The Science, The Art, and The Magic of Wine Fermentation

The Science, The Art, and The Magic of Wine Fermentation

Basic to winemaking is the process of fermentation. But to regard fermentation as simply a formality in the production of wine is to miss one’s opportunity to put his or her unique signature on a new wine, to forfeit the artistic license to create something truly unique. What distinguishes really good wines from ordinary wines is attention to every small detail in the winemaking process. No single step in the winemaking process is responsible for producing the indescribable aromas and flavors found in a memorable wine. Premium winemaking involves doing all of the little things right so that, when taken together, the sum of all of them is greater than any single part of the process. Since it all begins with fermentation, it seems reasonable to take a detailed look at the process, paying attention to all of the little details involved in the amazing transformation of grapes and juice to wine.

Some Basic Wine Chemistry

The chemical reaction representing fermentation was first formulated by Gay-Lussac in 1815, and looks like this:

Glucose/Fructose = Ethyl Alcohol + Carbon Dioxide

But this is just the science behind alcohol production. Obviously, wine is much more than the production of alcohol. There are many, many more reactions occurring during fermentation and afterwards that create the wonderful flavor and aroma compounds that, collectively, we call wine. In general terms, grape juice, pulp, skin and seed compounds are converted by numerous reactions to secondary compounds with different aromas and flavors than those present in grapes. Many of the chemical compounds that comprise wine have been characterized. A short list would include ethyl and other alcohols, fusel oils, glycerol, polysaccharides, amino acids, proteins, mannoproteins, phenols, tannins, anthocyanins, nitrogen, and many vitamins, minerals, enzymes, co-factors and anti-oxidants, just to name a few.

All About Yeasts

Yeasts are single celled fungi which can live under both aerobic (in the presence of oxygen) and anaerobic (oxygen-less) conditions. By definition, alcoholic fermentation is an anaerobic process carried out by yeasts and all yeasts share this ability. But to think of all yeasts as being created equal is to miss out on a key opportunity as a winemaker to take full creative advantage of the winemaking process. Commercially available wine yeasts are represented by three common species, Saccharomyces cerevisiae, Saccharomyces bayannus, and Saccharomyces uvarum. Among these species, however, there are literally hundreds of different cultured strains produced by several different companies and enological institutes. Most of these publish detailed literature on the various properties of each yeast strain that they offer.

Yeast strains vary in properties such as vigor, alcohol production and tolerance, SO2 tolerance, ability to ferment at cold temperatures, nutrient requirements, and hydrogen sulfide (H2S) production. Some yeasts display unique properties, such as the ability to reduce acidity, produce increased levels of aromatic ester compounds, and improve mouthfeel by producing increased levels of polysaccharides and mannoproteins. From my experience, claims about the particular properties of certain yeast strains by the companies that produce them are more than just good advertising, and the creative winemaker would be well-advised to experiment with various yeast strains to learn more about the properties of each in their own hands. If you have reached a plateau with a particular wine that you make, or you would simply like to add new dimensions (i.e. aromas and flavors) to an old standby, an easy experiment could be devised to assess the properties of several yeast strains simultaneously. I would suggest a trial using five yeasts selected for various properties that appeal to your stylistic goals based on the published properties of each. Purchase a good wine kit or fruit/juice sufficient to produce a typical five-gallon batch of wine, but instead of fermenting the wine in a single vessel, divide the must into five separate vessels and inoculate each with a different yeast strain, keeping all other things equal between the five lots. Once the finished wines are ready for formal evaluation, have someone help you prepare a blind tasting of each lot, making notes of the desirable, undesirable and unique properties of each experimental lot. In this way, you can test five yeast strains simultaneously, and decide which yeast or yeasts you like the best. I think you will be pleasantly surprised with the results. You could even take things a step further, performing blending trials using the five lots to see if a blend of lots made from different yeasts produces an even more appealing wine than any particular single strain. This is the art and fun of winemaking. The possibilities are endless.

Emulating the French model of winemaking, some commercial wineries and home winemakers are going to natural or native yeast fermentations, taking advantage of the local yeast strains colonizing their vineyards and grapes. The powdery covering on the skins of grapes, known as the “bloom”, is composed of a waxy substance containing several naturally occurring yeasts. Although Saccharomyces species may be found naturally on grapes, other yeasts such as Kloeckera, Pichia, Hansenula, and others may dominate fermentations when cultured commercial yeast strains are not inoculated into musts. These yeasts may produce unique flavors and aromas expressive of local terroir, but they are also notorious for their low SO2 tolerance and a propensity to “stick” or become sluggish, leaving residual sugar and an incompletely fermented must, a situation conducive to spoilage. Some naturally occurring yeasts such as Candida and Brettanomyces are generally considered spoilage yeasts, imparting aromas and flavors that are unappealing to many wine drinkers. Because of the uncertainty and unpredictability of natural yeast fermentations and the potential for spoilage yeast and bacteria to ruin a wine, most winemakers inoculate a carefully chosen commercial yeast strain with predictable qualities. My personal preference is for the latter, and I would recommend that home winemakers inoculate with a commercial yeast strain, at least until sufficient winemaking experience has been gained to venture into native yeast fermentations.
 
Building the Basic Yeast Starter

“A person’s a person, no matter how small…” from Horton Hears a Who by Dr. Seuss

OK, yeasts aren’t people, but they still deserve the respect of the winemaker. In my previous life as a home winemaker, I used to just pitch the dry packet of yeast into the must or juice, or mix it into some hot water like Mom used to do with her yeasts when she made bread. But I became wise to my ways after a sluggish (and eventually spoiled) batch of wine, and began to realize that yeasts are living creatures, and required gentler handling than I was giving them.

