Sugar is the major soluble solid in the must (grape juice) and it's concentration can therefore be measured by assessing the density of the must. This is done by using a refractometer, which measures the deviation occurring when a beam of light moves from air to the sugar solution, or by using a hydrometer, which measures relative density or specific gravity.
The acidity of grapes is monitored by changes in the titratable acidity and/or the pH of the berries. Titratable acidity is measured by adding a strong alkali to the must and measuring how much is required to neutralise the solution. Both sugar levels and acidity can be adjusted during the wine making process but ideally you want the right levels in the berries.
During the last few weeks we have been measuring sugar levels and acidity every few days. The Indian summer last week certainly helped us towards the right balance and as a result we are planning to harvest the grapes this weekend. Whilst this is a little early for the Chardonnay it is important to pick the grapes before botrytis (grey rot) takes a hold, which is much more prevalent in cool and damp weather conditions.