Effects of temperature on the aroma composition of hydrolysates from grape polyphenolic and aroma fractions (PAFs)

Abstract

AIM: The aim is to assess whether fast anoxic aging hydrolysis (75ºC x 24 h) can satisfactorily predict aroma developed from grape aroma precursors at milder conditions (50ºC x 5 weeks).

METHODS: Twelve different phenolic and aromatic fractions (PAFs) extracted, six from Grenache and the other six fromTempranillo, were reconstituted in model wine and aged in duplicate in two different anoxic conditions. Mild conditions were at 50º for 5 weeks in complete anoxia. Harsh conditions were at 75ºC for 24 hours in complete anoxia. Hydrolysates were further extracted and analysed by four different volatile analysis: esters and major alcohols were analysed by GC-FID, free norisoprenoids, terpenoids, phenols and lactones by GC-MS, Strecker’s aldehydes by GC-MS and varietal thiols were analysed by GC-NCI-MS.

RESULTS: A total of 32 compounds was quantified with RSD between 2 and 15%. Some compounds have concentrations very relevant from the sensory point of view, like TDN (1,1,6-trimethyl-1,2-dihydronaphthalene), β-damascenone, massoia lactone and 3-mercaptohexanol (3MH). Grenache samples presented higher concentrations than Tempranillo samples, of 4-vinylguaicol, 4-vinilphenol, linalool, β-citronellol and TDN.Most key aroma compounds can be satisfactorily predicted from the accelerated hydrolysis at 75ºC. Levels of eugenol, massoia lactone, 3MH, β-ionone and vainillin derivatives, released at 75ºC are significantly correlated with those found after 5 weeks at 50ºC, and the slopes do not significantly differ from 1. Levels of β-damascenone and α-terpineol in samples hydrolysed at 75ºC, were however, lightly higher than those found at 50ºC, but were strongly correlated. On the contrary, levels of Riesling acetal were a little higher at 50ºC, but were also strongly correlated.However, levels of TDN, linalool and geraniol in both hydrolytical conditions were poorly correlated. Data suggest that linalool and geraniol can be best represented by the accelerated hydrolysis, while TDN are best represented by the milder hydrolysis.

CONCLUSIONS

Harsh hydrolysis at 75ºC represents a good compromise which makes it possible to get a satisfactory representation of the grape aroma potential. Labile compounds, such as linalool and geraniol are in fact best measured at this temperature. However, long-aging aroma compounds, such as TDN, will require longer hydrolysis times and other relevant aroma compounds with different precursor molecules, such as 4-vinylguaiacol, are also poorly predicted at 75ºC.