Metabolic Profiling of Grape and Wine Aromas
Most aroma compounds exist in grapes as glycosidically bound precursors and the aglycones (‘free’ aroma compounds) are released by enzymatic or acidic hydrolysis during crushing, fermentation and wine aging. We evaluated procedures for measuring the glycosidically bound volatiles using both acid and enzyme hydrolysis conditions followed by analysis of the free volatiles by Headspace Solid-Phase-Microextraction coupled with gas chromatography-mass spectrometry (HS-SPME-GC-MS). Acid hydrolysis of a mixture of standard compounds at pH 1 and 100º C for 1 hour released 20-60% of the bound volatiles, however, these conditions also resulted in significant degradation (>50%) of the free volatiles. Enzyme hydrolysis was generally more effective at releasing glycosidically bound compounds with minimal artifactual changes in the concentrations of the free volatiles. However, esterase and oxidase activity was still observed resulting in artifactual degradation of the free volatiles. Most previous procedures for evaluating volatile glycosides employ an initial sample preparation step using column chromatography to isolate the glycoside fraction from the grapes prior to hydrolysis and analysis of the free volatiles. We observed that different types of sample preparation columns were not equally effective at retaining the glycosides and no column-type was effective for all the glycosides in our mixture. Using optimized conditions obtained from the previous studies with model systems, we compared direct enzyme hydrolysis of Cabernet Sauvignon and Chardonnay grape homogenates with and without prior isolation of the glycoside fraction. Both procedures resulted in new ‘free’ volatiles. However, results varied depending on the type of treatment. This research is providing important insight into approaches for rapid estimation of ‘aroma potential’ of grapes.