May 24, 2013

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Keys to Successful Fermentation: Part 1

May 17, 2013 by

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….

 

Filed Under: Enology, Fermentation Tagged With: ,

Biological Reduction of Total Acidity

May 7, 2013 by

A wine is technically considered dry when the sugar level falls below 0.5%. A dry wine that also has a total acidity level above 10 g/L will likely be harsh and astringent to the consumer. Granted, there are exceptions to this generalized idea of wine chemistry. The perception of dryness can vary based on other aspects of the wine, such as acidity, dry extract, and aroma.  A low tannin red wine can tolerate higher acid levels than a tannic red, and a white wine high in dry extract may tolerate higher acidity than one with less body. The level of “dryness” will also play factor in the perception of harshness in the wine, as an increase in sweetness can help to balance acidity (think lemonade).

University of Minnesota-developed wine grapes are rarely harvested with a TA under 10 g/L. It is not uncommon to see total acidity at harvest of 15-18 g/L in Frontenac.  Even the newest cultivar, Marquette, sees total acidity ranging from 0.9-1.3% – numbers almost unheard of for red V. vinifera cultivars – especially when Brix at harvest on these same grapes falls within normal to high levels for dry wine production (24-27). Thus, if a winemaker desires to produce a dry wine using University of Minnesota grape cultivars, he or she must always be concerned with mitigating the harshness of these high acids.

Wine Grape Acids and their Reduction. There are three general methods one can use to lower high acidity in wine: physical methods (blending and amelioration), chemical methods (bicarbonates), and biological methods (yeast and bacteria). A winemaker may also choose to produce a sweet wine to counter-balance the acidity. This may actually be the best approach for high-vigor vineyards, vineyards on cool sites, or in short growing seasons. Nonetheless, dry table wines are desirable for many winemakers and consumers. For the acid levels seen in Minnesota vineyards, the best approach is most likely a combination of all three methods if the wine is going to end up dry.

Two acids make up over 90% of the acids found in grapes: tartaric acid and malic acid. From a wine stability standpoint, tartaric acid is extremely important. It helps a wine to maintain a low pH, thus inhibiting microbial spoilage. Chemical methods of reducing acidity all act mainly on the reducing the amount of tartaric acid in the wine, while biological methods all reduce the amount of malic acid in your wine. Those familiar with consuming green apples such as ‘Granny Smith’ are familiar with the sour flavor coming from the malic acid. Thus, a wine that contains high quantities of this acid can have a sour character.  All living cells consume malic acid (malate) through their respiration cycle either by breaking down another molecule (usually a carbohydrate such as sugar), or by consuming malic acid directly from their environment

Biological Deacidification. The most common method of biological deacidification is the addition of lactic acid bacteria to the wine. The bacteria will directly consume malic acid and convert it to lactic acid, an acid with a softer less sour taste, in a process known as malolactic conversion or malolactic fermentation (MLF). Though not a true fermentation because no sugar is involved, carbon dioxide is a byproduct of the conversion, so the wine looks as though it is fermenting. Nearly all red wines around the world undergo MLF and some white wines also benefit from acid reduction of this practice. Traditionally, wines were left without sulfur protection following alcoholic fermentation to undergo MLF naturally – a process that could take several months.  The risks involved with leaving the wine unprotected by sulfur dioxide (spoilage by other organisms and oxidation), have pushed many wineries to use a malolactic bacteria starter culture.

Nonetheless, yeast also have the capability to consume malate during cellular respiration and convert it to eNanoVinificationthanol. It has long been known that certain yeast, especially several non-saccharomyces yeasts (Schizosaccharomyces pombe, Hanseniaspora occidentalis, Issatchenkia orientalis) are especially efficient at consuming malate. Because these yeasts have poor alcohol tolerance, they must always be used in conjunction with Saccharomyces species in order to complete fermentation in wine. While  S. pombe has been available commercially for some time for use  wine production, the development of other non-Saccharomyces yeasts for commercial use is a hot topic at the moment. We will likely see more of these yeasts available in an active-dry form to use in sequential yeast inoculations for wine.

Until then, we decided this winter to look at some of the commercially available yeast strains that have some reported ability to reduce malic acid, and trialed them with University of Minnesota cultivars. After consulting with several enological product suppliers, we came up with a list of several yeast with reported malate-reducing capabilities: Lalvin C (Lallemand), Exotics (Anchor), Lalvin ICV Opale (Lallemand), and Uvaferm VRB (Lallemand). We also trialed a non-Saccharomyces yeast that Lallemand has made available in an active dry form for sequential inoculations: Torulaspora Delbrueckii (sold commercially as Level 2TD). Although its malate-consumption hadn’t been verified, a technician at Lallemand had recommended it because they had observed some softening of the acidity in wines that had been fermented using it.

