Aroma characterization based on aromatic series analysis in table grapes

Aroma is an important aspect of quality in grapes and wines and, hence, for consumer acceptance1,2,3,4,5,6,7,8. Several families of compounds are responsible for the aroma of grapes. Among them, esters and terpenes are known to contribute to fruity/floral characters9,10; C6-aldehydes and alcohols possess green leafy aroma characters11; methoxypyrazines are strongly linked to green capsicum descriptors8,12. Meanwhile, C13-norisoprenoids generally contribute to many flavours13,14 in fruits and wines, such as berry, tobacco, honey, balsamic and violet aromas. Aroma compounds are usually located in both pulp and skin as free and bound glycosides1. Bound glycoside forms, which can be transformed by hydrolysis into odour-active forms (free fractions), increasing the aromatic characteristics of the grape, are non-volatile compounds with no direct contribution to the aroma of the grape15. In contrast, free forms are volatile compounds that can be sensed and tasted directly and are involved in the aroma of grape berries. Obviously, free forms are vital ingredients and completely determine the flavour of table grapes. However, not all volatiles contribute with the same intensity to aroma. The concentration-odour threshold ratio, known as the “odour activity value” (OAV), must be considered as the only principle to estimate the contribution of each compound to aroma, although interactions (antagonistic and additive effects) among different aroma components occur in the matrix9,12. Because an individual compound generally has several flavours16, it is difficult to establish or evaluate global aroma profiles only using the odour activity values (OAVs) of volatiles. Grouping the OAVs of the aroma compounds with similar descriptors into aromatic series makes up the organoleptic profiles (OAV Aroma Wheel). This process relates quantitative information acquired by chemical analysis to sensory perceptions12 and thus simply and effectively evaluates and compares the aroma characters9. This method has recently been employed to distinguish wine grape varieties4,12,17 and wines9,16. Currently, most of the studies on aroma have focused on wine grapes and wines and have made noticeable progress in many aspects, including varieties12, regions18, training systems19, developmental stages11,20, aromatic maturity21 and positions of berries4,17. Regarding table grapes, Yang et al. evaluated the volatile compounds in certain grapes at the germplasm level22 and optimized the harvest dates of three grapes23; Fenoll et al. predicted the Muscat aroma in table grapes via analysis of rose oxide24 and determined the evolution of aromatic compounds in Muscat Hamburg during ripening10. There remains a lack of research on the aspects of aromatic series and aroma profiles in table grapes, as well as a lack of effective indicators for evaluating or determining the aroma quality. Table grapes account for 80% of grape production in China22. It is very important to fill the research gaps in the evaluation system for table grape aroma qualities. In the present study, the 20 most popular table grape cultivars were selected for determination and analysis of the free aroma compounds in both skin and pulp juice by automatic solid-phase microextraction (SPME) combined with gas chromatography-mass spectrometry (GC-MS). Then, the aromatic series and aroma profiles of table grapes were constructed, and their similarity or dissimilarity was revealed by multivariate data analysis. Furthermore, the preferred series and their corresponding aroma components were confirmed by an additional sensory evaluation, which can be used as an important indicator for the improvement of the breeding and cultivation measures of table grapes. Finally, the aroma characters among table grapes, wine grapes and wine were compared.