Impact of smoke exposure on the chemical composition of grapes

Abstract

Vineyard exposure to smoke can lead to grapes and wine which exhibit objectionable smoky and ashy aromas and flavours, more commonly known as ‘smoke taint’ [1, 2]. In the last decade, significant bushfires have occurred around the world, including near wine regions in Australia, Canada, South Africa and the USA, as a consequence of the warmer, drier conditions associated with climate change. Considerable research has subsequently been undertaken to determine the chemical, sensory and physiological consequences of grapevine exposure to smoke. The sensory attributes associated with smoke-tainted wine have been linked to the presence of several smoke-derived volatile phenols, such as guaiacols, syringols and cresols [2]. These volatile phenols have been shown to accumulate in grapes in glycoconjugate forms, following grapevine exposure to smoke [3, 4]. However, their mode of entry, and therefore the factors influencing their uptake from smoke by grapevine leaves and fruit, have not as yet, been adequately investigated. This study aimed to investigate the extent to which berry physiology, in particular, the development of the berry cuticle and epicuticular wax, influences the uptake of volatile compounds from smoke. Potted Chardonnay and Shiraz grapevines were exposed to smoke for 60 minutes, at approximately one week prior to maturity. Fruit samples were collected immediately after smoke treatment, and again at maturity (i.e. one week later), for imaging (using an environmental scanning electron microscope) to identify any differences in the physiology of control and smoke-affected berries. Fruit sampled at maturity was also analysed by gas chromatography-mass spectrometry and ultrahigh performance liquid chromatography-tandem mass spectrometry to determine concentrations of volatile phenols and their glycoconjugates, respectively. The potential for a commercial particle film, i.e. kaolin, to act as a protective barrier against smoke, was also investigated, by comparing the volatile phenol and glycoconjugate concentrations of fruit harvested from grapevines treated with kaolin and/or smoke.

Literature: 1. Kennison, K.R., et al., Smoke-derived taint in wine: The release of smoke-derived volatile phenols during fermentation of Merlot juice following grapevine exposure to smoke. Journal of Agricultural and Food Chemistry, 2008, 56(16): 7379-7383. 2. Parker, M., et al., Contribution of several volatile phenols and their glycoconjugates to smoke-related sensory properties of red wine. Journal of Agricultural and Food Chemistry, 2012, 60(10): 2629-2637. 3. Hayasaka, Y., et al., Identification of a beta-D-glucopyranoside precursor to guaiacol in grape juice following grapevine exposure to smoke. Analytica Chimica Acta, 2010, 660(1-2): 143-148. 4. Ristic, R., et al., The effect of winemaking techniques on the intensity of smoke taint in wine. Australian Journal of Grape and Wine Research, 2011, 17(2): S29-S40.