Yeast deacidification trial. We did a small trial with these yeasts in which we used juice from the 2012 vintage that had been frozen. For each MN cultivar, we trialled 3 different yeasts, and used a fourth yeast as a control. One lot of juice was divided into 20 micro-vinification lots of 500 mL each. Thus each yeast was replicated in 5 fermentation lots. For this initial trial, we were concerned with monitoring mainly the chemistry change using each yeast. For white wines we used Lalvin DV10 as control, and for red wines we used ICV GRE (Lallemand) as a control yeast. Both are considered reliable fermenters with no reported malate degradation.  The unusually hot weather in 2012 caused initial brix levels to be extremely elevated, so initial malate numbers reflect juice that had been diluted to bring the sugar concentration down to 25° Brix.

microvin FGRIS

With Frontenac Gris, we started with an ameliorated juice that had a total acidity of 9.92 g/L, a pH of 3.00, and 5.1 g/L of  malic acid. Although all the added yeasts showed some reduction from the initial malate levels in the juice, the acid reduction seen in the Lalvin C, Exotics, and the combination of Torulaspora delbrueckii with Exotics all were significantly lower than the control yeast (p <0.05). We used Lalvin C in a larger lot following this trial in order to evaluate the sensory impacts of this yeast. It’s worth noting that in all 10 micro-vinifications in which Exotics was used, the wines exhibited some stuck fermentations. Thus, some care may be needed when using this yeast in order to complete fermentation in low pH juices.

 

microvin LC

The La Crescent juice that we divided up for the micro-vinification trials was ameliorated to 25 Brix, which left the starting malate levels at 5.3 g/L. The decrease in malic acid during fermentation was less pronounced than what we saw with the Frontenac Gris fermentation. In fact, only the vinification lots in which Exotics was used showed a statistically significant drop in malic acid (p< 0.05). ICV Opale is advertised to lower malate levels by 0.1 to 0.4 g/L. Our trials show that it exceeded this levels in high malate musts, however, this decrease was not significantly lower than our control yeast which has no reported malate reducing properties. We also saw stuck fermentations when using Exotics in the fermentation lots with La Crescent.

 microvin frontenac

 Our Frontenac was pressed and fermented as a rosé. Again, it was necessary to ameliorate to reduce the high sugars that we achieved in 2012, however, the initial malate concentration of the juice was still relatively high at 4.6 g/L. Interestingly enough, all yeast used for this trial caused a decrease in the final malic acid concentration of the wine. All observed differences in malate reduction were statistically significant (p<0.05), except for the two lots that were fermented with Lalvin C. There is essentially no difference between the observed malate reduction when using Lalvin C in conjunction with T. delbrueckii yeast. This (along with the other results seen when using T. delbrueckii) suggests that any impact on the perception of acidity due to this yeast is likely not related to malate degradation. All the Frontenac fermentations finished dry with no stuck or sluggish characters.

 

microvin Marq

 

Marquette was also pressed immediately and fermented as a rosé. The ameliorated juice had an initial malic acid concentration of 4.1 g/L. Exotics and VRB showed identical malate reduction capabilities, and even though the difference between these two yeasts and the control (ICV GRE) was only slight, the difference is statistically significant (p=0.046). Nonetheless, this difference is probably negligible in the sensory profile of the wine. Once again, Lalvin C proved to have the greatest potential for malate reduction, with a 1.10 g decrease in malic acid from the juice.

It is important to keep in mind that there are many different tools available to a winemaker to manage high acidity in their wines. The selection of yeasts that we looked at here are only a small example of what is available on the market. It is important to talk with technicians who supply your winery in order to get a better idea of what products might help with managing your acidity. This coming vintage (2013) will likely promise to be a challenging one, as we have yet to see bud-break in Minnesota this first week of May. Unless we see a warm summer and an extended warm fall, growers and winemakers need to start thinking now about how they might manage the vines and prepare for a high acid harvest.

 

 

 

 

Filed Under: Acidity, Uncategorized Tagged With: ,

Winemaker Roundtables

December 21, 2012 by

I’m excited to announce that in 2012, a series of regional Winemaker Roundtables will be held at several host wineries around the state. A review of common wine faults will be given at each roundtable, along with methods to prevent and fix these problems. Winemakers who attend will have the opportunity to have several of their wines evaluated blind by a panel of their peers. The enology lab at the University of Minnesota will also present wines from research trials to winemakers in the state who would like to have some examples with new techniques and practices with cold-hardy grapes.

Winemakers from bonded wineries in Minnesota are eligible to attend these sessions. Space will be limited to 10 winemakers at each session.

 

 

 

Filed Under: Uncategorized

High Total Acidity AND high pH?! How to handle it…

October 17, 2012 by

One of the reasons that grapes have been used to make wine for thousands of years is that they are one of the few fruits in the world that contain large concentrations of tartaric acid. The strength of acids is measured by their ability to shed protons – or more specifically, hydrogen ions (H+). Without going too deep into a chemistry lecture (which I’m sure will lose most of you in a few sentences), when you measure the pH of your wine, you are measuring the concentration of these ions – that’s what the big ‘H’ in pH stands for. The tricky thing to remember is that while pH is a measurement of H+, the formula for its calculation causes the pH to be inversely proportional to the H+ concentration. Thus, as the H+ concentration increases, your pH decreases.

