Impact of Frozen Storage on the Free Volatile Compound Profile of Grape Berries

Fruits stored at temperatures above 4 degrees C are subject to rapid deterioration due to microorganisms, enzymes, or oxidation reactions. Although freezing grapes is not part of the usual winemaking process, freezing grape berry samples during veraison and harvest to postpone time-consuming analyses is common practice. In this study, the free volatile compound composition, including C-6 compounds, monoterpenes, C-13-norisoprenoids, and benzene derivatives of fresh juice from two grape varieties (Thompson and Flame), was compared with that of frozen juice and juice extracted from previously frozen grapes (Juice-FRG) using gas chromatography-mass spectrometry solid-phase microextraction. Results showed that freezing the juice or extracting juice from previously frozen grapes had a cultivar-dependent impact on the free volatile compound profile as compared with fresh samples. Freezing of fruit juice or the whole berries affected most C-6 compounds, monoterpenes, and C-13-norisoprenoids to different extents, but it had little impact on benzene derivative volatiles. Results suggest that care should be taken when performing free volatile compound analyses from frozen grape berries or juice, and that other preservation methods should be investigated to preserve grape berries to be used for volatile metabolite studies.