Sometimes producers can have too much of a good idea. If a little extra freshness can lift the aromatic profile of a young white wine, it's not necessarily wise to go for a little more. Adding a touch of semillon to sauvignon blanc can bring out some of the more nettley flavours and also fill out the mid-palate.

The law permits the inclusion of up to 15% of another variety, without the need to disclose this on the label. In other words, a wine labelled simply "sauvignon blanc" may indeed have 15% semillon in it without the obligation to market it as a blend. Producers everywhere use this loophole to make a better wine.

A vast number of seemingly pure shirazes have a few drops of viognier, the variety which traditionally adds "polish" as well as a little perfume to Cote Rotie. Semillon helps to "fix" the more austere sauvignons and merlot (more in France than in SA) and adds a plush, rich note to the often quite unfriendly young cabernets of Bordeaux.

In every one of these examples, the trick lies in the restraint: the moment you can taste the viognier or the semillon, the shiraz or the sauvignon ceases to be what at first appeared to be on offer. It can become another perfectly good wine - but it is then a blend and it competes for your attention and approval in a different way.

It's an increasingly safe bet that many sauvignons contain at least a drizzle of semillon, shirazes of viognier, and cabernets a splash of shiraz. It's difficult to imagine why a winemaker seeking to make the best possible bottle from the wine he has in his cellar would leave out a component, when the mere addition of 2% or 3% would make a huge difference to the final product.

However, sometimes, in their enthusiasm, winemakers add too much. A year or two later, when the wine ages, what at first seemed like an eas y to integrate addition remains a jarring reminder of why less is often more.

I recently tasted the 2007 Lomond Snowbush blend, a wine that initially swept all before it in the sauvignon- semillon classes. Sadly, the small percentage of nouvelle, a crossing designed to lift the aromatics of slightly overripe sauvignon, has been doing too good a job - especially for a wine that never needed the extra freshness. The result is that now the other thatchy, grassy notes have integrated, the nouvelle sticks out like a Cape Dutch homestead in the Napa Valley. It's out of place - an unnecessary construction that has spoilt the lie of the land.

Sometimes I think the market must bear some of the responsibility for this kind of cosmetic manipulation. There is an undoubted pressure to improve on natural beauty by enlisting the assistance of Max Factor: the authentic, unflashy charm of a wine is deemed to be insufficient.

It is the purity of the Bouchard Finlayson Kaaimansgat Chardonnay 2009, for example, which makes it so attractive. It's still too young, so the CO2 presents as a distinct bubble (Peter Finlayson could hardly have imagined it needed a little fizz to go with all the other fresh notes).

Still, it has all the right elements. There's very bright, quite limey citrus fruit, with a tang on the finish as well as a slight creaminess to balance out the austerity.

Fortunately it hasn't been overdone, and while it is drinking well now, it will reward whoever is patient enough to let it age.

Sometimes the wine-making interference will vanish over time. The Vrede en Lust Mochalate Malbec 2009 is showing too much oak at present - clearly its producer aimed at getting a little of the business the coffee-chocolate pinotage brigade has been enjoying. Happily, the basic fruit quality is wonderful and the wood aromas are already evaporating. The wine never needed the oak and the winemaker should have had more faith in the grapes that came into his cellar.