Protein melanoidins

The initial products of the reactions between sugars and proteins may enter a cascade of reactions yielding fluorescence, browning, and polymerization of proteins ("cross-linking"). The brown pigments, so-called melanoidins, are polymers whose composition has not yet been established completely. Melanoidins bind calcium and may thus interfere with de- and remineralization in caries. 

Lipid oxidation products react with proteins and other amino compounds to form brown substances, similar to melanoidins. The formation of such brown substances was reviewed already at the first Maillard Symposium.150 The pigments formed are partly soluble in chloroform-methanol and partly insoluble, whereas true melanoidins are largely water-soluble. As most brown pigments of fish muscle are soluble in benzene-methanol and only to a lesser extent in water, the implication is that here oxidised lipid-protein interactions are more important than Maillard browning due to ribose-amino acid interactions. 

Much attention has recently been paid to arginine as a melanoidin precursor. In a model system, lV -acetylarginine was first allowed to react with glyoxal in aqueous solution at pH 7, followed by furfural. Repeated chromatography of the aqueous residue after ethyl acetate extraction led to a 1% yield of the deep red 29.180 Such a compound provides an example of a potentially coloured crosslink in proteins derived by the Maillard reaction (see Chapter 8). 

Casein heated in aqueous solution with furfural rapidly turns orange-brown. Melanoidins (> 10 kDa), isolated by ultracentrifugation, were enzymically hydrolysed and the product was separated by HPLC. Two peaks led to red compounds in the ratio 1 7, which were unequivocally identified by Hofmann191 as the lysine analogues of 24a and 24b (MM = 0.476 kDa), respectively. This is the first demonstration of the attachment of a specific type of coloured Maillard product to a protein... 

Addition of 1.5% glucose plus glycine or corresponding melanoidin Addition of 5% glucose to dough to replace an equal amount of sucrose Addition of amino acids or protein hydrolysates to dough... 

M. Lindenmeier, V. Faist, and T. Hofmann, Structural and functional characterization of pronyl-lysine, a novel protein modification in bread crust melanoidins showing in vitro antioxidative and Phase Eli enzyme modulating activity, J. Agric. Food Chem., 2002, 50, 6997-7006. 

Protein bound melanoidins (casein + U-14C-glucose) low-molecular-weight fraction... 

The browning reaction can be defined as the sequence of events that begins with the reaction of the amino group of amino acids, peptides, or proteins with a glycosidic hydroxyl group of sugars the sequence terminates with the formation of brown nitrogenous polymers or melanoidins (Ellis 1959). 

However, this reaction cannot possibly account for the inhibitory effect of sulfite. It is thought that sulfur dioxide reacts with the degradation products of the amino sugars, thus preventing these compounds from condensing into melanoidins. A serious drawback of the use of sulfur dioxide is that it reacts with thiamine and proteins, thereby reducing the nutritional value of foods. Sulfur dioxide destroys thiamine and is therefore not permitted for use in foods containing this vitamin. 

Yamamoto S. and Ishiwatari R. (1989) A study of the formation mechanism of sedimentary humic substances II. Protein-based melanoidin model. Org. Geochem. 14, 479-489. 

Aimed at removing proteins and high molecular weight, melanoidin-type material, the de-fatted beef bouillon was separated by means of ultrafiltration with a molecular weight cut-off of 1 kDa. Sensory analysis demonstrated that only the low molecular weight fraction (MW< IkDa) exhibited the typical complex, sweet, brothy taste. 

The main classes of Maillard reaction end products of practical interest inelude volatile heterocyclic molecules responsible for aromas of thermally processed foods colored oligomeric and polymeric molecules that determine non-enzymatic browning of thermally processed foods protein and lipid cross-linking and other advanced glycation end products in vivo, which are implicated in pathogenesis of comphcations in diabetes and aging. Here we only consider the contribution of D-finctosamines implicated in the formation of these end products. A number of reviews containing detailed considerations of the formation of volatiles, melanoidins, and... 

---