So what is the big deal about pH? Because tartaric acid is relatively strong, it works to keep a wine’s pH near 3.0, which in turn keeps the wine stable against microbes. This is one of the reasons why wine made from grapes has flourished around the world: it doesn’t spoil easily, and acts as an antiseptic. The combination of ethanol and the acidic environment are extremely inhospitable to most microbes. In an indigenous yeast fermentation, after the wine hits 5-6% alcohol, one yeast will dominate the fermentation: Saccharomyces cerevisiae or S. bayanus. After the sugar is depleted, there isn’t much left in the wine to act as a food source for microbes that are capable of surviving in those harsh conditions. Lactic Acid bacteria, if present, will begin to consume the malic acid (transforming it to lactic acid), while Acetobacter species are capable of turning ethanol into acetic acid (vinegar). However, Acetobacter needs oxygen in order to do this, so as long as you keep your containers full, you don’t need to worry much about them.

This year, like in 2010, we saw problems with high pH in many of our wines, but we saw it especially in Marquette. The most likely explanation is that Marquette grown under certain conditions has an excess of potassium, which can drive up the pH. Malic acid concentration likely also plays a role in increasing the pH, since it is a weaker acid that in turn is converted to an even weaker acid (lactic acid) in red wine vinification. In any case, the high pH is worrisome and steps need to be taken to ensure that the wine remains stable.

Sulfur Dioxide Addition. While it is still possible to limit microbes with sulfur addition when the pH creeps up to 3.8, you need to use substantially more SO2 as your pH increases. Most of the sulfur you add to wine becomes bound to sugars and other compounds in your wine. The rest of the sulfur exists as “free” or unbound SO2. At a pH of 3.4, you should aim for 35 mg/L of free sulfur in your wine in order to be sure that it’s protecting your wine against microbial spoilage. However, at a pH of 3.8, you’d need nearly 90 mg/L of free sulfur to get the same protection. Considering that the legal limit for TOTAL sulfur in your wine cannot exceed 400 ppm, one can see how maintaining a high free SO2 rate can quickly make it possible to exceed that limit. Though it’s possible to keep your wine clean with a high pH, it isn’t easy. One should consider a pH greater than 3.8 the breaking point where acidification becomes necessary.

Wine Sensory. The pH has a huge effect on the color of red wine, as it affects the colored pigments. If you start to keep track of your wine color and corresponding pH, it becomes almost possible to predict your wine’s pH based on color alone. A high pH wine will lose the vibrant red tones, and become more of an eggplant purple color. Low pH wines will have a bright pink rim and vibrant red hue. Differences occur between grape cultivar, of course, but generally if you observe the rim of color at the edge of the wine when you tilt your glass, if it’s purple then the pH is high. High pH wines also have a tendency to be described as “flabby” or “flat,” however it is difficult to say whether or not that holds true when the wine has a corresponding high total acidity, like we often see in Marquette. In Riesling, wines with equal sugar/acid ratios can taste sweeter at a higher pH.

Cold Stabilization. Wines with a pH greater than 3.65 should not be cold stabilized. When wines are cold-stabilized, the goal is to precipitate potassium bitartrate crystals so that they don’t fall out of solution in the bottle. Above pH 3.65, this salt acts like an acid. So, by removing an acid from the solution, it causes your pH to increase. However, if the wine’s pH is LESS THAN 3.65, cold stabilization will help to LOWER your pH. Below this point, potassium bitartrate acts as a base, so removing from solution causes the solution to become more acidic. Pretty cool, huh?

What we were faced with this year. The Marquette grapes that were harvested this year arrived at the winery with a pH of 3.6, but also had a total acidity of almost 1.0%! Knowing that the pH would increase during skin maceration (potassium is extracted from the skins), and again during malolactic fermentation, I acidified the must at harvest with tartaric acid at a rate of 0.2%. This brought the pH below 3.5. During Malolactic fermentation, we saw the pH creep up again to 4.0, so we were forced to once more acidify the wine to make it stable.

So here’s where a decision needed to be made: how much tartaric acid should we add? The total acidity was around 0.65%, which is pretty good for a red wine. Adding too much tartaric acid would make the wine tart and unpalatable. If I was working in a commercial winery, these are the options I’d see:

1) Acidify with Tartaric Acid. Aim to get the pH to 3.8, and hope that the tartaric acid additions didn’t make the wine too tart, then avoid cold-stabilization. A rule of thumb to use when acidifying:  1.0 g/L of tartaric acid will generally lower the pH by 0.1 (this is a guideline, of course… to be accurate, always perform bench trials before making a large addition).

