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New World Wine Maker Blog

Going Ape for the Non-sulphur Grape

It’s in everything, from jam to fruit juice. Sulphur, as a simple molecule, is seriously abundant. Some people even refer to humans as sulphur-DNA based life, owing to the glue-like role it plays in keeping our DNA wound up. Its role is not arbitrary. Its highly powerful, life-giving chemical properties are also what give it it’s extreme antimicrobial abilities. It’s nuclear for yeast and bacteria; the gestapo of must, showing little mercy or discrimination.

As much as you might not want to accept it, we primates are pretty similar to fungus. We’ve both got proton pumps, ATP pumps and supercoiled DNA coding. We both love sugar, and yes, believe it or not, alcohol is toxic to us both. And so is sulphur. The same biological pathways that sulphur devastates in yeast are damaged in us too when we ingest this controversial substance. Unsurprisingly, many would agree it’s a repulsive compound, and for those of you blessed enough to have come close to the pure form – it’s certainly not a fresh ocean breeze that caresses your inner nostrils. It’s more like sandpaper spinning on the end of a drill-bit forced up your nose.

It’s no surprise people go ‘ape’ for anything lacking it. Biodynamic wines fly off the shelves locally and internationally; people hate the stuff so much they’ve convinced themselves it’s an “allergen”. Actually it’s just quite nasty.

The problem is, it’s not easy to make a non-sulphured wine. Sulphur dioxide is as ubiquitous in winemaking as the wooden barrel; more so, in fact. It serves a plethora of purposes in protecting the wine against microbes, oxygen and flavour degradation etc. Since these risks appear daily in a winery, it’s necessary to have the ultimate prevention. It’s a bit like seat belts in your car, if your car happens to be driving the Dakar Rally.

It’s not impossible though. An esteemed winemaker, or two, has said “good grapes, good winemaking”. To clarify: good grapes are healthy, non-rotten grapes – preferably with a nice low pH (a nifty natural wine protectant); and good winemaking includes cellar floors, surfaces, pumps and pipes that are clean enough to perform surgery on. After that, if it survives the first week in bottle it’ll go all the way … or so I’ve been told. And there’s plenty of examples to show.

It’s generally a bit of a dicey argument to suggest zero sulphur content in your wines. A great many wineries work hard to protect their wine and simply add the utter minimal, which acts as a failsafe more than anything to prevent flavour loss from oxygen contact. This seems to work, resulting in a severely reduced sulphur content compared to the norm. Of course, this becomes incredibly difficult when working with large quantities of wine, and not everyone has the luxury of working with healthy grapes.

It’s all, however, an unseen hypocrisy; a perversion of ‘the ignorance is bliss’ scenario. Sure, don’t spray chemicals on my lettuce, chia seeds and paleo diet. Fine, don’t ruin my weight loss, low blood pressure and increased self-esteem with sulphur. Wine has a fraction of a percentage of sulphur, please get rid of that, it’s harmful for me and my unborn baby. But…you can leave that toxic 13% alcohol in there – no problem there!

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Reviewing YAN and Hydrogen Sulfide: Part 2

By: Denise M. Gardner

In a previous post, we discussed ways in which nutrient management during primary fermentation can affect hydrogen sulfide formation and the overall “health” of the wine.  This week, we’re going to explore how to mediate hydrogen sulfide aromas and flavors in a finished wine.

Sulfur-Containing Off Aromas

In general, many wine sensory scientists and wine experts will agree that is relatively a bad habit to use the term “sulfur” to describe off-odors associated with hydrogen sulfide or “stinky” aromas that are usually described by the term “reduced.”  One of the main arguments for avoiding “sulfur” as a description term for an aroma is due to the fact that there are actually several forms of aromatic sulfur-containing compounds found in wine, and they can have very different aromas (smells, odors) associated with that one compound.  The most common groups of aromatic sulfur-containing compounds in wine are:

  • Sulfur dioxide (SO2)
  • Hydrogen sulfide (H2S)
  • Mercaptans or Thiols
  • Disulfides

Additionally, many sensory experts will advise further to avoid using the chemical names as descriptors for describing an aroma found in wine (e.g., using the term “hydrogen sulfide” to describe the hard-boiled or rotten egg aroma).  It is typically recommended to use an actual descriptor when describing an aroma (e.g., using the term “rotten eggs” when that smell exists in wine).

