Amino acids reaction with reducing sugars

During the processing of foods, several chemical reactions affect the structure of some amino acids reactions with reducing sugars and polyphenols, interaction of the side chains of the amino acids, oxidation, etc. In some cases, these chemical modifications do not seem to change the nutritional availability of the amino acids involved, while in other cases, they do. In all cases, the presence of the newly formed molecules induces biochemical and physiological changes. 

Proline is a particular case in the Maillard reaction because it is, unlike other amino acids, a secondary amine. It represents an important precursor for popcom-like aroma compounds in the reaction with reducing sugars such as glucose [46]. The relevant aroma compounds in this reaction are 2-acetyl-l-pyrroline (22), l-(l,4,5,6-tetrahydro-2-pyridyl)-l-ethanone (23) and its isomer l-(3,4,5,6-tetrahydro-2-pyridyl)-l-ethanone (24) [47], The formation pathway (Fig. 3.25) shows the important intermediates 2-oxopropanal (7) and 1-pyrroline (25) [48, 49[. Another amino acid precursor for 22 is ornithine, an amino acid in yeast or yeast extracts [49[. 

Despite the current popularity of the polyphenol theory, a completely satisfactory scheme for the occurrence of humic and fulvic acids in diverse geologic environments has yet to be established. In practice, all pathways may be operative, but not to the same extent in all environments or in the same order of importance. A lignin pathway may-predominate in wet sediments, such as peats and swamps. The drastic conditions existing in soils under a harsh continental climate (e.g., some Mollisols) may favor humus synthesis by sugar-amine condensation. The disappearance of amino acids from buried sediments has been attributed to the formation of brown nitrogenous polyelectrolytes by reaction with reducing sugars (Stevenson, 1974). 

The reaction with reducing sugars, the Maillard reaction, is the most important reaction that takes place in foods during processing. Reducing sugars are very reactive compounds. Their reactions with amino acids are described in detail in Section 4.7.5.1. For example. 

Carbonylation occurs by the oxidation of some amino acid side chains into ketone or aldehyde derivatives by reactions with compounds of lipid oxidation or by glycoxidation with reducing sugars. These protein-carbonyl compounds are markers of protein oxidation, and recently, several carbonylated proteins and protein oxidation sites in milk (96), meat (97), and fishes (98) have been identified using a classical bottom-up proteomics approach based on 2-DE and MS/MS. Specific labeling of protein carbonyls using fluorescein-5-thiosemicarbazide has been developed and combined with 2-DE and... 

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). 

During baking there is an evaporation of water from the loaf, this is particularly marked near the surface of the loaf, and this evaporation plus the occurrence of the Maillard reaction cause a characteristic dark brown crust to be formed on the exterior of the loaf. The Maillard reaction is a complex set of chemical reactions in which the amino acids in proteins react with reducing sugars such as glucose and fructose, and which are very important to our perception of flavour in baked bread. 

Some Amino Add-Specific Compounds. As listed in Table I, there were three pyrrolizines and one pyridoimidazole identified in the model systems. Pyrrolizines are a very in rtant class of volatile conpounds generated by the reaction of proline with reducing sugars. 2-Acetyl-pyrido(3,4-d)imidazole was another amino acid-specific Maillard reaction product identified in the model systems, which was derived fi om the reaction of histidine with dicaibonyls. 

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