Chemical Conversion of α-Keto Acids in Relation to Flavor Formation in Fermented Foods

Formation of flavor compounds from branched-chain α-keto acids in fermented foods such as cheese is believed to be mainly an enzymatic process, while the conversion of phenyl pyruvic acid, which is derived from phenylalanine, also proceeds chemically. In this research, the chemical conversion of α-keto acids to aldehydes with strong flavor characteristics was studied, with the main focus on the conversion of α-ketoisocaproic acid to the aldehyde 2-methylpropanal, and a manganese-catalyzed reaction mechanism is proposed for this conversion. The mechanism involves keto−enol tautomerism, enabling molecular oxygen to react with the β-carbon atom of the α-keto acid, resulting in a peroxide. This peroxide can react in several ways, leading to unstable dioxylactone or noncyclic intermediates. These intermediates will break down into an aldehyde and oxalate or carbon oxides (CO and CO2). All the α-keto acids tested were converted at pH 5.5 and in the presence of manganese, although their conversion rates were rather diverse. This chemical reaction might provide new ways for controlling cheese flavor formation with the aim of acceleration of the ripening process or diversification of the flavor characteristics. Keywords: α-Keto acids; 2-oxo acids; flavor; leucine; aroma formation; 2-methylpropanal; dairy; cheese