Effects of soil characteristics on manganese transfer from soil to vine and wine

Aim: In recent times the export of Beaujolais wines has been jeopardised due to a limit of manganese content (Mn) in wine implemented by China (2 mg/L), related to suspicions of potassium permanganate fraud. Nevertheless, soil Mn content may be high in some soil types in Beaujolais. The aim of this study was to improve knowledge of manganese transfer from soil to vine and wine because data on this subject is scarce.

Methods and Results: Recent pedologic mapping of Beaujolais vineyards has enabled a Mn monitoring network to be set up in order to study Mn transfer from soil to vine and wine. Three soil types were considered. Two of the soils can be very high in EDTA Mn: soils from clays with cherts (soil type 7) and former piedmont deposits with leached soils (soil type 8). The third soil, though low in Mn, is the most important and symbolic of Beaujolais: granitic soil. Fifteen plots of Gamay were monitored during 3 years (2015-2017). Besides soil analysis made from pedologic pits, Mn content of petiole, must and wine (red standard wine-making of 40 kg grapes) were determined, as well as grape yield and biomass (pruning weight). Results show that Mn in petioles is better correlated with Mn in wine than Mn in must. Mn content of wine is little in relation with EDTA Mn in soil. It increases when soil pH or cation exchange capacity decreases.

Conclusions: 

This study has shown that Mn concentration in wine can be naturally very high (maximum of 14.6 g/L in this study). Soils with low cation exchange capacity and/or low pH, i.e. soil types 1 and 8, resulted in higher Mn content in wine. Low cation exchange capacity does not allow a great Mn fixation on clay-humic complex and low pH soil solubilizes metal generally and Mn in particular, so it can be taken up by the vine. Mn petiole content is a very good indicator of Mn content in wine. Maceration in red wine-making is also an element to take into consideration.

Significance and Impact of the Study: Mn content in Beaujolais wine can be very high because of soil type, rather than fraud. It is important to highlight this for wine exportations. Mn content in wine can be reduced by correcting the soil pH.