Identification of key odorants in fresh-cut watermelon aroma and structure-odor relationships of cis,cis-3,6-nonadienal and ester analogs with cis,cis-3,6-nonadiene, cis-3-nonene and cis-6-nonene backbone structures

Abstract

The scope of this study involved the identification of key odorants in fresh cut-watermelon, and the synthesis and evaluation of esters with potential watermelon-like aroma attributes. Aroma formation in fresh-cut watermelon is a dynamic enzymatic process, with the characteristic aroma components being formed immediately after cutting. The characteristic fresh-cut aroma is not long lasting due to further enzyme action that modifies the fresh-cut aroma components. The key to identifying the key components responsible for fresh-cut watermelon aroma was the application of a suitable volatile isolation strategy based on static headspace analysis (SHA). In this study, SHA was used to collect the headspace volatiles one minute after initial cutting of the fruit. This enabled a chemical ???snap shot??? of fresh-cut aroma to be taken. The most potent odorants in the headspace were determined by gas chromatography-olfactometry of decreasing headspace volumes (GCO-H) with confirmation achieved by application of a complimentary method based on GCO and aroma extract dilution analysis (AEDA) of fresh-cut watermelon aroma extracts prepared by solvent-assisted flavor evaporation (SAFE). Eight unsaturated nine-carbon aliphatic aldehydes and one six carbon unsaturated aldehyde were detected by GCO-H. These included cis-3-hexenal, cis,cis-3,6-nonadienal, cis-3-nonenal, cis-6-nonenal, trans-2-nonenal, cis-2-nonenal, trans,cis-2,6-nonadienal, trans,trans-2,4-nonadienal, and trans,trans,cis-2,4,6-nonatrienal. This finding is contrary to previous beliefs that alcohols are the main contributors to fresh-cut watermelon aroma. Most importantly, it was found that cis,cis-3,6-nonadienal was not only a potent odorant, but this compound alone possessed an aroma reminiscent of fresh-cut watermelon. Use of sensory evaluation, including ranking test, revealed that the aroma attributes of purified synthetic cis,cis-3,6-nonadienal closely matched those of fresh-cut watermelon. This ???watermelon aldehyde??? can be considered a potent odorants since its estimated odor detection threshold (in water) is very low (0.2 ppb). Unfortunately, cis,cis-3,6-nonadienal is a labile compound and is prone to both trans isomerization and oxidation of its aldehyde end group. For this reason, an attempt was made to create an ester with watermelon-like aroma attributes. Esters have been widely used in the food and beverage industries as flavoring agents because they are both stable and possess relatively low thresholds. To develop a watermelon-like ester, alcohol (formate, acetate, propionate, and butyrate) and carboxylic acid (methyl, ethyl, propyl, and butyl) esters with cis,cis-3,6-nonadiene backbones were synthesized. To achieve a more thorough understanding of the structure-odor relationship of these esters, the same types of alcohol and carboxylic acid esters were created with cis-3-nonene and cis-6-nonene backbones. The general structure/threshold trend was that threshold increased with increasing carbon number. In addition, threshold was also dependent on the number and position of the cis double bond. Descriptive sensory analysis was used to evaluate the effect of structure on the odor properties of the selected (low threshold) esters; however, no clear trend was found. Finally, further sensory analysis by ranking test was employed to determine, which, if any of these esters might be a suitable replacement for cis,cis-3,6-nonadienal for use as a watermelon flavoring. Although all esters had a ???fruity??? element to their aroma descriptions, none were significantly close in terms of their aroma attributes to cis,cis-3,6-nonadienal