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New World Wine Maker Blog - Technical Articles

Oral Bacteria Shown to Produce Aromatic Volatiles from Glycosidic Precursors: Implications for Perception of Aromas and Flavors in Wine

Article by Becca Yeamans of ‘The Academic Wino’

How a wine tastes is dependent upon many factors, including (but not limited to) the variety, the vintage, where the grapes are grown (soil, climate, etc), as well as the viticultural and winemaking techniques employed during processing.  The compounds responsible for how wine tastes are known as free volatile compounds, as well as aromatic precursors, the latter of which are present at much higher concentrations.  Non-volatile sugar-bound conjugates (a.k.a. “glycosidic compounds) have been well studied and have been shown to be released over time during wine aging or by using specific winemaking techniques. Specific glycosidic compounds known to be released over time, thus affecting how a wine develops and tastes, include terpenes, C13 norisoprenoids, benzenic derivatives, volatile phenols, and C6 compounds.  All of these glycosidic compounds have low odor thresholds, thus requiring very little to elicit a sensory response.

While wine aromatics have been extensively studied, it is not well known exactly how compounds responsible for aromatic character in wines interact with the physiological make-up of the human mouth. In addition to environmental and chemical sources, it is possible that the perception of different wine aromas can be altered by physiological factors like mouth temperature, saliva composition, or the oral microbial community present in each individuals’ mouths. Studies focusing on onions, bell peppers, and grapes found that the microbial community in the human mouth hydrolyzed odorless compounds into their corresponding volatile aromatic compounds, giving reason to believe something similar could potentially happen with wine.  Perhaps the microbiota living in the human mouth can hydrolyze these odorless precursors and convert them into their corresponding aromatic compounds, just like it’s been shown with other foods.

A 2015 study in the journal Food Chemistry aimed to evaluate whether or not human oral microbiota can convert odorless aromatic precursor compounds in wine into their corresponding aromatic glycosidic compounds. The results could potentially have a profound impact on our understanding of how we taste and evaluate wines.

Brief Methods

Experiment 1: In vitro

For the in vitro experiments, three microbes commonly found in the human mouth were cultured on sterile growth media.  Different concentrations of grape extract were added to the microbes, with bacteria growth measured after 24-48 hours, depending upon the specific microbe.  From these growth measurements, inhibition of growth was also calculated.

Experiment 2: Ex vivo:

For the ex vivo experiments, fresh saliva was collected from three volunteers (ages 28-31).  Prior to collection, the volunteers had not taken any antibiotics or other medications, and were non-smokers.  Volunteers did not consume any food or beverages within two hours of the saliva collection time.

The saliva from each volunteer was divided up into four different treatments: 1) fresh saliva in an aerobic culture, 2) fresh saliva in an anaerobic culture, 3) sterilized saliva (pasteurized), and 4) non-enzymatic saliva (heated).  Microbe counts were measured after 24-48 hours at 37oC.

Grape extract (comparable to 40g of grapes) was added to the saliva cultures and bacterial growth was monitored.  Glycoside hydrolysis by the microbes was measured by monitoring and analyzing the volatile compounds released after four time periods (0hr, 2hr, 24hr, and 72hr).

To test the ability of human oral microbiota to hydrolyze glycosidic compounds in general, a standard solution of octyl-β-D-glucopyranoside was cultured with the saliva samples and monitored over time.