Sulfur Dioxide (SO2)

Sulfur dioxide is an antioxidant and antimicrobial preservative frequently used in wine production.  However, it is also produced by yeast during primary fermentation, which is why wines (and other fermented products) cannot be sulfur dioxide-free (commonly referred to as “sulfite free” in the mass media).  The aromatic descriptor commonly associated with a high concentration of sulfur dioxide is termed “burned match,” but a high concentration of sulfur dioxide can also cause a nasal irritation that many will describe as nasal burning.

Hydrogen Sulfide (H2S)

Hydrogen sulfide is an aromatic compound that is commonly described as having a “rotten egg” or “hard-boiled egg” aroma.  Like many sulfur-containing compounds, hydrogen sulfide has a low sensory threshold (<1 – 1 part per billion, ppb), indicating that about 50% of the population could sense this compound at that concentration without being able to identify it, specifically, as hydrogen sulfide.

As we saw in our previous post, hydrogen sulfide development can result as a component of poor nutrient management during primary fermentation.  Residual elemental sulfur from pesticide sprays has also been linked to latent development of hydrogen sulfide in wines.  In a 2016 edition of Appellation Cornell, Dr. Gavin Saks’ lab provided a detailed and practical report on how hydrogen sulfide can be a problem for winemakers post-bottling and the potential links to hydrogen sulfide development as a function of residual sulfur from the vineyard (Jastrzembski and Saks, 2016).

Occasionally, winemakers may also experience hydrogen sulfide formation during a sur lie aging period; a time in which the finished wine remains on the lees when lees are stirred in the wine.  It is also common for sparkling wines, produced in the traditional method, to exhibit a small perception of hydrogen sulfide when the bottle is first opened.

Mercaptans/Thiols and Disulfides

Finally, mercaptans or thiols, sulfur-containing compounds that contain the functional group –SH, and disulfides, sulfur-containing compounds that contain a S-S bond, can also be problematic for winemakers when found at high concentrations.

The presence of sulfur-containing volatile compounds is not always considered detrimental to wine quality.  For some wine grape varieties (e.g., Sauvignon Blanc), these classes of compounds can make up their varietal aroma.  In very small concentrations, sulfur-containing compounds can also be aroma enhancers, indicating that their presence can actually make the wine smell fruitier than if they were not present in the wine.  However, when at substantial concentrations, volatile sulfur-containing compounds can also produce various “stink” aromas that mask a wine’s fruitiness, freshness, and make the wine generally unappealing.  This is phenomena is dependent on the concentration of the sulfur-containing compound and the chemical makeup of the solution (i.e., wine) it is in.

Mercaptans or thiols and disulfides have a variety of descriptors associated with them, and their perception is largely based on concentration.  When we’re discussing the negatively-associated descriptors, common terms include: garlic, onion, canned asparagus, canned corn, cooked cabbage, putrefaction, burnt rubber, natural gas, and molasses amongst others.

Are There Sulfur-Containing Off-Aromas in Your Wine?

To identify if hydrogen sulfide, mercaptans/thiols, or disulfide-based off-odors exist in your wine, it may be best to use a copper screen as a bench trial.  While analytical identification of these compounds is possible, it is often expensive and leaves the winemaker guessing on what to do next.

For a quick assessment of a wine’s aroma, winemakers can drop 1-2 pre-1985 copper pennies into a glass of wine to see if the aroma freshens.  The freshening aroma is due to the fact that the copper from the penny is reacting with the sulfur-containing compounds in the wine and making them aromatically inactive.

The "penny test" is often used to quickly determine if a wine is suffering from reduction, the presence of several types sulfur-containing off-odors. Photo by: Denise M. Gardner

The “penny test” is often used to quickly determine if a wine is suffering from reduction, the presence of several types sulfur-containing off-odors. Photo by: Denise M. Gardner

A technical copper screen takes a bit more work and should be conducted in a quiet and aromatically-neutral environment.  It is recommended to do this outside of the cellar.

Copper addition, in the form of copper sulfate, is often used to remediate aromas/flavors associated with hydrogen sulfide. One-percent and 10% copper sulfate solutions can be purchased through your local wine supplier.  The basic protocol associated with a copper screen is as follows:

  1. Add 50 milliliters of wine to two glasses.
  2. Label one glass “control” and the other “copper addition” (see image below).
  3. Add 1 mL of 1% copper sulfate to the “copper addition” glass.
  4. Cap both glasses for 15 minutes.  Sniff the aroma of each wine.

