Amadori compounds, Maillard

Scheme 12.1 Initial steps in the Maillard reaction showing the formation of an Amadori compound...

Amadori compounds (N-substituted-l-amino-l-deoxy-2-ketoses) are potential precursors to the formation of many of these heterocyclic volatile products. The secondary nitrogen in most Amadori compounds is weakly basic and is therefore a likely site for rapid nitrosation reactions via normal reactions with nitrous acid, under mildly acidic conditions. However, purified Amadori compounds are usually obtained only after tedious isolation procedures are invoked to separate them from the complex mixtures of typical Maillard browning systems. Takeoka et al. ( 5) reported high performance liquid chromatographic (HPLC) procedures to separate Amadori compounds in highly purified form on a wide variety of columns, both of hydrophilic and hydrophobic nature. They were able to thus demonstrate that reaction products could be followed for kinetic measurements as well as to ensure purity of isolated products. 

Only recently have N -nitroso Amadori compounds been characterized chemically. The first description of an -nltroso derivative of an Amadori compound reported the formation of 1-deoxy-l-(N -nitroso-3,4-xylidino)-D-f ructose to confirm that a secondary amino group had been formed in an Amadori compound ( 6). Coughlin et al. ( 7) and Heyns et al. ( 8) described the formation of nitrosated Amadori compounds. Since Amadori compounds are weakly basic secondary amines and occur widely in Maillard browned foods and beverages ( 5) and unburned tobacco ( ), the genotoxic potential of these compounds is of interest. 

It is well known that the Maillard reaction in foods is initiated by the formation of colorless and tasteless intermediates, which preferentially are formed in low-moisture systems ( ,5.). In this way by reaction of glucose with amino acids fructose-amino acids are formed via Amadori rearrangement of the primary glucosyl-ami-no acids (1 ). Fructose-amino acids e.g. have been isolated from freeze-dried apricots and peaches ( 6,7,8j. Amadori compounds arising from aldoses and amino acids are formed during drying of foods of plant origin and can be easily detected by amino acid analysis (j>). 

It becomes clear that analytical methods based on the evaluation of the end products of deteriorative reactions will not be satisfactory. Therefore in our own experiments amino acid analysis of Amadori compounds and gas chromatography of volatile Strecker aldehydes were applied to detect the onset of the Maillard reaction well before detrimental sensory changes occurred. 

In former experiments (5) we have shown that chemical analysis for Amadori compounds (mainly consisting of fructose-glutamic acid) and isovaleralde-hyde, formed by Strecker degradation of the amino acids leucine and isoleucine, can be used for an early detection of undesirable quality changes caused by the Maillard reaction. In order to demonstrate the usefulness of these compounds as indicator substances for quality improvement of dried products, we performed drying experiments with carrots as an example of plant products. 

Table I shows that an increase of Amadori compounds occurs parallel with an increase of isovaleraldehyde formed by Strecker degradation of the amino acid leucine (18 ). It becomes evident from Table I that the flavor impression "burnt arises if certain concentrations of isovaleraldehyde are exceeded this flavor change is increased by increasing isovaleraldehyde concentrations. By this means an analytical control of undesirable sensory changes caused by the Maillard reaction in carrots is available.

The influence of the carrot variety on Maillard reaction Samples of six freeze-dried carrot varieties were equilibrated at room temperature to a water activity of 0.33 (J 7), corresponding to an average water content of 6.3 % (related to wet matter). Then the samples were heated to 55 °C for 30 h and the concentrations of Amadori compounds as well as the corresponding sensory changes were determined. The results are listed in Table IV. The amount of Amadori compounds formed by the heating process seems to be correlated... 

The initial stages of the Maillard Reaction deals with the condensation of amino acids, peptides or proteins with reducing sugars. The reaction occurs with the application of heat with the formation of an "Amadori compound" (See Chapter 1 of this book). 

Scheme 2.3 Maillard reactions the two major pathways from Amadori compounds to melanoidins (based on Hodge213)...

The Maillard reaction is initiated, as assessed by the formation of some Amadori compounds by means of ion-exchange amino acid analysis of aqueous extracts without prior hydrolysis. 

K. Eichner, M. Reutter, and R. Wittmann, Detection of Amadori compounds in heated foods, in Thermally Generated Flavors Maillard, Microwave, and Extrusion Process, T. H. Parliment, M. J. Morello, R. J. McGorrin (eds), American Chemical Society, Washington, DC, 1994, 42-54. 

F. Gerum, M. O. Lederer, and T. Severin, Cross-linking of proteins by Maillard processes Model reaction of an Amadori compound with W -acctyl-l -arginine, in F. 1998, 409. 

J. Hirsch, V. V. Mossine, and M. S. Feather, Detection of some dicarbonyl intermediates arising from the degradation of Amadori compounds (the Maillard reaction), Carbohydr. Res., 1995, 273, 171-177. 

Nutritional Effects Due to the Presence of the Maillard Products. Many physiological or antinutritional effects have been attributed to the Maillard products. Specific effects have been attributed to the Amadori products deoxyfructosylphenylalanine (a model substance not likely to be present in large quantities in foods) appears to depress the rate of protein synthesis in chicks (32) and to partially inhibit in vitro and in vivo the absorption of tryptophan in rats (33). The compound e-deoxyfructosyllysine inhibits the intestinal absorption of threonine, proline, and glycine and induces cytomegaly of the tubular cells of the rat kidneys (34) as does lysinoalanine. In parenteral nutrition the infusion of the various Amadori compounds formed during sterilization of the amino acid mixture with glucose is associated with milk dehydration in infants and excessive excretion of zinc and other trace metals in both infants and adults (35,36,37). 

Figure 1.6. Formation of the Amadori compound from D-glucopyranose (D-glucose) and glycine. Note that a molecule of water is split off. Tlie Amadori compound is an intermediate product in the Maillard reaction, which is perceived as one of the pathways in humification. (From Mossine et al., 1994.)...

In mixtures of amino acids and sugars, meat flavor can be developed via different pathways. Cysteine does not only generate meat flavor via keto-L-cysteines but it reacts also with (the decomposition products of) the Amadori compounds of other amino acids to produce meat flavors that are of comparable intensity (5). This means that there are at least two major pathways to meat flavor development in such Maillard reaction systems ... 

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