Diurnal temperature shifts and wine quality

Diurnal temperature range (DTR) refers to the difference between the highest daytime temperature and the lowest nighttime temperature.

Source: http://apollo.lsc.vsc.edu/classes/met130/notes/chapter3/daily_trend4.html

As it relates to wine, many estates/regional promoters tout the fact that their estates are located in areas of high diurnal variation because of, as they see it, the benefits conferred on grapes grown in the region.  For example, washingtonwine.org states "One of the greatest natural phenomena for growing grapes which end up balanced between ripe sugars (which will equate to alcohol in the wine) and crisp acidity is a difference between day time and night time temperatures ..."  The site goes on to say that Washington State has some of the most dramatic fluctuations in the world with differences of as much as 40℉ between the high day and low night temperatures.  The organization sees the cool evenings as preserving malic acid in the grape which "translates through fermentation to wine and adds freshness and balance."  It should be noted here that the organization sees wine balance as a mediation between sweetness and acidity, a position that runs counter to the definition that has been established on this blog.

Concha y Toro, one of the leading Chilean producers, sources grapes for its Don Melchor label from the Puente Alto vineyard in Maipo Valley.  According to the winery's website (conchaytoro.com), "It is the sharp diurnal temperature differences that assist tannin ripening and the development of aromas as well as fixing exceptionally high quantities of polyphenol compounds in the grape."  In their discourse on the benefits of high diurnal variation, Concha y Toro focuses exclusively on its perceived benefits for the secondary metabolites versus the primary metabolite arguments of washingtonwine.org but, in both cases, the argument is "high DTR is beneficial for the grape and its end product, the wine."  That is not a universally held position.

There is a second school of thought which decries significant DTRs and views balanced wines as a product of balanced temperatures.  The principal of this school is John Gladstone (Wine, Terroir, and Climate Change) but philosophical adherents are sprinkled throughout the industry (see, for example, Climate and the ripening process, wine business.com, August 2007; What makes LI wines "cool", Suffolk Times, March 31, 2011).  The basic tenet of this school is that a low temperature range during the growing season creates wines of the best quality and that sites with this characteristic tend to be in maritime or high-latitude locations.

According to adherents, the factory that is the vine operates on all cylinders during the daytime with the presence of light allowing for photosynthesis and the accumulation of sugars into the berry.  Photosynthesis ceases with the onset of darkness, but, according to this school, if the nighttime temperature does not fall below a certain level, respiration and flavor and tannin synthesis will continue, resulting in more rapid and complete phenolic ripening of the fruit at lower sugar levels.  Diurnal variation in arid regions, according to this school, allows production of high levels of sugar during the daylight hours and the cool nights take the vines out of the effective metabolic range.  This slows the phenolic ripening process thus allowing the accumulation of higher sugar levels over the longer ripening period and an unbalanced wine.

At this moment let us take a step back and look at what is known regarding temperature effects on berry development:

Higher temperatures

• increased rate of sugar accumulation
• increased rate of organic acid degradation
• inhibition of anthocyanin development

Lower temperatures

• reduction in the rate of sugar accumulation 
• reduction in the rate of organic acid degradation.

We also know that phenolic ripeness is highly temperature dependent (Mark Greenspan, wine business.com) and that phenolic ripening processes will operate sub-optimally or shut down as you move further away from the optimal (Mark Greenspan; Louisa Hargrove, Suffolk Times).

Based on the foregoing, the following Greenspan adages still hold true:

• In arid regions, cool nights are essential for the prevention of rapid acid metabolism
• Different wine styles are produced in high-DTR versus narrow-DTR areas
• Areas with narrow-DTRs generally gain phenolic ripeness at lower sugar levels than do their high-DTR counterparts

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