Three grape health checks
As we approach the beginning of harvest you will see an increasing number of men and women in white coats plucking unripe berries from grapevines. One convention says they must pluck three berries from the top, middle and bottom of bunches from all parts of the vineyard until they have a statistically significant sample totalling 200 berries, at least that’s the way they do it at Kumeu River.
Winemaker, Michael Brajkovich MW, says picking whole bunches is quicker and easier and there is not much difference between the final results, but they continue to do it the way they’ve always done it.
The purpose of this premature harvest is to measure three things that reveal the health and state of maturity of the precious grape harvest – sugar, acid and pH.
Careful monitoring increases in frequency as harvest approaches. If a “weather event” threatens, a knowledge of current sugar, acid and pH levels is invaluable in deciding whether to cut and run or to ride it out.
Sugar is easy to measure in the vineyard using a hand-held refractometer, although the results are a little more accurate in the lab. A typical range is 18° to 13° Brix, which is the percentage of sugar in the grape juice. The Aussies use the measurement of Baumé or potential alcohol in a dry wine. You can convert Brix to Baumé by dividing Brix by 1.8. Thus 18° Baumé is the equivalent of 23.4° Brix.
Sugar needs to be closely monitored to achieve an ideal target.
The target for acidity is usually between 7-10 grams/litre (g/l) expressed as tartaric acid. Acidity drops during fermentation, cold stabilisation (to remove unstable potassium bitartrate) and malolactic fermentation if that takes place.
The technical definition of pH is;
“The negative logarithm of the all-important hydrogen ion activity or concentration.”
But I prefer to think of it as the strength of acidity, which is overly-simplistic. Grape juice is acidic with a typical pH of between 3 and 4. pH is an important factor in wine stability and its ability to age. Two wines can have identical acidity levels and yet have a markedly different pH.
While acknowledging that sugar, acid and pH are the holy trinity of grape analysis, Michael says nothing beats walking through the vineyards and tasting grapes after checking their skin and seed colour.
“It usually backs up our laboratory analysis.”