Grapevine & Wine 02/05/2011

A reduction of herbal excesses in wine is possible

Methoxypyrazines are aromatic compounds which can be unpleasant in excessive quantities. It is better to intervene on the field than in the cellar

Pyrazine compounds are very important for food flavor. They were identified for the first time in a study on the volatile fraction of Cabernet Sauvignon grapes, and have been found in other varieties of white and colored grapes, such as Cabernet Franc, Merlot, Pinot Noir, Sauvignon Blanc, Chardonnay, Riesling.
These compounds in wine constitute a limited group of aroma compounds, which are extremely powerful because of their extremely low perception threshold (10 ng/L in wine).

Wine methoxypyrazines are in particular responsible for herbal, pepper, asparagus, cooked potato, cooked green beans, and earthy tones olfactory sensations. 2-Methoxy-3-isobutyl pyrazine is responsible for green pepper smells, while 2-methoxy-3-isopropyl pyrazine for a pleasant alcoholic smell. When these compounds are present in a not ripe fruit they are often unpleasant for the consumer. The most important of these compounds is 3-isobutyl-2-methoxypyrazine (IBMP). According to some authors, 3-isopropyl-2-methoxypyrazine (IPMP) can be the most important. Methoxypyrazines are present in relevant quantities in some French varieties (Sauvignon blanc, Cabernet Sauvignon, Cabernet Franc, Merlot, Carmenère etc.). IBMP is present in concentrations between 0.5 and 50 ng/l in Sauvignon Blanc and Cabernet Sauvignon, and in even higher concentrations in Carmenère. Concentrations up to 40 ng/l can be found in Sauvignon Blanc, thus determining its vegetable note.

Pyrazines synthesis is reduced as the fruit ripens, since these substances are degraded. The influence of weather on pyrazines has been analyzed too, revealing that it is instrumental in determining the concentration of these compounds during the development of grapes. A study on variations of pyrazines during the development of Sauvignon Blanc grapes in two different regions of New Zealand in 1987 (the first region added up to 2100 degrees, while the colder of the two regions added up to 1430 degrees), it was found that the content of pyrazines in the grapes of the two regions, as fruits reached the maturation state (21 Brix), was equal to 1.3 and 9.5 ng/l, respectively. In grapes from the warmer region, the concentration was lower than the perception threshold, while it was about 5 times larger than the threshold in grapes from the colder region.

The possibility to reduce the herbal note, which is highest at the véraison, and to reach acceptable levels for wines, is thus associated to the conditions of maturation. The role of agricultural practices, of environment and weather is important, as shown by the study finding that more herbal wines are obtained in colder regions.

Justin J. Scheiner and his staff carried out intriguing studies on Vitis vinifera L. cv. Cabernet Franc and Merlot to evaluate effects of the timing and entity of removal of basal leaves on the concentration of IBMPs in grapes. Treatments consisted in the removal of 50% or 100% of leaves from the area where fruits grew, 10, 40 or 60 days after anthesis. During the second year of the study on Cabernet Franc another treatment was practiced 15 days before véraison. In both years relevant reductions (between 28% ad 52% from the control values) of IBMPs were found before véraison in both treatments (50% and 100% removal) at day 10 after anthesis.

In 2007, all defoliation treatments significantly reduced (between 46% and 88% with respect to the control group) IBMP concentration in Cabernet Franc grapes at the harvest, with a more pronounced decrease in treatments entailing 100% leave removal, 10 and 40 days after anthesis.
In 2008, the 100% removal treatment 10 days after anthesis, and 50% and 100% treatments 40 days after anthesis significantly reduced IBPM concentrations (34% to 60%) in ripe Cabernet Franc grapes.

In the case of Merlot grapes, all defoliation processes determined a reduction in IBPM concentrations (38% to 52%) at the harvest.
Removal of leaves at the beginning of the season (10 or 40 days after anthesis) caused a diminution in IBPM accumulation before véraison in both experiments, thus suggesting that precocious defoliation represents a more effective strategy for the management of vineyards, as far as the reduction of IBPM in grapes is concerned, with respect to a later defoliation.

di Graziano Alderighi