From early July we test the sugar levels of the grapes every three days or so. This is done using a refractometer, which measures the grape juice by its refractive index. A shadow line is reflected on a glass plate inside the instrument, which is then viewed through a magnifying eyepiece. We use the Brix scale, where 1 degree Brix is 1 gram of sucrose in 100 grams of solution and represents the strength of the solution as per cent of weight.
We take samples from every third line and from the same six or so plants in those lines. This enables a good idea of where plants are doing the best and also an overall idea of when the whole vineyard will be ready for harvest. We aim to achieve an average reading of around 24, so using a factor of 0.55, we end up at a wine alcohol of 13-14 degrees. This is only an estimate of future alcohol level as solids in the juice can change the outcome.
Squeezing a drop of juice onto the bottom plate and then closing the top plate squashes the juice into a thin film which allows light to refract through onto a given and engraved scale. Each harvest is different but you get to know which plants are good and which areas of the vineyard are best.
Some of the bunches are almost perfect, a shame to cut them . . . almost!
We also taste the grapes for sweetness and flavour, chew the skin on the front teeth to get skin tannin taste, make sure the pips are brown for ripeness and feel the grape and skins for firmness and give.
As we get closer to our harvest, to allow maximum sun and ambient heat to increase the sugar levels, we cut away the lower leaves and wait. It is at the end a judgement call as to the actual day of the harvest, how much bird damage, how many grapes turning to raisins, verses ripeness and sugar levels.