Yeasts are mostly unicellular micro-organisms, but they can become multicellular micro-organisms through asexual reproduction. For winemaking purposes, only the yeast species Saccharomyces cerevisiae is important; this yeast species species turns sugar into alcohol through fermentation.
Saccharomyces cerevisiae is likely the first living organism domesticated by humans, albeit unknowingly. For clarity’s sake, when the term “yeast” is used on this page, it will refer to Saccharomyces cerevisiae, and none of the estimated 5,000 other species of yeasts.
As a general rule, all yeasts except wine yeast are undesirable in winemaking. Bacteria must be kept out of the fermentation environment, with a few exceptions (for example, bacteria for malolactic fermentation). Evidently, winemaking requires a high level of cleanliness and care.
From the above, we can conclude that only a single yeast species qualifies as wine yeast. Then, what separates wine yeast from baker’s yeast, or from brewer’s yeast, or from wild yeast? Before answering this question, a comment about microbiology is warranted.
In microbiology, many species have been re-classified because of new information. The author will stick to the terminology that is commonly accepted at the time of this writing, while being aware that this terminology may not stand the test of time.
Until a few decades ago, wine yeasts were simply variants of Saccharomyces cerevisiae, until it became customary to talk about Saccharomyces ellipsoides. The new terminology is that wine yeast is Saccharomyces bayanus, which developed out of the wild yeast types of Saccharomyces uvarum, cerevisiae, and eubayanus. North American fermentologists believe that wine yeast cultures are single species cultures, while European fermentologists believe that wine yeast cultures are mixed cultures of closely related yeasts.
In microbiology, genetic variants are usually referred to as “strains.” It seems that humans discovered strains of yeast as recently as 6,000 years ago, which is when the first historical evidence of leavened bread is dated to. The domestication of baker’s yeast resulted from humans inadvertently selecting a certain strain of yeast adapted for breadmaking.
Wine yeast presumably developed when early humans discovered that consuming over-ripe fruit had interesting side effects. This resulted in them collecting fruit for the purpose of of wild fermentation. Early humans were able to pass yeast from batch to batch by storing fruit in the same containers repeatedly. By using wine barrels repeatedly, wineries are able to develop special yeast cultures, and by extension, can give their wines a signature flavour. Most European strains of wine yeasts are still named after the winery where they were initially cultivated. With the advent of microbiology at the end of the 19th century, particularly desirable strains of yeast were developed and cultured, resulting in yeast becoming a commercial product.
A word about brewing yeast: Until chilling was practised in the late Middle Ages, the only type of brewing yeast that was known is what we know today as “top-cropping yeast,” yeast that floats in the top layer. With chilling, bottom-cropping yeast developed, giving beer a different taste. Classic ale-type beers used top-cropping yeast, while lager-type beers used bottom-cropping yeast. Today, baker’s yeast is a top-cropping yeast while wine yeast is a bottom-cropping yeast.
Having covered the basics of yeast above, there remains one strong recommendation that must be made: Always use a pure yeast culture and never rely on wild yeast. Wild yeast will only develop quality wine if you are working within a wine cellar that has been able to develop a unique culture. If your wine cellar is younger than at least 100 years, don’t even bother using wild yeast.
Pure yeast cultures are widely available and cheap. Unless you happen to be a microbiologist and have the equipment, please don’t try to culture yeast for purposes other than winemaking.
Commercial yeast is either freeze-dried or in liquid form. Once a package of yeast is opened, it must be used up immediately. Saving an opened package of yeast invites other micro-organisms to contaminate the package.
When wine yeast is added to induce fermentation, the yeast enters a microbiological battle.
The winemaker’s task is to set up the battle for the yeast to win. In order for the yeast to win, winemakers need:
An optimal level of acidity. Yeast is naturally acid-tolerant, but fungi and most bacteria are not.