Setting up a copper screen can help determine if a wine is suffering from aromas caused by sulfur-containing compounds. Photo by: Denise M. Gardner

Setting up a copper screen can help determine if a wine is suffering from aromas caused by sulfur-containing compounds. Photo by: Denise M. Gardner

Sniff (smell only!) both glasses. Most people start with the “control” and smell the treated wine (wine containing copper sulfate) second.  If the aroma/flavor of the “copper addition” glass has improved, or the hydrogen sulfide aroma has subsided, then a copper addition trial should follow to determine the exact concentration of hydrogen sulfide needed to clean up the wine in question.  Remember that the legal limit for copper allowed in a finished wine is 0.5 ppm.

Treatment of Sulfur-Containing Compound Off-Aromas

Sulfur-containing compounds are quite reactive, which can make dealing with them fairly difficult.  Many educators agree that the best way to treat sulfur-containing compounds, especially those that stink, is to prevent their existence as best as possible.

In the Appellation Cornell newsletter that focused on sulfur pesticide residues, Jastrzembski and Saks (2016) recommended that sulfur residue concentrations should not exceed 1 mg/kg at harvest in order to avoid latent hydrogen sulfide or sulfur-containing off-aromas later in processing and storage.  Additionally, many experts recommend appropriately treating fermenting musts with nutrient management strategies based on the starting YAN concentration to minimize the incidence of hydrogen sulfide formation during primary fermentation.

As described above, winemakers may also opt to treat the wine with copper sulfate to try to reduce the perception of hydrogen sulfide or other sulfur-containing aromas.  It should be noted that aromas caused by disulfides cannot be mediated with a copper sulfate addition.

There has been more conversation in the academic community regarding the reemergence of hydrogen sulfide or sulfur-containing off-aromas after a wine has been treated with copper and post-bottling.   The theory around this appears to circulate around residual copper initiating reactions in the wine that lead to more sulfur-containing off-odors.  This continues to be an ongoing discussion amongst researchers and will likely be a hot topic within with the wine industry.  For now, it is important for winemakers to understand that there may be a risk of off-odors reemerging post-copper treatment and post-bottling.  This topic will also be discussed to some degree at the 2017 PA Wine Marketing and Research Board Symposium on March 29, 2017 in State College, PA, and winemakers are encouraged to attend.

Some hydrogen sulfide or sulfur-containing off-odors can sometimes be mediated with use of fresh lees stirred in the wine or the addition yeast lees-like products.  Winemaking products like Lallemand’s Reduless, yeast hulls, or some cellulose-based products can help reduce or eliminate the intensity of these off-odors.  As with any other product additions, it is recommended that wineries always do bench trials first and before adding to the entire volume of wine.  Additionally, Enartis USA (Vinquiry) has previously distributed a fact sheet to help winemakers troubleshoot reduced wines and determine how to best treat a problem wine.

The incidence of reduction, sulfur-containing off-odors, or hydrogen sulfide can be a frustrating circumstance for winemakers.  However, adequate vineyard care and proper nutrient management during primary fermentation can help minimize the incidence rate of sulfur-containing off-odors from occurring in their wines.  Of course, problems with wines do occur, and we hope that the recommendations above will help winemakers solve wine problems pertaining to sulfur-containing off-odors.

 

Resources

Jastrzembski, J. and G. Sacks. 2016. Sulfur Residues and Post-Bottling Formation of Hydrogen Sulfide. Appellation Cornell, 3a.

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CHRISTO LE RICHE – WINEMAKER AT LE RICHE WINES

Q. Where and when were you born ?

“I was born in Belleville on 31st July 1984.”

Q. Where did you study and what qualifications do you have ?

“I studied at University of Stellenbosch where I achieved a BSc Agric Viticulture and Oenology followed by BSc Agric Hons Cum Laude Viticulture.”

Q. Do you consider your approach to winemaking to be different to others ? 

“Not particularly. Winemaking is an art-form, but for me that skill lies in your ability to anticipate the quality of a vineyard, know when to pick for a specific style and how your blends will react together. Every vintage is unique and requires a unique response. This is something that comes with experience as well as knowledge and is intuitive part of winemaking that is unique to every winemaker. In terms of the production method I don’t think it is necessary to reinvent the wheel. Most of the time your “unique” technique is simply a copy of a winemaker you have no knowledge of. Winemaking is simple, but having the correct gut feel is more difficult.”

