Yeasts are micro-organisms and mostly unicellular, but can become multicellular through budding, a process of asexual reproduction. For our purposes, only the yeast species Saccharomyces cerevisiae is important; it converts sugars to alcohol and carbon dioxide gas, a process known as “fermentation.” This yeast is likely the first organism domesticated by humans, although unknowingly. For the sake of clarity, when the term “yeast” is used in the following, it refers to Saccharomyces cerevisiae only, and none of the estimated 5,000 other species of yeasts.
The plural “yeasts” indicates that the plurality of various yeast species is meant. As a general rule, all yeasts except “wine yeast” (more specifics below) are undesirable in wine-making, and bacteria must be kept out, with few exceptions (for example, those for malo-lactic fermentation). This leads to the conclusion that wine-making requires a high level of cleanliness and care.
From the above, we can conclude that there is only a single species that qualifies as THE YEAST. Then, what distinguishes baker’s yeast from wine yeast, or from brewer’s yeast, or from wild yeast? Before approaching the answer, a philosophical comment about microbiology as a science is warranted.
In microbiology, a vast amount of species have already been re-classified, owing to newly revealed genetic information. Until the dust has settled over current disputes, and a new predominant opinion has been commonly accepted, this author will stick to the terminology that is commonly accepted at the time of this writing, while aware that it may not stand the test of time.
Until a few decades ago, wine yeasts were simply variants of the Saccharomyces cerevisiae subtypes, until it became fashionable to talk about Saccharomyces ellipsoides. The new terminology is that wine yeast is Saccharomyces bayanus, and has developed out of the wild types of Saccharomyces uvarum, Saccharomyces cerevisiae, and Saccharomyces eubayanus through hybridization. This sounds like a classical puzzle for geneticists. The discussion still goes on, with a noticeable North American – European divide. It is rather descriptive that even a scientific article published a couple of years ago, has the title “On the Complexity of the Saccharomyces bayanus Taxon: Hybridization and Potential Hybrid Speciation” (Laura Pérez-Través, Christian A. Lopes, Amparo Querol, Eladio Barrio; PPLoS ONE 9(4): e93729. doi:10.1371/journal.pone.0093729). Part of this divide also shows that the current dominant North American belief is that wine yeast cultures are single species cultures, while European oenologists hold the belief that wine yeast cultures are mixed cultures of closely related yeasts.
As one can regularly see with micro-organisms, they come in various genetic variants or subtypes, in micro-biology this is usually referred to as “strains.” It would appear that humans had unknowingly dug into the plethora of yeasts that surround us as early as 6,000 years ago, which is when the first archaeological evidence of leavened bread is dated to. It seems that this inadvertent domestication of a certain type of yeast resulted from humans selecting a certain strain that was especially adapted to the environment of bread-making. Assuming the dried dough crumbs from the bread with that specific strain were allowed to pass into sequential batches of dough, perhaps inadvertently, over the course of many centuries, and given the rapid growth of yeast, this would result in a strain that would be qualified as baker’s yeast. It may be of interest that in North America, bakers often relied on spontaneous leavening, meaning wild yeast, until Charles Louis Fleischmann introduced yeast as a commercial product in 1868. As a conclusion – yes, baker’s yeast does still know how to produce alcohol, but it is inadvisable to use it for that purpose.
It is plausible that a similar development took place when early humans discovered that consuming over-ripe fruit can have interesting side-effects. This eventually resulted in the collection of fruit for the purpose of letting nature takes its course in the form of wild fermentation. One could safely assume that wine barrels had repeated uses, so that yeast was able to pass from batch to batch. This also serves to explain why certain wineries have their special cultures, and by extension give their wines a specific flavour. Most European strains of wine-yeasts are still named after the place where they initially were cultivated in wineries. With the advent of micro-biology at the end of the 19th century, particularly desirable strains were developed and cultured, which has resulted in pure yeast cultures becoming a commercial product. Given the quality standards that can rightfully be expected in our century, only fermentation with a pure yeast culture is acceptable. One should never allow a wild fermentation to run its course; it is extremely unlikely that this would randomly lead to a super-yeast that outperforms cultures that have been developed over many centuries.
