Tuesday, 05 April 2016 11:55

Protecting grapes from UVs

Written by  Tessa Nicholson and Dr Damian Martin – Plant & Food Research, Marlborough
UV light is essential for fruit development – but too much can be detrimental to the ensuing wine. UV light is essential for fruit development – but too much can be detrimental to the ensuing wine.

We humans have to take measures to protect our skin, hence the campaign of slip, slop slap.

But when those long days, clear skies and rising temperatures appear, how do you protect your fruit from the dangers of UV rays? Do they even need protection?

The answer is quite simply – yes, especially if the variety you are growing is Sauvignon Blanc.

Dr Damian Martin from Plant & Food Research in Marlborough says UV rays can be as damaging to berries as they are to human skin. Too much exposure can also result in a change in wine style, that may not be wanted.

For example, take 1998 – the last major El Nino cycle experienced in New Zealand. In Marlborough in particular the summer was long, cloudless and very hot. The ensuing wines, which many winemakers had been excited about, didn't receive the rave reviews most were expecting. Martin says that had a lot to do with the dry conditions and intense sun between January and April, but it was also a reflection of viticultural practices at the time.

"In 1998 we were extremely excited because Sauvignon Blanc got to 23 or 24 brix which it had never done before. I was at Corban's and we were looking after the Stoneleigh vineyard. We were really excited about the wines. But we harvested too late at the end of the day. That was the lesson learned. In hot dry years you have to pick early so you can get more freshness, more acidity and a bit more herbaceousness in the wines to make a classic Marlborough style."

Martin says the effects of UVs on grapes are very specific to the formation of phenolic compounds.

"A grape berry exposed to UV will produce phenolic compounds to absorb that UV and protect it from damage. Too much UV and those phenolics will then come out in the juice and into the wine."

The end result tends towards a style that doesn't match a typical Marlborough Sauvignon Blanc.

With El Nino currently in full force and some of the hottest days of the year still to come, he says growers will need to take precautions to protect their fruit from over exposure. Which can be difficult if the ensuing drought continues. With irrigation likely to be turned off in some parts of the country, vines will be under stress.

"One of the effects of water stress is a much greater chance of basal leaf drop, which leads to more UV exposure and warmer bunches."

Martin says it is a double edged sword, given one of the ways to assist a water stressed vine is to remove some of the canopy to encourage fruit development. If the vine is too stressed, some of that canopy will automatically die off – again exposing the fruit to sunlight.

If you need to reduce leaf area, he suggests removing top canopy only, leaving the basal leaves for as long as possible.

In terms of red varieties, the issues with UV damage are not quite as high. Mainly because red fruit requires UVs to help with colour development and to ensure that colour remains stable during the wine making process.

"But even so," Martin says, "you can still over do it in terms of exposure. It's just they are much more forgiving and you get more gains from exposing the fruit than you get with Sauvignon Blanc."

Whereas in Australia sunscreen products such as Kaolin are often used to protect the fruit from UV damage, Martin says it isn't a practice usually employed in New Zealand. We can possibly thank our specific climatic conditions for that he says.

"One of the unique things about New Zealand is that we have very high energy from the sun but with a cool ambient temperature. Most places in the world where they experienced the equivalent energy that we get, would be 45 degrees outside. Your vines wouldn't produce quality in that.

"However that cooler ambient temperature is almost certainly a contributor to the type of New Zealandness of our wines. We have the cool ambient temperature combined with the high UV energy – so we are getting the best of both worlds."


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