Science in the Cellar

A cellar can be an incredibly creative and innovative place, and winemaking an artistic process. It’s all about imagining what you want to create, what steps you will take to get to that finished product, and then using all your senses to get there. In the vineyard, you’re tasting the berries and crunching the pips between your teeth, using the quality of the crunch and the sweetness of the juice to decide when to pick. During fermentation, you’re smelling the CO2 gas, feeling the heat of the fermentation with your hand, hearing the bubbling as the gas breaches the skins, and watching the colour of the juice change as the skins impart their burgundy or amber hues. All of these sensations tell you something about what is going on below the surface. Despite the astounding ability for winemakers to know what is happening in the microbiological realm based purely on senses, winemaking does also require a little bit of science – quite a lot of science, sometimes. There are many complex biochemical processes taking place, so to ensure we are creating a wine with the desired characteristics, or to ensure nothing is going wrong, we need to use a bit of Biology 101. To do this, we use various contraptions and meters (which often just look like yet another glass stick in the cellar) to observe the microscopic realm.


This goody gets its name from the fact that it uses the refraction of light to determine how much sugar is in the juice of the grapes. Knowing the sugar level, or Balling, before you harvest is super important, as you want just the right amount for the yeast to work as optimally as possible. Too little and you might get a stuck fermentation; too much and you might have too much alcohol at the end. For reds, it’s generally between 23-26B, and for whites it’s between 19-23B, but this can change based on the desired style (e.g., dessert wines will have a much higher sugar). So, we take the Refractometer into the vineyard with us while we’re walking and tasting the grapes to help us plan when we’re going to pick. It’s quite a fun game to try and guess how much sugar is in a grape, and then check to see if you’re right.

Balling Meter

 “Yeasts eat sugar, fart CO2, and pee alcohol”. Not the most charming of pictures, but it’s what Billy told me as a child to help me remember the basic principle of yeast metabolism, and, not surprisingly, I have never forgotten. So, in order to know when the fermentation is finished, we want to know when all the sugar has been eaten by the yeasts.

Similar to the Refractometer, a Balling meter (or Hydrometer) also measures sugar, but in a different way. In simple terms, the Hydrometer is displaced by the sugar in the juice and floats. To measure, we fill up a measuring cylinder with 500ml juice, place the Hydrometer, and watch it bob up and down until it stabilises and we can read the number (which takes some practice). The more sugar, the further outside of the juice it floats, and the less sugar, the deeper it sinks. We use this to measure the progressive loss of sugar in the wine as it ferments. Once it hits zero, we know fermentation is done.


Temperature is a key indicator in winemaking. Fermentation – the process of yeast (and sometimes bacteria) metabolising sugar and nutrients to produce alcohol and CO2 – is exothermic, i.e., it produces heat. As the yeasts start to multiply and more sugar is metabolised, the temperature of the must (grape juice) gets higher and higher, sometimes going from 10 – 15ºC to as much as 30 – 40ºC. When you stick your hand in, it’s like dipping into a jacuzzi. When the temperature comes down again, we start to detect that the fermentation is finishing. Monitoring the temperature of a fermentation helps us not only decide when it has started or stopped, but whether it might be too hot, causing the yeast stress, and we then need to cool it down to avoid off flavours or incomplete fermentation.

Laboratory Analysis

In addition to the basics like sugar and temperature, getting an in-depth analysis of certain key components in wine is an essential part of winemaking. This gives us insight into what the final product will taste, smell, and feel like, and informs us if we need to take any emergency action if the wine is unstable. For these, we’ll send samples to a laboratory.

Alcohol and residual sugar are the first things we’ll measure, which tells us how the yeasts performed and whether the fermentation is complete. In addition, we’ll analyse the amount of malic and lactic acid which tells us if the secondary bacterial fermentation, known as malolactic fermentation, is complete.

Measuring pH is also really important, as this tells us how stable the wine will be going forward. High pH can create an environment in which unwanted bacteria can thrive, causing spoilage or unwanted flavours, one of which is VA. Although a normal byproduct of the winemaking process, a high volatile acidity (VA) can also be an indicator of bacterial activity, so it’s important to measure this. The pH and VA also tell us how much SO2 we need to add to protect the wine, as SO2 is less effective at a higher pH.

Finally, we measure total and free SO2. SO2 is a natural byproduct of yeast metabolism, and shows up in small quantities even before we add any. Free SO2 is that which is free in solution in the wine, while total is the sum of the free SO2 and SO2 which is bound to other compounds in the wine. Only the free SO2 is available to perform antimicrobial activity, so it’s important we have enough of this in the wine before we put it in the barrel. Once we know these values, we can look at the pH and decide how much SO2 we want to add to get the desired final quantity. 

Wine is a complex ecosystem, with multiple biochemical pathways happening simultaneously. This is even more so with natural winemaking, as the contributions of wild yeasts present other challenges. While the analyses above are by no means all the science one needs to make wine, these are the ones we use most often to understand what is happening at a microscopic level. This informs our activities in the cellar, and helps us ensure we are making high quality, natural wines that people love, while pushing the boundaries to make them even better.