The Role of Yeast in Wine Making
Yeast play the most important role in the wines we drink. Without them, nothing would ferment and we would have no wine at all.
Yeasts are everywhere, at all times. They are in your house, on your skin and inside your body. We have a symbiotic relationship with them and the part they play in producing our wines is exciting and tragic at the same time.
Types of Yeast
Yeasts live in nature everywhere, for winemakers the most important type of yeast is ‘Saccharomyces’ (pronounced sack-row-mice-ease). In Latin, this word means “sugar fungus”.
The Saccharomyces species of fermenting our food and drink include:
- Saccharomyces pastorianus - For beer and lager making
- Saccharomyces bayanus - For cider and wine making
- Saccharomyces cerevisiae - Bread, beer, sake and wine making
For wine, there are good yeast or ‘vinification yeasts’ and bad yeast called ‘spoilage yeasts’
But the truth is, that even what winemakers term ‘good yeast’ can have some pretty nasty results, if the winemaker doesn’t keep their activity under control.
All yeast, good and bad, will be present on the grapes that go to the winery to get made into wine. Even the winery itself is home to yeast strains and different species will affect the wine at certain times. This happens when the conditions are just right for them to metabolize and multiply, this is why there are some that dominate the vineyards and some that like the conditions of the winery.
Types of Fermentation in Wine Making
Winemakers can add the grape juices (the official term is ‘inoculate’) with a commercial yeast strain to start fermentation, or they can choose to leave the juice and wait for the natural or indigenous vineyard yeasts already present in the juice to start fermentation on its own.
Waiting for the natural yeasts to start fermentation is called a ‘wild ferment’.
For a wild fermentation to work well, a winemaker must have excellent quality fruit. This means it can’t be moldy, damaged or split before it reaches the winery. Wild fermentations usually take more time than commercial yeasts and can be tricky to get right.
Sometimes the fermentation can get ‘stuck’ or incomplete, as the yeast may not have enough nutrients to continue fermenting sugar to alcohol, or they get too hot or too cold.
Because of these issues, wild fermentation is less predictable for commercial wine production and takes too long.
Advantages of wild fermentations
- It’s a greater reflection of the vineyard and the grapes that are grown there
- The resulting wine changes year to year
- Different yeasts present in the juice will add complexity to the wines
Using a commercial yeast strain will help winemakers control the wine they produce every year, with a consistent style and quality. For the commercial wines we see today, the wine is expected to taste the same year after year and consumers don’t like when the wine they like so much suddenly doesn’t taste the same.
Advantages of Commercial Yeasts
- They start fermenting quickly
- Fermentation happens evenly, unlikely to get stuck.
- Are tolerant of fungicide on the fruit, moldy fruits, and high acid in the grapes from unripeness.
- Don’t mind lower temperatures
Yeasts are picky little creatures. They will die if they are too cold and if they are too hot.
- Too cold -They start to die or ‘sleep’ at 9 degrees Celsius or 48.2 degrees Fahrenheit
- Too hot -They can’t survive more than 34 degrees Celsius or 93.2 degrees Fahrenheit
- Just right -The perfect temperature is between 15 - 25°C and 59 - 77°F
For the perfect inoculation of the grape juice, the temperature should be exactly 16°C (or 60.8°F)
Once the fermentation has started, the speed of fermentation will double for every 10°C = 18°F rise in temperature.
Red wine fermentation require temperatures to reach 26 - 30 °C = 78.8 - 86°F for the pigments to be extracted from the grape skins. It is common to warm the fermenting juice artificially to help this happen. This has to be done carefully, as yeast die quickly in the heat.
White wine fermentation may require the fermenting juice to be cooled to 12 - 15°C = 53.6 - 59°F to help preserve the delicate varietal characteristics. These flavor and aromatic compounds are destroyed in high temperatures
Nutrients for Yeasts
Like all living organisms, yeasts need some basic nutrients to survive. They don’t just survive on sugars in the juice, they need nutrients to do the job of fermenting sugars into alcohol.
When yeast don’t have the nutrients they need, when they need it, they can produce some nasty products and they can start smelling pretty nasty.
Again, winemakers have to keep an eye (and nose) on the yeast activity, this is when they will modify the juice to make sure the yeast have the nutrients they need.
