Thermally Generated Flavors - American Chemical Society

Chapter 11

Dicarbonyl Sugar Derivatives and Their Role in the Maillard Reaction

Downloaded by UNIV OF NEW SOUTH WALES on April 15, 2016 | http://pubs.acs.org Publication Date: November 30, 1993 | doi: 10.1021/bk-1994-0543.ch011

Milton S. Feather Department of Biochemistry, University of Missouri, Columbia, MO 65211

The Maillard reaction involves the interaction of reducing sugars with protein amino groups to give 1-amino-1deoxy-2-ketose derivatives (Amadori compounds), followed by their degradation. During this degradation, deoxy-dicarbonyl sugar derivatives are formed, which play an important role in subsequent stages of the reaction. The role of "3-deoxyglucosone", as well as other dicarbonyl intermediates produced from Amadori compounds are discussed herein. Ascorbic acid also undergoes Maillard type reactions with amino acids. This is probably due to the fact that it undergoes degradation to give carbonyl and dicarbonyl intermediates. Among the degradation products detected are L-threose as well as two 5-carbon dicarbonyl compounds. Data are presented with respect to the identification of these compounds as well as how they arise from ascorbate or its oxidation products. Finally, the results of some studies of aminoguanidine (guanylhydrazine) as an inhibitor of the Maillard reaction are presented.

The Maillard reaction represents a complex series of degradation reactions that is initiated by the interaction of a carbonyl compound, usually a reducing sugar, with an amino group, usually a protein or an amino acid. The reaction is a degradative reaction with respect to both the amine and the reducing sugar, both of which eventually disappear from the reaction solution or the food preparation, if the reaction is allowed to proceed sufficiently long. During the Maillard reaction, polymeric pigments are formed (Maillard polymers), which contain carbon atoms derived from both the sugar and the amine; an increase in the ultraviolet absorbance is observed (a UV maximum gradually develops at about 300 nM); food flavor and aroma

0097-6156/94/0543-0127$06.00/0 © 1994 American Chemical Society Parliment et al.; Thermally Generated Flavors ACS Symposium Series; American Chemical Society: Washington, DC, 1993.


128

THERMALLY GENERATED FLAVORS

constituents are produced; and an increase in the amount of carbonyl groups is observed. The latter is due, in large part, to partially dehydratedsugar-derived dicarbonyl compounds that are produced as intermediates in the reaction. In addition, proteins undergo functional modifications, since some of the free amino functions on them have reacted with reducing sugars, and extensive protein crosslinking is also observed. The modification of protein significantly alters the nutritional value of food proteins, especially the availability of lysine, which has a free epsilon amino group, that can easily react with reducing sugars. The reaction is of considerable and obvious interest to food chemists and technologists, but has also recently been found to be an in vivo reaction, which may have deep seated health related consequences. A number of international symposia have been held on the subject and the proceedings have been published (1-4). In addition, the excellent reviews by Ledl (5) and by Danehy (6) are noteworthy. The latter is of special interest with respect to the generation of food flavors and aromas in foods and the influence of amino acids on this reaction.

Initial Stages of the Maillard Reaction From the standpoint of carbohydrate chemistry, the initial stages of the reaction are reasonably well understood, when aldose sugars, which are the most common reactants in food systems, serve as participants. As shown in Figure 1, using glucose as an example, the sugar initially reacts with an amine to give an intermediate glycosyl amine, which rapidly rearranges to a 1-amino-l-deoxy-2-ketose. The conversion of a sugar into an Amadori compound constitutes the Amadori rearrangement, and the product of the reaction is commonly referred to as an Amadori compound. Amadori compounds are conformationally unstable, and in solution, exist in both the pyranose and furanose forms as well as measurable amounts of open chain form. This has been shown to be the case for a number of Amadori compounds that were prepared from a variety of amino acids and was demonstrated by 13C NMR spectroscopy (7). Compared to the parent aldose sugar, Amadori compounds are unstable (8), and they undergo a variety of degradation and dehydration reactions, which depend on the solution pH, temperature, water content of the reaction mixture, and the duration of heating time. The difference between the rate of decomposition of an Amadori compound and the parent aldose sugar is large. Amadori compounds, for example, when heated at pH 2.0, will completely decompose in a matter of hours, while a sugar, such as D-glucose is stable at these conditions for weeks, or even months. Thus, the formation of an Amadori compound provides a pathway for a relatively stable sugar, such as D-glucose, to undergo a variety of degradation reactions at relatively mild conditions, as would be encountered during the cooking or processing of foods. Amadori compounds may be expected to form any time reducing sugars (or compounds that contain them) are found in the presence of protein or amino acids. They can be expected to be formed in almost all types of foods, particularly when they are heated, a process that increases the rate of reaction between sugars and amino groups.

