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.
Part 1: Yeast Hydration and Addition
1) Choose the correct yeast (Account for Osmotic Shock).
Grapes are naturally high in sugar. When yeast encounter this high sugar environment, there is a certain amount of osmotic pressure placed on the outside of the yeast cell wall. Since the cell wall is permeable, the yeast expend energy to ensure that they maintain an equilibrium between the the pressure on the inside and the outside of the cell. To do this, they tend to produce more glycerol inside the cell, but they also will produce acetic acid to try to decrease the viscosity of the fluid outside of the cell (the grape juice). This phenomenon is well known in ice wine production, and is why these wines tend to have higher levels of volatile acidity than table wines. In this type of environment, the yeast need an array of micronutrients and amino acids to form the fatty acids and sterols that will strengthen their cell membrane. A winemaker can also minimize damage to the yeast by making sure it isn’t exposed to further stress such as cold temperatures and excess SO2.
The initial osmotic pressure placed on the yeast will impact the physiology of the cell for the duration of its life, that is, until the end of fermentation. The resistance of yeast to alcohol in the final stages of fermentation depends on the initial osmotic pressure placed on the yeast and its ability to resist this stress. If a winemaker knows that the potential alcohol of the juice is greater than 13%, it is important to choose a yeast that has the ability to resist higher alcohol levels. Late harvest or ice wine styles should be fermented with a yeast that is intended for high sugar musts in order to minimize the potential problems with volatile acidity, and to ensure that the fermentation begins in a timely fashion.
2) Proper yeast re-hydration practices (resistance to other shock factors).
As mentioned above, sterols and polyunsaturated fatty acids are important factors that the yeast need to create a strong cell membrane. When one rehydrates the yeast in water (along with yeast nutrient), the yeast metabolism is in a respiratory state (consumes oxygen) which allows it to more easily synthesize these resistance factors in its cell wall. If yeast is rehydrated in juice, the yeast are more inclined to have a fermentative metabolism from the get-go, which makes it difficult to synthesize the products necessary to strengthen its cell wall to provide protection from stress during fermentation. The initial content of these resistance factors will become diluted with each generation during the multiplication phase.
The yeast multiplication phase corresponds to the consumption of the first 30-40 grams of sugar. Once the initial population of yeast cells reaches 100 million cells/mL of must, the juice is considered completely colonized. This level of colonization does not depend on the initial population of the yeast. So, in order to arrive at 100 million cells/mL, the greater the initial population of yeast, the less they need to replicate to reach their maximum population. Thus, their resistance to stress becomes less diluted, and the yeast are more able to survive in the high alcohol environment near the end of fermentation. This isn’t to say that you should double or triple the recommended dose for yeast in your fermentation. This dilution of stress factors is only seen if the initial amount of dried yeast used is less than 300 mg/L. Thus, the recommended quantity of 400 mg/L on the package of active dry yeast accounts for this.
3) Yeast Nutrition.
During the multiplication phase, yeast need amino acids/nitrogen, fatty acids, and micro-nutrients (vitamins and minerals). Some of these elements aren’t bioavailable in the juice at this critical moment when the yeast need them the most. By adding nutrients that make these elements immediately bioavailable to the yeast, it diminishes the risk of added stress to the yeast due to a nutritional deficit. Adding yeast nutrients during rehydration and at the moment of yeast addition to the must allows the yeast to multiply in the best conditions. However, the different enological yeasts all have different needs when it comes to nutrition. The dose necessary during yeast addition depends on which yeast you use, along with other factors: potential alcohol, maximum fermentation temperature, oxygenation, and the initial temperature of the must during addition.
Have you ever jumped into water that is just above the freezing point? You know then, how yeast might feel if they are immediately dumped into a cold tank of juice – something that is common in white and rosé fermentations. It is easy to evaluate the potential for cold shock to the yeast: the greater the temperature difference between the water at the end of yeast hydration and the juice in the tank, the greater the stress to the yeast. If the temperature difference is greater than 10ºC, the stress on the yeast caused by the cold shock will have physiological consequences to the yeast that will affect it throughout the fermentation. When it is known that there is a high potential for this cold shock during yeast addition, it is important to take some steps to compensate for these risks. The most important is to slowly acclimate the yeast to the juice temperature by adding some of the juice to the hydration water to bring down the temperature. The temperature decrease should not be more than 10ºC over a 20 minute period. When the yeast is added to the tank, the temperature difference should not be greater than 10ºC. Other ways to compensate for this stress are by adding a higher dose of active dry yeast, and ensuring adequate nutrition.
5) Compensation for the elimination of fatty acid sources (white and rosé wines).
In all white and rosé fermentations the juice is racked 24-48 hours after pressing to eliminate suspended solids. The degree of clarification can be enhanced by using fining agents and enzymes in the juice – an important step if the grapes arrived in poor sanitary state. Ideally the turbidity of a juice following the first racking falls between 100 and 250 NTU. Nonetheless, while eliminating pectin particles and insoluble solids, you are also removing poly unsaturated fatty acids that are important for yeast survival. If the juice clarification is less than 200 NTU, it is important to take steps to reduce stress on the yeast. Adding yeast nutrients rich in fatty acids, or increasing the initial yeast population are ways to ensure yeast survival through the end of fermentation.
To Be Continued with Part 2: The first quarter of fermentation….