August 23, 2014

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Nitrogen in the Winery

 

Winemaking begins in the vineyard, and so does nitrogen. Nitrogen is one of the most common elements in the universe. On Earth, in its elemental form, it exists as a gas that forms 80% of our atmosphere. However, it is also a chemical constituent of many important components essential to life. Nitrogen makes up the building blocks of DNA, and it is also an important element in the composition of amino acids. When linked together, amino acids form the enzymes that drive all of life’s biochemical reactions. They are the building blocks to all proteins, hormones, and some plant metabolites that are responsible for wine flavor. Plants draw mineral nitrogen from the soil and convert it to amino acids and other compounds. Animals who consume plants in turn ingest the nitrogen that the plants have drawn from the soil. Even single-cell organisms, such as yeast, need nitrogen for survival.

 

Many of us are well aware of the effects of nitrogen on the growth of plants. Nitrogen is the most important nutrient involved in regulating vine growth, morphology, and tissue composition. Soils that are high in nitrogen cause an increase in vigor, which can lead to shaded canopies and high yields of unripe fruit in vineyards. However, it is also important to understand how the nitrogen that is in fruit at harvest can have an effect on fermentation.

 

What’s your YAN, man? When grapes or other fruits are harvested, they contain nitrogen in many different chemical forms. The most important nitrogen-containing compounds for fermentation are free amino acids (FAN), ammonium ions (NH3), and small peptides. These compounds can, for the most part, be consumed by yeast during fermentation and are collectively called yeast assimilable nitrogen, or YAN.

 

YANThe free amino acid content (FAN) of the grape juice can be measured by a variety of different methods, but the most commonly accepted way to measure it is the NOPA assay. I won’t detail the procedure here as there are plenty of resources available, but it is worth noting that a spectrophotometer is needed in order to interpret the results. For wineries looking to upgrade their lab, I’d highly recommend investing in this piece of equipment.

 

The ammonia (NH3) content of juice (which is 83% nitrogen) is measured enzymatically, and the results are also determined by a spectrophotometer. The sum of the FAN and the NH3 collectively give us the amount of YAN in the juice.

 

Another method for measuring YAN is called the Formol titration method. While it is a simpler method, involving only a titration, it does involve using a Formaldehyde solution. In order to mitigate health and safety risks with this method, the titration must be performed under a fume hood – which is a much greater investment for a winery than the cost of a spectrophotometer. Newer methods of measuring YAN are also available, but require highly specialized lab equipment.

 

Nitrogen and fermentation. After sugar, nitrogen is the most important macronutrient for yeast. When juice is lacking in nitrogen, the yeast can exhibit sluggish fermentations, create off-odors, and eventually expire before consuming all the sugar resulting in stuck fermentations. Yet, while every winemaker I know carefully tracks the ºBrix (sugar) in their fruit, many winemakers don’t always measure the nitrogen content of the juice. Why? Well, many simply add a set amount of nitrogen (in the form of commercial yeast nutrients) as part of their regular fermentation protocol. Or, perhaps they don’t add a standard addition at the start of fermentation, but as soon as the wine starts smelling “stinky” (sulfide aromas like cooked cabbage or rotten eggs), they add nitrogen in the form of salts such as diammonium phosphate (DAP). When yeasts lack amino acids in their diet, they start to synthesize their own. Unfortunately, yeasts’ recipe for amino acids includes adding a bit of sulfur to create cysteine and methionene. When they then metabolize these amino acids, hydrogen sulfide is a byproduct.

 

Nonetheless, although a minimum amount nitrogen is important in preventing fermentation difficulties, it is possible to have too much of a good thing. When the nitrogen concentration in the grape must is too high (>450-500 mg/L YAN), it can stimulate the yeast to start overproducing undesirable aroma compounds such as ethyl acetate – an acetate ester with a nail polish aroma. Acetic acid production is also increased, as well as other aroma compounds that can be both beneficial and/or detrimental to a wine’s character. Even more disconcerting is the fact that wines made from high nitrogen juice contain greater amounts of the possibly carcinogenic compound ethyl carbamate. Bacteria can transform any excess amino acids following fermentation into biogenic amines like histamine and phenylethylamine – compounds which can cause headaches, nausea, or extreme reactions such as heart palpitations and shortness of breath in those who are sensitive. Thus, knowing the quantity of nitrogen at the start of fermentation can help prevent some of the undesirable consequences of adding more nitrogen than necessary (not to mention the added cost of using these nutrients!).

 

How much YAN do I need? The minimum amount of YAN needed for fermentation depends on a variety of factors such as the initial sugar concentration of the must, the fermentation temperature, and the strain of yeast used to ferment the wine. Nonetheless, it is generally accepted that juice with YAN less than 140-160 mg/L should be supplemented. Recommendations for initial YAN based on Brix levels have also been reported and used with success (table 1). Winemakers wishing for a fruitier style wine may wish to adjust their YAN to 300-350 mg/L, as at this level the maximum production of fruity ester aromas is obtained.[1] YAN levels above 450-500 mg/L can lead to the production of off-aromas and flavors.

