The red wines from Bordeaux contain one or more of several grape varieties: Cabernet sauvignon, Cabernet franc, Merlot, Malbec and Petit verdot. (They used also to contain Carménère, but that grape is now rare in Bordeaux.) When Robert Mondavi and Philippe de Rothschild decided to make a Bordeaux-style wine from Napa-grown grapes, they naturally used these same varieties.
This wine has been known as Opus One, with its first vintage in 1979. It was the first ultra-premium wine from the USA, the California equivalent of a Bordeaux first growth, intended as a benchmark for the wines produced from cabernet grapes in the Napa Valley. It has struggled to maintain that reputation, as it has been persistently criticized for inconsistency from vintage to vintage. Certainly, other wines have surpassed it in price and/or reputation (e.g. Ridge Monte Bello has a similar Bordeaux-style aim), although they all sell considerably fewer than the 25,000 annual cases of Opus One.
This inconsistency bears looking into. I contend that it has at least something to do with the variation in grape varieties.
The wine started out as a blend of mainly cabernet sauvignon, along with some cabernet franc and merlot. Then, malbec was added to the blend in 1994, and petit verdot was added from 1997 onwards. The proportion of these grape varieties in the wine has varied from year to year, as determined by the winemakers. The winemakers were Tim Mondavi and Lucien Sionneau from 1979–1984, and Tim Mondavi and Patrick Léon from 1985–2000, with Genevieve Janssens assisting from 1991–1997. Michael Silacci has been the chief winemaker since 2001, early on with either Tom Mondavi or Philippe Dhalluin, but alone since 2004.
In this blog post I wish to look at the variation through time in the diversity of the grape varieties within the wine. A number of mathematical measurements of diversity have been developed in science, for making precisely this sort of comparison. The idea is to reduce the proportions of the various grape varieties down to a single number (for each vintage) that quantifies their diversity, from a single grape variety at one mathematical extreme to equal amounts of each grape variety at the other extreme.
The one I will use here is called the Shannon Diversity Index (see Wikipedia). This Index will be a number between 0 (for a single grape variety) and the natural logarithm of 5 (for equal amounts of each of the 5 varieties). The data for each vintage come from the Opus One web site. The variation in Shannon diversity is shown in the first graph, with the vintages plotted horizontally and the diversity plotted vertically.
This graphs shows that there was a lot of variability between the first few vintages, while the winemakers worked out what wine style they were aiming for. Furthermore, from the early 1990s onwards the diversity has steadily increased. This has been partly the result of using five grape varieties, as opposed to the original three, but it is mainly a result of using greater proportions of the minor varieties. In the early years, there were vintages composed of >95% cabernet sauvignon, but over the past 10 years it has been closer to 80%. For the rest of the grapes, it has been c.7% merlot, c.6% cabernet franc, c.6% petit verdot, and c.1% malbec.
Having established that the winemakers have been moving towards a greater diversity of grape varieties in Opus One, we can now ask whether this has improved the wine quality in the eyes of the drinkers. There have, of course, been a number of retrospective tastings of the vintages of Opus One, which is getting closer to its 40th vintage. It therefore seems worthwhile to see whether the quality scores given to these wines are associated in any way with the particular mixture of grape varieties that have been included in the wine over the years.
The most complete vertical tasting that I have been able to find is that of Antonio Galloni, from 2013, which included all of the vintages from 1979–2010. Sadly, the best vintage of all has been suggested to be the 2013, which misses out. In the next graph I have plotted Galloni's quality scores (vertically) against the Shannon diversity (horizontally), with each point representing a single vintage.
The graph shows a general increase in quality score with increasing diversity, with four exceptions (as labeled in in the graph). Excluding these four vintages for the moment, a correlation analysis shows that 42% of the variation in the wine quality score is associated with the grape diversity score. That is, increasing the diversity of the grape varieties in the wine has generally improved the quality, which is presumably what the winemakers have intended.
The four exceptions are instructive. The 1980, 1984 and 1987 vintages consisted almost entirely of cabernet sauvignon (>95%), and this has obviously been a very erratic strategy in terms of wine quality (sometimes it worked and sometimes it didn't). Furthermore, the 1980 and 1984 vintages consisted solely of cabernet sauvignon and cabernet franc, with no merlot at all. On the other hand, the 2006 vintage had the highest proportion of merlot yet, at 12%, which is double the usual amount. This created a high diversity value but obviously not a high quality score from Galloni. The winemakers again tried such a high proportion of merlot for the 2011 vintage (11%), and Galloni's preliminary score for the resulting wine (not yet released) indicated that he didn't think it had worked then, either.
This pattern could, of course be unique to Antonio Galloni — I have repeatedly pointed out that wine critics rarely agree much with each other about wine quality (see How large is between-critic variation in quality scores?). However few of the other professional commentators have conducted extensive vertical tastings of Opus One. So, by way of comparison, let's look at the opinions of a group of non-professionals.
In 2002, Bob Henry, a wine marketer from California, conducted a group tasting of the first 20 vintages of Opus One (1979–1998). Each of the 23 tasters was asked to rank their top three wines, with 3 points being assigned to the top wine, 2 points to the second wine, and 1 point to the third wine. These scores were then summed across the tasters, in order to rank the quality of the vintages. These results are compared to those of Galloni in the next graph, with each point representing one of the 20 wines.
As you can see, only nine of the wines scored any points (ie. was a top-3 wine for any of the tasters). Most of these wines were also high-scoring wines for Galloni, and so we can treat this as a general confirmation of his scores. However, note that the 1980 vintage, which had a low grape-diversity score but still received 95 points from Galloni anyway, was not a high-scoring wine for the tasting group. This means that only the 1987 wine scored points but had a low grape-variety diversity. Indeed, the 1991 vintage was the only high-scoring wine before the introduction of malbec and petit verdot to the mix.
In biology (including agriculture), diversity is considered to be a Good Thing. Here, the Opus One wine seems to support this idea, as increasing diversity of grape varieties is associated with higher quality wines. Furthermore, the winemakers have been steadily increasing this diversity with each succeeding vintage. This is a strong argument for varietal diversity in wines. If nothing else, this helps explain the wine's reputation for inconsistency — poor vintages have generally arisen from reliance on too few grape varieties.