Q. How involved do you get in the vineyard ? 

A prompt reply. “As much as possible. I am the consultant to all my producers and I try and get to the vineyards at least once every two weeks during the important months of October, November, January and February. Obviously I visit the vineyards more regularly during harvest to establish the harvest date.”

Q. Do you have any varieties you prefer to work with ? 

“There is no doubt that Cabernet Sauvignon is my first love.”

Q. Have you been influenced by any particular winemaker or region ?

“Andy Ericson, the Terroir Capital consultant winemaker, and Carl  Schultz stand out to me. My father (Etienne Le Riche) has also played  a major role in my development. In terms of regions I look to Napa and Bordeaux for quality inspiration, but viticulturally I look much wider. I am not a fan of one dimensional viticultural approach.”

Q. What would you consider your greatest achievement as a winemaker ?

“I am still way too young for the question !!  It was great to receive  a Platter 5  Star for  my 2011 Reserve Cabernet Sauvignon, but I think some of my other wines have been better. I value the friendships and connections I have made much higher than the awards.”

Q. What “secrets” have you “developed” that make your wines different to others ?

“Intuition. My “optimal ripeness” window is unique to my experience based on my travels and the vineyards I work with. That and the use of manual open top fermentation techniques.”

Q. How important is modern winemaking  equipment in your winemaking ?

“Not very important. The equipment changes every couple of years  with a newer and better model. Once you have the equipment that works, stick to it, but be aware of what is on the market in case you need a change ! Take berry sorting equipment for example. It has been refined to optical jet selection, but I have lately been moving away from berry sorting because I believe it reduces the quality of my wines.”

Please give me a brief personal history and a look into the future.

“I was raised on Rustenberg estate on the Simonsberg, where my Dad was the winemaker and it was there my love for wine and vineyards was kindled. Working with my father  provided me with a basis to understand the industry and know what a career in winemaking  would demand from me.  After my studies I travelled  to Napa and Bordeaux to learn about Cabernet Sauvignon production, but also to enlarge my wine knowledge and life experience. I also worked for a few South African wineries which provided me with a strong base to join the family business from. After my travels I started working full time for Le Riche while furthering my studies with an Honours degree in Viticulture.

My hobbies include surfing, spearfishing, trail running  and training my dog for wing shooting. On most weekends you will find me outside in nature. The future, for me , includes finding new and better Stellenbosch vineyards to work with. It would also be great to find a few small vineyards of outstanding quality to own. I will keep perusing the Le Riche vision of producing top Stellenbosch Cabernets and striving toward finding the best expression of our vineyards.”

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Supermarket Saves

Have you ever experienced that immensely overwhelming feeling of exponential confusion when standing in the wine aisle at your local supermarket? Yeah, me too. The kaleidoscope of labels, hieroglyphic cellar and estate names, endless cultivars and blends and price ranges as wide as the Amazon River can easily have you flat on your back with TMI (too much information). To tell you the truth, there is no easy way of choosing a “good” wine from such a great variety. However, here are some of the nifty tips and tricks that I use when I’m facing the wine aisle gauntlet.

The first one is a no-brainer: specials. That is one of the best perks of buying at a supermarket. Stores regularly have specials and promotions and many times the prices can compete with those that you might find at the cellar-door. And yes, sometimes you might buy six bottles of wine where you only needed one, but its wine so who’s even counting?

Secondly, there is the all-important factor of price. For most people this is probably the variable that carries the most weight when deciding which wine to buy. And this is also very dependent on personal preference. For instance, I myself am very comfortable with drinking wines that are in the price range of R50 – R70 and I will even splurge a bit more when it is for a special occasion. I generally prefer to stay under the R100 mark when buying from a supermarket- but that’s just me.

Then of course there is the ever-confusing label. I personally do not care much for reading back labels. It has happened too many times that the back of a wine label has been completely misleading. Instead, I look for back labels that DON’T give much information on the specific bottled wine. Labels that have a story of the history of the farm or estate or even the block of vineyards the wine is made from are much more intriguing to me and does not fill my mind with expectations that are most likely not going to be met. Also, a bottle of wine with a creative front label has rarely disappointed me before. If they put as much effort into making the wine as they have into designing the label, you must be in for a treat (fingers crossed).