Selected Results

  • No human oral microbiota was found in the sterile or non-enzymatic saliva treatments (as expected).
  • Adding octyl-β-D-glucopyranoside to human saliva resulted in hydrolysis and the release of the volatile compound aglycone.
  • None of the oral microbes were inhibited by the glycosidic extract.
  • Every oral microbe tested was able to hydrolyze the glycosidic compounds in the grape extract, resulting in the release of terpenes, benzenic derivatives, and C6 alcohols.
    • Note: Many of these compounds can be associated with various aromatic characteristics in wines (e.g. terpenes can produce flowery or citrus aromas and certain benzenic compounds such as β-phenylethanol can produce rose aromas.)
  • While some aromatic compounds were found as a result of the oral microbes hydrolyzing the glycosidic compounds in the grape extract, other common compounds were not present (i.e. C-13 norisoprenoids, vanillins, and volatile phenols).
  • The ability to hydrolyze and the resulting aromatic compounds produced from this hydrolysis depended upon the type of oral microbe present.
    • A. naeslundii was the producer of the most linalool and its oxides, which is associated with floral notes in grapes and wine.
    • E. faecalis, A. naeslundii, and S. mutans produced the most aroma-causing aglycones.
    • G. adiascens, V. dispar, and F. nucleatum produced the fewest aroma-causing aglycones.
      • NOTE: Some of these bacteria have difficulty growing in culture media, so it isn’t clear whether their lack of glycosidic hydrolysis is real or a function of being unable to grow under the experimental conditions.
    • There were significant differences between the three volunteers in terms of the species make up of their mouths. No person had the same species in the same amounts.
    • While the microbial counts were statistically the same for each volunteer, the aromatic aglycones produced were significantly different, which is likely due to the different species present in each individual’s mouth.


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Addition of exogenous tannins during winemaking

By Wineland Magazine

Tannins are especially associated with their astringent taste. The perception of astringency is the result of the reaction between tannins and the proteins in saliva. Tannic red wines form a precipitate in the mouth, but due to the lack of tannins in white wines the precipitation will not occur. The tannins occurring in wine, can originate from the grapes or the stems, but they can also be added as exogenous tannins during winemaking.

Tannins are phenolic compounds which occur in many plants. The ability of tannins to react with proteins, is important during winemaking, because enzymatic reactions are inhibited by them and they can also contribute positively to the protein stability of wine. As result of the structural characteristics, they can also contribute to the oxidative and colour stability of wines. They also play an important role in the sensory characteristics of wine. A variety of tannin products became available over recent years and it is important that winemakers are informed about the characteristics of the different products. Available tannins include hydrolysable tannins (ellagic and gallic basis) and condensed tannins (catechinic basis). Ellagic tannins originate from oak and chestnut wood, gallic tannins from oak wood wounds, tara and myrobalan exotic trees, while catechinic tannins originate from grapes and quebracho trees. The traditional tannins for winemaking (exogenous tannins) are extracted from the nutgalls of certain trees ….


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Starting your fermentation right: nutrient supplementation

By: Denise M. Gardner

Based on the number of questions I have received this year about yeast assimilable nitrogen (YAN), it looks like more winemakers are taking it upon themselves to measure YAN on pre-harvested fruit or on incoming juice.  This can be a great step in improving wine quality!  Measuring YAN offers several benefits to winemakers, including:

  • Minimizing the incidence of hydrogen sulfide development in the wine.
  • Enhancing varietal character by producing cleaner wines with adequate and specific nitrogen supplementation throughout primary fermentation.
  • Minimizing excessive nutrient supplementations, in which left-over nitrogen (after primary fermentation) may act as nutrient sources for spoilage yeast and bacteria.
  • Reducing unnecessary work for your employees by minimizing problematic production situations (e., fixing wines with hydrogen sulfide). Such actions could have economic benefit (i.e., reduction in supplies, reduction in time/labor)

Below is a quick refresher for those that may have questions about YAN.

The Basics

  • YAN = Ammonia Concentration + Primary Amino Acid Concentration given in the units: mg N/L (read: milligrams of nitrogen per liter)
  • Most suppliers (g., Lallemand, Scott Labs, Enartis, Laffort) will providerecommendations on what to add in low, medium, or high YAN situations. Make sure you consult your handbooks or supplier websites for their product-specific recommendations.
  • At the start of fermentation, you want to avoid adding diammonium phosphate (DAP) or complex nutrient additions that contain DAP (g.,Fermaid K) when hydrating your yeast. Use hydration-specific products like GoFerm or Nutriferm Energy.
  • Most suppliers recommend making 2 additional nitrogen supplementation additions during primary fermentation and after inoculation. If only making 1 nutrient addition after inoculation is practical for you, add your nitrogen supplement at about 1/3 of the way through primary fermentation (e., 1/3 drop in sugar depletion).