°Brix of must or juice

Target YAN concentration (mg/L)

21

200

23

250

25

300

27

350

Table 1 – Recommended YAN concentrations as a function of sugar concentration[2]

 

YAN and Cold-Hardy Hybrids. In general, University of Minnesota-developed hybrids contain high quantities of YAN, though variations in total YAN concentration can be seen depending on the geographic area of the vineyard. A recent survey of YAN in cold-hardy grape cultivars across the Eastern US conducted by Amanda Stewart as part of her phD dissertation at Purdue University found that 19 of the 20 highest reported YAN values were for University of Minnesota-developed cultivars. In fact, the highest ever reported YAN value for grapes (938 mg/L) was recorded in Frontenac Gris grown in Iowa.[3] Her study also confirmed that YAN is highly variable and dependant not only on grape cultivar, but also by geographic location and vintage. This is confirmed by YAN data compiled at the Horticulture Research Center in Excelsior, MN. We have found YAN to be highly variable in Minnesota grapes. Because it is impossible to predict YAN concentrations, even from fruit grown in the same vineyard, it is recommended that winemakers always have their YAN quantified by a reputable lab prior to addition of any yeast nutrients.



[1] Ugliano, M., P. Henschke, M. Herderich, I.A. Pretorius. 2007. Nitrogen Management is critical for wine flavor and style. Australian Wine Research Institute. Wine Industry Journal. (22)6: 24-30.

[2] Bisson, L.F., C.E. Butzke. 2000. Diagnosis and rectification of stuck and sluggish fermentations. Am. J. Enol. Vitic. 51:168-177.

[3] Stewart, Amanda. 2013. Nitrogen composition of interspecific hybrid and Vitis vinifera wine grapes from the Eastern United States. Doctoral Dissertation. Retrieved from Proquest Dissertations and Theses (Accession order No. AAI3592130)

Keys to Successful Fermentation: Part 1

facebook_32Fermentation is a natural process by which yeast consume sugar and convert it to ethanol.  A successful fermentation is one in which the winemaker ensures that the conditions are met to enable a population of yeast to live and thrive until the winemaker wishes – generally until all the sugars have been depleted. All this needs to be done while minimizing the production of volatile acidity and sulfur off-aromas, and maximizing the desirable aromas and flavors produced during fermentation. It sounds easy enough, but for anybody who’s been around the industry can attest, stuck and sluggish fermentations happen more often than you might wish.  So, I present, the key points to a successful fermentation in four parts: yeast hydration and addition, the first quarter of fermentation, mid-fermentation, the last quarter of fermentation.

Yeast Population Kinetics

There are four main stages that a population of yeast will go through in a typical wine fermentation as illustrated in figure 1 below.

1) Lag phase – this is a very short period of time in which the yeast become acclimated to the juice or must. The duration of the lag phase is less than a few hours, until the yeast realize that they are in a sugar and nutrient-rich environment and they begin to multiply by budding (yeast division).

2) Exponential growth phase – yeast multiply rapidly. The yeast population can double every 4 hours until a maximum population density is achieved. There is an increased demand for oxygen as yeast cells replicate.

3) Stationary phase – The yeast population has reached a critical mass. This is the longest phase of fermentation in which the yeast are actively converting sugar to alcohol through anaerobic fermentation. At this point oxygen isn’t necessary for yeast survival, but a winemaker may choose to aerate a wine for other reasons (reduction aromas, color stability, etc.)

4) Yeast Death – Over time, the yeast will slowly deplete the nutrients available in the juice (sugar), and will also be producing waste products that are toxic (ethanol). Dead yeast cells will break apart (lyse) as they fall to the bottom of the tank and release more toxins that will kill surviving yeast. Thus, the decline of the yeast population is a rapid, exponential decline.

By understanding the important steps that  winemaker needs to take during each of these phases of fermentation, one can be assured that the risk of a stuck or sluggish fermentation is minimized. The first part begins with hydrating active dry yeast, and adding the yeast to juice or must.

 

yeast growth

Part 1: Yeast Hydration and Addition

 

1) Choose the correct yeast (Account for Osmotic Shock).

Grapes are naturally high in sugar. When yeast encounter this high sugar environment, there is a certain amount of osmotic pressure placed on the outside of the yeast cell wall.  Since the cell wall is permeable, the yeast expend energy to ensure that they maintain an equilibrium between the the pressure on the inside and the outside of the cell. To do this, they tend to produce more glycerol inside the cell, but they also will produce acetic acid to try to decrease the viscosity of the fluid outside of the cell (the grape juice). This phenomenon is well known in ice wine production, and is why these wines tend to have higher levels of volatile acidity than table wines. In this type of environment, the yeast need an array of micronutrients and amino acids to form the  fatty acids and sterols that will strengthen their cell membrane. A winemaker can also minimize damage to the yeast by making sure it isn’t exposed to further stress such as cold temperatures and excess SO2.

The initial osmotic pressure placed on the yeast will impact the physiology of the cell for the duration of its life, that is, until the end of fermentation. The resistance of yeast to alcohol in the final stages of fermentation depends on the initial osmotic pressure placed on the yeast and its ability to resist this stress. If a winemaker knows that the potential alcohol of the juice is greater than 13%, it is important to choose a yeast that has the ability to resist higher alcohol levels. Late harvest or ice wine styles should be fermented with a yeast that is intended for high sugar musts in order to minimize the potential problems with volatile acidity, and to ensure that the fermentation begins in a timely fashion.

 

2) Proper yeast re-hydration practices (resistance to other shock factors).