Yeast need oxygen to repair their cell walls or skin. If the oxygen is limited, they cannot repair themselves and when alcohol starts to form in the juice, it can kill them easily. When grapes are initially processed, there is some oxygen in the juice, this is enough for about five generations of yeast. As the oxygen is used up, if no more is available, they cannot divide their cells any further and no more generations are created.
Nitrogen is required by yeast to manufacture proteins for the construction of cells.
Winemakers call the nitrogen required by yeast to manufacture proteins for the construction of cells 'Yeast Available Nitrogen' or YAN
The amount of this yeast available available nitrogen depends on:
- The grape variety (for example, Riesling naturally have lower levels of nitrogen)
- Rotten grapes reduces nitrogen
- The growing season. Hotter temperatures reduce the amount of nitrogen in the grapes.
In the first 36 hours of fermentation, yeast will consume most of the available nitrogen in the juice.
Fermentation takes longer than this depending on the quantity, so a nitrogen supplement must be added for the yeast to survive.
Winemakers will add ‘diammonium phosphate’ or DAP
If the fermentation lacks nitrogen for too long, hydrogen sulfide is created.
If hydrogen sulfide is detected at any time; smells like an open sewer, adding DAP is best done a little at a time. If you add too much too quickly, the ferment can happen too rapidly, and this creates an imbalance in the compounds created.
Beware, there are legal limits for DAP levels in wine and excess DAP reduces all the delicious floral and fruity aromas in the finished wine.
The main sugars in the grape juice are ‘fructose’ and ‘glucose’. They are in there at about equal proportions.
When yeasts are present, they will first utilize the glucose.
When the glucose is mostly metabolized, they will start consuming the fructose. This will usually happen at the end of the fermentation.
4. Vitamins and Minerals
Commercial nutrients blends such as Fermax, Fermaid, Startup and Superfood contain vitamins, minerals and yeast extracts for growth.
Yeast extracts or ‘Autolyzed yeast’ means that a batch of yeast was heated, initiating autolysis, which means their cell walls are broken and contents leaks out killing the yeast. The dead yeast and the resulting fluid are now a source of nutrients. Yeast hulls (cell membranes or dead yeast skins) are also of protein for fresh yeast to make into new cell walls (membrane or skin).
- Fully Autolyzed - means a rich, fully soluble nurtient product where all the yeast were killed in 'inactivated'
- Partial Autolyzates - means not all the yeast were inactivating before drying
Other nutrients in these supplements include a blend of magnesium, zinc potassium and other trace minerals. The supplements are usually added half way through the primary fermentation where the yeast in the juice are fully activated and working happily.
The Product of Fermentation
With all going well, yeasts are busy breaking down sugars that exist in the grape juice. They do this because the sugars provide them with the energy they need for their own existence.
This process by which they break down the sugars is not efficient, as a lot of their energy the use to break the sugars down, is lost as heat. But never the less, they are producing ethanol (alcohol) as the byproduct of consuming the sugars. With a limited amount of grape juice to float around in, the yeast soon find themselves reaching toxicity. The alcohol content is too high and they die from its high levels of it, or they have run out of consumables.
The Life Cycle of Yeast
There are five phases in which the cycle of fermentation occurs.
1. Lag phase
The lag phase is when yeast are added and are adapting to their new environment. This take up to 48 hours. They are adapting to the temperature, pH and pressure.
2. Log phase
This is also called the exponential growth phase. The yeast population is multiplying rapidly and actively expanding.
3. Deceleration phase
The nutrient levels begin to fall, as this happens, the yeast population slows in growth; as their cells can’t divide without the nutrients they need.
4. Stationary phase
The yeast are still active and consuming sugar, but the active yeast population is now equal to the death rate and remains constant.
5. Decline phase
This is also called the death phase. The yeast cells eventually die.
Yes, by the end of the winemaking there will be no live yeasts left.
Their bodies or ‘cells’ are usually removed by fining and clarifying the wine, so it is crystal clear. Some wines are left unfiltered, leaving the ‘yeast hulls’ in there, so you can drink them too.
Remember, yeasts are only a cell. They do not feel pain or emotion so although you may feel sad for their lifecycle, they do the job that they do, whether we care about it, or not. Respect is given where due, and they certainly deserve that.