BREW 204

"If there is an art to winemaking then it is the art of controlling yeast. It is the art of selecting the appropriate yeast, introducing it at the correct moment, feeding and nurturing it so as to coax it into living, reproducing and dying in a prescribed manner, and then cleaning up after it so as to preserve the fruit of its labor. It is the art of controlling its temperature, the amount and kind of air it is allowed to breathe, and feeding it the sugar and other nutrients it needs to serve man. Controlling yeast is the real art of making wine." - Jack Keller, Jr.

Yeast are single celled fungi. Merrian-Webster defines fungi as "any of a major group of saprophytic and parasitic spore -producing organisms usually classified as plants that lack chlorophyll and include molds, rusts, mildews, smuts, mushrooms, and yeasts." These single cell plants, of the genus Saccharomyces, consume sugar and transform it into approximately equal parts of alcohol and carbon dioxide through fermentation. It is really the yeasts that are the winemakers; brewers are just technicians providing for the welfare of the yeasts.

History

In 1680 Anton van Leeuwenhoek, inventor of the microscope, discovered that some kind of microscopic life existed while he was looking at a drop of his wine. His microscope was not quite powerful enough to realize that he had just discovered yeast, however. Of course, there was still no connection between yeasts and fermentation, but it was the foundation for future discovery. In 1785, the French chemist Antoine Laurent Lavoisier hypothesized that alcohol fermentation was a chemical process. Sugars had already been identified as the raw material of fermentation. Lavoisier noted that between the alcohol and carbon dioxide gas produced during fermentation, pretty much all fermentable sugar was accounted for. He concluded that fermentation was caused by the sugar molecule being chemically split. He was wrong, but he was the first to offer a reasonable explanation for how alcohol was made. In 1878, Wilhelm Kuhne, coined the word enzyme to describe the catalyst [a substance that enables a chemical reaction to proceed at a usually faster rate or under different conditions than otherwise possible] that he thought had to be in the yeast. This was closer to true than previous hypothesi. The word enzyme comes from the Greek - en (in) and zume (yeast). It is the enzymes secreted by yeast cells that act upon sugar molecules and create the process known as fermentation. Over 2 dozen enzymes are involved in creating about 30 different chemical reactions that transform sugar molecules into alcohol and carbon dioxide. These reactions are strictly ordered, one dependent upon the preceding one to succeed. Protease works only on proteins; invertase breaks down sucrose; and so on until zymase creates alcohol itself. So far this process isn't fully understood and it has not been reproduced by man. Fermentation is completely unique to yeast.

Yeast Strains

There are thousands of millions of types of yeast. Of those, only 250 will produce fermentation. And of those, only 24 are "good" yeast. The good yeasts (Saccharomyces cerevisiae) for winemaking include the strains Saccharomyces bayanus, Saccharomyces uvarum, Saccharomyces oviformis, Saccharomyces carlsbergiensis, Saccharomyces logo, Saccharomyces chevalieri, Saccharomyces diastaticus, Saccharomyces fructuum, Saccharomyces italicus, Saccharomyces hispanica, Saccharomyces oxydans, Saccharomyces pasteurianus, Saccharomyces prostoserdovii, Saccharomyces sake, Saccharomyces sterineri, and Saccharomyces vini predominantly.

A word on natural (spontaneous) fermentation

Winemakers of Europe have for hundreds of years been returning the pressed-out skins (pomace) of the wine grapes to their vineyards as fertilizer. Over time, the selected yeast strains become dominate so much so that the winemaker can achieve precisely predictable results by simply encouraging spontaneous fermentation of newly pressed wort. For the winemakers in the United States, planting new vines where none have grown before, this is an envious situation. Even in the vineyards of the USA that have been around for 150 years, such domination by a primary Saccharomyces is centuries away. It is argued not only in the SCA but in the Mundane World as well that you could allow spontaneous fermentation in winemaking. . . I do not recommend this method. Your odds of getting the "right" yeast are 24 in 250 or 10.4:1.

How it works

When yeasts enter a new environment, they exist for a while in what is known as a lag period to determine whether or not that environment is suitable for colonization. If they determine that it is, they begin reproducing. Yeast reproduce at an amazing rate in a favorable environment. This rate is typically one logarithmic unit per hour, meaning that 1 yeast cell can become 1,000,000 in only six hours. Luckily, when their population density reaches about 5,000,000 per drop of liquid, they settle down and maintain a relatively steady population. When you innoculate your wort the yeast you chose gains the advantage and grows from about 1000 cells per mL to between 10,000,000 and 100,000,000 cells per mL within 6-12 hours. Below 500,000 per drop, your wort may appear clear. This is a good indicator for many things, including when fermentation is done.

A strange phenomenon in pitching yeast is that if you innoculate a wort with too much yeast, your end product will taste distinctly of yeast. But if you allow the yeast to naturally grow from a normal inoculation, then all should be well. Sometimes it takes a long time for yeast to get started. If you properly started your yeast, and you pitched it at the correct temperature, you will more than likely get fermentation. If you don't get any action out of it for 48 hours, remove the lid, cover with a towel, and come back in 12 hours. If it still looks, and sounds, like you don't have anything going on, then re-pitch.

How to "pre-measure" your alcohol content and control your final Specific Gravity

Attenuation refers to the percentage of sugar converted to alcohol. The attenuation is determined by the composition of the wort and the yeast strain used. Each yeast strain ferments sugars to different, varying degrees. This results in higher or lower final specific gravities (SG). This will affect the residual sweetness and body. This will also help you figure out how sweet a drink will be, and wether you are going to pop any corks or explode any bottles. To properly measure SG you will need a hydrometer. Most hydrometers are calibrated to about 70F. The hotter a liquid, the thinner is becomes. Consult with the directions of your particular hydrometer for temperature variance.

So, if you had a 10% (by weight) sugar solution (about 1.040), and got 100% attenuation, the resulting specific gravity would be about 0.991 (corresponding to about 4% alcohol by weight).

The trick is to match your yeast to your application. If for instance you are wanting to make a Sweet Mead, with a moderate alcohol percentage (12%) you wouldn't want to use a champagne yeast that is going to ferment out to 16-18%. You might want to use a Pasteur Red or RC212 (see chart), with a starting SG of 1.125. But it you are trying to make a dry mead with a high alcohol content you use a 1118 or 3021 with a starting SG of 1.125. See how the same SG is used for two totally different meads just by changing the yeast? By knowing what yeast you are using, you can tailor your beverage to the correct style and type you are wanting.

Some Class Notes