As mentioned above, sterols and polyunsaturated fatty acids are important factors that the yeast need to create a strong cell membrane. When one rehydrates the yeast in water (along with yeast nutrient), the yeast metabolism is in a respiratory state (consumes oxygen) which allows it to more easily synthesize these resistance factors in its cell wall. If yeast is rehydrated in juice, the yeast are more inclined to have a fermentative metabolism from the get-go, which makes it difficult to synthesize the products necessary to strengthen its cell wall to provide protection from stress during fermentation. The initial content of these resistance factors will become diluted with each generation during the multiplication phase.

The yeast multiplication phase corresponds to the consumption of the first 30-40 grams of sugar. Once the initial population of yeast cells reaches 100 million cells/mL of must, the juice is considered completely colonized. This level of colonization does not depend on the initial population of the yeast. So, in order to arrive at 100 million cells/mL, the greater the initial population of yeast, the less they need to replicate to reach their maximum population. Thus, their resistance to stress becomes less diluted, and the yeast are more able to survive in the high alcohol environment near the end of fermentation. This isn’t to say that you should double or triple the recommended dose for yeast in your fermentation. This dilution of stress factors is only seen if the initial amount of dried yeast used is less than 300 mg/L. Thus, the recommended quantity of 400 mg/L on the package of active dry yeast accounts for this.

 

3) Yeast Nutrition.

During the multiplication phase, yeast need amino acids/nitrogen, fatty acids, and micro-nutrients (vitamins and minerals). Some of these elements aren’t bioavailable in the juice at this critical moment when the yeast need them the most. By adding nutrients that make these  elements immediately bioavailable to the yeast, it diminishes the risk of added stress to the yeast due to a nutritional deficit. Adding yeast nutrients during rehydration and at the moment of yeast addition to the must allows the yeast to multiply in the best conditions. However, the different enological yeasts all have different needs when it comes to nutrition. The dose necessary during yeast addition depends on which yeast you use, along with other factors: potential alcohol, maximum fermentation temperature, oxygenation, and the initial temperature of the must during addition.

 

4) Accounting for cold shock in low temperature juice.coldshock

Have you ever jumped into water that is just above the freezing point?  You know then, how yeast might feel if they are immediately dumped into a cold tank of juice – something that is common in white and rosé fermentations. It is easy to evaluate the potential for cold shock to the yeast: the greater the temperature difference between the water at the end of yeast hydration and the juice in the tank, the greater the stress to the yeast. If the temperature difference is greater than 10ºC, the stress on the yeast caused by the cold shock will have physiological consequences to the yeast that will affect it throughout the fermentation. When it is known that there is a high potential for this cold shock during yeast addition, it is important to take some steps to compensate for these risks. The most important is to slowly acclimate the yeast to the juice temperature by adding some of the juice to the hydration water to bring down the temperature. The temperature decrease should not be more than 10ºC over a 20 minute period. When the yeast is added to the tank, the temperature difference should not be greater than 10ºC. Other ways to compensate for this stress are by adding a higher dose of active dry yeast, and ensuring adequate nutrition.

 

5) Compensation for the elimination of fatty acid sources (white and rosé wines). 

In all white and rosé fermentations the juice is racked 24-48 hours after pressing to eliminate suspended solids. The degree of clarification can be enhanced by using fining agents and enzymes in the juice – an important step if the grapes arrived in poor sanitary state. Ideally the turbidity of a juice following the first racking falls between 100 and 250 NTU. Nonetheless, while eliminating pectin particles and insoluble solids, you are also removing poly unsaturated fatty acids that are important for yeast survival. If the juice clarification is less than 200 NTU, it is important to take steps to reduce stress on the yeast. Adding yeast nutrients rich in fatty acids, or increasing the initial yeast population are ways to ensure yeast survival through the end of fermentation.

To Be Continued with Part 2: The first quarter of fermentation….

 

Yeast Selection Trials for Cold-Hardy Grapes*

One of the questions winemakers in northern climates ask most often is what yeast strains are recommended for fermenting various cold-hardy grape cultivars. While I understand why this question is asked – most catalogs selling yeast don’t list ‘Marquette’ or ‘Frontenac’ as recommended cultivars for a particular strain – it is also difficult to give a recommendation based on grape cultivar alone. Variables such as growing conditions of the grapes, winemaking conditions in the cellar, and stylistic goals are all important factors in determining what yeast should be used for making a certain wine. Vintage variation (especially in northern climates) can mean that a certain outcome with a commercial yeast strain one year doesn’t necessarily mean that we will have the same outcome the following year. Yeast can’t enhance the spicy character of Marquette, for example, if the aroma compound(s) responsible for that character aren’t in the grapes when they are harvested. Complicating matters is the fact that we are just beginning to learn what aromatic compounds might be involved in varietal aroma for these grapes!

Development of new yeasts.  Before a new commercial yeast strain is released, it undergoes extensive fermentation trials, from lab-scale to commercial scale and with various grape cultivars, to understand its impact on the wine. These trials require a great deal of costly research in order to be certain that the yeast activity will be fully understood once it is released. Unfortunately, the costs of this research guarantees that more obscure grape cultivars are not typically used in these trials. You are about as likely to see yeast recommendations for Picpoul or Vermentino as you are Marquette or Frontenac Gris. Fortunately, with the assistance of the Northern Grapes Project, researchers in the Midwest and Eastern US will be able to perform small-scale yeast trials this year for our cold-hardy grape cultivars.