2) Acidify with Tartaric Acid. Aim to get the pH below 3.65 and KNOW the wine was going to be very tart, but then cold-stabilize. With this option, the cold-stabilization will further lower the pH another 0.1 to 0.2 points (depending on the potassium bitartrate concentration). Then, working at a pH of 3.4-3.5, we will have room to remove the tartaric acid using chemical deacidification methods. Chemical deacidification comes with the worry of losing some of the aromatics, so bench trials should be performed to determine the amount of additive works best for the individual wine.

3) Blend the wine with a lower pH wine (of course do bench trials to see if you like the blend). This of course is still an option if you choose option 1 or 2, especially if you find the wine is still too tart. Blending is one of the the real arts in winemaking.

4) Use an anion exchanger. However, while an ion exchanger is available on the commercial scale for wineries, the cost of the equipment isn’t practical unless your last name is Mondavi.

We went with option #2. Since we are an experimental winery, blending is not an option. If I went with the first option, the amount of tartaric acid needed to get the wine under a pH of 3.8 made the wine too tart.  The wines were acidified with 4 g/L of tartaric acid, which brought the pH down below 3.6 (and the TA above 1.0%), and they are now chilling  at 28°F. I’m hoping that cold stabilization removes 1-2 g/L of total acidity, and we can use potassium bicarbonate to remove an additional 1-2 g/L.  In the end, I’m hoping that nearly all of the added tartaric acid that was added to the wine can be removed, and we’ll be left with a wine that has a healthy pH between 3.6-3.8, with a palatable TA around 0.6%.

 

Filed Under: Acidity, Enology, Uncategorized Tagged With: , , ,

Edelweiss Kabinett

August 28, 2012 by

Today we harvested some Edelweiss. I’ve struggled a bit on what we can do for vinification trials with this grape. For those who are unfamiliar with Edelweiss, it was originally developed as a table grape by Elmer Swenson back when he was working for the University of Minnesota. Although it is not seedless, which is a problem for the table grape market; it has some aromas and flavors similar to Concord grapes and can be used to make a nice aromatic white wine. We tend to simply refer to the grape as having a labrusca character (grapes from the species Vitis labrusca  have this distinct aroma), though in most wine circles this aroma is called “foxy” – an unfortunate term that really does nothing to describe the flavor to most people.

Early European settlers, upon eating the wild grapes that grew along the riverbanks in the Eastern US, decided they had an “animal-den” aroma and nick-named them fox grapes.  Perhaps our early ancestors were more familiar with fox aroma than most Americans are today, but apparently they were onto something. Methyl anthranilate, the compound that is most often cited as the compound responsible for the characteristic aroma of V. labrusca grapes such as Concord, is used as a flavor additive in candy to give it a “grape” aroma. However, researchers also point to another compound present in Concord and other V. labrusca grapes that has a similar ‘foxy’ or ‘grapy’ aroma:  ortho-amino acetophenone (OAP).[1] Athough present in grapes in much smaller quantities than Methyl anthranilate, humans are able to detect it at a lower threshold, thus it is believed that it may play a greater role in the distinctive foxy aroma of V. labrusca grapes.[2]  Coincidentally, OAP is also found in the scent glands of certain weasels,[3] so perhaps our early ancestors weren’t so far off in relating the aroma to an animal-den. However, perhaps due to our undying love of peanut butter and jelly sandwiches (or perhaps our failure to adopt fox-hunting as enthusiastically as our Aristocratic ancestors), Americans will almost always describe the aroma of OAP as grape-like or candied.

While the grapey aroma of Edelweiss and other V. labrusca hybrids isn’t necessarily off-putting to most people, it is also not an aroma that wine-drinkers associate with high-quality wine. It’s a candied-fruit aroma that is more reminiscent of candied strawberries, Jolly Rancher candy, or Welch’s White Grape Juice – not exactly flavors that go well with that roasted chicken dinner. However, on the patio on a hot summer day, Edelweiss wine can be quite refreshing. In fully-ripe Edelweiss, the candied fruit aromas can make for a tasty grape, but it can overpower the flavor of the wine. Thus, most people growing Edelweiss for wine production will harvest it before it reaches full ripeness to keep the wine aromas more subdued.

Here is a look at some of the harvest numbers we’ve had for Edelweiss over the past few years:

Harvest Date

Brix

Total Acidity (g/L)

pH

9/9/2002

17.8

8.68

3.16

8/30/2005

18.8

10.57

3.05

9/5/2007

17.0

7.02

3.23

9/4/2008

17.2

12.03

2.97

9/10/2009

17.4

9.62

3.1

8/25/2010

15.4

7.43

3.29

9/8/2011

18.1

8.25

3.2

Although Edelweiss isn’t a high-sugar grape to begin with (remember, it was developed as a table grape), one can see that we’ve never harvested it much higher than 18°Brix, so the potential alcohol of the wine will likely not be greater than 10% in any given year. Thus, many winemakers will add sugar to the juice in order to make a wine with a more “acceptable” table-wine level of 12-14%. In my opinion, the higher alcohol level tends to overpower much of the delicate aroma and flavor of the wine. There’s a disconnect between the fresh acidity and light flavors of a grape harvested underripe with the alcohol level of a grape that was left to soak up the sun a bit longer. There is precedent in the world for harvesting grapes early for winemaking. In German-speaking countries, wines made from early-picked grapes are given the designation ‘Kabinett.’ I’m a huge fan of Kabinett Rieslings. Often they are made in a semi-sweet fashion by stopping fermentation early – at say 7-8% alcohol. They are wonderfully delicate, easy-to-drink, and refreshing – something I admire in a well-made Edelweiss. By law, Germans harvest grapes for Kabinett wines between 17-19 brix, and they are not allowed to add any sugar. This year I intend to make a wine in that style. Our numbers this year will work perfectly: 17.7 brix, 7.8 g/L total acidity. We’ll try cool-fermenting it with high terpene releasing yeast (Laffort VL1) to see if we can enhance some of the delicate floral aromas, then we will arrest fermentation with about 1% residual sugar to keep it slightly sweet. A perfect wine for summer.

Happy Harvest!


[1] Shure, K.B. and T.E. Acree. 1995. In vivo and in vitro flavor studies of Vitis labruscana Cv. Concord. ACS Symposium Series 596, American Chemical Society, Washington. pp. 127-133

[2] Acree, T.E., E.H. Lavin, R. Nishida, and S. Wantanabe. 1990. The serendipitous discovery of ortho-amino acetophenone as the ‘foxy’ smelling component of Labruscana grapes. Chem. And Eng. News 9:80

[3] Brinck, C., S. Erlinge and M. Sandell. 1983. Anal sac secretion in mustelids: a comparison. Journal of Chemical Ecology. 9(6): 727-745

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Yeast Selection Trials for Cold-Hardy Grapes*

August 21, 2012 by

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

 

Filed Under: Fermentation Tagged With: , , , , , ,

Of Marquette and Tannin…

June 15, 2012 by

Tannins are found throughout the plant world, and at least one of their properties has been known for some time. The word ‘tannin’ is derived from the process of using plant extracts to cure leather (tanning). This highlights one of the principal chemical aspects of tannins – they are highly reactive with proteins. Tannins play an important role in both grapes and wines. In wine, the perception of astringency on the palate is attributed to tannins.  In your mouth they bind with salivary proteins and cause the proteins to precipitate. The end result is that your mouth will lack the lubrication that saliva provides. Thus, astringency caused by tannins are very much a tactile sensation in your mouth. This is why we often will describe the sensation of tannins as silky or rough. The British learned long-ago that a splash of milk in their black tea can make it more palatable. This works because instead of reacting with the salivary proteins in the mouth, the tannin extracted from the tea leaves reacts with milk protein (casein), resulting in a beverage that is less astringent. The same thing occurs when one consumes red wine with cheese. The proteins in the cheese react with the tannins in the wine, making the wine seem less harsh. Tannins are what make drinking red wine with a high-protein food like steak such an enjoyable experience.

In nature, one role they play is a protective role against predation. The puckering sensation one feels from eating unripe fruit is usually the result of under ripe tannins. In grapes, the early formation of tannins coupled with the high acidity in the green grapes causes them to be unpalatable to birds and other animals until the seeds within the grape are ripe. The color change, sugar accumulation, diminishing acidity and astringency all work to make the berries more enticing so that animals will eat the fruit and help distribute the seeds far and wide!

In wine,  tannins undergo several different physical and chemical changes and are important factors in the mouth-feel and astringency of a wine, its color, as well as its aging potential.  However, the purpose of this post is to not discuss the properties of tannins in detail (which would take an entire post in itself), but  to discuss different methods that we trialled this past year to try to enhance tannin concentration in Marquette wine. While Marquette does have some perceptible tannin on the palate, the overall concentration is quite low – much less than even Pinot Noir (which is one of the few red wines that can be enjoyed with fish due to its low tannin concentration). While this isn’t necessarily a bad thing, an increase in tannin can help add structure to an otherwise thin and/or weak wine. It may also help to make the wine more stable (ie: age-able) over the long-term.

Tannin essentially exists in the skin, seeds, and stems of grapes. Fermenting wine with oak chips or aging wine in oak barrels can also be a source for tannins in wine. There are also a number of different commercial tannin additives that winemakers can use that contain tannins derived from grapes, oak, and/or exotic woods.

During the 2011 harvest, I was more interested in determining how different vinification techniques might enhance the tannin concentration in Marquette wines without any addition of commercial tannins or oak. In summary, I used three different techniques: 1) Fermentation with 50% whole clusters, 2) Saignee – I removed 20% of the juice before fermentation started, 3) I froze the grapes prior to fermentation (I would have preferred to try a heat treatment, but it seems like our industry is far from investing in thermovinification or flash-detente systems, and I couldn’t simulate it in our winery anyway. It seemed like freezing was a technique relative to the current state of our industry).

