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 5Below 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 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. 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