My next point is quite a controversial one: cork versus screw cap. This one is 100% completely up to personal preference. There is no right or wrong and the jury is still out on which one is better for the wine- if there even is a slight advantage for either. I tend to choose a screw cap when I am buying wine that I am going to take to a braai or a party at some else’s place and I choose corked red wines when I am cooking red meat for my family and close friends at home- there is nothing like the sound of a cork popping to start a wonderful evening of merriment and chatter. Whereas screw caps work better for picnics and braais (no searching for a bottle opener or a cork stopper to close the bottle back up again).

If you do have a bit of wine knowledge it will definitely count in your favour. Especially when it comes to familiar or “trustworthy” wine cellars. Most people have a favourite winery and if you particularly like one of their wines, the chances are quite good that you will like some of their other wines as well.

Another simple solution when choosing a wine: phone a friend! I have relied on wine recommendations from my friends countless times. And we all have that one friend with whom you share the exact same taste in wine. So next time you know you are heading to the wine aisle, pick their brain a little. Nobody knows your taste in wine better than your friends.

And if all else fails, do what I do- experiment. Every time I go into a supermarket to buy wine, I buy a different bottle. I make sure to try different wineries, wine styles and cultivars or blends. I see the immense variety of wines we find in supermarkets as a great privilege, instead of a daunting mass of information. In South Africa we are absolutely spoilt for choice when it comes to wines and now we don’t even have to drive to the estates to buy exclusive wines at cellar-door prices- your nearest supermarket has done the job for you.

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Reviewing YAN and Hydrogen Sulfide: Part 1

By: Denise M. Gardner

Yeast assimilible nitrogen (YAN) is the sum of the amino acid and ammonium concentrations available in the grape juice at the start of fermentation.  Typically, the amino acid proline is not included in the reported amino acid content as it is not readily utilizable by yeast cells.

The amino acid component of YAN is often referred to as the “organic” YAN form.  In contrast, the ammonium ion content is referred to as the “inorganic” YAN form and may be written in its ionic abbreviation: NH4+.  Due to the fact that ammonium is only connected to a series of protons (H+ions), it tends to be easier to move in and throughout the yeast cell to be consumed during fermentation (Mansfield, 2014). When these two components (organic + inorganic) are added together, the resultant value is the YAN, written with the units: mg N/L.

The winemaking challenge associated with YAN is the fact that it is quite variable, and current research has not identified ways to change the YAN, predictively, in fruit through the manipulation of vineyard practices.  YAN varies by vintage year, grape variety, cultivar, and with the use of various vineyard management practices.  In Penn State’s research vineyards, ~1 acre in size and containing 20 different wine grape varieties, YAN values ranged dramatically each vintage year amongst the various wine grape varieties.  On any given vintage year YAN values ranged from low (<100 mg N/L) to high (>300 mg N/L) amongst the varieties grown in that one site.

The variability associated with YAN provides a secondary challenge to winemakers: the lack of predictability associated with hydrogen sulfide formation during primary fermentation due to unfulfilled nitrogen needs by wine yeasts.

What does YAN have to do with Hydrogen Sulfide?

Winemakers often talk about YAN in relation to hydrogen sulfide (H2S) as the two have been associated with one another throughout primary fermentation.  Although there are several potential causes of hydrogen sulfide formation during wine production, some of which we will talk about in our Part 2 series, nitrogen imbalance has been one of the factors that winemakers can influence through production.  Unfortunately, there is no way to ensure that a wine will not produce hydrogen sulfide by the end of fermentation, but treating wines with proper nutrient supplementation can help minimize the incidence of hydrogen sulfide production during primary fermentation.

Hydrogen sulfide is produced by the yeast cell via the sulfate reduction pathway (Figure 1).  While I know this figure looks scientifically daunting, we can try to simplify its purpose to discuss how hydrogen sulfide is released into wine.  Sulfate (SO42-), naturally abundant in grape juice (Eschenbruch 1974), is transported into the yeast cell for amino acid (cysteine and methionine) development, which are naturally lacking in concentration in grape juice (Bell and Henschke, 2005).  Energy is used by the yeast (represented as ATP in Figure 1) to chemically alter the structure of sulfate in order to make it useable by the yeast cell.  This useable form can be seen as sulfide (S2-) in the image below.  Using nitrogen, which is required to make an amino acid, the sulfide content is depleted as cysteine and methionine amino acids get produced.  Therefore, as sulfide reserves are depleted, cysteine and methionine contents generally increase to be used for building proteins that will be needed by the existing or new yeast cells.

Figure 1: A simplified version of the sulfate reduction pathway.

