John Gladstones has constructed his wonderful new book, Wine Terroir and Climate Change to examine the effect of each element of terroir (the environment) on the stages of growth and fruit ripening of the grape vine. He has proposed hypotheses around the vine physiological responses to these terroir signals and the ultimate effect on grape composition and wine quality and style.
Hence his chapters sequentially look at the effect on the vine of; atmospheric temperature and all of the other climate components; the geography, topography and soil; and the geology of the terroir. The book is largely constructed as an examination of the logic of terroir. It is terroir centric.
In Review 1 of the Wine and Terroir part of the book I tried to tease out of each of the chapters the vine physiological response as the central theme. Instead of looking at the atmospheric temperature affect on all components and phases of vine physiology, I have tried to look at an aspect or phase of vine physiology such as fruit ripening and attempted to summarise the effect of all of the terroir components on that physiological process.
The first review is directed at vine logic rather than terroir logic. It is vine centric.
There is one very important vine physiological response to terroir that was not included in the first review.
The missing physiological response is the formation and retention of aroma and flavour compounds in grapes and their expression in wine, which is at the very core of wine quality
At the time I did the first review I found it difficult (my problem) to extract the John Gladstones’s flavour formation and retention hypothesis from the many places flavour development was mentioned in the book.
Throughout the book, John Gladstones attempts to explain those environmental and genetic factors at work in the biosynthesis of the tertiary compounds at the core of grape flavour. Further he examines the environmental influences involved in the wine quality defining balance of the retention of these aroma and flavour compounds in the grape and their potential loss by vaporisation and chemical degradation. And finally he speculates about the role of these compounds in wine style and quality and the circumstance of appreciation of different wine styles.
The Formation and Retention of Grape Flavour and Phenols
Pervasive throughout the book are references to the author’s hypotheses about grape flavour formation and perhaps of equal importance to wine style and quality, the optimal environmental conditions required to retain those flavours in the berries and their transfer into the wine.
Here John Gladstones is referring to the non sugar ripening process which he defines as,
“The biosynthesis and storage of aroma and flavour compounds (engustment), loss of malic acid and other acids; continued skin anthocyanin synthesis in red varieties; loss of herbaceous aromas and flavours and very importantly the maturation of tannins in seeds skins and cluster stems.” (page 54).
A central argument of the book is that regional/site differences in heat summation and temperature equability largely define site suitability for varieties of different maturation groups.
He identifies the climate regime of the last 30 days of the ripening period as the most influential for the production and retention of the aroma and flavour compounds and the polymerisation and extractability of the phenols of the skins and seeds (page 20).
Climate is not the only terroir attribute at play in the last 30 days of ripening. As discussed in the previous review the role of abscisic acid is essential to the continuity of the ripening process and the synthesis of these tertiary aroma and flavour compounds. Responding directly to soil environmental conditions, the roots are the main source of abscisic acid (page 54).
However the quantum and quality of the tertiary aroma and flavour compounds synthesised is profoundly influenced by atmospheric temperature.
John Gladstones identifies the optimal mean temperature of the last 30 days of ripening for the synthesis of flavour and pigment in red varieties as 18-22ºC and for the best attributes of delicate white and sparkling wines the mean can be as low as 12-15ºC (page 20).
In table 3.1 (page 36) he summarises the ideal temperature regime, sunshine hours, rainfall and afternoon humidity for the final 30 days of ripening for the different wine styles ranging from sparkling wines, through light, medium and full-bodied table wines and then fortified wines.
The synthesis of the volatile aroma and flavour compounds is optimised when the abscisic acid ripening signal is strong, the light is diffused by cloud or the low sun angle at high latitude to maximise photosynthesis without harmful radiation heating, the day temperature is cool, the night temperature is warm and the afternoon humidity is moderate to high to restrict transpiration. He argues these low daily temperature range attributes are characteristic of the great terroirs (pages 38&39).
John Gladstones’s argument for the importance of the temperature equability (low daily temperature range) of a site, feeds into the aroma and flavour story. The low day temperatures at moderate humidity favour the direct synthesis of these aroma and flavour compounds and the accumulation of sugars as starch during the day. At night the surplus assimilate is translocated to the berries as sugar the substrate for aroma, flavour and phenol formation during the warm nights. The warm nights of temperature equable sites potentially allow 24 hour synthesis of aroma, flavour and phenols essential to wine quality (page 26).
Flavour formed in the berry skins at night has the least possibility of evaporative loss and the maximum possibility of being conjugated with sugar and transported to the interior of the berry where it is protected against degradation and evaporation (page47).
Importantly the moderate-cool day temperatures of temperature equable sites also limit the evaporative loss of volatile aromas.
