Yeast concentration (wine)

So once you know the yeast cell concentration, how can you use the data to improve your fermentations?

Knowing the yeast cell concentration, expressed in millions of cells per milliliter, is crucial for vintners. It is a key parameter that can affect the fermentation process, and it is especially of paramount importance during the pitching stage. How much yeast you pitch depends on a few factors, such as the type of wine you’re fermenting, the type of yeast you’re using, and what you’re aiming at. However, while experimenting may lead to variations, consistency is often the goal. Therefore, it is essential to measure yeast cell concentration and yeast viability accurately to ensure consistent inoculation rates in each batch of wine. Underpitching can result in stuck or longer fermentations which translate into added costs, while overpitching can lead to a high risk of yeast autolysis, which can cause off-flavors. And while you can, of course, choose to play with these off-flavors that, when properly handled, can impart to a wine characteristics such as creaminess and richness, there is simply no way to control them unless you know exactly what you started off with.

The consequences of underpitching and overpitching can greatly impact the final product. And while many winemakers will say that it is always better to overpitch than to underpitch, the truth is this approach is just choosing the lesser evil. Underpitching can lead to an increase in diacetyl, esters, and volatile sulfur compounds, and, of course, stuck or slow fermentations. On the other hand, overpitching can cause the wine to ferment too quickly, reduce ester production, cause a loss of flavor complexity and affect the mouthfeel of the wine. It is also, of course, far from ideal in terms of cost savings, as you’re basically wasting yeast.

Stuck or sluggish fermentation, however, is not only caused by underpitching, but it can happen at any time during the fermentation process. The yeast may begin to struggle to convert sugar to alcohol and carbon dioxide or it may become completely dormant. This may be caused by a number of reasons, from a poor inoculation technique or a lack of oxygen at the start of the fermentation to insufficient nutrients, such as nitrogen deficiency, a temperature that’s either too high or too low, a lack of oxygen, sugar / alcohol levels that are too high, a pH that’s too low or bacterial contamination that may affect the yeast’s ability to consume sugar. And while a fermentation becomes sluggish or stuck, a myriad of problems can arise, from off-flavors caused by bacteria taking over the fermentation or by autolysis, to oxidation and residual sugar ultimately leading to undesirable sweetness in a wine otherwise destined for dryness. By monitoring the yeast cell concentration at all times during the fermentation process, you can easily notice the first signs that something is amiss with your fermentation and you can anticipate and rectify a sluggish or stuck fermentation before any of the aforementioned effects take hold.

Verifying the yeast manufacturers’ claims is another important use of yeast cell concentration data, particularly before or during the pitching process. There have been several instances where lower yeast concentrations were found than what was specified on the packaging, which can, of course, negatively impact the fermentation process when you’re actually inoculating an insufficient number of yeast cells. Utilizing the yeast cell concentration results from the Oculyze Fermentation Wine App you can check whether your supplier’s claims are accurate and that you’ve chosen the best yeast supplier, to begin with.

Last but not least, keeping an eye on yeast growth by monitoring your yeast cell concentration allows you to harvest and propagate your yeast efficiently, which can significantly reduce your production costs, with yeast being such a costly ingredient. It also, of course, allows you to propagate and reuse the yeast that you know has already performed well for you.