Bruce relates that the average pH has been steadily rising from a decade ago where the average Cabernet pH would have been 3.3. This has risen to a cool year average of 3.4 and in the warmer years edging up to 3.7.
Obviously this is problematic because it necessitates higher and higher levels of acidification. But things are complicated by the fact that the decrease in total acidity isn't a spread across the different naturally occurring grape acids. In fact it is overwhelmingly the Tartaric acid which is being lost. This means that the grapes arrive into the winery with proportionately raised Malic acid levels. Malic acid is not microbiologically stable, and is almost always lost after fermentation through the action of Malolactic bacteria. This has a knock on effect on the final acidity of the wine. Meaning that Bruce then has to decide whether to acidify the must to a pre malolactic fermentation level which is more suitable for fermentation, or to a post malolactic ferment goal.
Incidentally when it comes to correcting acidity, it's pretty much established practice across South Australia. 3.57 is the magic number, pH's above this figure leave a fermenting must wide open to oxidative problems, and will likely lead to heightened volatile acidity or worse. So obviously a must with a pH above 3.57 will need acidifying, but the other benefit of acidity is that it will balance the final wine, acting like a frame work for the fruit and tannin to hang upon. It is here where things get difficult because the wine maker needs to anticipate how much acidity will be needed, but there is always some acidity consumed during fermentation. The final complicating factor is that post fermentation acidity fixing never really integrates into the texture of the wine and often appears on the palette as a kind of disjoint between the fruit of the wine and the acidity alongside it.
There are several reasons why the average pH values could be increasing, however Bruce attributes it to increasing vine stress, which may be caused by any of several factors. Obviously, eight years of drought is almost certainly the main factor as the increased temperatures, accompanied by shortage of water put great stress on the vines, however the is evidence of concurrent draining of both sodium and potassium levels in the soil, which may very well impact on the vines ability to ripen in a slow and balanced fashion.
There has been some research into Cabernet clones that are better able to cope with the hotter weather, however it appears that most of this was done some time ago, and really with vines planted in 1968 from cuttings taken from Jock Redman's vineyards, there is little that can be done on that front in the near future.
Hopefully the Lynn's will adapt to the new conditions, because a tasting of their wines showed some beautiful wines, their entry level Musician 08 came across as an excellent value example of Coonawarra fruit, more seriously their 2008 Cabernet Sauvignon positively glowed with minty, eucalypt and cassis aromas, medium weight chewy tannins had me thinking naughty thoughts about serving it lightly chilled of a summers evening. Their 2006 Shiraz was showing slightly maturing red berried fruit,a delicate earthiness and boasted great drinkability. Of their top end wines the youngest, 2007 Malleea was a riot of sophisticated oak notes, balsamic vinegar, cassis, dark cherries and cream. The 2004 of the same wine was easing itself into its drinking window still prodigiously full bodied but with complex secondary notes, earthiness and lots of deep dark brooding fruit, though still balanced and drinkable.