Bioclimatic indices of the ripening period (2) : Thermal conditions at night
The Cool Night Index
The Cool Night Index was created to study nocturnal conditions of grapes ripening according to the different wine-making regions (Tonietto, 1999 ; Tonietto et Carbonneau, 2004). By convention, this index is also called nycthermal index and is equivalent in the North Hemisphere to the minimum temperature of the harvest month which is most of the time September. In the South Hemisphere, the minimum temperature of March is taken into account. There are 4 classes for night conditions as shown below in table 1.
Nighttime coolness and Wine characteristics
The nocturnal coolness is an important factor during the grapes ripening period. Vaudour (2003) explains that « this Cool Night Index of Tonietto is based on the relationships between cool temperatures and the biosynthesis of aromatic compounds during the ripening period, within the 30 days before harvest ».
A good synthesis of sugar in berries would usually be associated with wine regions experiencing a minimum temperature below 10°C (50°F) and a wide diurnal temperature range, during the period preceding harvest. Fregoni et Pezzutto (2000) explained that the last 10 days before harvest are crucial for quality and pointed out that « grapevines feel difficulties to ensure the transfer of compounds from photosynthesis, when temperatures stay high at night (like in climates with small thermal variations) ». For all these reasons, « the best organoleptic characters in grapes are obtained under climates with important diurnal thermal differences during the period preceding harvest » (Fregoni et Pezzuto, 2000).
Many other authors showed the relationships between minimum temperatures and/or diurnal contrasts and wine Characteristics. For instance, Winkler et al. (1974) showed that a small difference in temperatures between day and night give less marked colors on berries skin than if differences were greater. Jackson and Lombard (1993) reported that cool nights associated with warm day temperatures, i.e., great daily thermal amplitude, were able to decrease acid levels in grapes compared to cooler daytime and warmer nighttime conditions. More recently, Yamane et al., 2006 and Koshita et al., 2007 showed that « a large temperature range between nighttime and daytime allows a good biosynthesis of anthocyanins and abscisic acids giving grapes their skin color and wine its quality. »
Towards a new approach of the Cool Night Index
The Cool Night Index can be criticized since it is only based on the minimum temperature of one month, the one before harvest, specially if local scales are investigated. It seems more judicious to actually go into deeper details by calculating the amount of time during which grapes are exposed to different ranges of temperatures rather than the value of the monthly minimum temperature. This approach takes better into account the climate conditions within a complex terrain and specially under radiative weather conditions (Bonnardot et al., 2012). Indeed, spatio-temporal fine scale variations such as thermal inversions, common during clear sky nights, would be better considered using this approach. This still has to be further investigated in relation with berries composition.
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