Figure 1: A simplified version of the sulfate reduction pathway.

Sulfur dioxide (SO2) plays a role in the sulfate reduction pathway in that it bypasses the transport mechanism required to bring sulfur into the yeast cell.  It other words, it can diffuse across the cell membrane and into the internal parts of the yeast cell.  Sulfur dioxide will get chemically altered to be made into the useable sulfide , S2-, form as well.  Therefore, fermentations that contain a high concentration of sulfur dioxide at the start of fermentation have the potential to increase the utilization of sulfur dioxide during yeast metabolism.

These processes function normally until a depletion of nitrogen (from the nitrogen pool) or an accumulation of sulfide develops in the yeast cell.

If there is not enough nitrogen (low YAN fermentations) available to make the sulfur-containing amino acids (cysteine and methionine) then, eventually, the yeast cell will not be able to continue manufacturing these amino acids.  In this situation, the sulfide concentration generally starts to increase within the yeast cell.

The chemical form sulfide, however, is toxic to the yeast cell and thus, the yeast will try to eliminate it from its internal structures.  Therefore, when sulfide concentrations get too high, the yeast will diffuse this across its cell membrane into the surrounding media: the fermenting juice.  When hydrogen sulfide concentrations get high enough in the fermenting juice, winemakers can often sense the rotten or hardboiled egg aroma associated with the compound.

What if there is too much nitrogen?

In contrast, too much nitrogen (high YAN fermentations) can also be problematic.  Higher concentrations of the inorganic component of YAN can lead to a high initial biomass (population) of yeast.  The rapid increase in yeast populations can lead to nutrient starvation by a majority of the yeast when the wine is about almost finished completing fermentation.  With a large biomass of yeast incapable of obtaining the proper nutrient (nitrogen) content to grow and reproduce, hydrogen sulfide development can result.  This is due to the fact that there is a large population of yeast in situations in which there is not enough nitrogen to support their growth (i.e., there is not a lot of food to go around for all of the yeast cells).  With hydrogen sulfide development occurring late in primary fermentation, it is obvious that the winemaker would become concerned with hydrogen sulfide retention by the time fermentation is fully complete.

Too much nitrogen can also cause other quality problems.  Due to the excess amount of available nutrients, yeast can grow and reproduce quickly, which often leads to very rapid or very hot fermentations.  The speed of fermentation, of course, can affect the aromatics and quality of the wine (i.e., fast fermentations often lead to simpler aroma and flavor profiles).  This may not be an issue with some styles of wine, but for many white wine or fruit (other than grapes)-based fermentations, aromatic retention is often a priority by the winemaker.

Due to the fact the initial YAN is so high, all of the nitrogen contents may not be utilized by the yeast population by the end of fermentation, and could remain in suspension in the finished wine.  As yeasts begin to autolyze, all of their inner components, including the remaining nitrogen content, will become available in the wine.  The excess “food” could be available for other microorganisms (like acetic acid bacteria, lactic acid bacteria, or Brettanomyces), which could potentially lead to spoilage problems if the wine is not properly stabilized.  Such spoilage is, obviously, detrimental to wine quality and undesirable by the winemaker.  Alternatively, remaining nutrients could be utilized by malolactic bacteria or those wines that will be given tirage for sparkling production (Bell and Henschke, 2005).

Finally, higher YAN concentrations can lead to an increased risk of ethyl carbamate production in wine; ethyl carbamate is a known carcinogen that can give susceptible individuals headaches, or even respiratory illness.  Ethyl carbamate is produced in a reaction between ethanol and urea (Bell and Henschke, 2005).  The heavy use of DAP has also been linked to a higher potential risks of ethyl carbamate due to the fact that DAP inhibits the transport of amino acids into the yeast cells, and therefore, leaves a higher concentration of amino acids available that can potentially be altered into urea, a precursor for ethyl carbamate (Bell and Henschke, 2005).

The fact that excess nitrogen can be problematic during wine production should provide insight to winemakers to avoid over-supplementing their fermentations.  Hence, it is often recommended to that winemakers measure and identify their starting concentration of YAN and supplement accordingly.

Nitrogen Supplementation

Nitrogen (nutrient) management and supplementation is not uncommon during primary fermentation as nutrients are an important component of yeast cell growth and metabolism.  In the yeast cell, nitrogen is a required nutrient in the synthesis of amino acids and to build proteins that are used in the yeast cell walls and organelles, as discussed above.  Without protein development, the yeast cell cannot live.