Parliment et al.; Thermally Generated Flavors ACS Symposium Series; American Chemical Society: Washington, DC, 1993.


2

CH OH

I

CIO

Chain

Form)

C omp o u n d

Form)

Ring

Form)

Compound

(Pyranose

Ama d o r i

C ompound

(Furanose

Ama d o r i

2

HOH C

Glucosylamine

HOXOH

Figure 1. The formation of Amadori compounds involves the initial interaction of a reducing sugar (D-Glucose) with an amine to give a glycosylamine intermediate, which quickly rearranges into the product. The Amadori compounds are depicted in the pyranose, furanose, and open chain forms, all of which exist in solution.

(Open

Ama d o r I

I

H-C-OH

I

H-C-OH

I

HO-C-H

H.MR


1

so

I

1

I I a.

a.

a.

a.

I

!

130



The Degradation of Amadori Compounds Our knowledge of all the degradative reactions that an Amadori compound can undergo is incomplete at present. Molecular oxygen probably plays a part in some of these degradation reactions, as do solution pH, temperature, and the basicity of the amino group of the Amadori compound that is involved in the reaction. The present thinking suggests that Amadori compounds initially undergo enolization and dehydration to give partially dehydrated dicarbonyl sugar derivatives. It is these intermediates that serve as the precursors for production of a variety of food flavor and aroma constituents, production of the UV absorbing materials, and possibly as reagents that modify proteins in later stages of the reaction. Interestingly, it is these compounds that we know the least about in terms of their overall participation in the reaction. Formation of 3-Deoxy-1,2-Dicarbonyl Intermediates. 3-Deoxy-Den/fhro-hexos-2-ulose (hereafter referred to as 3-deoxyglucosone) represents an important dicarbonyl intermediate and is also the most well studied of the dicarbonyl derivatives. It was originally isolated from a glycine-derived Amadori compound by Anet (9), and its preparation, also described by Kato (10\, results from the reaction of D-glucose with N-butylamine. More recently, we have reported a practical synthesis of it (11) starting from the crystalline bisbenzoylhydrazone, which was originally prepared and characterized by El Khadem (12) and his co-workers. 3-Deoxyglucosone can now be prepared in reasonably pure, gram sized quantities with relative ease. It is a highly reactive intermediate and represents one of the first formed dicarbonyl compounds from an Amadori compound. It is converted into 5-(hydroxymethyl)-2-furaldehyde (HMF), in acidic solution (13). HMF is a strong UV absorber (A=18,000, k=280 nM), which probably accounts for much of the UV absorption normally associated with Maillard reactions. 3-Deoxyglucosone is produced via an initial 1,2 enolization of an Amadori compound, followed by elimination of the amine substituent, as shown in Figure 2. It represents an initial dehydration product derived from a hexosederived Amadori compound. Experiments using 3-deoxyglucosone as a starting compound indicate that its reactivity is consistent with it being an important Maillard reaction intermediate. When tested with amino acids, it gives brown colors (Maillard Polymers) at a much faster rate than D-glucose itself (14), and has been shown to function in Strecker degradation reactions with phenylalanine, converting it to phenylacetaldehyde in high yield (15). In in vitro experiments, 3deoxyglucosone has been shown to be involved in protein crosslinking (16,17), and has been detected in serum and urine of human subjects (18). It is noteworthy that, in these latter experiments, both 3-deoxyglucosone and 3-deoxy-D-fructose were found to be present in both urine and serum, with the latter in the highest yield. It is probable that the fructose derivative is produced from the deoxyosone via enzymatic reduction by aldose reductase.