Yeast trials.  While we may already have some ideas of how certain yeasts behave with cold-hardy varieties, we have yet to perform a study that includes statistical analysis of sensory data in replicated wine trials. This will allow us to evaluate whether a certain aroma or flavor can be attributed to a difference in yeast, grape cultivar or to the growing conditions. Although we do not fully understand the key aromatic compounds involved in the varietal aroma of cold hardy wines, we are able to build on knowledge gained from studies of of wine aroma and yeast metabolism to make educated yeast and cultivar matches. After several years of trials, we will be able to give confident recommendations for yeast strains to winemakers desiring a certain style wine from their cold-hardy grapes.

Grape aroma vs. Wine aroma.  Wine primary aroma compounds, which are also described as the varietal aroma of grapes, are the key aroma compounds that are used to distinguish wines made from one grape cultivar over another. In the grape berry, they are present in both volatile and non-volatile forms. The term ‘volatile’ simply refers to the fact that these compounds can be found in the headspace above the wine in a glass. In other words, this is what you smell when you stick your nose in a wine glass. Some volatile or ‘free’ aromas that are present in the grape berry are also present in the wines. When this occurs, drinking a wine made from that cultivar may remind you of how the grapes tasted when you picked them ripe off the vine.

However, the grape berries are also full of bound aroma compounds that can’t be tasted when you eat a grape, but are transformed into their free form by the action of yeasts, bacteria, and enzymes over the course of vinification. These bound compounds are often present in much higher quantities than the free volatile compounds, and are also considered an important component of the varietal aroma of wine. This is one of the reasons why the aroma and flavor of a wine is much more complex than the juice from which it was made. Yeast can play a key role in liberating these bound aromatic compounds so that they can contribute to the overall bouquet of a finished wine.[i] Thus, using a compatible yeast when vinifying a certain grape cultivar can help to enhance the varietal aroma of the wine. Some of the most important primary aromas that scientists have identified in grapes, and which yeast play a role in releasing during winemaking, are thiols and monoterpenes.

Thiols.  Volatile thiols are one of the most potent groups of compounds found in wine. Some can impart a negative aroma, while others contribute positively to a wine’s bouquet. They are almost non-existent in grape juice, and tend to only develop during fermentation. In Sauvignon Blanc, they are responsible for the box tree, passion fruit, grapefruit, and guava aromas that give the wine its varietal character. However, they have also been identified in wines made from other grape varieties such as Colombard, Merlot, Riesling, Semillon, and Cabernet Sauvignon. In grape juice, researchers have been able to identify the thiols in their bound form, also called an aroma precursor. Because of this, they have been able to understand the biochemical processes that yeast use to break apart the glycoside bonds with the thiols. Although all yeasts are capable of cleaving these bonds, certain strains of yeast have been shown to be better at it than others. Just as human metabolism varies according to a person’s genetics, so does yeast’s. Those that can efficiently release thiols are typically marketed as yeast that will enhance the varietal aroma of Sauvignon Blanc. This year we will be trialing two strains of yeast that are known thiol-releasers to see how they effect the overall aroma wines made from Frontenac gris. We suspect that perhaps some of the tropical fruit aromas found in wines made from this cultivar could be due to thiols.

Monoterpenes.  The second class of primary aroma compounds released by yeast are monoterpenes. Often simply referred to as terpenes, they are potent aromatic compounds found throughout the plant world. In grapes, they are found in large quantities in aromatic varieties like Gewurztraminer, Muscat and Riesling. Monoterpenes such as geraniol and linalool are often used as a fragrance in everything from soaps to air fresheners due to their rose or rose-like aromas. Other terpenes (cintronellol and nerol) can smell like citrus or lemongrass. Unlike thiols, monoterpenes often exist in a free, or volatile, form that can be detected in the grapes themselves. Nonetheless, a significant portion of monoterpenes found in grapes exist in a non-volatile, bound form. Yeast, bacteria, and enzymes in the grapes themselves are all capable of cleaving glycoside bonds and enhancing the varietal aroma of a wine. Knowing that La Crescent heady floral aromas are similar to a Muscat or Gewurztraminer, one can suspect that monoterpenes play a role in its varietal aroma. Using a commercial yeast strain that is a good terpene releaser can help intensify the primary aromas found in the grape. For our trials with La Crescent this year, we are using two yeast strains intended for aromatic white wine production, but are especially interested in yeast that will help with terpene expression. Vitilevure Elixir and Cross Evolution are two yeasts that we hope will show off the varietal attributes of La Crescent.

Enhancing spicy aromas. Spicy aromas exist in many different grape cultivars, however the chemical basis of these aromas isn’t completely understood. Although the compound responsible for black pepper aromas in Syrah and other cultivars has recently been discovered, researchers are still trying to identify if there is a biological method (yeast) of expressing it in wines.[ii] Nontheless, through sensory analysis of wines fermented with different yeast strains, we know that some are better able to enhance spicy characters than others.We aren’t certain what aromatic compound(s) is(are) involved in that spicy character, but we know that it exists. We know that sometimes Marquette wines can have a spicy character, even though we don’t know what causes it Thus,we will be trialing two yeasts that are known to enhance spice in two different cultivars. The strain D254 has been used in Rhone varietals, whereas the strain BRG has been used successfully in Burgundian varietals to enhance spicy characters. We are hoping that both can be used with success to enhance the varietal aroma of Marquette.

