throwing some light on heat >

2003
throwing some light on heat

Ripeness is generally seen as a function of flavour - the desirable endpoint of a flavour progression as the grapes hang on the vine in sufficiently warm conditions to effect change. Conventional wisdom has been that the warmer the grape growing region, the more quickly the vine is able to ripen grapes. Early attempts to measure the physical parameters of regions led to the classification of California into five climatic regions based on heat-summation units (Amerine and Winkeler, 1944). The units, called degree-days, are calculated by multiplying the number of days during which a vine is growing by the average number of degrees for each day above 10ºC (i.e. after subtracting the 10º from the average). The theory is that significant growth starts only at around 10ºC. Martinborough would be considered a very cool growing region, with a heat summation of around 1,150 degree days, and is very similar to Dijon in Burgundy and Geisenheim in Rheingau (Germany). Other classification systems exist for grape growing regions, and these may confer more precision than that of Amerine et al., but in my view they still exhibit the fundamental flaw that they consider the ripening process to be a function of heat alone.
Literally hundreds of flavour compounds are found in ripe grapes. These include many from the important terpene family, a large number of which are found in other types of fruit and food. However, explanations about nature are never as simple as they first appear. Consider Riesling. If we observe the ripening process, flavour changes are taking place as the sugar rises. At lower sugar levels, the flavours are mostly floral; if this process continues, the character of these flavours transforms to citrus (lemons etc through to grapefruit) and finally into more luscious fruit (white peach, yellow peach and even some tropical fruits). In the slightly warmer areas of Germany such as the Rheingau, flavours can be of ripe peach, or even guavas, but not too far away in the cooler Mosel and the even cooler Saar, wines are harvested during an earlier part of the flavour spectrum, so that they display much more of the earlier floral character (depending on the warmth of the vintage). It could at first be considered rather odd that many consider the Mosel and Saar wines to be the finest expression of this variety, as they are less flavour ripe, and have lower sugars. If fruit from the warmer Rheingau were picked earlier in an effort to produce a wine more like that from the Mosel, we might end up with a wine that shows more flavour similarities, but it would inherit something which is slightly mean, maybe green, and which could be considered to taste 'unripe'. Thus we arrive at the notion of 'physiological ripeness' or, as others might describe it, 'phenolic ripeness', and this has to be considered at least as important as the degree of flavour ripeness and levels of sweetness.
A large number of compounds from the phenol family that occur in grapes and wine are not ‘flavours' as we commonly conceive of them. But these compounds do affect our palate and can appear astringent or bitter, or have some tactile effect. Earlier in the phenolic ripening process there might be ‘green flavours' associated with them, but these green flavours will later be less obvious or transformed to other less strident vegetal characters. Another important factor in their impact on flavour is the way they ‘tighten' the traditional flavour profile, being able to make the wine seem less generous and less acceptable, even astringent. It is these sensations which can cause tasters to ascribe a lack of ripeness to a wine, or, to describe it as ‘lacking generosity' or simply as less attractive when compared to another version which is phenolically riper.
I would therefore divide the ripening process of grapes into three elements - the production of sugar, the ripening of traditional flavours (terpenes and the like) and the ripening of the phenolics. There is an interdependence between these three groups which means that under given conditions, they tend to ripen hand in hand; however they can do it more quickly or slowly than each other depending on variables such as the heat/light input, crop levels, soil temperatures, available moisture, wind speeds and canopy management. It is in the winemakers interests to restrain the sugar levels (i.e. not allow the final wine to become too alcoholic), to find the appropriate flavour profile (not necessarily the ripest), and to secure phenolics which are ripe enough for the wine to be considered generous but not so ripe that the wine loses the other benefits of protection against oxygen in the ageing process, nor its structure - 'tactile form' - for when it is tasted.
Based on heat summation, one would expect Martinborough to have similar ripening characteristics to both Burgundy (for Pinot noir) and Rheingau (Riesling). In fact it appears that Martinborough achieves flavour ripeness and higher sugar levels in its grapes with considerably greater ease than its continental counterparts. On the other hand, it is also noticeable - particularly in reds - that in some vintages we have more difficulty in achieving phenolic ripeness in the wines, and so tannins in reds (in particular) can be rather green, making wines appear less ripe. My suspicion was that the ripening of these compounds requires the higher mid-season temperatures of the continental climate rather than our drawn out, milder maritime climate. Our problem therefore became how to compensate for the mid-season energy deficit without accelerating flavour ripening and sugar accumulation. There is a considerable body of work which points to the benefit of direct sunlight onto the grapes later in the season, but much of this research has been carried out in continental climates and it espouses caution because of grape sunburn under peak temperatures and the more extreme conditions.
We have found that light exposure right through berry development toughens the grape skin, heightening resistance to disease and sunburn, and that the effect of the light enhances that of the heat on the phenolics from mid-season, advancing the ripeness of the tannins and phenolics while leaving the true flavour profile and sugars relatively less advanced. The result is more generous-tasting wines; a better, riper fruit tannin structure that offers the potential for the wine to be longer lived; a flavour profile that is largely unchanged - and all this in a wine with a lower alcohol than it would otherwise have. And there is more...For our latitude, we would expect to have 15% more light than rural Germany or Burgundy but instead the increase is 40-50%. Pigmentation in most white and red cultivars is restricted to the outer hypodermal layers of the skin. In the white grapes, the colour comes from several groups of compounds including the important flavonoids, the most prolific being quercetin which is a plant defence against sunburn. The production of this compound in New Zealand whites can be dramatically enhanced by exposure to our heightened levels of sunlight, and it appears to be no coincidence that research at UC Davis by Professor Waterhouse found in a very large survey of quality wines that the best correlation between wine quality and any of the constituents of the wines was in the level of this compound. In reds, similar pigments occur but here the predominant pigments are anthocyanins.
We have had similar results. In all but the most extreme years, completely exposed fruit inevitably produces better white wine. In reds, there is some argument about the correlation between wine quality and colour (the colour compounds are also phenolics), but our experience is that: for fruit from a given site, if exposure enhances the phenolic colour compounds, the quality of the final wine is also enhanced. Our viticultural management enhances the colour of the red wines as well as the ripe phenolic structure, and this is the core of what we are aiming for in the Dry River style - rich colours, generous-tasting wines, lower alcohols, and a fruit tannin structure that we hope will sustain the wine for a long and positive maturation.

 

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