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Yeasts & Winemaking – Part I…

A Brief History:

Yeast microbes are probably one of the earliest domesticated organisms, and may be one of mankind’s earliest industrial microorganisms. Archaeologists digging in Egyptian ruins found early grinding stones and baking chambers for yeasted bread, as well as drawings of 4,000-5000 year-old bakeries and breweries. It is believed that these early fermentation systems for alcohol production were formed by natural microbial contaminants from fruit or other juices containing sugar. Such microbial flora would have included wild yeasts and lactic acid bacteria that are found associated with cultivated grains and fruits. Wild yeast spores are constantly floating in the air and landing on uncovered foods and liquids, and have been doing that for eons. Even today many winemakers prefer natural yeasts to commercial yeasts. These natural yeasts are indeed floating in the air, especially the air of wineries that has been making wine for some time in the same location, often with grapes from the same vineyards.

It was not until the invention of the microscope followed by the pioneering scientific work of Louis Pasteur in the late 1860’s that yeast was identified as a living organism, and the agent responsible for alcoholic fermentation.

Only in the last 150 years since the Pasteur’s experiments, have scientists begun to explore how yeast works. Pasteur first proposed the production of carbon dioxide from yeast as responsible for raising a loaf of bread in 1859. Shortly following these discoveries, it became possible to isolate yeast in pure culture form. With this new found knowledge that yeast was a living organism and the ability to isolate yeast strains in pure culture form, the stage was set for commercial production of yeast that began around the turn of the 20th century. Since that time, bakers,winemakers, scientists and yeast manufacturers have been working to find and produce pure strains of yeast that meet the exacting and specialized needs of the baking and wines industries.

Wine Yeasts:

We have used commercial yeasts to make wine for many years at De Angelis Wines. While we are amazed at their efficiency, variety, and ability to convert the sugars glucose and fructose to alcohol, understanding how this happens is a daunting task. [For convenience, we will refer to the mix of glucose and fructose in grape juice as sugars.] Many yeasts are what we will term here “Good Yeasts” and some are – like it or not – “Bad Yeasts”. In this politically correct world in which we live some will object to labeling a fungus as either good or bad.  We have no such problem, because while many types of yeast used in wine making are good yeasts, some are even better than others.  Ditto for those we call bad yeasts. Some are troublesome, and some cause great misery to a winemaker.  Why we differentiate yeasts this way will become clear as we develop this series of articles about Wine yeasts. [After exploring Good Yeasts, we will explore Bad Yeasts.]

It is also important to point out that we are not trying to make readers into microbiologists, or chemists, or anything but more informed wine lovers. If along the way this increases your interest in these fungi, and you develop an abiding interest in wine yeasts, and learning what some may call arcane facts, so much the better.  Along the way, we will offer opinion on certain matters related to yeasts. These will be based on our experiences as winemakers, rather than on well defined research. We will always let the reader know when we are doing that – offering opinions – so you can argue with us, and our opinions, as well as our conclusions.

Yeasts are the stars of a process called Fermentation. No, I am not dismissing the grapes, or all the other hundred of chemical components in grapes and grape juice. I am telling you that during fermentation most of a winemaker’s concern is developing and maintaining a healthy population of yeast cells in the brew, called “must” by wine people. Your first question may be “What’s a healthy population?” Assuming a winemaker can read, and further assuming they follow instructions –learned either long ago, or newly learned – yeast that will be used to inoculate the must should have a yeast cells population density in the must equal to 10^6 or 10^7/milliliter (ml).  That’s a 1 followed by either 6 or 7 zeros. [i.e. 1,000,000 or 10,000,000 cells/ml]. Once the yeast becomes active and reached maximum activity that number increases to about 10^8.

So far we have not mentioned the specific genus or family of commercial yeasts that are usually used in winemaking. The most often used Genus where we make wine is Saccharomyces and the Species are more often than not Cerevisiae and Bayanus.  There are hundreds of Sub-Species with names like Montrachet, Pasteur, ICV – D21, ICV-D254, Assmanshausen, Zymaflore FX -10, Syrah, VIN 2000, and many, many more.  Each of these does primarily one thing – they convert sugars in the juice to alcohol.  Secondarily they promote flavors, aromas, mouthfeel, mid-palate and finish in a wine.  While a yeast will probably ferment any must, there are specific yeasts that do a better job when used with one variety must wine over another, and when fermentation conditions are such that one yeast may not perform well, but another will. For example, ICV-D21 will work well under low nutrition conditions, while ICV-D254 needs more nutrients to complete fermentation.  Some yeast will complete fermentation up to 16% alcohol while others cannot function at alcohol levels greater than 14-15%.  A winemaker must consider each fruit must as a different challenge, and find the correct yeast that will ferment grape juice so that the resultant wine  has both the characteristics, and the look and feel of the wine, he or she envisaged when the decision was made to ferment that particular wine must.

So what are some of the things that a winemaker has to consider when choosing yeast?  Let’s take a look.  Prior to doing that however, we must give credit to Yair Margolit’s book “Concepts in Wine Chemistry“  Dr. Margolit presents all of the concepts and details one needs to understand what yeast bio-chemistry is all about.  Another reference book for the very serious wine geek is “Principles and Practice of Winemaking” by Boulton et al.  Actually, if you have nothing to do one day, and want a challenge, read the 90 pages on fermentation in Dr. Boulton‘s book!

Part II and III will be forthcoming…

 

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