Yeast-derived aroma and flavors.  While we are looking for certain yeasts that may help to express the varietal aroma of cold-climate grapes, yeast also produce a number of aromatic compounds as a by-product of fermentation that will affect overall wine bouquet. Of course, the most important job of yeast is the production of alcohol from sugar. The presence of ethanol is essential to enhance the other sensory attributes of a wine. However, excessive ethanol can mask the aroma and flavors in a wine and give the wine an overall impression of “hotness” that is undesirable. While there are many important yeast by-products that contribute to the overall aroma and flavor of wines (fusel alcohols, glycerol, sulfides, volatile phenols, succinic acid, acetic acid…), perhaps the most important aromatic compound to consider when selecting a commercial yeast strain is its ability to synthesize esters. The esters produced by yeast will contribute to the fruity and floral aroma of a wine. These compounds can have aromas ranging from pear drops to flowers, honey, and bananas. Often they are used in the food industry to give artificial fruit flavors to candies.

Esters characterize young wine aroma.  While ester producing yeast strains aren’t typically associated with enhancing the varietal aroma of a wine, it has been shown that their production can be influenced by grape variety. For example, in Pinot Noir wines, the characteristic fruity aromas of plum, cherry, strawberry, raspberry, blackcurrant and blackberry characters were shown to be influenced by esters. These esters are synthesized by the yeast, but from aroma precursors found in the grape berry.[iii] Nonetheless, these compounds are some of the first to disappear during wine aging. The fruity and banana aromas that you smell in the winery during fermentation are typically associated with esters which disappear quickly in finished wine.. Mixed yeast cultures containing non-Saccharomyces yeast can also have a positive impact on the production of esters in wine. If a winemaker wishes to guard these aromas in a wine, they should be sure to ferment the wine cold and limit oxygen uptake. Ester-producing yeast strains should typically be used only if the wines are meant to be bottled and consumed while they are still young. In years where poor growing conditions (rot or botrytis) make it difficult to get fruity aromas from the grapes themselves, esters from yeasts may help make up for lack of varietal character. There is also some market demand for wines with this fruity aromatic profile. We will be using two high-ester producing yeasts in trials with Frontenac this year: Rhone 4600 and ICV Opale.

Selecting a yeast.  Think of yeast as one tool in your toolkit to help direct a wine to what you want it to be. The first step a winemaker needs to take when deciding what yeast to use is to determine the stylistic goal he or she has in mind for a wine. Is it going to be fresh and fruity with some residual sugar, or will the wine undergo a significant aging period in new oak and made into a dry wine? Perhaps you are making wines in both those styles. You probably wouldn’t want to use the same yeast for both of those wines. A wine that is meant to be fresh, young, and fruity should probably be fermented with yeast that will add some fruity esters to the wine. However, if you put that wine into a barrel, those ester aromas will quickly disappear due to their high volatility. You are better off trying to get the most fruit flavor out of the grapes themselves by using yeast that enhances varietal character.

Vineyard environment.  Sometimes the stylistic goal the winemaker has in mind may not even be possible depending on growing conditions of the grapes. In a warm year, if the Brix is greater than 25, yeast that only tolerate 14% alcohol should not be used (assuming you want a dry wine). We battle with high acidity in all our wines, but growers in the most extreme growing regions of the north may have to face the fact that their grapes may have too much acid to ever turn them into a palatable dry wine. This may also be true in short growing seasons where it is difficult to get the acid numbers down prior to harvest. Sometimes trying to force a wine to be something that it is not is a sure way to end up with a mediocre wine. It is important to remain realistic and understand that no matter how hard you try, you probably will never be able to make a “big” Bordeaux-style wine from Marquette or Frontenac.

Winemaking environment.  Winemaking conditions are also important. While Saccharomyces yeast tolerate  the harsh conditions in grape juice and wine, each strain has their own special range of ideal conditions for growth. The yeast cell wall is made up of fatty acids in a lipid bilayer. Think of it as a layer of oil. Just as some fats react differently to extreme temperature changes, so does this lipid bilayer surrounding the yeast cell. Really cold temperatures can make it stiff and hard to move, while really hot temperatures make it thin and runny. The yeast cell wall  is also sensitive to alcohol and osmotic pressure. The cell wall needs to transport nutrients into the cell and export waste products out of the cell, and both can make it difficult for the yeast to do so. The sugar concentration of the  juice ormust can make it difficult for the cell to get rid of waste, as it’s pushing against the osmotic pressure of the solution against its cell wall. A buildup of waste inside the cell will lead to cell death. Also, each strain of yeast varies in how efficiently it uses nutrients. Although all winemakers should be checking the YAN levels of their juice or must, this becomes even more important when using a yeast strain that has higher nutrient needs.

In the end, selection of a commercial yeast strain can have a significant impact on your finished wine.  Yeast can play an important role in ensuring that a fermentation finishes clean and dry with a predictable outcome to a wine, which is crucial to successfully marketing cold-hardy cultivars.

*This article was published in the Nothern Grapes Project newsletter on August 17th, 2012


[i] A. Zalacain, J. Marín, G.L. Alonso, M.R. Salinas. 15 March 2007. Analysis of wine primary aroma compounds by stir bar sorptive extraction, Talanta 71:4, 1610-1615

[ii] Logan, Gerard. University of Auckland, New Zealand. 5 August 2012. Personal communication

[iii] Moio, L. and Etievant, P.X. (1995) Ethyl anthranilate, ethyl cinnamate, 2,3-dihydrocinnamate, and methyl anthranilate – 4 important odorants identified in Pinot Noir wines of Burgundy. American Journal of Enology and Viticulture. 46, 392-398


 

Measuring Sugar in wine

Learning how to measure sugar in your grapes, juice, and wine is the most fundamental analysis that winemakers learn. It is sugar that will be converted to alcohol by your yeast, so an accurate measurement in the vineyard and in the juice or must at harvest can give you a good estimate of your wine’s potential alcohol. It is rarely the case that your wine will have too little alcohol – early harvest Riesling in Germany often has final alcohol levels between 7 and 10%. Even with alcohol levels this low, the wine’s low pH helps to keep it stable against microbes. Proper sanitation through the vinification process will ensure a clean and crisp wine. If your potential alcohol is too high, on the other hand (> 14%), your fermentation may struggle towards the end, depending on the type of yeast involved.

