Early Maillard reaction, amino acids

The extent of heat-induced changes in protein-rich foods can be measured by determining some early Maillard reaction products (O Brien and Morrissey, 1989). Acid treatment of protein-bound or free A -fructoselysine liberates lysine, with a yield of 50%, and two other amino acids, furosine (20%) and pyridosine (10%) (Figure 13.1). The three products that result from hydrolysis of A -lactuloselysine are formed in the proportions of approximately 5 3 to 4 1 to 2, however the yield of different derivatives is variable. Therefore, in order to use these unique amino acids as indicators of changes in lysine content, the hydrolysis should be carried out in strictly-defined conditions. Furosine is present in various food products in a very wide range of concentrations (Table 13.2). 

Radical reaction mechanisms during the early Maillard reaction were first detected by Namiki et al. (7, 2). He identified iV.A -dialkylpyrazine-cation radicals that originated from the primary Schiff base formed by reaction between glucose and amino acids. The glycolaldehyde alkylimine formed by a reverse aldol reaction of the Schiff base leads to a dialkylpyrazinium radical cation after self-condensation. The formation of dialkylpyrazinium radical cations, which could be detected by EPR spectrometry, represents an alternative pathway of the Maillard reaction it starts at the very beginning of die reaction, well before the formation of Amadori rearrangement products and depends on the pH value it starts around pH 7 and increases up to pH 11. 

In heat treated or stored food products several amino acids are not fully available because of derivatization or crosslinking reactions. Since 30 years furosine is known as a useful indicator of early Maillard reaction which is applied in food science, nutrition and medical biochemistry. Recently more sensitive analytical methods for furosine determination are available which have again increased the attractivity of this important indicator. Lately, N -carboxymethyllysine (CML) became available as another marker of special interest, because CML is a more useful indicator of the advanced heat damage by Maillard reaction than furosine. In addition, CML has the advantage to indicate reactions of lysine with ascorbic acid or ketoses such as fructose. Indicators for protein oxidation of sulfur amino acids are methionine sulfoxide and cysteic acid. An established marker for cross-linking reactions is lysinoalanine, which also indicates protein damages due to processing under alkaline conditions. Other markers formed as a consequence of alkaline treatment are D-amino acids. 

Reduction with borohydride (Lys J), This method, developed by Hurrell and Carpenter first stabilises the bond between the e-amino group and the sugar derivative by hydrogenation and then applies the usual acid hydrolysis. Blocked lysine from an early Maillard reaction, stabilised in this way, does not regenerate lysine upon acid hydrolysis. 

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. 

As a result of the very wide chemical interest which the Maillard reaction has attracted since the early years of this century, and of its diverse commercial and biological implications, a vast amount of work thereon has been published. In this article, the chemical aspects of the reaction between simple amino acids and mono- and di-saccharides will be surveyed the reaction with peptides and proteins will be discussed only when it contributes to our understanding of the mode of interaction between simple amino acids and sugars. The significance of the reaction in the preservation of various foodstuffs has now been reviewed many times2 9 and continues to be the subject of much investigation.10 16 The reaction is also... 

Many other compounds have been identified as products of the sugar-amino acid reaction. Amongst these are 2,3-butanedione,195 formaldehyde,196 pyruvaldehyde,20,195, 247, 248 3-hydroxy-2-butanone,248 hydroxy-2-propanone,248 and acetaldehyde.73 Imidazoles,196, 249-251 especially 4(5)-methylimidazole, have also been isolated from the reaction under mild conditions, and could have been formed from a-hydroxy ketones or aldehydes plus amino compounds. Following the early identification247 of pyru-valdehyde as a product of the Maillard reaction, much work was done in an attempt to show that this reactive compound plays a major role in the reaction,249 262 254 but, that this is so is by no means certain.7... 

The early scientific discoveries relating to heat induced aroma development can be traced to the work of Louis—Camille Maillard at the University of Nancy during the period of 1912 to 1936 (2). He published at least 8 papers on the subject of the reaction of sugars with amino acids. The Maillard Reaction, or so-called, non enzymatic browning reaction chemistry, has become the focus on a great amount of scientific work (3). 

As well as the amino and carboxy groups, the amino-acid side-chain is also involved early in the Maillard reaction, so the side-chain functional groups become incorporated into the products from Maillard reactions. Cysteine, in particular, reacts with glucose to generate numerous reaction products, some of which introduce attractive tastes and aromas to foods associated with sulphur functional groups. 

G. Westphal and L. Kroh, Meehanism of the early phase of the Maillard reaction. Part 1. Influence of the straeture of the carbohydrate and the amino acid on the formation of the A-glycoside, Nahrung, 29 (1985) 757-764. 

---