a problem because it is only rarely they are lacking in diet. However, free or protein-bound essential amino acids may be damaged irreversibly; the amount of free amino acids in food is always very low and they are important as constituents of proteins. This means that the most relevant nutritional effect of the Maillard reaction is non-enzymatic glycosylation of proteins which involves mostly lysine, whose bioavailability may be drastically impaired. This should be distinguished very clearly from enzymatic glycosylation, a normal step in the biosynthesis of glycoproteins, in which oligosaccharides are bound to serine or asparagine through a glycosidic bond. The first glycation products are then converted to the Amadori product, fructosyllysine, that eventually can cross-link with other amino groups intramolecularly or intermolecularly. The resulting polymeric aggregates are called advanced glycation end products (AGEs). Lysine availability is an important nutritional parameter especially in foods for particular classes of consumers, such as infant formulas (Ferrer et al, 2000). Statistically significant losses of available lysine (about 20%) with respect to raw milk have been reported as a consequence of the thermal treatment applied in the preparation of these foods. Because the reactions of lysine are so relevant in nutrition, over a period of time different MRPs have been proposed as markers of protein glycosylation. Thermal processing and nutritional quality 273 Volatile compounds Maillard reaction Antioxidants Metal chelating agents Toxic compounds Tasty compounds Brown compounds Fig. 11.8 Functional classification of Maillard reaction products
274 The nutrition handbook for food processors H R'OC-C-NHR (CH2)4 lysine (CH2)4 N-(CH2)4 HO→H (CHa) CH2OH H3C、N Fig. 11.9 Compounds derived from fructosyllysine decomposition. Fructosyllysine is unstable in the acid conditions of protein hydrolysis, produc- ing about 30% furosine, pyridosine(a minor cyclisation product) and about 50% lysine(Fig. 11.9). Furosine was first detected in foods by Erbersdobler and Zucker (1966)and can be easily analysed by HPLC: thus furosine quantification is con- sidered a good estimate of nutritionally unavailable lysine Milk proteins, owing to their nutritional relevance, have been considered with particular attention Owing to the presence of lactose, the Amadori compound in this case is lactulo- syllysine and furosine is again a useful marker of lysine unavailability. For this reason several authors have used the furosine method for determining the progress of the Maillard reaction in different foods( Chiang, 1983: Hartkopf and Erbersdobler, 1993 and 1994, Henle et al, 1995; Resmini et al, 1990). However, today very powerful analytical techniques are disclosing new possibilities permitting, for example, the direct determination of fructosyllysine (Vinale et al, 1999) by the use of a stable isotope dilution assay performed in liquid chromatography- mass spectrometry(LC-MS). This method overcomes the problems of hydrolytic instability of the analyte and the incompleteness of the enzymatic digestion technique Other possible markers of lysine transformation are N-E-carboxymethyllysine (CML) and 5-hydroxymethylfurfural (HMF)(Fig. 11.10). CML was detected for the first time in milk by Buser and Erbersdobler (1986)and an oxidative mech- anism was proposed for its formation(Ahmed et al, 1986).The formation of HMF in foods has been explained in two ways: via the Amadori products through eno- lisation (in the presence of amino groups)and via lactose isomerisation and degra- dation, known as the lobry de Bruyn-Alberda van Ekenstein transformation (Ames, 1992). Because of this, it has recently been proposed to measure sepa- rately the HMF formed only by the acidic degradation of Amadori products and
Fructosyllysine is unstable in the acid conditions of protein hydrolysis, producing about 30% furosine, pyridosine (a minor cyclisation product) and about 50% lysine (Fig. 11.9). Furosine was first detected in foods by Erbersdobler and Zucker (1966) and can be easily analysed by HPLC: thus furosine quantification is considered a good estimate of nutritionally unavailable lysine. Milk proteins, owing to their nutritional relevance, have been considered with particular attention. Owing to the presence of lactose, the Amadori compound in this case is lactulosyllysine and furosine is again a useful marker of lysine unavailability. For this reason several authors have used the furosine method for determining the progress of the Maillard reaction in different foods (Chiang, 1983; Hartkopf and Erbersdobler, 1993 and 1994, Henle et al, 1995; Resmini et al, 1990). However, today very powerful analytical techniques are disclosing new possibilities, permitting, for example, the direct determination of fructosyllysine (Vinale et al, 1999) by the use of a stable isotope dilution assay performed in liquid chromatography – mass spectrometry (LC–MS). This method overcomes the problems of hydrolytic instability of the analyte and the incompleteness of the enzymatic digestion technique. Other possible markers of lysine transformation are N-e-carboxymethyllysine (CML) and 5-hydroxymethylfurfural (HMF) (Fig. 11.10). CML was detected for the first time in milk by Büser and Erbersdobler (1986) and an oxidative mechanism was proposed for its formation (Ahmed et al, 1986). The formation of HMF in foods has been explained in two ways: via the Amadori products through enolisation (in the presence of amino groups) and via lactose isomerisation and degradation, known as the Lobry de Bruyn-Alberda van Ekenstein transformation (Ames, 1992). Because of this, it has recently been proposed to measure separately the HMF formed only by the acidic degradation of Amadori products and 274 The nutrition handbook for food processors CH2 O HO H H OH H OH CH2OH NH (CH2)4 H R'OC C NHR NH2 (CH2)4 H HOOC C NH2 O H2 C N H (CH2)4 O COOH NH2 H3C N (CH2)4 O NH2 COOH lysine furosine pyridosine Fig. 11.9 Compounds derived from fructosyllysine decomposition