There are a number of ways of doing this.
The most obvious criteria is sugar ripeness. The amount of sugar in the grape juice determines approximately the alcoholic content of the finished wine.
Lower sugar content eg. around 11% maybe suitable for delicate white wines eg. Semillon but totally unsuitable for full bodied whites like wooded Chardonnay and reds which need to be around 14%.
We determine sugar content by use of a refractometer.
Juice, squeezed on the instrument, is read via a scale. We use the Baume scale where 13° Baume would equal 13% sugar. Other scales in use are Brix and Oechsle.
Hydrometers can also be used but we find this instrument better for tracking the decreasing sugar content of the fermenting juice.
Initially grapes are tested randomly during walks through the vineyard.
But when things start to get serious, a more formal approach is needed to determine the ripeness of the whole vineyard.
|pH meter, Hydrometer, Refractometer|
Obviously the ripeness of the grapes throughout the vineyard will vary according to environmental factors and when the bunch was formed after bud burst. There are ways of ensuring an even bud burst by applying a chemical eg. Dormex, but not here.
Even grapes berries within a bunch will have varying sugar levels.
So our method is to ignore end of row vines (they tend to be the ripest) and take grape berry samples from two to three bunches of every third vine. Grapes are taken from the bottom, top, outside and inside of each bunch. They are then combined, crushed and the juice left to settle. We then determine the sugar level from the clarified juice. Rule of thumb is the result will be approximately one degree higher than that of the actual harvest.
We also look for pH and titratable acid (TA) levels. As grapes ripen, pH rises and acid content drops. Both together give us an idea of the wine’s acidity which is important from a processing and taste point of view. But, as we can adjust this during the wine making process, it is not too important at this stage.
|Refractometer with scale showing juice at 12° Baume (21° Brix)|
But there are also other non scientific methods to be used in conjunction with sugar ripeness.
The colour of the bunch stems and grape seeds change as ripeness progresses turning from green to brown.
The berries will plump up as sugars increase and will be easier to pull from bunches the riper they get.
The grape seeds are easily chewable when ripe.
Ripe grapes are sweet, with no hint of bitterness in the flesh or seeds.
Also, experienced winemakers will look for the ultimate “varietal” flavors to start showing through during taste testing.
It all might sound a little complicated but it isn't.
We find the most difficult thing is weather related. When substantial rain over a few days is predicted, do we harvest when ripeness is not at a premium or do we wait and take the risk that juice dilution, fruit splitting and subsequent disease may result in reducing yield and quality.
In our warm maritime climate (where the harvest months, February, March and April, on average are some of the wettest of the year) this is a decision we have to make almost every vintage.
Sometimes we have won, other times not.