Climacteric Respiration and Fatty and Amino Acids Drive Volatile Ester Biosynthesis in Monthong Durian Aril during Fruit Maturation

The distinctive flavor of durian fruit is increasingly characterized, yet the metabolic pathways during ripening are not fully understood. This study investigated the volatile dynamics in ripening Monthong durian, revealing clear correlations between ripening stages and aroma compound production. Fruit ethylene production peaked one day after the first climacteric peak, initiating the second climacteric peak, pulp softening, and ester and sulfur-containing volatile emission. Ethanethiol, ethyl-2-methyl butanoate, ethyl-2-methyl propanoate, ethyl butanoate, and ethyl hexanoate dominate the odor, along with somewhat fermented ethanol and acetaldehyde. Ethanethiol and ethyl-2-methylbutanoate intensely elevated during late ripening, although D-limonene prevailed early. Short-chain fatty acid esters were performed early on, whereas branched-chain amino acid esters increased significantly in overripe fruit. Valine, leucine, isoleucine, and methionine increased 1.07–1.29-fold from unripe to overripe, while cysteine and glutamic acid decreased 1.11–1.25-fold. Propionic acid (C3) was found throughout, whereas acetic acid (C2) increased significantly 22-fold from ripe to overripe stages. The main free fatty acids throughout maturation were palmitic (C16:0) and Z-9-oleic (C18:1). Linoleic acid (C18:2) fell by 25% from unripe to ripe pulp, but linolenic acid (C18:3) increased 44%. Lipoxygenase activity increased throughout maturation, notably in overripe pulp, promoting fatty acid breakdown and volatile generation. From unripe to overripe, aldehyde dehydrogenase and alcohol acetyltransferase activity increased 1.6- and 1.8-fold, respectively. These findings demonstrate that the coordinated metabolism of fatty acids and amino acids, mediated by key enzymes, drives ester biosynthesis during durian fruit ripening.