We all have different thresholds of perception be it for aromas, flavours or tactile sensations. I was reminded of this frustration – ‘what, you mean you don’t smell the TCA in this wine?’ – at an eye-opening tasting organised by De Grendel’s Charles Hopkins. The topic on this occasion was greenness in wine, cabernet sauvignon in particular. The original impetus for such exploration was a comment by Australian winemaker, Sue Hodder of Wynns. She had pin-pointed a red wine in an international line up as South African; when Hopkins asked her why she was so sure, she answered because of  its greenness.

Wanting to be on a little surer ground about thresholds, I delved into Emile Peynaud’s The Taste of Wine, which I was sure would bring some clarification and, sure enough, there’s a whole, detailed chapter on The Senses.

Thresholds come in more than one level, Peynaud explains. There’s the threshold of sensation, explained as ‘the weakest concentration of stimulus which will give rise to a sensation’, or ‘the minimum quantity of a given substance (e.g. sugar for sweetness) needed to produce a sensation which cannot however be identified at that level.’ A step up from that is the threshold of perception, ‘identification and recognition’, again requiring a minimum quantity of a substance for it to be recognised and identified. While recognition can be intuitive, identification can require training or experience.

The chemical compound we were looking for in Hopkins’ tasting was 2-Methoxy-3-isobutylpyrazine, which provides grassy, herbaceous and green pepper aromas.

This is obviously a very strong compound, as it can be detectable at a concentration of 2 ng/l in water. ‘n’ is a nanogram, which is 1/1000th of a milligram, or something unimaginably miniscule.  Anything over 10ng/l is considered problematic, while Dr Bruce Zoecklein, emeritus professor at Virginia tech, believes a wine with a level of over 30ng/l is faulty. (See here For Dr Zoecklein’s occasional and interesting notes on various aspects of winemaking, available free.)

Hopkins tuned our palates with two vintages of his Koetshuis Sauvignon Blanc, the much-awarded 2010 and the 2011. Analysis, at considerable cost, reveals the former has an IBMP level of 55ng/l (so faulty according to Zoecklein and, I can imagine the French judges on the Classic Wine Trophy, who usually lambaste our sauvignons for their greenness!), the latter 17 ng/l. The difference is vintage related, the latter wine still with a thread of greenness but juicier and the one I could drink more comfortably.

Switching to cabernet, two from Firgrove and one from De Grendel, I was intrigued to note the different way I experienced greenness; in the sauvignons it was more of a flavour, but in the cabernets it was more of a tactile sensation of tannic austerity.

According to empirical research Hopkins has carried out, site plays an influential role; cooler spots at higher altitude and closer to the ocean produces cabernets with more greenness; cabernets from warmer spots, further from the ocean, are less green. It goes much deeper than that; it’s subject that would make a worthy topic for a Masters thesis.

Finally, Hopkins presented blind six cabernets or Bordeaux-style blends, with the request we select our favourite and the wine with the highest IBMP level.  This exercise re-inforced my like of tannin, grape rather than oak, which gives form but which is missing in those lush, over-ripe – and over-oaked – reds. Was it an over-reaction then to Chateau Pichon Longueville 2008, which several of us considered over-ripe but which had the lowest IBMP level of 4 ng/l?  The balance, all South African, clocked in between 10ng/l (Rust en Vrede Cabernet 2009) and 42 ng/l (Meerlust Rubicon 2007); half of the tasters showed a preference for the Rubicon and Hillcrest Hornfels 2008 (41 ng/l), the two wines with the highest IBPM levels, which, if nothing else, indicates a high tolerance for tannins.

What are the implications of this exercise? Apart from perhaps the incorrect siting of cabernet, it could well explain why many of our cabernets and Bordeaux-style blends age rather than evolve, that green element never disappears, nor does it allow for the development of those more interesting secondary and tertiary flavours.

This is a subject that deserves much more research. As always, wine is a journey rather than a destination.