Yeast hydration nutrients are an important component of re-hydrating freeze dried yeast. Winemakers should make sure to avoid DAP additions at this stage. Inoculation photo by Denise M. Gardner

Yeast hydration nutrients are an important component of re-hydrating freeze dried yeast. Winemakers should make sure to avoid DAP additions at this stage. Inoculation photo by Denise M. Gardner

A Review: Why to not add DAP at yeast hydration/inoculation

YAN is composed of inorganic (ammonium ion) and organic (primary amino acid) nitrogen components.  Amino acids are brought into the yeast cell through transport across the cell membrane.  The presence of alcohol and ammonium ions (i.e., DAP) inhibit amino acids from being brought into the cell.  This is why winemakers are advised NOT to add DAP at inoculation or at the beginning of fermentation, as yeast can actively absorb organic nitrogen in the juice (aqueous) environment.

Once alcohol concentrations begin to increase, as a result of primary fermentation progression, transport of amino acids from the wine into the yeast cell will be inhibited.  Therefore, the primary source of nitrogen will then come from inorganic sources, such as DAP.  A more thorough summary of how nitrogen is utilized by yeast can be found at the following pages:

In general, winemakers can select from three different kinds of nitrogen-based products to add during fermentation:

  • Hydration Nutrients (g., GoFerm, Nutriferm Arom, etc.)
  • Complex Nutrients (g., Fermaid K, Nutiferm Advance, Superfood, etc.)
  • Diammonium Phosphate or DAP

Need more direction on when to add which nutrients?  Look no further!  We have a practical fact sheet waiting for you at the Penn State Extension website.  As a general rule of thumb, remember to make your YAN additions based on the volume of wine that you are treating.  For whites, roses, and some reds (e.g., hot pressed Concords), YAN additions will be made based on the juice volume.  For most other reds, YAN additions should be based on the must volume.

Dealing with Low YAN Fermentations

Low YAN fermentations are defined as having less than 125 mg N/L in the must/juice at the start of fermentation.  In these situations, it’s essential for the winemaker to provide enough “food” for all of the yeast during primary fermentation.

Depending on the reference, most scientific literature will recommend adding up to 200 – 250 mg N/L.  This concentration of nitrogen should provide adequate supplementation for the entire biomass throughout the duration of fermentation.

Be aware that if you are using a HIGH NITROGEN DEMANDING YEASTstrain (e.g., BM45, ICV-GRE, among others), however, you may be required to add additional supplementation.  If you are starting with a low YAN situation and would like to use a high nitrogen requiring yeast strain, we recommend contacting your supplier for specific nutrient addition instructions.

Dealing with High YAN Fermentations

Many suppliers define a high YAN fermentation anywhere above 250 mg N/L.  However, some YANs from Pennsylvania grown grapes are at concentrations greater than 400 mg N/L!  This YAN concentration can create a challenging fermentation and processing situation for the winemaker.

Due to the excess amount of available nutrients in these situations, yeast can grow and reproduce quickly, which often leads to very rapid and hot fermentations.  The speed and temperature of fermentation can affect the aromatics and quality of the wine (i.e., fast fermentations often lead tosimpler aroma and flavor profiles).  This may not be an issue with some fermentations, but for many white, rosé, or fruit (other than grapes)-based fermentations, aromatic retention should be a priority by the winemaker.

Higher concentrations of the inorganic component of YAN can lead to a high initial biomass of yeast.  This is a problem because the rapid increase in yeast populations can lead to starvation by the majority of the yeast by mid- to late-fermentation, especially if there is not enough nutrition to fulfill all of the yeast during fermentation.  Yeast starvation leads to yeast stress, and one of the stress responses by yeast is the production and release of hydrogen sulfide.  Therefore, having a high YAN at the start of fermentation may cause hydrogen sulfide issues in the wine by the time fermentation is complete.

What should you do if you have a high YAN?