A word about brewing yeast may be in place. Until chilling became a practise in brewing in the late Middle Ages (ice blocks were cut from lakes during winter, and stored underground in ice cellars for brewing during summer), the only type of brewing yeast that was known is what we know today as “top-cropping,” where the yeast floats in the top layer. With chilling, bottom-dwelling yeast (“bottom-cropping”) was developed, which serves to give beer a different taste. A classical ale-type beer would be brewed with top-cropping yeast, while a lager-type beer uses bottom-cropping yeast. Why is this significant in this context? Baker’s yeast is top-cropping, while wine yeast is bottom-cropping, which has to do with fermentation temperature, and not just the particular strain that is used.
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. The saying “it will ferment by itself anyway” will only lead to a quality wine if you are working within a wine cellar that has over the past few centuries been able to develop a specific culture. If your wine cellar is younger than at least 100 years, or even 200 years, then don’t even bother trying.
Yeast cultures of cultivated strains are available commercially, and they are cheap. Unless you happen to be a microbiologist, and thus, have the equipment, don’t try to culture yeast for purposes other than using it in your wine-making. The commercially available yeast is either freeze-dried and grainy, or in liquid form. Once the package is opened, one must use it up immediately. Storing an opened package invites contamination with other micro-organisms.
Yeast has intentionally been cultured in Europe for more than a century, and unintentionally in traditional wine cellars for several centuries. In Europe, it is still common practise to name yeast strains after their location where they dominated originally. Depending on the strain, a yeast can have varying degrees of alcohol tolerance, the sugar tolerance, … and the list goes on.
When a yeast culture is added to the batch to induce fermentation, the yeast gets tossed into the midst of a microbiological fight that had started long before its introduction, all because the batch for fermentation has not been prepared under sterile conditions, and probably never will. There is a fight for survival already going on, and it is at this point that the main task of the vintner comes to light, to have the right conditions prepared for the cultured yeast to win the battle. In order for the cultured yeast to come out triumphant, we will need at minimum:
An optimal level of acidity. Yeast is naturally rather acid-tolerant, but fungi and most bacteria are not.
A low level of oxidation-reduction potential (ORP) to create the anoxic conditions under which the cultured yeast can thrive. This is traditionally done by adding “ sulfur” (sulfur dioxide or sulfurous acid) prior to adding the yeast. Cultured yeast is somewhat tolerant to sulfur dioxide, but “wild” yeasts are not. Actual ORP measurements, and consequently ORP control, have yet to make it as the new standard. If “sulfur” is used, the concentration should not exceed a sulfur dioxide equivalent of 40 mg/l. Osorno offers two products for this purpose. These are Sulfite Solution and Sodium Metabisulfite powder. The first product is more simple for use.
As high a population density of cultured yeast we can get, to overwhelm the competition through sheer numbers. Therefore, it is good thinking to prepare a pre-culture that is added, rather than using yeast directly out of the package.
An optimal level of glucose, the preferred sugar, as the main yeast nutrient; all other sugars will have a delayed effect. The osmotic pressure of the fermentation batch is initially almost completely determined by the sugar concentration. Because a high osmotic pressure is detrimental and possibly lethal to micro-organisms, it is important to start with a lower sugar concentration than what is needed in total.
Aside from glucose as the main nutrient, yeast nutrient mixture containing minerals, amino acids, and vitamins, as will be covered in a separate section below.
A carrier material on which yeast cells can grow. This is usually not a critical point due to the fruits providing so many cell fragments that the yeast cells will have plenty of choices for settlement. However, for many wines, honey wine (mead) being one of the most notorious, the ingredients come without a natural carrier. In this case, it is advisable to add a carrier of some sort, for example a small amount of apple sauce, or oat flour (flakes are too big).
Selection of a Yeast Strain
The North American market for pure cultured yeast for hobby vintners seems to be dominated by Lalvin Yeast, a division of Lallemand (https://www.lallemandbrewing.com/en/canada/), selling yeast under the Lalvin brand. The Lallemand company was founded in Montreal at the end of the 19th century by Fred “Lallemand,” an immigrant from Alsace, France, a region that has historically shifted between French and German control and was part of the German Empire during Lallemand’s founding. Fred came to be known as “l’allemand,” the German, and so the name stuck. The available selection for hobby vintners is poor but sufficient, and uses cryptic strain names (their professional selection is much wider, with a clear indication of the origin of the strain), and are for high-alcohol wines. The website gives access to detailed information about each strain, downloadable in PDF format.
Recipe of Fermentation Starter
The recipe is calculated for 100 – 200 l drum. If targeted production volume of wine is 20 – 40 litres, than only 1/3 of the ingredients amounts should be used for preparation of the fermentation starter.