11. FEATHER

Dicarbonyl Sugar Derivatives and the Maillard Reaction

131

Formation of 1-Deoxy-2,3-Dicarbonyl Intermediates. Figure 3 shows a mechanism wherein an Amadori compound undergoes an initial 2,3 enolization, a process that ultimately produces a 1deoxyglucosone intermediate. For many years, this intermediate was a hypothetical one, having never been isolated or shown to be present during a Maillard reaction. The intermediate was predicted to serve as the precursor for maltol and isomaltol, important pastry flavor and aroma constituents, as shown in Figure 4. Ledl (19) and his coworkers have recently confirmed that the 1-deoxyosone is actually produced during Maillard reactions; trapping and identifying it as the quinoxaline derivative. Clearly it is produced in the reaction, but little is known about its reactivity or role in Maillard reactions, other than the fact that it is, as expected, produced and appears to be a precursor of a number of food flavor and aromas. It is noteworthy that the 5 carbon analog, also a hypothetical intermediate, which could be produced from pentoses or hexuronic acids (the latter via decarboxylation) probably serves as the precursor (20) of 4-hydroxy5-methyl-3(2H)-furanone, an important component of cooked meat flavor and aroma, as shown in Figure 5. Indirect evidence (14C tracer studies) supports this hypothes is in the sense that the methyl group of the furanone is derived from C-1 of the reducing sugar and from C - l of the Amadori compound (21). Formation of 1,4-Dideoxy-2,3-Dicarbonyl Intermediates. Lastly, the 2,3 enolization of an Amadori compound could also give rise to a 1,4 dideoxy-2,3-dicarbonyl derivative that remains attached to the amino group. This type of derivative is also now known to be produced from an Amadori compound, having been isolated by Ledl and his co-workers recently from a synthetically produced Amadori compound (22). Some years ago, a substituted lysine derivative was shown to be present when dried milk preparations were subjected to hydrolysis, prior to amino acid analysis. Furosine (23), the isolated derivative (Figure 6) would be expected to arise from the 1,4 dideoxyosone derivative, which, in turn, would be expected to be formed from the Amadori compound, produced as a result of reducing sugar reacting with a Lysine residue during the spray drying of milk. This represents a concrete example of how the nutritional availability of lysine in proteins is decreased as a result of the Maillard reaction.

The Role Of Ascorbic Acid In The Maillard Reaction. Ascorbic acid also serves as a very active participant in Maillard reactions. We have recently initiated some studies relative to the compounds that are produced during the degradation of it and what their role is in the Maillard reaction. At pH 7.0 and at 37 °C in the presence of oxygen, ascorbic acid is very unstable, even in the absence of amino groups. This suggests that it may well decompose >ounds that are capable of to give carbonyl-containing interacting with amines to give Maillard reaction products. Some of the compounds produced during the degradation of ascorbic acid are shown in Figure 7, and include threose, glyceraldehyde, as well as xylosone and 3-deoxyxylosone (24, 25). The latter would be expected


132


0

H CH N H R

CHNHR

CIO

C-OH

2

I Downloaded by UNIV OF NEW SOUTH WALES on April 15, 2016 | http://pubs.acs.org Publication Date: November 30, 1993 | doi: 10.1021/bk-1994-0543.ch011

CIO

I

HO-C-H

I

2

I

-

H-C-H

I

H-C-H

H-C-OH

I

I

I

CH 0H

CH 0H 3-Deoxyg I ucosone

CH 0H

2

7

I

2

I

H-C-OH

CH

NH R

H-C-OH

I

I

H 0

HO-C-H

H-C-OH

y

2

2

Figure 2. The dicarbonyl intermediate 3-deoxyglucosone (shown above) appears to be produced from an Amadori compound via 1,2 enolization, followed by loss of one molecule of water and hydrolysis of the schiff base substituent.

CH NH R 2

H0-

CH,

C-OH

CIO

2

II

H

H-

CH NH R

C-OH

I

OH

I

OH CH OH 2

-

H-C-OH CH OH 2

H 0 2

I CIO

-

H-C-OH H-

I

NHR

I H-C-OH H-C-OH

I CH 0H 2

1 -D e o x y g I u c o s o n e

Figure 3. The dicarbonyl intermediate 1-deoxyglucosone (shown above) appears to be produced from an Amadori compound via initial 2,3 enolization, followed by elimination of the amine substituent.



2

2

H H

HO

H. .OH

CH,

.OH

V 2

•2H 0

2

-2H 0

Mai

t o I

I s oma I t o I

Figure 4. 1 -Deoxyglucosone appears to serve as the precursor of isomaltol and maltol, prominent food flavor and aroma constituents. The mechanism for the formation of these compounds are shown above.

1-Deoxyg I ucosone

CH 0H

I

H-C-OH

I

CIO H-C-OH

I

CIO

I

CH,

1-Deoxyg I ucosone

CH 0H

I

H-C-OH

I

H-C-OH

CIO

CIO

CH,


OH

5

r

I

I

5


-

NHR

-

*

I

HO-C-H

Compound

Amadorl

2

CH OH

H-C-OH

i

I

H-C-OH

2

2H 0

Furanone

Form

>

2

CH OH

i

H-C-OH

I

CH

II

C-OH

I

CIO

CH.NHR

Thermally Generated Flavors - American Chemical Society

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