Refractometer

The first tool often used to measure sugar is a refractometer. I won’t go into too much detail on it’s use as it’s pretty straightforward.  A drop of juice is placed on a quartz surface at one end of the instrument, and you look through the sight glass on the other end. The sugar in the juice will cause light to bend at a certain angle, depending on the quantity. The refractometer measures this angle and contains a scale corresponding the the quantity of dissolved sugar in the mixture. The scale is typically given in °Brix measurement (% sucrose by mass – ie grams sucrose/100 g of solution). It is important to realize that this tool will only give you an accurate measure of your sugar when used in juice. Once your wine starts fermenting, any reading will be inaccurate due to the fact that alcohol has a higher refractive index than water. If there is any alcohol present when using a refractometer, your brix reading will be artificially high. On more than one occasion, a winemaker will discuss their vinification process with me using the term “brix.” Often it’s used when discussing residual sugars in their wines, or perhaps the level of sugar remaining when the wine was pressed. This always causes me to cringe a bit inside, because I know that if they are using Brix to measure remaining sugar in their wine, there is no doubt that the measurement is incorrect.

Once fermentation begins, a hydrometer should be used to measure the specific gravity of your wine. Often hydrometers come with more than one scale on the side. Many times, there is a scale used to measure Brix. Again, °Brix is a measurement of the percentage of sugar by weight in your solution. Because alcohol weighs less than water, measuring your °Brix by specific gravity will give you an incorrect measurement of the actual amount of remaining sugar if there is alcohol in the solution. A hydrometer is not capable of determining the amount of alcohol present in a solution. So, depending on the sugar you started with, the percentage of alcohol can vary by a few degrees with the same quantity of sugar remaining. If alcohol is present when you measure °Brix by specific gravity, the number you get for your brix measurement will be lower than it actually is. If you have a hydrometer with a Brix scale, it should only be used when measuring the sugar quantity in grape juice. You should not be using it to track the fermentation of your wine.

During fermentation, one should use the hydrometer’s specific gravity scale. Tracking your specific gravity will help you determine how quickly the sugar in your wine is being converted into alcohol. All hydrometers are calibrated at 20°C, so you should also measure the temperature of your wine and correct your specific gravity based on the temperature. Your hydrometer should come with a temperature correction chart. Take your reading by looking at the bottom of the meniscus and line it up with the corresponding numbers on the scale. Another common error is measuring a must that contains lots of particles of skins or pulp. This will interfere with your measurement. Carbon dioxide can also push the hydrometer up in your graduated cylinder, so be sure to take your reading quickly if your wine is fermenting.

Once the s.g. falls below 1.0, you know that there is less sugar in the wine than alcohol. It DOES NOT mean that your wine is now “dry,” but it is getting close to dryness. This is another mistake that I’ve come across over and over again. I’ve had people come to me wondering why their wine started re-fermenting in the bottle. They insist that the wine was dry when they bottled it, and when I ask how they measured the residual sugar I’m told that the specific gravity was less than 1.0. Remember that the specific gravity is the result of a mixture of mainly water, alcohol, and sugar.  If your alcohol is very high, you can still have quite a bit of residual sugar left in your wine and still have the s.g. fall below one. In most cases, there is still 2% residual sugar – a sufficient quantity to cause re-fermentation at a later date. Another serious issue is starting malolactic fermentation (MLF) with this much residual sugar. The bacteria responsible for the conversion of malic acid to lactic acid also love to munch on sugar (like most any living creature). The problem is, unlike yeast, bacteria will convert sugar to acetic acid, which increases the wine’s volatile acidity. Thus if you have a wine destined to undergo MLF, you should be certain that it is dry.

Once the specific gravity drops below 1.0, another test is needed to measure residual sugar. Many home winemakers use Clinitest for this purpose. Some commercial winemakers may also use it as a quick way to estimate remaining sugar in the wine. It was once an important tool used to measure residual sugar in the urine of diabetics. It is a fairly simple test: a few drops of wine are placed in a test tube with a tablet that reacts strongly with the liquid. The tablet’s reaction with the sugar causes a color change that is then compared with a standard color strip that indicates the percent of sugar in the solution. The downside of Clinitest is that it can be difficult to measure the color change in red wines.  Also, because an eye-dropper is used to measure your wine sample, you cannot count on your results to be accurate. It is a good idea to run the test several times so you can be confident in your results. Wine with a residual Sugar that is < 0.5% can be considered dry. It is rare for a wine to have zero sugar at the end of fermentation.

If you have access to a spectrophotometer, enzymatic analysis of residual sugar is one of the best and most accurate ways to determine the quantity of sugar left in your wine. When sending a sample to a lab for analysis, this is likely the method that they use. I highly recommend that any winery interested in doing their own lab analysis invest in a spectrophotometer. It is one of the most important pieced of equipment for wine analysis. Click the link above to get a great article from Cornell University on the many uses of a spectrophotometer. A basic model for wine and juice analysis can be purchased for less than $1000, and will open the door to a whole new range of testing capabilities.

