Fermentation is a natural process by which yeast consume sugar and convert it to ethanol.  A successful fermentation is one in which the winemaker ensures that the conditions are met to enable a population of yeast to live and thrive until the winemaker wishes – generally until all the sugars have been depleted. All this needs to be done while minimizing the production of volatile acidity and sulfur off-aromas, and maximizing the desirable aromas and flavors produced during fermentation. It sounds easy enough, but for anybody who’s been around the industry can attest, stuck and sluggish fermentations happen more often than you might wish.  So, I present, the key points to a successful fermentation in four parts: yeast hydration and addition, the first quarter of fermentation, mid-fermentation, the last quarter of fermentation.

Yeast Population Kinetics

There are four main stages that a population of yeast will go through in a typical wine fermentation as illustrated in figure 1 below.

1)     Lag phase – this is a very short period of time in which the yeast become acclimated to the juice or must. The duration of the lag phase is less than a few hours, until the yeast realize that they are in a sugar and nutrient-rich environment and they begin to multiply by budding (yeast division).

2)     Exponential growth phase – yeast multiply rapidly. The yeast population can double every 4 hours until a maximum population density is achieved. There is an increased demand for oxygen as yeast cells replicate.

3)     Stationary phase – The yeast population has reached a critical mass. This is the longest phase of fermentation in which the yeast are actively converting sugar to alcohol through anaerobic fermentation. At this point oxygen isn’t necessary for yeast survival, but a winemaker may choose to aerate a wine for other reasons (reduction aromas, color stability, etc.)

4)     Yeast Death – Over time, the yeast will slowly deplete the nutrients available in the juice (sugar), and will also be producing waste products that are toxic (ethanol). Dead yeast cells will break apart (lyse) as they fall to the bottom of the tank and release more toxins that will kill surviving yeast. Thus, the decline of the yeast population is a rapid, exponential decline.

By understanding the important steps that  winemaker needs to take during each of these phases of fermentation, one can be assured that the risk of a stuck or sluggish fermentation is minimized. The first part begins with hydrating active dry yeast, and adding the yeast to juice or must …

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