Enhancing tropical fruit avour in Chardonnay and Shiraz through foliar nutrient sprays

TROPICAL FRUIT CHARACTERS IN WINE Tropical fruit-related characters such as passionfruit, grapefruit, pineapple, mango or guava, are an attractive feature of many white wines, and are particularly common in Sauvignon Blanc. The tropical nature of Sauvignon Blanc is driven by the presence of potent compounds commonly known as thiols. Recently these compounds have also been shown to be significant contributors to Australian Chardonnay wines (Capone et al. 2017, Capone et al. 2018). For Chardonnay, two thiols were found to be important: 3-mercaptohexanol (3MH) and 3-mercaptohexyl acetate (3MHA)1. In a set of young unoaked wines made from juices sourced from vineyards across Australia, these compounds were associated with enhanced fruity aroma, and at higher concentration with a ‘grapefruit’ character or, in some cases, clear ‘passionfruit’ or ‘box hedge’ flavour, characters more usually associated with Sauvignon Blanc. While thiols are generally considered most important in white wines, they can also contribute to the flavour of red wines; for example, thiol concentration has been linked to enhanced ‘blackcurrant’ aroma in red wine (Rigou et al. 2014). The thiols 3MH and 3MHA have been observed in Bordeaux red wines (Bouchilloux et al. 1998) and a recent survey of commercial Australian red wines by the AWRI commonly found them above their sensory threshold (Siebert et al. 2019). The generation of thiols in wine is somewhat unusual among wine flavour compounds in that it requires several steps. The first step involves metabolism within the grape; the second step requires the action of grape crushing; and, finally, the third step occurs through yeast metabolism. Non-odorous forms of these thiols are bound to amino acids in crushed grape berries and are broken down and released by yeast enzymes during fermentation. One well-known way of enhancing the level of thiols in wine is by yeast selection. Not only can choice of yeast strain increase or decrease the overall concentration of thiols but it can also influence the ratio of 3MH to 3MHA (Cordente et al. 2017), strongly affecting wine flavour. However, the effect of yeast is limited by the amount of precursors available in the grapes to be broken down. The length of time between machine harvesting or crushing and the start of fermentation can have a large influence on precursor concentrations. Manipulation of thiol concentrations in wine can also start in the vineyard by taking steps to increase the concentration of precursors in grapes. As the building blocks for the production of these precursors include nitrogen and sulfur compounds, vine fertilisation is one vineyard management option to achieve this. The use of foliar sprays in vineyards is commonplace, although vine nutrients are generally applied to the soil. French studies using foliar applications of nitrogen and sulfur resulted in wines higher in tropical thiols (Lacroux et al. 2008, Geffroy et al. 2016). However, there is limited information of the relevance of this technique in Australian vineyards, and limited reported sensory data.