Winemakers can supplement their fermentations with nitrogen by adding nutrient supplements in the form of:

  • Hydration nutrients (e.g., GoFerm, Nutriferm)
  • Complex nutrients (e.g., Fermaid K, Nutriferm)
  • Diammonium phosphate (DAP)

DAP is considered an inorganic form of nitrogen, while the complex nutrients may contain additional organic yeast components that contribute organic forms of nitrogen.  Recall, above, that the inorganic form of nitrogen is more readily consumed by yeast, and it can be easily absorbed by yeast cells even as alcohol concentrations rise during primary fermentation.  Amino acids, on the other hand, require energy expenditure in order to be brought into the cell through transport proteins located on the cell membrane.  The presence of both alcohol and ammonium ions inhibit the transfer of amino acids from the juice into the yeast cell (Santos, 2014).  Therefore, it is often recommended to avoid the addition DAP or products that contain DAP (i.e., Fermaid K, Nutriferm Advance) at inoculation and until after yeasts have the opportunity to best absorb amino acids.

Starting YAN Concentrations

Nonetheless, nutrient supplementation strategies are often based on starting YAN concentrations in the fruit.  Due to the regular variability of YAN concentrations, winemakers are encouraged to measure YAN for each lot of grapes every year.  This is often problematic for winemakers whom do not have the time to run the appropriate analyses associated with YAN or the financial resources to send samples to an analytical lab.  Such challenges force many winemakers into a situation in which all fermentation lots are treated with the same repeated nutrient supplementation regardless of the starting concentration of YAN.

In previous Extension workshops, research from Cornell University on Riesling wine grapes found that they could accurately predict the harvest YAN when good field samples were taken within 2 weeks from harvest (Nisbet et al., 2013).  In 2016, Cornell released a second publication that focused on YAN prediction models for Cabernet Franc, Chardonnay, Merlot, Noiret, Pinot Noir, Riesling, and Traminette.  While the prediction models were not recommended for regions outside of the Finger Lakes (where the data was sourced from for this study), they found that in some cases, YAN data could be obtained within 5 weeks of harvest (Nisbet et al., 2014).  This extra flexibility in time can aid in obtaining accurate YAN results before the grapes reach the crush pad, which ultimately helps winemakers prepare for nutrient supplementation before the start of fermentation.

Until further research can provide predictive modeling for other wine regions, it is generally accepted that winemakers should measure YAN at or as close to harvest as possible.

YAN can be measured using the following the analytical procedures:

  • Enzymatic methods for both primary amino acids and ammonium.
  • Probe for ammonium ions.
  • Formol titration

While the Formol titration is often preferred by many small wineries due to the lower start-up investment, the use of formaldehyde, a known carcinogen and lung irritant, in this protocol does require some consideration for laboratory safety.  Additionally, the proper disposal of formaldehyde, a hazardous substance, can be an issue for many wineries.

Enzymatic methods by spectrophotometer definitely require a bit of experience in order to become more efficient in their use, which can be problematic for those operations that find measuring YAN too timely.  Additionally, enzymatic kits have to be purchased fresh and have a small shelf life.  The advantage of investing in a spectrophotometer, however, is that other enzymatic kits can be purchased to measure additional wine components including residual sugar, malic acid, and acetic acid.

Nonetheless, measuring YAN should be a consideration for wineries that struggle with hydrogen sulfide aromas by the end of primary fermentation.  It is through the starting numerical value that winemakers can better manage and adjust nutrient supplementation strategies to help minimize the reoccurrence of hydrogen sulfide at the end of fermentation.

Nutrient availability during primary fermentation is only one potential contributor to hydrogen sulfide formation in wines.  In the next blog post, we’ll explore other potential causes of hydrogen sulfide formation and how to best mediate the problem when it exists.

 

References

Eschenbruch. R. 1974. Sulfite and sulfide formation during winemaking – a review. Am. J. Enol. Vitic. 25(3): 157-161.

Bell, S.-J. and P.A. Henschke. 2005. Implications of nitrogen nutrition for grapes, fermentation and wine. Aust. J. Grape and Wine Res. 11:242-295.

Mansfield, A.K. Are you feeding your yeast?: The importance of YAN in healthy fermentation. Webinar. Feb. 2014.