Significantly John Gladstones introduces this new hypothesis that the low boiling point aroma compounds of grapes are volatilised from the berries at elevated temperatures and in the case of very high berry temperatures they are chemically degraded.
He uses the measured monoterpene loss from the leaves of forest trees and its exponential response to temperature to justify his berry aroma evaporation hypothesis in the absence of direct grape experimental evidence (page 20). Aroma and flavour evaporation from berries is a concept, which carries through the book.
The flavour story John Gladstones weaves is complex. He explores the well established science of volatile aroma compounds (terpenes) being trapped in berries by their conjugation with sugar into non aromatic and non volatile form and for the thiols which are important to Sauvignon Blanc and the Cabernet family aroma, their conjugation with the amino acid cysteine achieves the same result (page 21).
Thus the carbohydrate (sugars) and the nitrogen (cysteine) economies of the vine become important in retaining synthesised aromas and flavours, their conjugation protecting them from loss by evaporation.
The terpene glycosides undergo hydrolysis late in ripening and in fermentation to re-release the aromas and the thiols are released as aroma from cysteine bondage during fermentation allowing expression of these latent volatile aromas in the finished wine (page 21).
The importance of moderate-cool bunch temperature to the retention of volatile aromas is contrasted with the evaporative loss of berry aromatics and the “dead fruit” character of berry shrivel which is caused by abnormal heat stress during final engustment. (page 24).
The incidence of strong sunlight directly onto bunches creates an accumulative heating effect because of bunch geometry and the low number of transpiring stomates on berries which can raise berry temperatures 12 to 15º C above ambient temperature. This effect is especially accentuated in black skinned grapes and results in loss of volatile aromas (page 27).
Paralleling the volatile aroma and flavour story is the formation of tannins and colour (anthocyanins) in the ripening berry.
As discussed in the previous review the maximum amount of phenol has formed in the berry by the end of the lag phase of berry development (page 104).
The lag phase is the period before veraison in which seed development is completed under the control of auxin (page 102). The length of the lag phase largely defines whether a variety is early or late ripening (page 103).
In late ripening varieties, John Gladstones hypothesises that the long lag phase allows the accumulation of more tannin and colour (anthocyanin) than in early maturing varieties. This fits with universal observation that later ripening Cabernet Sauvignon has more colour and tannin than early ripening Pinot Noir (pages 106-107).
He argues, “early maturing varieties, phenologically adapted to short and cool climates ripen best under cool conditions.” (page 110).
The short, cool ripening season of early maturing varieties allows for maximum retention of the low boiling point aromas and flavours and restricts the development of tannin astringency and bitterness.
The prolonged lag phase and ripening season of late ripening varieties in warm climates allows the volatilisation of the low boiling point aromatics and the accumulation of higher boiling point aromatics and astringent tannins (page 106).
On the much discussed subject of tannin maturity, John Gladstones hypothesises that achieving tannin maturity is a result of a reduction in the total amount of tannin which can be extracted from the berry after veraison.
There is a larger reduction of the extractability of the smaller molecular weight tannins (catechins) in the seeds due to seed hardening than the larger molecular weight polymers (flavonols) from the skins. The catechins are responsible for bitterness and the flavonols for astringency, so the overall organoleptic effect is of a softening of total tannin astringency and the disappearance of bitterness (pages 104 to 106).
Early ripening varieties have less of all tannins but especially of those causing bitterness.
Finally John Gladstones speculates on what he calls “the ecology of winemaking and drinking”. (page 110).
To paraphrase this concept, wines made from early ripening varieties in cool regions are aromatic, relatively high in acid and low in tannins. Grape composition is responsible for these wine characteristics but also the low fermentation temperature in the cool autumn/winter of cool regions preserves and enhances these aromatic qualities.
John Gladstones extends the argument to what he terms “the co-adaptation in grape-growing, winemaking and wine drinking”, beyond the grape and fermentation “to the regional perceptions of wine style and quality.”
He makes the case that the northern European people and cultures naturally value the cool climate wine styles as the best terroir expressions of their regions. These wines are usually consumed cold to optimise the expression of the low boiling point volatile aromas and flavours.
By contrast John Gladstones attributes the room temperature consumption of the natural wines of late maturing varieties and warmer areas, to the need to “bring up the high boiling point aromatics that have survived warm ripening and fermentation and soften perception of high tannins.” (page 110).
The following is my comment
Comparing and contrasting the preference of the northern Germanic wine growing regions and cultures for aromatic low tannin, predominantly white wines with their Gallic neighbour’s preference for earthy flavoured, high tannin mainly red wines it can be inferred that vine terroir has imprinted on human preference and culture, not as is sometimes claimed the other way around.