Keys to Successful Fermentation: Part 1
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.
4) Accounting for cold shock in low temperature juice.
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….
Biological Reduction of Total Acidity
A wine is technically considered dry when the sugar level falls below 0.5%. A dry wine that also has a total acidity level above 10 g/L will likely be harsh and astringent to the consumer. Granted, there are exceptions to this generalized idea of wine chemistry. The perception of dryness can vary based on other aspects of the wine, such as acidity, dry extract, and aroma. A low tannin red wine can tolerate higher acid levels than a tannic red, and a white wine high in dry extract may tolerate higher acidity than one with less body. The level of “dryness” will also play factor in the perception of harshness in the wine, as an increase in sweetness can help to balance acidity (think lemonade).
University of Minnesota-developed wine grapes are rarely harvested with a TA under 10 g/L. It is not uncommon to see total acidity at harvest of 15-18 g/L in Frontenac. Even the newest cultivar, Marquette, sees total acidity ranging from 0.9-1.3% – numbers almost unheard of for red V. vinifera cultivars – especially when Brix at harvest on these same grapes falls within normal to high levels for dry wine production (24-27). Thus, if a winemaker desires to produce a dry wine using University of Minnesota grape cultivars, he or she must always be concerned with mitigating the harshness of these high acids.
Wine Grape Acids and their Reduction. There are three general methods one can use to lower high acidity in wine: physical methods (blending and amelioration), chemical methods (bicarbonates), and biological methods (yeast and bacteria). A winemaker may also choose to produce a sweet wine to counter-balance the acidity. This may actually be the best approach for high-vigor vineyards, vineyards on cool sites, or in short growing seasons. Nonetheless, dry table wines are desirable for many winemakers and consumers. For the acid levels seen in Minnesota vineyards, the best approach is most likely a combination of all three methods if the wine is going to end up dry.
Two acids make up over 90% of the acids found in grapes: tartaric acid and malic acid. From a wine stability standpoint, tartaric acid is extremely important. It helps a wine to maintain a low pH, thus inhibiting microbial spoilage. Chemical methods of reducing acidity all act mainly on the reducing the amount of tartaric acid in the wine, while biological methods all reduce the amount of malic acid in your wine. Those familiar with consuming green apples such as ‘Granny Smith’ are familiar with the sour flavor coming from the malic acid. Thus, a wine that contains high quantities of this acid can have a sour character. All living cells consume malic acid (malate) through their respiration cycle either by breaking down another molecule (usually a carbohydrate such as sugar), or by consuming malic acid directly from their environment
Biological Deacidification. The most common method of biological deacidification is the addition of lactic acid bacteria to the wine. The bacteria will directly consume malic acid and convert it to lactic acid, an acid with a softer less sour taste, in a process known as malolactic conversion or malolactic fermentation (MLF). Though not a true fermentation because no sugar is involved, carbon dioxide is a byproduct of the conversion, so the wine looks as though it is fermenting. Nearly all red wines around the world undergo MLF and some white wines also benefit from acid reduction of this practice. Traditionally, wines were left without sulfur protection following alcoholic fermentation to undergo MLF naturally – a process that could take several months. The risks involved with leaving the wine unprotected by sulfur dioxide (spoilage by other organisms and oxidation), have pushed many wineries to use a malolactic bacteria starter culture.
Nonetheless, yeast also have the capability to consume malate during cellular respiration and convert it to e
thanol. It has long been known that certain yeast, especially several non-saccharomyces yeasts (Schizosaccharomyces pombe, Hanseniaspora occidentalis, Issatchenkia orientalis) are especially efficient at consuming malate. Because these yeasts have poor alcohol tolerance, they must always be used in conjunction with Saccharomyces species in order to complete fermentation in wine. While S. pombe has been available commercially for some time for use wine production, the development of other non-Saccharomyces yeasts for commercial use is a hot topic at the moment. We will likely see more of these yeasts available in an active-dry form to use in sequential yeast inoculations for wine.
Until then, we decided this winter to look at some of the commercially available yeast strains that have some reported ability to reduce malic acid, and trialed them with University of Minnesota cultivars. After consulting with several enological product suppliers, we came up with a list of several yeast with reported malate-reducing capabilities: Lalvin C (Lallemand), Exotics (Anchor), Lalvin ICV Opale (Lallemand), and Uvaferm VRB (Lallemand). We also trialed a non-Saccharomyces yeast that Lallemand has made available in an active dry form for sequential inoculations: Torulaspora Delbrueckii (sold commercially as Level 2TD). Although its malate-consumption hadn’t been verified, a technician at Lallemand had recommended it because they had observed some softening of the acidity in wines that had been fermented using it.