Nisbet, M.A., T.E. Martinson, and A.K. Mansfield. 2013. Preharvest prediction of yeast assimilable nitrogen in Finger Lakes Riesling using linear and multivariate modeling. Am. J. Enol. Vitic. 64(4): 485-494.

Nisbet, M.A., T.E. Martinson, and A.K. Mansfield. 2014. Accumulation and prediction of yeast assimilible nitrogen in New York winegrape cultivars. Am. J. Enol. Vitic. 65(3): 325-332.

Santos, J. Getting Ready for Harvest: Yeast Nutritional Needs. Workshop Seminar. July 2014.

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Meet Pierre Wahl – Winemaker Rijks

Q. When and where were you born ? 

“Port Elizabeth on 24th March 1974.”

Q. Rather  unique place for a winemaker to come from ?

“Yea, I know that Bruwer Raats was born there but did his schooling in Bloemfontein.”

Q. What made you go for wine ?

“I always had a passion for nature and my Dad was in citrus so as a youngster I spent hours in orchards. However I wanted something more creative and wine seem to tick all the boxes. Outdoors, creative, natural and potential to offer lots of satisfaction”

Q. Where did you study and what qualifications did you have ?

“I received my diploma in Oenology in 1995 at Elsenberg Agricultural College .”

Q. Do you consider your approach to winemaking to be different to others ?

“I work on gut feel most of the time. I think this is something you master  only with experience. I want to show purity of fruit and bring out the terroir in the wines .”

Q. How involved do you get in the vineyard ?

“I get involved from budburst until harvest.  The viticultural guys  and I work together as a team to achieve the quality of grapes needed to achieve the quality of wines we make.”

Q. Do you have any varieties you prefer to work with ?

He answers with a smile:  “I love Chenin Blanc and Pinotage. Chenin because it is such a forgiving cultivar and Pinotage  because it understands me! You want the variety to understand the winemakers thoughts, not the other way round. That’s why winemaking starts in the vineyard.”

Q. Have you been influenced by any particular winemaker  or by a region ?

“I have never worked under any winemaker so created my own signature of wine styles. If I think back to a time that may have influenced my way of thinking, it must have been the two weeks I spent in Napa and Sonoma in 1999. I learnt how they go about picking at tannin ripeness and also how they made Pinot Noir. A lot of that I implement today in making my Pinotage.”

Q. What would you consider your greatest achievement as a winemaker ?

“No doubt that being accepted as a Cape Winemakers Guild member.  To be seen as one of the elite  winemakers in my country. For me it is more important to be consistent than being remembered for one trophy or a couple of double gold medals.”

Q. That is quite a statement from a guy who has just won the Diner’s Club Winemaker of the Year for 2016 ?

“Well, that is undoubtedly a great honour but being invited by your peers to be a member of the Guild is still the greatest.”

Q. What “secrets” have you “developed” that make your wines different to others ?

Again with that infectious smile “If I tell you it won’t be a secret anymore ! No secrets but basically just making wine with minimum interference in the cellar. Minimum fining and filtration. pH management is of utmost importance, especially with pinotage.”

Q. How important is modern winemaking equipment in your winemaking ?

“ Not really important. To have whatever equipment in working condition and to have all equipment sterilised after working is more important. And, of course, adequate cooling for cooler fermentations.”

Q. You say you never really worked under a winemaker. How come ?

“When I graduated in 1995 I commenced my winemaking career at Niel Joubert Wines in Paarl as Chief Winemaker !”

Q And then ?

“I accepted a challenging appointment in 1998 at Moreson in Franschhoek. It was here I that I gained most of my winemaking experience and began to travel the world.”

Q. When did you move to Rijks ?

“After experience in Paarl and Franschhoek I headed for Rijks in Tulbagh and into totally unknown territory in 2002.”

Q. You have helped other cellars ?

“Yes , during the past 14 years at Rijk’s I have consulted for various wineries in the Tulbagh area in an effort to improve all the wines of the area.  During this time I won  numerous awards  and then in 2007  I went to the Northern Rhone in France  to work for a vintage to gain a different perspective.” He carried on “Despite the French visit I believe to this day that my wines have a unique signature which is a direct result of never being an assistant and working under prescribed winemaking conditions.”

Q. To wrap up ?”

“When Rijks started out in 2000 we made wine from10 different cultivars. In 2006 we took out all but Chenin Blanc, Pinotage and Shiraz. We believe that by doing that we can keep on being consistent in making quality wines and become one of the icons in the South African wine industry.”

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