Thank you Nancie Hemminger and inderpal singh for your contributions last week on Rosé Winemaking!
This week: Diurnal Shift
What is “diurnal shift” and why is it important for quality winegrowing regions?
Sweet! A weather question! "Diurnal shift" or diurnal temperature variation describes the change in temperature from its maximum, typically in the late afternoon, to its minimum, typically just before sunrise. This temperature range is important for viticulture in that given ideal conditions, the fruit can maintain a good balance between ripeness and acidity. During daylight, photosynthesis provides the fuel (sugar) for growth and ripening. If nighttime temperatures are sufficiently warm, the vines will continue to try to ripen. With the loss of fuel from photosynthesis, the vines will use its own malic acid for fuel. So, the lower the nighttime temperatures, the more malic acid that remains. There is a lot of interplay between diurnal temperature variation and the locality of the vineyard. Far northern climates (think the Mosel) will have lower daytime high temperatures, but more daylight hours given its latitude, so sugar accumulation would be lessened by lower temp, but increased by more hours of photosynthesis. Mountain valleys will have lower nighttime temps owing to drainage and cold air damming etc. As with most everything in winemaking, there are no hard and fast rules, but there are locations that provide, in general, better growing conditions than others. Climate change also plays a role here as diurnal ranges are generally getting smaller as the planet warms. Again, I'm broad-brushing here, but a smaller global diurnal range is likely not a good thing for those of us that like high acid wines.
Diurnal shift's effect on the preservation of malic acid is probably more important in hotter regions (regions closer to the Equator) where hotter days need colder nights to stop malic acid consumption.
However, in cooler regions, where daytime temperature is not that high, there is no need for a big diurnal shift .There’s no benefit because if both day and night temperatures are moderate, malic acid consumption will be moderate as well.
And in cold/marginal regions,, maintaining acidity is usually not an issue as growers are more concerned with getting sufficient ripeness. Losing acidity is generally not a problem there.