Nonenzymatic Browning and Formation of α-Dicarbonyl Compounds by Ascorbic Acid in Lemon Juice

Citrus juice quality is significantly reduced by nonenzymatic browning, in which ascorbic acid (ASA) plays a crucial role. ASA is converted to dehydroascorbic acid (DHA), which degrades into reactive α-dicarbonyl compounds (αDCs). This study investigated the effects of ASA and DHA on browning and αDCs formation in lemon juice. In the model solution, ASA had a greater effect on browning than sugars and amino acids. Under nonoxidative conditions, ASA retarded browning, whereas DHA continued to promote browning. DHA degradation results in the formation of αDCs, including xylosone, 4,5-dihydroxy-2-oxopentanoic acid, 3-deoxythreosone, methylglyoxal, and glyoxal. In lemon juice, eliminating the headspace inhibited the degradation of ASA but accelerated the degradation of DHA and did not inhibit αDCs formation, as observed in the lemon juice model solution. These results suggest that both DHA and αDCs formation must be inhibited to effectively prevent the browning of lemon juice.