Here is a graph displaying the difference in the tannin concentration of the 3 different fermentations (Quantified using HPLC):

The only technique that really had an impact on the tannin concentration was the 50% whole cluster fermentation. None of the different methods showed a difference in anthocyanin (colored pigments) concentration. We saw similar results with our tannin trials in 2010.

Using whole clusters when fermenting wine is a fairly common practice in certain cultivars, such as Pinot Noir and Grenache, with low quantities of phenolics (tannins and anthocyanins). Besides increasing the tannin content of the finished wines, using whole clusters when fermenting can benefit the wine in other ways as well.

One caveat to using this technique is that the stems must NOT BE GREEN. Green stems will release compounds that will taste green and herbaceous. Ripe clusters with brown, lignified stems are what you’re looking for. The grape clusters should be placed whole into the fermentor without crushing.

Grape stems have the ability adsorb/absorb certain compounds, as well as releasing other components into the wine over the course of fermentation. They will release tannins, but may also bind some of the colored compounds of your wine, which can result in a wine that has a lower color intensity. However, whole cluster fermentations can also help to aerate the cap as well as provide channels for wine to traverse during pumpovers. Oxygen helps bridge tannin-anthocyanin complexes, which can stabilize the color of the wine, while the channels provided the stems helps facilitate extraction of anthocyanins from the skins. Both will result in a wine with an intensified color. Another benefit of this increase in oxygen is that the temperature of the cap is easier to manage, and fermentation time is shorter.

Nonetheless, perhaps the most important thing to consider when deciding whether or not to employ this technique (besides making sure stems are not green) is the effect it can have on the total acidity and pH of your finished wine. Generally, wines fermented with whole clusters will see a decrease in total acidity, and an increase in pH. In years when high pH is already a concern, you may not want to use this technique unless you are prepared to acidify your wine.

All-in-all the tannin concentration in Marquette is very low – so low that we have had difficulties quantifying it using classic methods.  So far, it seems as though whole cluster fermentation could be an option for increasing the structure in Marquette wines. This coming vintage, we will continue to trial this method. In the next few years we will learn much more about the the types and location of the tannins in Marquette, as well as how they develop over the course of ripening thanks in part to work in the Northern Grapes Project. This will help guide recommendations for winemaking protocols over the next few years.

 

 

I would like to give a special thank you to Dave Manns, a post-doctoral research fellow at Cornell University who assisted in HPLC analysis of this tannin trial.

 

Further Reading:

Hashizume, K. and Samuta, T. 1997. Green Odorants of Grape Cluster Stem and Their Ability To Cause a Wine Stemmy Flavor. J. Agric. Food Chem. 45, 1333-1337

Weston, Leslie. Grape and Wine Tannins and Phenolics – Their Roles in Flavor, Quality and Human Health

http://www.wineanorak.com/tannins.htm

http://www.princeofpinot.com/article/865/

 

Filed Under: Enology Tagged With: ,

Frontenac Gris Rosé

April 19, 2012 by

I realize I am WAY behind in updating this blog. I will try to remedy this in the coming weeks.

I have a lot to write about, as we recently finished our tasting evaluations of our 2011 wines. Although the majority of the wines we evaluated are Minnesota selections that haven’t been released, we were also able to do some evaluations of our trials with Minnesota cultivars. Today I’ll talk about one of our trials: Frontenac gris rosé.

There are two methods one can employ to make a rosé wine. The first, which I mentioned in my Marquette vinification trial post last year, is the saignee method or “tank bleeding.” Essentially you fill your tank with red grapes, and do a cold soak for anywhere between 6 and 24 hours. This allows time for some of the color from the skin of the grapes to seep into the colorless juice. The longer you let them soak, the darker the color. After the desired soaking time has passed, you open the racking valve at the bottom of your tank (with a hose attached, of course), and pump 5-10% of the volume of your tank into another tank. Then, you ferment your red grapes to make a red wine, and your saignee juice is fermented as a rosé. Of course, this method is typically employed with Vitis vinifera grapes, of which most have colorless  pulp. Most of our hybrid grapes have colored pulp and skin, so this maceration step is unnecessary if you wish to make a rosé from Frontenac or Marquette. Often the problem with Frontenac rosé especially is that its color is more of a claret rather than a rosé – even without any skin contact!

So that brings me to the second method of making a rosé. The French would argue that this is the only way to make a rosé (unless you’re in Champagne). It’s the direct press method. This how I would recommend rosé made from Frontenac or Marquette should be done. With the saignee method, it may be difficult to achieve a lighter-colored wine. With the direct-press method you essentially treat the red grapes as if they were white grapes.  You press the grapes right after harvest and can crush/de-stem, or press them whole-cluster. If you whole-cluster press you may be able to achieve a lighter color because of adsorption of anthocyanins to the stems.  Of course if you were using Vitis vinifera like they do in Provence, you would need a short maceration time to achieve some color extraction. Traditionally, the grapes would be crushed, de-stemmed, and macerated for a short period of time. Maceration often takes place directly in press.

