Impact of heating must before fermentation on Chardonnay wines

Abstract

Prefermentation steps of white winemaking are very important for controlling the stability and the sensory attributes of wines. Usually musts are clarified by cold settling to prevent the start of the fermentation, before racking big lees and thus limiting the appearance of vegetable or reduction off flavour while favouring an aromatic expression with low turbidity. Besides, to reach the protein stability, some white wines further require a bentonite fining, sometimes associated with negative effects on the sensory quality. This study aims to know the impact of musts heating after pressing on a Chardonnay wine in northern conditions by comparison with a classic cold racking of the must. This technique appeared a few years ago to treat spoiled grape harvest or sub-maturity ones. After pressing and SO2 addition, a Chardonnay must was separated in two glass containers: one stored at 6°C during 12 hours and the other one heated at 65°C during 12 hours. A racking of the clear juice was then realized before temperature adjustment to 18°C for starting the alcoholic fermentation. All the other wine making stages, ageing and bottling were similar for both conditions. Without impact on the alcoholic fermentation, must heating gave a more expressive and aromatic wine with citrus fruits notes. The latter appeared early in the wine making process and remained after bottling, suggesting a likely higher concentration of volatile thiols. Two months after the bottling, the wine from the heated must was considered significantly fruitier, more persistent and was more appreciated in quantified descriptive analysis by a wine professionals panel. Must heating also led to chemical changes in the wine. Untargeted analysis by 3D fluorescence showed phenolic and protein component evolution for the heated must modality. Acid phenols and derivatives analysis showed that they were present in lower concentration in the heated must modality, contrary to caftarique or coutaric acids. Oxidation and esterification reactions with tartaric acid could have been favoured. Steric exclusion chromatography allowed to confirm hypotheses for the protein fraction. At the end of alcoholic fermentation, the high molecular weight fraction (> 200 kDa) seemed to decrease in the heated must modality. At the end of malolactic fermentation, fractions between 20 and 30 kDa, potientially associated with heat instable proteins, were also reduced in the heated must modality. A heat test realized after bottling confirmed that the heated must modality was stable, but not the control. This trial confirms the interest for this technique especially for cellars, which already have must heating equipments for red grapes (thermo process). This technique seems to favour the expression of fruity white wines. In a context of reduction of oenological waste production, a further advantage is that bentonite treatment seems to be no longer required.