How to manage in vine the green aroma in wines
Abiotic factors such as light and temperature at the bunch level, vine water status and various viticultural practices can influence the concentration of 3-isobutyl- 2-methoxypyrazin in the berry and the wine
The green aroma descriptors of Sauvignon blanc grapes and wines originate from 3-alkyl-2-methoxypyrazines (MPs). The most important MP found in grapes and wines is 3-isobutyl- 2-methoxypyrazine (IBMP), whereas 3-isopropyl-2-methoxypyrazine (IPMP) is rarely present in grapes at harvest.
Abiotic factors such as light and temperature at the bunch level, vine water status and various viticultural practices can influence the concentration of IBMP in the berry and the wine. It has been shown that grapes and wines from cooler climatic regions contain higher concentrations of IBMP than grapes produced in warmer regions. Furthermore, recent studies have shown the importance of the timing and severity of leaf removal on the final concentrations of IBMP in mature grapes. Scheiner have shown that early leaf removal, performed 10 days after flowering, intensely reduced IBMP concentration in Cabernet Franc grapes, while the same treatments applied 40 and 60 days after flowering had a less significant effect. Leaf removal performed after v?raison had little or no effect on the IBMP concentration in the grape berries. Pre-vraison bunch exposure is therefore crucial for reducing IBMP concentrations in grape berries at harvest, whereas light exposure after v?raison does not influences IBMP degradation.
The experiment was performed in a commercial Vitis vinifera L. cv. Sauvignon blanc (clone 316 grafted on 101-14) vineyard located in the Overberg region of the Western coastal area, South Africa.
To study the influence/role of light and temperature, a control canopy (shaded treatment) with 100% shaded bunches was compared to an exposed treatment with 100% exposed bunches on the morning side of the canopy. To accomplish this, complete leaf- and lateral shoot removal was performed at the phenological stage of berry pea size (19 December 2012). Leaves and lateral shoots were removed from the bunch zone on the morning side of the canopy to a height of 0.3-0.4 m above the cordon. As a control, no leaf- or lateral shoot removal was performed on the shaded treatment. The stem water potential (SWP) was used to determine the vine water status. The vines did not experience any water constraint during the growing and ripening season with a mean SWP-450 kPa.
The choice of the site for Sauvignon blanc was done considering the positive effect of the sea breeze, coming from the Atlantic Ocean, on the bunch microclimate.
The treatments did not affect the berry fresh mass and the main grape berry maturity parameters at harvest . The concentration of IBMP was analysed in whole grape berries at intervals starting at 53 days after flowering (12 January 2012) and ending at harvest (113 days after flowering; 13 March 2012). Leaf and lateral shoot removal in the bunch zone resulted in lower IBMP concentrations during ripening. Significantly higher concentrations of IBMP were observed in the shaded treatment in comparison to the morning exposed treatment at 51, 59 and 73 days after flowering. At harvest, no significant differences were noticed between the treatments. A faster decrease in the IBMP concentration in the grape berries for the first three sampling dates of the morning exposed treatment were observed. This can be due to a lower IBMP accumulation in the grape berry which is most probably a consequence of early leaf and lateral shoot removal. From day 94 after flowering to harvest, little change in the IBMP concentration in the morning exposed treatment occurred, whereas the concentration of IBMP in the shaded treatment decreased slowly up to harvest. Regardless of the loss of significance in the IBMP concentration in grape berries at harvest, leaf and lateral shoot removal does offer an option to harvest Sauvignon blanc grapes at a lower total soluble solids level with less methoxypyrazine when compared to the shaded treatment.
The IBMP concentrations in the wines were higher in the shaded treatment when compared to the morning side exposed treatment. The IBMP concentration in the wine from the shaded treatment was above the detection threshold (2 ng/L), corresponding with the attributes detected during the sensory evaluation of the wines.
Tropical descriptors (grapefruit, passion fruit and guava) were mainly grouped with the varietal thiols such as 3-sulfanyhexan-1-ol (3SH) and 3-sulfanyhexyl acetate (3SHA) and some esters. By applying leaf and lateral shoot removal, we were able to produce a more complex (fruitier) style of Sauvignon blanc in comparison to the one dimensional (greener) style of the wine produced from the shaded treatment.
To reduce the pyrazine level in the berry and consequently in the wine, canopy manipulation can be applied in the bunch zone by removing all the leaves and lateral shoots at a height of 30 to 40 cm above the cordon. ? Removing all leaves and lateral shoots in the bunch zone on one side of the canopy is an effective tool to reduce the IBMP concentrations in the wines. The choice of the canopy side to remove leaves has to be reasoned properly according to the row orientation and site location.
By removing only the laterals in the bunch zone, and keeping the basal adult leaves of the primary shoot intact, sufficient light will be allowed into the canopy at the bunch level. This selective opening of the bunch zone would, however, be more expensive than just removing both laterals and basal leaves simultaneously.