Although I mentioned Frontenac and Marquette as two red grapes that can be used to make a rosé, there is a third option: Frontenac Gris. Frontenac Gris does not contain anthocyanins (red pigments) in the pulp like Frontenac. However, it still retains some red color in the skin. If you press the grapes immediately after harvest, it yields a gold to amber-colored juice. But, if you allow a certain amount of skin contact (or if you over-extract during pressing), you can extract some of the color from the skins. Thus, it is really the only grape we have that can be handled as one would handle V. vinifera when making a rosé.

Knowing that Frontenac Gris isn’t as highly colored as a red grape, our skin contact time needed to be longer than the 6-24 hours traditionally needed for making a rosé from red (vinifera) grapes. We decided to do two trials: a 3-day pre-fermentation maceration, and a second where we actually fermented the grapes on the skins. We already knew that fermenting Frontenac Gris on the skins (when we made a FG port last year) gave us a really pretty dark pink wine, so I wasn’t too worried about too much color. The idea was to see what we could achieve with maximum anthocyanin extraction during alcoholic fermentation. It’s important to remember that a certain percentage of color will be lost immediately after fermentation. Another percentage is lost with sulfur addition. So, if the color of your wine doesn’t resemble the color of your juice, then this is why.

So here’s a picture of the color difference between our two trials. See if you can pick out which was a 3-day cold soak prior to fermentation and which was fermented on the skins:

If you couldn’t figure it out, the wine on the left was macerated (cold soaked) on the skins for 3 days, while the wine on the right had a 3-day cold soak plus spent a week on the skins during alcoholic fermentation. While the color from a photograph isn’t always indicative of what it looks like in real life, it gives you a good indication of the final color difference in the wines. The 3-day cold soak was more of an orange/salmon color. It wasn’t exactly rosé, but it wasn’t terribly unattractive either. It all depends on what the winemaker is looking for in their final color.

While Frontenac Gris doesn’t have anthocyanins in the pulp, there still tends to be a high amount of other colored molecules. I think the high quantities of these yellow/gold pigments mixed with a small amount of red yielded a wine that had more of an orange/salmon color.

Another great thing about using Frontenac Gris to make a rosé wine is that there are almost no tannins in the grape, thus by fermenting on the skins you don’t extract heavy amounts of tannins. Nonetheless, there can be bitter and herbaceous elements that are extracted from the seeds, or from the skin of fruit that is underripe.

Here’s the breakdown of the chemistry in the finished wine

TA  (g/L )                  pH                  Alc. %

Frontenac Gris – AF on skin

            9.20

            3.50

       15.4

Frontenac Gris – 3-day

          10.45

            3.41

       15.4

An interesting note from the fermentation on skins is the decrease in total acidity and the increase in pH. This could be due to some excess potassium extracted from the skins that may have facilitated tartrate precipitation as well as increasing the pH. Since we didn’t measure potassium, this is only a guess. However, the final chemistry of the two wines is pretty close.

As for how the wines taste, I’ll leave you with some of the tasting notes from our evaluation. The wines were tasted blind by our viticulture and enology crew.  Both of these wines were fermented to dryness and no adjustments were made post-fermentation. This was to ensure that they followed our standard protocol for winemaking. Some slight adjustments to the acidity or sweetness may have yielded wines that were a bit more balanced on the palate. You can see that there was some herbaceous character noted in the grapes fermented on the skins. Some tasters found it off-putting, while others enjoyed it. It is also possible that some fining could help remove some of these bitter compounds. In the end, I hope this trial at least gives you some tools to use in your own wineriess.  Cheers to some tasty rosé wines… just in time for summer!

 

Color (3-day cold soak pre-fermentation) salmon/orange
Aroma white chocolate, apricot, fruity, red fruit, artificial cherry, strawberry, berry, banana, hybrid, plum, soapy, some bakers spice, dried apricot, concentrated raisin, petrol/chemical
Palate acid, good citrus/peach flavors, some bitterness, tart, hot, different, red fruit, tart, berry, nutty, sour, peachy, berry, cloves

 

Color (Fermented on skins) dark pink, vibrant red, rose, pretty garnet
Aroma cherry, oregano, more riparia, lots of red hybrid, Frontenac flavors, herbaceous, blackberry, camphor, green pepper, cherry Robitussin, raspberry, cherry
Palate acid, hot, chemical, cherry, bitter, takes on more hybrid flavors, blackberry, black currant, herbaceous, thin, hybrid, underripe, red currants, cherry, plum, chokecherry, some bitterness, hot, cherry, raspberry, spice

 

 

Filed Under: Enology Tagged With: , , , ,

Measuring Sugar in wine

January 24, 2012 by

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.

