FIGURE 1.

Decomposition of protein and metabolism of amino acids (Smit et al., 2005). Proteolysis in lactic acid bacteria is initiated by cell envelope proteinase (CEP), which degrades proteins into oligopeptides. The second stage of protein degradation is the transfer of dipeptides, tripeptides, and oligopeptides into cells. Three transport systems have been found in lactic acid bacteria, namely oligopeptide, dipeptide and tripeptide transport systems (Opp, DtpP, and DtpT, respectively). Finally, the Pep family hydrolyzes dipeptides, tripeptides, and oligopeptides into amino acids. The metabolism of amino acids includes deamination and decarboxylation. The deamination reaction produces various α-carboxylic acids, which are involved in various metabolisms in lactic acid bacteria cells. The amino acid decarboxylation reaction produces biogenic amines, which mainly includes the transport of amino acids into the cell, decarboxylation, and transport outside the cell after being converted into biogenic amines. Transamination of amino acids leads to the formation of alpha-keto acids. Alpha-keto acids can be converted to aldehydes by decarboxylation. Aldehydes are converted to alcohols or carboxylic acids by dehydrogenation. The direct dehydrogenation of alpha-keto acids leads to the formation of hydroxy acids. 1: cell envelope proteinase, 2: peptidases, 3: biosynthetic enzymes, 4: dehydrogenase, 5: aldolases, 6: lyases, 7: acyltransferases esterases, 8: dehydrogenase, 9: aminotransferases, 10: decarboxylase, 11: deiminases decarboxylase, 12: dehydrogenase complex, 13: biosynthetic enzymes.