A low level of oxidation-reduction potential (ORP) to create anoxic conditions under which yeast can thrive. This is usually done by adding sulfur (sulfur dioxide/sulfurous acid) before adding the yeast. If you add sulfur, the concentration of sulfur dioxide should not exceed 40 mg/L. Osorno offers two products for adding sulfur to wine. These are Osorno’s Sulfite Solution and sodium metabisulfite powder.
As high a population density of cultured yeast as possible to win the battle through sheer numbers. Therefore, it is a good idea to add a pre-culture of yeast, rather than adding yeast directly.
An optimal level of glucose as the main yeast nutrient. The osmotic pressure of a fermentation batch is almost completely determined by the initial concentration of sugar. Because a high osmotic pressure can kill micro-organisms, it is important to start with a low sugar concentration.
A yeast nutrient mixture containing minerals, amino acids, and vitamins.
A carrier material in which yeast cells can grow. This is usually unnecessary because fruit provides so many cell fragments, however, for wines without a natural carrier (e.g: honey wine/mead), we advise adding a small amount of apple sauce or oat flour.
Recipe of Fermentation Starter
This recipe for a Fermentation Starter is calculated for a 100 – 200 L drum. If the targeted volume of wine is 20 – 40 litres, then only a third of the ingredient amounts should be used.
- Dissolve 150 g of sugar in 1 L of freshly boiled water.
- Add 30 g tartaric acid.
- Let the solution cool.
- When the solution is slightly warm, add 1 g of yeast nutrient.
- Mash some raspberries and add 2-3 packets of dried yeast to the solution.
- Let the solution stand for a few hours.
- When foaming begins, add 500 ml of wine, and mix.
- When foaming resumes, add another 500 ml of wine, and mix again.
- Let the solution stand until foaming resumes once more.
- When the water/wine mix is actively foaming, add to drum with wine.
Although sugar is the yeast’s main source of energy, yeast needs additional nutrients.
The main nutrients yeast needs are phosphate and nitrogen. Nitrogen is so important to yeast that measuring nitrogen before fermentation has become standard practice in wineries.
Adding diammonium phosphate (DAP) has also become standard practice in wineries. It is a good rule of thumb to add 2 g of DAP to a 10 L batch of wine before fermentation. In commercial winemaking, Australia and the EU recommend adding 3 g of DAP per 10 L batch of wine.
Unfortunately, supplementing yeast is not as simple as adding these nutrients to a batch of wine. To begin with, DAP already contains nitrogen and phosphorous, but yeast still requires extra nitrogen.
The easy solution is to add a nitrogen fertilizer like urea. Urea is cheap, easily available, and safe for human consumption. In fact, there are yeast nutrient products in the North American market that mix DAP with urea. However, yeast can only use urea partially, and will convert it to carbamate. Carbamate is a known carcinogen, so the reader should not use these products.
We strongly advise against using yeast nutrient products that contain urea or yeast nutrient products that do not declare its composition. Unfortunately, these products are still sold in Canada and the United States to hobby winemakers. In commercial winemaking, these products are prohibited. Nevertheless, you can still use urea as a fertilizer, as opposed to a nutrient for fermentation.
Instead of using urea as a source of nitrogen, you can purchase Osorno’s Composite Yeast Nutrient. To purchase this product, please visit the Osorno Store.
Yeast also has a need for inorganic components like potassium, calcium, and magnesium, and in trace amounts, copper, iron, manganese, and zinc. If good well water is used, the need for calcium, magnesium, iron, and manganese is taken care of. Mineral additives are available for winemaking to supplement mineral-poor water.
In addition to the inorganic substances discussed above, certain organic compounds are essential for yeast, such as thiamin (vitamin B1), riboflavin (vitamin B2), biotin (vitamin B7), pantothenic acid (vitamin B5), nicotinic acid (vitamin B3), and inositol.
Yeast itself is actually an excellent source of these vitamins. However, at the time of fermentation, these vitamins are not available in a sufficient amount for the yeast to grow, unless more is added.