Changes in Free Amino Acid Content in the Flesh and Peel of ‘Cavendish’ Banana Fruit as Related to Branched-chain Ester Production, Ripening, and Senescence

Abstract

The concentrations of free amino acids in the peel and pulp of banana (Musa sp., AAA group, Cavendish subgroup, cv. Valery) fruit during ripening at 22 °C were measured. All 20 amino acids were quantified at seven distinct ripening stages as defined by measures of internal ethylene, O2, and CO2 concentrations, aroma volatile emissions, and peel color. Volatile production commenced 2 days after the peak in ethylene production and 1 day following the climacteric peak in internal CO2. The maximum rate of branched-chain ester synthesis occurred 2 to 3 days after its onset. Production of 2-methylpropyl and 3-methylbutyl esters was much higher in the pulp compared with the peel, confirming that the pulp, rather than the peel, is the primary site of banana aroma synthesis. Of the amino acids measured, only leucine, valine, and cysteine increased concomitantly with ester formation. This was observed in the pulp, but not in the peel. The data suggest the metabolic pathways for valine and leucine formation also support, respectively, the synthesis of 2-methylpropyl and 3-methylbutyl esters. It is not clear how leucine and valine can accumulate despite the fact that they act as feedback inhibitors of their respective synthetic pathways. There was a slight peak in the formation of several other amino acids in the pulp (e.g., alanine, arginine, asparagine, glutamine, and methionine) coinciding with the climacteric respiratory peak in CO2, but a similar pattern was not seen for the peel. These data are the first to demonstrate distinct differences in amino acid metabolism in the peel and pulp of banana related to their role in ripening and aroma biosynthesis.