“I felt a great disturbance in the force, as if millions of voices cried out in terror and were suddenly silenced…” Obi-Wan Kenobi, from StarWars IV: A New Hope

It wasn’t until a 1998 Purdue University Wine School lab session taught by the wonderful wine microbiologist Ellie Harkness that I began to realize how many yeasts’ unnecessary deaths I had been responsible for through the years. Then and there I resolved to change my ways, and Ellie showed me the light. Commercial wine yeast is usually supplied in dehydrated form in 5 gram packets or 500 gram bricks. In this state of “suspended animation”, yeast cells are very delicate and are vulnerable to thermal shock and rehydration (osmotic) stress injury. It is therefore important to rehydrate yeast slowly and gently, and at the right temperature to ensure the least possible cell mortality. It is not difficult to rehydrate yeast effectively, but performing this initial step in fermentation is absolutely critical to all that will occur later in the life of the yeasts and the wine.

The basic ingredients are:

Dehydrated yeast, 1 gram/gallon of must or juice
Clean, 104°F water; vol. of water in milliliters (ml, cc) = grams of yeast X 10
Balanced Yeast Nutrient, 1 gram/gallon of must or juice

Run the water into a container sufficiently large to accommodate the yeasts as they swell and froth during hydration. For a 5 gram yeast starter, a medium to large tumbler works fine. For larger batches, pitchers and even 5-gallon buckets may be more adequate. Sprinkle the dehydrated yeast evenly over the surface of the water and allow it to sit undisturbed for 5-10 minutes, then vigorously stir the yeast into the water, introducing plenty of air into the mixture. Remember, at this stage, the yeast cells need a lot of oxygen to get started. The yeast nutrient can now be stirred into the yeast/water mixture (commonly-used brands are Fermaid-K and Fermax; some winemakers like to add diammonium phosphate (DAP) to provide additional nitrogen). Next, allow the yeast starter to rest undisturbed in a cool room, providing enough time so that the yeast starter slowly cools down to within 10°F of your must/juice temperature. Once this temperature has been reached, your healthy yeast starter can be stirred into the juice or must, again with enough vigor to introduce oxygen into the must. The larger the volume of the starter, the longer this cool-down process takes, and vice-versa. This procedure should avoid creating any unnecessary “disturbances in the force”, and will ensure the safest and happiest start for your young fermentation.

Keep Your Cool

Stylistically speaking, one of the most important controls that the winemaker can exert on a fermentation is the temperature at which the process is conducted. All of the processes that turn juice and grapes into wine involve complex chemical and enzymatic reactions, and these processes occur more rapidly at higher temperatures and more gradually at lower temperatures. Generally speaking, white wine fermentations are carried out at cooler temperatures to preserve fruity aromas and flavors that quite literally may “bubble out” if a fermentation is allowed go rapidly to completion. In the extreme, white wine fermentations may be carried out at temperatures as low as 40°F. At these low temperatures, a fermentation may take weeks to months to complete. Although flavor and aroma preservation are the desired result in such low temperature fermentations, undesired effects may occur such as a stuck fermentation, sulfur dioxide (rotten egg, burnt match) production, or the appearance of strange, medicinal aromas and flavors when yeast are stressed in this manner. Usually, a more moderately cool fermentation temperature in the range of 50-65°F is chosen to maintain fruity character while avoiding the problems associated with excessive yeast stress at temperatures cooler than 50°F. Certain yeast strains have been specially cultivated and selected for their ability to ferment without excessive stress responses at low temperatures, so a winemaker should do his or her homework to learn which of these are best suited to the style of wine desired. If you have never tried to control the temperature of your white wine fermentations, I think you will be pleasantly surprised with the results of your next batch of white wine if you do so. A cool basement or a refrigerator is ideal for carrying this out successfully at home.

On the other hand, red wine fermentations are usually carried out at significantly higher temperatures to aid in extraction of all of the “goodies” from the skins and seeds that make red wine what it is. With the notable exception of perhaps a carbonic maceration style Nouveau fermentation, which represents an attempt to make a fruity, “white-style” wine from red grapes, I can think of no other red wine fermentation that would benefit from fermenting the must at cooler temperatures. Premium red winemakers always talk about wanting to generate heat in their musts when producing full-bodied red wines. If a fruitier dimension is desired, a percentage of whole clusters is often added to the larger mass of crushed and de-stemmed fruit to maintain this lighter, fruitier style of red wine. Pinot Noir is often fermented in this fashion. Heat can also be overdone, however, and must temperatures must not be allowed to exceed 90°F by any great margin, or the risk of a stuck fermentation will be increased. Red wine musts sometimes must be cooled to avoid risking a stuck fermentation from heat stress on yeast. Excess nitrogen addition to a must may actually precipitate this situation, and should be avoided. A complete discussion of the many ways to manage a red wine fermentation is beyond the scope of this article, but suffice it to say that the way in which the cap of skins and seeds is managed will dictate to a large extent the style of red wine produced. As with white wine fermentations, yeast strains specifically chosen for their red wine production properties are available to the creative winemaker. These should be sought out and experimented with as described earlier in this article to place one’s own signature style on a red wine fermentation.

Control of fermentation is one of the key factors in creating wines with style and distinction. I hope that this article at least gives you a new perspective on your creative winemaking efforts, and that you will enjoy using all of the tools at your disposal as a winemaker to craft memorable wines that you, your family and friends can enjoy for years to come. Remember to be kind to our yeasty little friends, and they will repay the kindness many-fold in the great wines that they help you to make.

 Chris Nelson, MD

Master  Winemaker
Chrisman Mill Vineyards and Winery