Yeast deacidification trial. We did a small trial with these yeasts in which we used juice from the 2012 vintage that had been frozen. For each MN cultivar, we trialled 3 different yeasts, and used a fourth yeast as a control. One lot of juice was divided into 20 micro-vinification lots of 500 mL each. Thus each yeast was replicated in 5 fermentation lots. For this initial trial, we were concerned with monitoring mainly the chemistry change using each yeast. For white wines we used Lalvin DV10 as control, and for red wines we used ICV GRE (Lallemand) as a control yeast. Both are considered reliable fermenters with no reported malate degradation. The unusually hot weather in 2012 caused initial brix levels to be extremely elevated, so initial malate numbers reflect juice that had been diluted to bring the sugar concentration down to 25° Brix.
With Frontenac Gris, we started with an ameliorated juice that had a total acidity of 9.92 g/L, a pH of 3.00, and 5.1 g/L of malic acid. Although all the added yeasts showed some reduction from the initial malate levels in the juice, the acid reduction seen in the Lalvin C, Exotics, and the combination of Torulaspora delbrueckii with Exotics all were significantly lower than the control yeast (p <0.05). We used Lalvin C in a larger lot following this trial in order to evaluate the sensory impacts of this yeast. It’s worth noting that in all 10 micro-vinifications in which Exotics was used, the wines exhibited some stuck fermentations. Thus, some care may be needed when using this yeast in order to complete fermentation in low pH juices.
The La Crescent juice that we divided up for the micro-vinification trials was ameliorated to 25 Brix, which left the starting malate levels at 5.3 g/L. The decrease in malic acid during fermentation was less pronounced than what we saw with the Frontenac Gris fermentation. In fact, only the vinification lots in which Exotics was used showed a statistically significant drop in malic acid (p< 0.05). ICV Opale is advertised to lower malate levels by 0.1 to 0.4 g/L. Our trials show that it exceeded this levels in high malate musts, however, this decrease was not significantly lower than our control yeast which has no reported malate reducing properties. We also saw stuck fermentations when using Exotics in the fermentation lots with La Crescent.
Our Frontenac was pressed and fermented as a rosé. Again, it was necessary to ameliorate to reduce the high sugars that we achieved in 2012, however, the initial malate concentration of the juice was still relatively high at 4.6 g/L. Interestingly enough, all yeast used for this trial caused a decrease in the final malic acid concentration of the wine. All observed differences in malate reduction were statistically significant (p<0.05), except for the two lots that were fermented with Lalvin C. There is essentially no difference between the observed malate reduction when using Lalvin C in conjunction with T. delbrueckii yeast. This (along with the other results seen when using T. delbrueckii) suggests that any impact on the perception of acidity due to this yeast is likely not related to malate degradation. All the Frontenac fermentations finished dry with no stuck or sluggish characters.
Marquette was also pressed immediately and fermented as a rosé. The ameliorated juice had an initial malic acid concentration of 4.1 g/L. Exotics and VRB showed identical malate reduction capabilities, and even though the difference between these two yeasts and the control (ICV GRE) was only slight, the difference is statistically significant (p=0.046). Nonetheless, this difference is probably negligible in the sensory profile of the wine. Once again, Lalvin C proved to have the greatest potential for malate reduction, with a 1.10 g decrease in malic acid from the juice.
It is important to keep in mind that there are many different tools available to a winemaker to manage high acidity in their wines. The selection of yeasts that we looked at here are only a small example of what is available on the market. It is important to talk with technicians who supply your winery in order to get a better idea of what products might help with managing your acidity. This coming vintage (2013) will likely promise to be a challenging one, as we have yet to see bud-break in Minnesota this first week of May. Unless we see a warm summer and an extended warm fall, growers and winemakers need to start thinking now about how they might manage the vines and prepare for a high acid harvest.
Winemaker Roundtables
I’m excited to announce that in 2012, a series of regional Winemaker Roundtables will be held at several host wineries around the state. A review of common wine faults will be given at each roundtable, along with methods to prevent and fix these problems. Winemakers who attend will have the opportunity to have several of their wines evaluated blind by a panel of their peers. The enology lab at the University of Minnesota will also present wines from research trials to winemakers in the state who would like to have some examples with new techniques and practices with cold-hardy grapes.
Winemakers from bonded wineries in Minnesota are eligible to attend these sessions. Space will be limited to 10 winemakers at each session.
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