Identification and quantification of 2,5-diketopiperazine platform biochemicals along with pyrazines and pyridinols in the dissolved organic matter phase after hydrothermal carbonization of brewer’s spent grain

2,5-diketopiperazines have been known to be formed as a result of thermal treatment of foodstuffs (cacao, coffee, cooked meat) in the absence of water. Herein, it was demonstrated that an array of 2,5-diketopiperazines was formed upon hydrothermal carbonization of protein-rich brewer’s spent grain. Identification of individual diketopiperazines was performed by GC/MS analysis of the solvent extracts from the lyophilized dissolved organic matter phase. Structural assignment was facilitated using an array of diketopiperazine surrogates which were prepared hydrothermally from amino acids in an environmentally friendly way. The most abundant surrogates were formed from proline, phenylalanine, and leucine. Functionalized amino acids played a minor role under hydrothermal conditions. Other N-organic compounds from hydrothermal treatment of brewer’s spent grain included functionalized pyridines and alkylated pyrazines. The potential of hydrothermal carbonization to produce value-added compounds such as diketopiperazines from (waste) biomasses should be further exploited for biorefinery purposes.