Following Fermentation

Fermentation is underway in many parts of the region… the madness begins :)

I love this fermentation chart available from Anchor Yeast. Sugar levels and temperature should be monitored daily during fermentation. Notice how during the first week of fermentation this chart has a place to note the drop in sugar twice daily. This is how often you should be monitoring your fermentation. Temperature should be taken during each of your measurements for two reasons – one is to make sure that your fermentation isn’t getting too hot (or too cold – did somebody turn the cooling jacket on too high?), and secondly you have to adjust your density reading according to the temperature difference. I love that this chart also has the temperature range indications right on the side for you to look at every time you are monitoring your fermentation.

There is also a space for noting sensory information of the fermenting wine. This should also be done daily during fermentation. You can monitor for the start of reduction odors (cooked cabbage, or canned bean smells) so that you can fix these problems early on in fermentation with either a nutrient addition or splash racking. You can note on which days you make your nutrient additions and have a visual reference for at what stage in fermentation you were (DAP additions at 1/3 sugar depletion are often recommended).

If you aren’t using a chart like this to monitor your fermentations, now’s the time to start using one! Luckily this one is ready to print – one less thing to worry about at this busy time of year.


Happy Fermentation.P.S. Here is the same chart “en français” for those who might be interested

What yeast should I use?

One question I am asked again and again are my recommendations for which commercial yeast strain I prefer for a certain grape variety. There has been some work on this by the U of MN in conjunction with Scott Labs and Fieldstone Vineyards. The chart that they came up with for yeast recommendations is posted here. (click on the photo to see it enlarged). However, there are new yeast varieties that are released on the market every year, and we can’t possibly test all of them at a time. We can have an idea as to which yeast selections might work well based on what we know about certain varieties, and this is how we choose which varieties to trial. I’m hoping to give insight as to how I might choose a yeast to trial with a particular cultivar, so perhaps more wineries can think about trying different yeast, too.
The first question I usually have when someone asks my recommendation is “what style are you shooting for?” Though yeast in itself won’t help you achieve a certain style, it can be an important tool. Often it is the quality of the grapes coming in that will determine what type of wine you will make, and using a yeast that promises to enhance certain aromas such as “spiciness” will be of no use if the grapes themselves lack this character. In areas and/or vintages where grapes are affected by rot, it may be wise to choose a yeast that will ferment your red grapes quickly, so you can press the fruit, and get it off the skins and filtered as quickly as possible. Are you going to cold-ferment the wine, or barrel-ferment? What characters do you want to enhance or diminish?  Add the fact that in Minnesota we are working mainly with hybrids that haven’t been given complete chemical and sensory analysis, and one can see how quickly the answer to the question “What yeast should I use?” becomes increasingly complicated. The advancements made in commercial yeast strains make it an important tool for winemakers, and knowing how to select the yeast you use will have a postive effect on your final wine.
History

Even though alcoholic beverages have been made since antiquity, it wasn’t until 1863 when Louis Pasteur first described yeast as being responsible for the process of alcoholic fermentation. With that knowledge, we had a better understanding of winemaking.  Muller-Thurgau was the first to introduce the concept of inoculating wine fermentations with pure yeast cultures in 1890. Of the 100 different genera that represent over 700 different species of yeasts, only 15 are associated with wine: Brettanomyces/Dekkera, Candida, Cryptococcus, Debaryomyces, Hanseniaspora/Kloeckera, Kluyveromyces, Metschnikowia, Pichia, Rhodotorula, Saccharomyces, Saccharomycodes, Schizosaccharomyces, and Zygosaccharomyces (Pretorius, 2000). While all these yeast genera have been found in the wine industry and are capable of fermentation, only Saccharomyces is able to ferment wine to dryness. Nearly all commercial yeast strains intended for fermenting wine are genetic variants of Saccharomyces cerevisiae. While hundreds of different commercial yeasts exist, they all have been selected for certain properties to ensure good fermentation, while adding positive aromatic and gustatory properties.

When looking through a catalog of commercial yeast strains, one can quickly become overwhelmed by the choices of yeasts available, especially when we aren’t working with the recommended grape varieties. Although some of them have been proven to work well with certain hybrids, others may work well too, but we just haven’t proven it.

Fermentation properties

Ethanol Tolerance – All commercial yeast strains have good tolerance to alcohol, though some can tolerate higher levels than others. This is an important factor to consider for several reasons:

  • Many hybrid grapes are high in sugar at harvest – on par with sugar levels in warm climates (26-28 brix). If you plan on fermenting a wine to dryness, make sure it will tolerate high ethanol levels. Ethanol weakens the cell walls of yeast, but so does an acidic environment. High acid wines coupled with high alcohol are not an ideal environment for yeast ot survive. You may find that a yeast tolerant to 14% has trouble finishing a wine to 14% alcohol when the wine also is high in acid.
  • If you are not planning on fermenting a wine to dryness, you want to make sure the yeast isn’t TOO hardy. This is especially important when making a port-style wine in which you plan on muting fermentation with an alcohol addition. If you have a yeast that is known to be tolerant to 17% alcohol, you may find that it adapts to 18% in your port. Also, if you are making a sweet, late-harvest style with a potential alcohol of 20% at harvest, the wine may be out of balance if you allow the yeast to ferment most of the sugars.

