Glycosidically Bound Aroma Precursors in Tamarillo Fruit (Solanum betaceum Cav.): Towards a Comprehensive Understanding of Flavour and Ripening-Related Chemistry

Abstract

Tamarillo (Solanum betaceum Cav. syn. Cyphomandra betacea Sendt.) is a small native South American shrub having egg-shaped fruit with smooth skin and juicy seeds. Currently, New Zealand is one of the main commercial producers of tamarillo fruit. Tamarillo fruit contains good nutritional values and health benefits but remains an underutilized species, mainly due to its unpleasant flavour properties that are described as bitter, sour, and astringent. Noteworthy, tamarillo aroma is less studied compared to other bioactive phytochemicals such as anthocyanins, carotenoids, and phenolic acids. The knowledge concerning its volatile compounds is limited to several articles published about 10 years ago. Furthermore, as an important reservoir of fruit aroma, the glycosidically bound volatiles (GBVs) of tamarillo have yet to be identified. Therefore, this study was conducted to fill the above knowledge gaps. The present work focuses on the glycosidically bound aroma precursors of three commercial tamarillo cultivars that are widely grown in New Zealand, namely Laird’s Large, Mulligan, and Amber. A comprehensive understanding of glycosidic aroma precursors of tamarillo fruit is achieved from the aspects of flavour chemistry and the associated fruit ripening chemistry. The ripening-related chemistry reveals the molecular and biochemical processes during tamarillo fruit ripening, while the flavour chemistry underlines the potential contribution of GBVs to the overall aroma of tamarillo fruit and its associated products. Ripening-related Chemistry: key metabolites and flavour-regulation pathways in tamarillo were investigated to explore the development of free and glycosylated volatile compounds during fruit maturation. Changes in the physical parameters, concentrations of flavour precursors, and activities of key endogenous enzymes were monitored. Results revealed the lipoxygenase (LOX) pathway as a crucial biosynthesis mechanism for the generation of C6 free alcohols and their glycosidic precursors during tamarillo ripening. This biosynthesis pathway was highly correlated with the activities of key enzymes (lipoxygenase, alcohol dehydrogenase, and β-glucosidase) and the contents of substrates, especially linolenic acid (p < 0.01). Flavour Chemistry: GBVs of tamarillo fruit were isolated through solid-phase extraction (SPE) and the released aglycones were characterized applying gas chromatography-mass spectrometry (GC-MS). During SPE, the matrix effect was evaluated based on the recovery rate of GBVs containing multiple aglycone classes. Two hydrolysis strategies were compared, i.e., enzymatic hydrolysis (using β-glucosidase at 5.6 nkat/mg) and acidic hydrolysis (at pH 0 and 3). The free volatile compounds (FVOCs) were analysed using headspace solid-phase microextraction (HS-SPME) combined with GC-MS. Flavoromics and chemometric approaches were applied to screen the GBVs and the FVOCs to identify any volatile markers that were capable of differentiating the juice samples. This is followed by applying the sensomics approach (including aroma extract dilution analysis (AEDA), aroma omission, and aroma recombination tests) to elucidate the key aroma aglycones. Their aroma contribution was then evaluated through aroma profile comparison analysis. Finally, the intact GBVs of tamarillo fruit were characterized by HPLC-qTOF-MS/MS analysis. Enzymatic hydrolysis induced fewer rearrangements than acid hydrolysis and was favourable for the release of aglycones with a pleasant scent, such as prenol, linalool, and α-terpineol. A total of 33 free and 49 bound volatiles were identified from the Laird’s Large fruit with a complementary profile. Furthermore, the chemometric approach identified 11 free and 22 glycosidic volatile markers (with Variable Importance in Projection > 1) that could distinguish among the tamarillo juice samples, with most of them having odour activities (OAV) > 1. From the molecular sensory approach, the key endogenous aromas of tamarillo fruit identified were glycosidic geraniol, α-terpineol, 2,5-dimethyl-4-hydroxy-3(2H)-furanone (DMHF), 4-vinylguaiacol, and vanillin. Among these compounds, the enzymatically released DMHF from the Laird’s Large sample has an OAV of 6059. It is worth to highlight that eight aroma-glycosides were putatively identified from the Mulligan fruit for the first time, including two monoterpenediols, two benzenoids, one volatile phenol, two alkenols, and one norisoprenoid glycosides.