Lactic acid bacteria: A possible aid to the remediation of smoke taint?

Abstract

With climate change, the occurrence of wildfires has increased in several viticultural regions of the world. Subsequently, smoke taint has become a major issue, threatening the sustainability of the wine industry. Indeed, following smoke exposure, ripening grapes absorb volatile phenols (VPs) arising from the burning of lignified plant material and store them as odourless glycosylated compounds. During the winemaking process, these precursors can be liberated through acidic hydrolysis or through the glycosidase activities of yeasts and lactic acid bacteria (LAB), thereby leading to undesirable odours such as ash tray, burnt, and smoky. Various remediation strategies have been proposed, including spraying kaolin on grapes to prevent absorption of VPs by grapes or various wine treatments by reverse osmosis or carbon addition to remove introduced VPs in wine, but these are currently employed with moderate success. LAB, including Oenococcus oeni, possess glycosidase enzymes, but studies have mainly been limited to the breakdown of artificial substrates such as p-nitrophenyl-β-glycosides, and their efficacy against smoke-derived VP glycoconjugates in real grape juice has never been truly assessed. Therefore, this study aimed to investigate the ability of a large number of wine-related LAB species to release VPs in real smoke-exposed grape juice. Indeed, the use of LAB with strong glycosidase activity could be used to ameliorate smoke taint, when combined with already proposed remediation strategies that are more effective on free VPs than on glycosylated VPs.

Forty-two LAB isolates were screened for glycosidase activity in sterile smoke-exposed Pinotage grape juice. Volatile phenols were quantified using Gas Chromatography – Mass Spectrometry. All isolates were found capable of releasing free-VPs to various extents. Four LAB isolates, namely Lactiplantibacillus pentosus 65.1, Latilactobacillus sakei 115, Lentilactobacillus hilgardii 87.2 and O. oeni 192, released more overall VPs than other investigated isolates in smoked Pinotage juice. Amongst the VPs quantified, isolates of Lac. plantarum and Lac. pentosus were found to release high levels of 4-ethyl phenol (4-EP), which was then demonstrated to originate not only from the release of smoke-derived glycosylated 4-EP, but also from de novo intracellular production. Overall, the study showed that selected LAB isolates secrete glycosidases, with good potential to release free-VPs prior to their removal using agents such as charcoal.