Osmotolerance – the high sugar environment of late-harvest wines is difficult for survival of some yeasts. If you have a very high-sugar must, make sure you are innoculating with a yeast that is designed for it. This is especially important when making ice wine.

Temperature tolerance – Again, this goes back to the wine style you desire. Often white and rose wines are cold-fermented to enhance fruity characters in the finished product. If you are planning on cold-fermenting, make sure the yeast is tolerant to colder temperatures. If your plan is to cold-ferment your wine, you want to be sure that the yeast is tolerant of cold temperatures. On the other hand, if chilling the tank is how you wish to stop fermentation, you might want to choose a less cold-tolerant yeast.

Fermentation efficiency – Another thing to think about is the efficiency of fermentation. On average, it takes 16.8 g/L of sugar to make 1% alcohol. Some yeasts have been selected for being less efficient at fermenting - they require more sugar (18g/L) to make 1% alcohol. In places where high potential alcohol is a problem, these yeasts may help make the final wine lower in alcohol, and thus more balanced.

Flavor Characteristics

Perhaps the most important factor in deciding which yeast to use is the flavor characteristics of that yeast. This is also one of the most complicated factors to discuss without giving a detailed lesson in biochemistry, but I’ll try my best.

Low sulfide/DMS

These compounds are responsible for cooked vegetable and onion/garlic aromas in wine.  All commercial yeasts are designed to be low-producers of these compounds.

Thiol Production

While certain thiols smell like burnt rubber or even skunk, other thiols are responsible for desirable odors such as grapefruit and passionfruit. These are the main compounds that give Sauvignon Blanc wines their distinctive aroma. Often yeasts are selected to be “thiol producers” for varieties which contain the positive thiol precursors. Because it is unknown what type of volatile precursors are present are not present in grape juice as free thiols, but are released from non-volatile precursors during alcoholic fermentation. The characteristic boxtree and grapefruit aromas of Sauvignon Blanc is due to thiols, and it  precursers have been identified in the juice.

Other aromatic grape varieties like gewurztraminer, muscat, and viognier contain another type of aroma precursor that produces terpenes during fermentation. Using a yeast that cleaves the bound terpenes from sugar will help improve the aroma of the finished wine from these aromatic varieties. Because we know that La Crescent is a relative of muscat, and has similar heady aromatics, one might consider using a yeast designed for these aromatic varieties.

Yeast not only helps to release aroma precursors already found in the grapes themselves, but it also produces aromas as a byproduct of metabolism. The most important positive aromas it produces are esters. They typically have fruity characters like banana and pineapple. If the goal is to make a young, fruity wine then you should choose a yeast that is a high ester producer. Typically rose is made in this style, as well as wines meant to be sold en primeur like Beaujolais Nouveau.

Besides aromatic contributions made by yeasts, they can also affect the mouthfeel of wine by glycerol production. Some yeast cells may also autolyse (break down) more readily after cell death to provide proteins that can enhance the palate. The glycerol producing property might be important in a white or rose wine that won’t undergo aging before bottling, while the enhanced autolysis may be important if you are aging the wine on lees (in barrel aged whites).

Other Yeast Properties

More than just affecting the taste and aroma of the wine, yeast can also be selected for technological properties. In wineries with limited tank capacity, a low-foaming yeast allows for optimization of tank space. This would also be an important property with wineries that do long pump-overs on their red wines. Low tolerance to sulphites, and low sulphite-binding properties can also be important for certain wines. People who develop yeast also want to be certain that they resist dessication, and are genetically stable. Health concerns have led to laboratories looking for yeast strains that are low sulphite and biogenic amine producers. In Minnesota, our high acid levels make us look toward yeast strains that can partially degrade malic acid. 

Yeast Recommendations in Minnesota?

We can quickly start to see how recommending yeast for a certain variety begins to be difficult. A single variety can be grown in a different manner, and harvested with different sugar and acid levels. The grapes from a single can have such different properties depending on where and how they are grown that using one yeast across the board can give quite different results. Today yeast catalogs contain a ton of information regarding the sensory and chemical impacts of certain yeast strains on various grape varieties. Unfortunately, not a lot of proper trials with trained sensory analysis panels have been carried out with our hybrid grapes. Nonetheless, it is easy to compare our varieties to wine “types” to get an idea of what yeasts may work well. La Crescent, for example, is an aromatic variety similar to a muscat or gewurztraminer. It would make sense, then, to use yeast varieties that are meant for aromatic whites. When making a late-harvest or ice wine, look for a yeast that has a high sugar tolerance and is designed for making dessert wines. Marquette has some nice spicy black pepper notes when fermented with yeast intended for Rhone varietals. It’s Pinot Noir background might lead us to look for yeast strains that are intended for this varietal. Also, the lack of tannin in our red varieties might make looking toward yeasts that will enhance mid-palate structure. Vitis labrusca based hybrids might benefit from a high-ester producing variety. The foxy aroma may be enhanced by other fruity notes. People producing fruit wines might also want to look for high ester producing varieties, as well as yeast that will enhance the mid-palate.

In the end, the possibilities are really endless for yeast trials in wineries. With new strains being released every year, we really have limitless options. It is up to individuals to decide what style they are shooting for, and do trials in their winery to see what works best for them.

Further Reading:

Scott Labs Yeast

yeast selection charts

Pretorius, 2000. Tailoring wine Yeasts for a New Millenium