Supply of oxygen is very important during fermentation of must to keep the yeast growing. On the other hand, once fermentation is complete, oxygen can lead to accelerated aging of wine. So we add oxygen during the early phases of fermentation and we control the exposure of wine to oxygen once it is in the barrel.
Oxygen dissolves in wine, as it does in water. Wine can dissolve oxygen up to around 8 ppm or mg/L at 60 dF (significantly more at lower temperatures). Measuring DO is tricky because sampling exposes wine to air
DO can be measured electromechanically (taking a sample and measuring a charge difference across a membrane) or optically (measuring absorption of light). The optical method is non-destructive (i.e. no need to touch or alter the sample), but it requires expensive tools. The electromechanical method is cheaper, but destructive and less reliable because a sample must be extracted and probed, and that exposes the sample to oxygen in the air. We are experimenting with measuring DO with a Portable Dissolved Oxygen Meter HI9146 from Hanna (see http://www.hannainst.com/usa/prods2.cfm?id=004002&ProdCode=HI%209146).
We measure DO in three situations
- In must before and during fermentation. Yeast growth and replication depends and absorbs a fair amount of oxygen, that is one reason for vigerous punch-downs and/or macro-oxidation (injection of air or pure oxygen) in the must during the first half of the fermentation. The DO measurement before and after each punch-down or injection should allow us to better manage the oxygen levels
- In wine during cellaring: The goal then is to minimize the dissolved oxygen. Oxygen (except when added in extremely small doses, i.e. controlled micro-oxidation in steel tanks) ages the wine prematurely. Wine is exposed to oxygen each time the barrel is topped up or racked. Consequently we measure DO before each topping up, before and after racking, and most importantly, before bottling.
- In wine to measure its oxygen absorption capacity: The more oxygen and the faster a wine is able to absorb oxygen, the longer its expected future life and thus ability to mature further. We measure absorption capacity in test-bottles by forcefully increasing the dissolved oxygen level to its maximum (by vigorously shaking the bottle thus dissoving air), measuring the DO, then cap the bottle and measuring the DO over time recording how fast and to what level it falls (i.e. how fast the dissolved oxygen gets absorbed)
When measuring DO with our electromechanical meter, we need to take the measurement instantly after taking the sample and taking the sample with as little turbulence as possible. Otherwise the sample’s exposure to ambient air will inflate the measurement.