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It’s Snowing… in my bottle!

January 12, 2012 by

I recently had a few wineries who brought me examples of bottles of wine that resembled snow globes. Though I realize that Minnesotans are missing snow this winter, I doubt that seeing it in a bottle of wine will make up for it! Haze formation or deposits in wine are caused by the wine not being properly stabilized, and are quite varied in origin. I had to brush-up on my detective skills on how to identify each different type of deposit, and thought I’d share them with you. I imagine that this problem might be more widespread than the examples I received this week.

I can’t stress the importance of stability testing before bottling your wines. Though “stability” is a fairly obscure term, in general, a wine that is stable would be expected not to undergo any undesirable physical or sensory changes from the time of bottling until it is consumed. Although it is impossible for a winemaker to imagine what the life of each particular bottle of wine will have or the conditions it will be exposed to, a series of stability tests have been devised that will predict physical changes to the wine throughout typical wine storage.

Stability tests are carried out pre-bottling in order to predict protein stability, tartrate stability, oxidative stability, color stability, and metal stability. It is important to learn more about the causes of each of these instability problems in order to prevent them from occurring in your wine. However, small wineries or home winemakers sometimes overlook the importance of doing these stability tests – not realizing their wine is unstable until a deposit forms in the bottle. Yes, I know they are time-consuming and tedious, but once a deposit forms in your wine there is really little you can do but decant each bottle, filter out the deposit, and re-bottle the wine (after having to do it on a few hundred bottles of wine, you will perhaps realize that stability testing isn’t a waste of time after all). However, understanding the source of the deposit may help you understand the underlying cause so that you can prevent the problem in the future.

Perhaps the easiest type of deposit to identify are crystalline deposits. These are either Potassium tartrate crystals or Calcium tartrate crystals. If the latter, the crystal may appear in a more rhomboid shape. However, the only real way to determine what mineral is involved is by a flame test. Calcium burns red, while potassium will burn violet.

The amorphous (or non-crystalline) deposits that form in wine are a bit more difficult to differentiate. There are four different origins of these amorphous deposits: proteins, tannins/pigments, mineral (copper or iron), or microbiological.

Protein precipitation is perhaps the most common cause of a hazy deposit. It’s typically light (white to tan) in color and consists of very fine particles that remain in suspension for a long period of time. To be sure, if you are able to collect the deposit by either decanting the wine or by centrifugation, then try to dissolve it in a 0.1 M sodium hydroxide (NaOH) solution.  If it dissolves, it is likely protein in origin.

If the deposit is due to tannin and/or pigment precipitation, the color of the deposit will often be darker in color – more brownish than white. If you can collect the deposit, it should be soluble in a 50% ethanol solution.  The source of this deposit is often due to oxidation of some of the pigments in the wine. However, it is also possible that if you overfined your wine with a protein fining agent, some of the excess protein can react with the tannins in natural cork closures when you bottle it. They will combine to form a precipitate in the bottle.

A haze that is microbial in origin will typically be accompanied by a wine that has excess carbon dioxide. It is fairly unmistakeable when a wine shows a bit of unintended fizz. Microorganisms (yeast and bacteria) are never the cause of protein instability in a wine. Haze due to microorganisms is usually due to improper filtration or sanitation of your bottling line. You can confirm this type of deposit by looking at it under a microscope.

Excess iron or copper can also form a deposit in your bottle. During bottling, exposure to oxygen causes iron to change from its soluble state to an insoluble state. In red wines, it interacts with the tannins to cause a precipitation that is blue in color. In white wines it interacts with phosphates. You need a significant quantity of excess iron for this reaction to occur – more than 7 mg/L for white wines, and more than 10 mg/L for red wines. Typically these levels are only obtained if a vineyard or winery uses old equipment constructed from iron. Another possible cause is if vineyard soils are treated with iron, and dust from the earth is somehow deposited on the grapes prior to harvest. Copper deposits are also rare, and form in conditions of reduction (opposite of oxidation) and are accelerated when the bottle is exposed to light. Again, this type of deposit is rare in modern winemaking. Older cellars might still have equipment made of brass, which can cause excess copper to form in the wine. Residual copper from vineyard sprays may also make their way into the winery if the spray was done too close to harvest. It’s also possible that when treating your wine with copper sulfate you added excess copper. If your wine contains more than 0.5 mg/L of residual copper, it should be treated with a protective colloid such as gum arabic. Note that the legal limit of residual copper in your wine is 1 mg/L. Thus, the importance of running bench trials prior to using copper sulfate to treat reduction odors! A deposit that is mineral in origin will dissolve in a solution of 25% hydrochloric acid (HCl).

Hopefully this short tutorial on identifying deposits in your wine will help with preventing them in the future!

 

Source:

Iland, P., N. Bruer, E. Wilkes. 2004. Chemical Analysis of Grapes and Wine: Techniques and Concepts. Patrick Iland Wine Promotions.

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