Characterisation of the bound volatile compounds of kiwifruit

Abstract

The genus Actinidia, collectively known as 'kiwifruit', includes over 70 species, of which only a few are commercially important. Kiwifruit is the subject of this research. At present, the knowledge of kiwifruit aroma is limited to the free volatile portion, and concentrated on the 'Hayward' cultivar. Information on the volatiles of other species/cultivars is limited, and information concerning the bound volatiles of kiwifruit consisted, until now, of one article published more than 15 years ago. Therefore, this study aimed to provide new knowledge on kiwifruit aroma by focusing on the bound volatile portion of four kiwifruit species, three of them being commercially important (A. deliciosa 'Hayward', A. chinensis 'Hort16A', A. arguta), plus one with low aroma (A. eriantha). Initially, a preliminary investigation of the bound volatiles of kiwifruit was conducted to select the best way to isolate and hydrolyse the glycosides, and extract the released aglycones. The bound volatile content and composition of the four kiwifruit species mentioned was elucidated, and the important odorants were identified by GC-MS/olfactometry. It was found that benzenoids, alcohols and terpenoids predominated in the bound volatile extracts of all the fruits analysed. The information gathered was compared with information currently available on the free volatiles of kiwifruit, to gain a deeper understanding of kiwifruit aroma. In A. arguta, 2,5-dimethyl-4-hydroxy-3 (2H)-furanone (DMH, furaneol) was the predominant bound volatile compound, as well as the bound volatile with the strongest smell. The DMHF glycoside was identified by LC-MS as DMHF-β-D-glucopyranoside. Glucose was identified as the major sugar moiety in A. arguta glycosides. The bound volatile composition of A. eriantha was unexpectedly diverse, considering the bland, green aroma of this fruit, with odorants such as 2-phenylethanol, (E)- isoeugenol and vanillin having been found. The bound volatile profiles of 'Hayward' and 'Hort16A' were similar, but compounds were found at different concentrations. Bound volatiles with green notes were found in both 'Hayward' and 'Hort16A', although they were more predominant in 'Hayward'. Finally, the effect of ripening on the bound volatiles of 'Hayward' and 'Hort16A' was evaluated, finding that bound volatiles tend to increase in the ripe stage, and then decrease in over-ripe