The reactions of amino acids under subcritical water conditions in the temperature range 503-563 K using a pressure value corresponding to the saturated vapor pressure of water at the employed reaction temperature (hereafter called saturated subcritical water) were investigated. As reactants, solutions containing a single amino acid or a mixture of 17 different amino acids were used, and the obtained results in both cases were compared. Glysine, alanine, valine, and proline were produced as intermediate products during the thermal transformation of other amino acids. Generally, the results showed that the existence of amino acids together in a mixture decreased the overall stability since the activation energy values of decomposition of individual amino acids reduced significantly due to their presence in a mixture of amino acids. Most of the amino acids were found to be labile at acidic and near-natural pH values and more stable at a highly basic pH value.

The reactions of amino acids under subcritical water conditions were investigated. All reactions were carried out under a pressure value corresponds to the saturated vapor pressure of water at the employed reaction temperature (hereafter called the saturated subcritical water condition). The reaction temperatures were in the range of 503-563 K. As reactants, seventeen different amino acids were investigated including, glycine, alanine, valine, leucine, isoleucine, methionine, phenylalanine, tyrosine, serine, threonine, cysteine, proline, lysine, histidine, arginine, aspartic, and glutamic. The obtained data revealed that the major decomposition pathway were the deamination and decarboxylation. It was found that glycine, alanine, valine, and proline were synthesized as intermediate decomposition products of other amino acids. Glutamic acid is the only amino acid that decomposed through lactamization process to pyroglutamic acid. Based on the experimental data a general decomposition pathway of all tested amino acids was developed by considering that the decomposition reactions extended to produce final decomposition products, carbon monoxide, water, and ammonia.