Dissolve in 1 litre of water 150 g glucose or in 1 l of just boiled water 150 g of sugar. Add 30 g tartaric acid. Let it cool if water is hot. When solution is slightly warm, add 1 g of yeast nutrient, mash a couple of raspberries, add 2-3 packets dried yeast. Let it stand for couple of hours. When foaming begins, add 500 ml of wine, mix. When foaming resumed add another 500 ml of wine, mix. Let it stand until foaming resumed. When water/wine mix is actively foaming add to drum with wine.
Yeast is cheap labour, but they do not work for free. The main wages that they collect is from sugar that they “eat,” meaning that a few percent of the sugar are in fact not converted into alcohol. Although this process is the yeast’s main energy source, they need additional nutrients as “nutritional supplements.” The main additional components needed are phosphate and one or more nitrogen sources. The latter is so important that it has drawn much scientific and commercial attention, to the extent that the term Yeast Assimilable Nitrogen (YAN) has been coined, and that YAN measurement prior to fermentation has become standard practice in wineries.
In most cases, the addition of some diammoniumphosphate (DAP) will satisfy the needs of the wine yeast, so that this has also become standard practise. It is a good rule of thumb to add 2 g of DAP to a 10 l batch prior to fermenting. The amount is not critical, and some excess of DAP will neither harm the yeast, nor the wine drinker, and has no adverse effect on taste, unless used in unreasonable quantities. In commercial wine-making, Australia and the EU have far stricter rules on DAP than US, where the recommended limit is 3 g of DAP per 10 l batch.
Unfortunately, things are not necessarily as easy as they may appear at first glance – and this is also the case with DAP. To begin with, it contains nitrogen and phosphorous in an atomic ratio of 2 : 1, but yeast requires more nitrogen than that. The easy remedy is to add something that provides additional nitrogen, and in an agricultural-heavy environment like North America, urea, as one of the classical nitrogen fertilizers, that comes to mind. Urea is cheap, easy to get, and not harmful to humans, in facr there are yeast nutrient products on the North American market that are mixtures of DAP with urea. The bad (and eventually fatal) news is that yeast can only use urea partially, and converts it to carbamate, which it then leaves behind. Carbamate is a known carcinogen, so that reader should abstain from using these products, unless there is a desire to develop cancer. We strongly advise against the purchase and use of yeast nutrients that contain urea or of yeast nutrients where the manufacturer does not declare its composition. Regrettably, these potentially fatal products are sold in Canada and the United States to hobby wine-makers; they are prohibited in commercial wine-making in these countries. It must be pointed out that there is nothing wrong with using urea as a nitrogen fertilizer in a vineyard, where it is used as a nutrient for the grapevines, as opposed to a nutrient for future yeast.
The relative nitrogen deficiency of DAP is often compensated by adding diammoniumsulfate (DAS) to the mixture, with or without additional amino acids (in the US, only glycine is a permitted additive amino acid for wine). DAS also serves as a sulfur source. Osorno produces Composite Yeast Nutrient, to purchase this product please visit Osorno Store.
Yeast has the additional nutritional need of inorganic components such as potassium, calcium, and magnesium, and in trace amounts (micro-nutrients) copper, iron, manganese, and zinc. If a good well water is used in the preparation of fruit wine, the need for calcium, magnesium, iron, and manganese is taken care of. If reverse osmosis water or distilled water is used, re-mineralization of that water is a necessity, much the same goes for water that is intended for human consumption. For wine-making, mineral additives are available to offset the deficiencies of mineral poor water, such as reverse osmosis water.
In addition to the inorganic substances discussed above, certain organic compounds are essential nutrients for yeast, such as thiamin (vitamin B1), riboflavin (vitamin B2), biotin (vitamin B7), pantothenic acid (vitamin B5), nicotinic acid (vitamin B3), and inositol, which can be considered as a “vitamin cocktail for yeast.” It is somewhat ironic that for most of these vitamins, yeast itself is an excellent source. However, at the time of kickstarting the fermentation, there is not a sufficient amount available for the yeast, unless it is added.
As a conclusion, there is a need to add yeast nutrients, preferably the complex type, and possibly also mineral additives if the water is poor in minerals. Osorno is planning to start production of Composite Yeast Nutrient with Vitamins and Mix of Mineral Additives for Mineral Deficient water. Please e-mail us if you are interested in purchasing of these new additives.