  • First, always reference your supplier recommendations. Each year, suppliers publish current guidelines for how and when to add various nutrients during fermentation.
  • I’ve found it helpful to document trends in high YAN fermentations. For example, if you notice that a variety with a routine high YAN year-to-year, note the years where hydrogen sulfide becomes an issue.  Good record keeping during primary fermentation can remind you what you did during production.  You may need to alter these practices for the following vintage year.
  • If all else fails, refer to Penn State’s Wine Made Easy Fact Sheet on Nutrient Supplementation during Primary Fermentation

Additionally, high YAN concentrations may leave some nitrogen left over by the end of fermentation and could remain in suspension in the finished wine.  This excess “food” could be available for other microorganisms (like acetic acid bacteria or Brettanomyces), which could potentially lead to spoilage problems if the wine is not properly stabilized.  In high YAN situations, it is especially important to ensure that the wine is stabilized with adequate sulfur dioxide additions and by minimizing other risk factors (e.g.,temperature control of the wine).

It is also be researched that high starting YAN values may led to increased concentrations of ethyl carbamate. Ethyl carbamate is naturally produced by fermentation, but it is a mild carcinogenic compound.  For this reason, many countries have legal maximum ethyl carbamate concentrations in wine.  For more information on ethyl carbamate, please see this guide published by UC Davis or this Extension report from Virginia Tech’s Enology Grape Chemistry Group.

Our Understanding of YAN is still Developing

Every year, YAN is a big topic of conversation amongst industry suppliers and academics.  Current investigations include:

  • The impact of primary amino acid uptake as a function of temperature, reported by Cornell University and discussed at the 2016 American Society of Enology and Viticulture (ASEV) – Eastern Section conference (Missouri) in a presentation by Scott Labs.
  • YAN recommendations for hybrid varieties produced in the Mid-Atlantic, a topic discussed by Dr. Amanda Stewart from Virginia Tech University during the 2014 PA Wine Marketing & Research Board Symposium. This includes looking at other nutritional factors beyond nitrogen supplementation, which was also discussed at the ASEV-Eastern Conference in 2016 by Scott Labs.
  • Optimal nutritional strategies for challenging fermentations, which is often reported in supplier catalogs like the Scott Labs 2016 Handbook
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Making red wine from fruit high in potassium

While the issue is quite challenging to address in an established vineyard, processing grapes from high pH fruit, or fruit that has the potential to create a high pH wine (>3.70), as a result of high potassium (K or K+) concentrations is also a challenge for the winemaker.

Concentrations of 22 – 32 mmol/L K+ (860 – 1,279 mg/L K+) are considered “normal” ranges for wine grapes (Somers 1977, cited byMpelasoka et al. 2003), while ranges in the 27 – 71 mmol/L K+ (1,056 – 2,776 mg/L K+) are considered “high” (Somers 1975, cited by Mpelasoka et al. 2003) and may lead to potential winemaking problems.  Grapes and juice that come in with high levels of potassium can lead to a series of difficulties for winemakers including:

  1. High potassium concentrations can cause large increases in pH during primary and malolactic fermentations, which drive the finished wine into a high pH (>3.70) range.
  2. Color hue, intensity, and stability of red wines can be negatively affected.
  3. High pH wines produced throughout the Mid-Atlantic may lead to negative perceptions associated with taste and mouthfeel of both white and red wines.
  4. As pH is a big driver in wine stability, higher pH’s will have impacts on the microbial stability (both in terms of microflora and inhibition of growth), sulfur dioxide levels and efficacy, color stability of red and rosé wines, stability of tartaric acid, and protein stability.
  5. Higher pH’s leads to an increase in oxidative potential, which may cause premature oxidation for young wines.

Figure 1: Color instability problems associated with 2013 Chambourcin wines. The sample on the right is from our Biglerville (Adams County) research site, which we later discovered is associated with high potassium in the fruit and wine. The sample on the left is from our North East (Erie County) site, which is more representative of the color hue and intensity associated with Pennsylvania-produced Chambourcin. Photo by: Denise M. Gardner
Figure 1: Color instability problems associated with 2013 Chambourcin wines. The sample on the right is from our Biglerville (Adams County) research site, which we later discovered is associated with high potassium in the fruit and wine. The sample on the left is from our North East (Erie County) site, which is more representative of the color hue and intensity associated with Pennsylvania-produced Chambourcin. Photo by: Denise M. Gardner
Although the articles listed below are not peer reviewed, previous attention has been given to high potassium winemaking issues.  Some of the content relayed in these 2 articles will not be discussed in this blog post:

  1. Really, really high pH remedies from Wines & Vines: a discussion on potassium concentrations increasing the pH of wine and utilization of ion exchange if the problem is not prevented
  2. High pH and high potassium wines produced in Colorado from White Hall Vineyards: Includes a discussion pertaining to malic acid concentration in high pH fruit. (Author’s note: This article discusses adding tartaric acid prior to fermentation, but not exceeding a TA of 8.0 g/L while hitting a pH of 3.60, ideally.  While the practice of analytically checking your additions is encouraged, and will be discussed throughout the duration of this blog post, please note that sampling procedures and tartaric acid settling time will greatly influence your juice TA after tartaric acid addition.)

A problem for winemakers is that unless potassium uptake and management is addressed in the vineyard, they will likely have to deal with having high potassium-based fruit for several years.  However, winemakers are encouraged to work with their growers, as this is a relatively newer viticultural issue that the Mid-Atlantic is facing, and it may take several years to stabilize before results are seen in incoming fruit from the vineyard.

In regions like Australia, which frequently experience high K concentrations in their fruit and wines, making tartaric acid additions to the juice, pre-fermentation is often recommended to lower the pH of the must/juice and precipitate some of the potassium as it binds to tartaric acid during primary fermentation.  While a 2 g/L of tartaric acid addition to must/juice is a common recommendation for acidulating musts, it may not be enough in order to alter the effects of high potassium concentrations in the fruit.  In these cases, a higher addition rate of tartaric acid, such as 4 – 6 g/L of tartaric acid, may not be out of the question.

It should be noted that must/juice acidification will have chemical and sensory implications to the finished wine.   If the winemaker is aiming to produce a specific style, making large tartaric acid additions pre-fermentation may not be conducive with the desired and finished wine style.  However, when dealing with high potassium issues, and hence, high pH issues, larger tartaric acid additions pre-fermentation seem to be helpful in stabilizing red wine color and improving the flavor of red wines.  For those that would prefer a lower TA (<6.0 g/L tartaric acid), deacidification following malolactic fermentation of red wines is recommended. While there are limitations on deacidification practices, including the degree to which a winemaker can deacidify, this action may help improve mouthfeel and decrease the perception of acidity (sourness) in the finished wine.

Wine Trials at PSU

During the 2015 harvest, our research team confirmed that a couple of our varieties that annually had high pH problems came from sites or locations with high potassium retention in the fruit.  This did not necessarily correlate with high potassium concentrations in the soil.

From our Biglerville (Adams County) research vineyard, our Merlot contained 1,682 mg/L K+ and Cabernet Sauvignon contained 1,668 mg/L K+ in the 2015 growing season.  Both samples were taken from the must and analyzed by atomic absorption analysis at Enartis USA – Vinquiry.

Based on previous research from Somers (1975), both musts were considered high in potassium.  The following (Tables 1 and 2) show additional harvest parameters for our Merlot and Cabernet Sauvignon musts in the 2015 season.

As previous yeast strain selection, malolactic bacteria selection, and standard (2 g/L) tartaric acid addition trials did not seem to improve color stability or flavor of the wines in past harvest years, we took the approach at comparing 3 different tartaric acid addition rates (2 g/L, 4 g/L, and 6 g/L) to the Merlot pre-fermentation and two rates (4 g/L and 5 g/L) of tartaric acid to the Cabernet Sauvignon based on previous recommendations made in the Australian literature.  There were fewer treatments on the Cabernet Sauvignon due to decreased yields in 2015.  Please note that these treatments were not replicated and, therefore, we have not provided any statistical parameters.

For the Merlot, the 2 g/L addition rate of tartaric acid acted as the “control,” as previous years indicated no differences in pH or TA by the end of MLF between wines fermented without tartaric acid added pre-fermentation and a 2 g/L addition treatment.  There was no designated “control” for the Cabernet Sauvignon fermentations.

Table 1: 2015 Pennsylvania Merlot must chemistries in 2015; pH and titratable acidity (TA) were adjusted pre-fermentation (i.e., pre-inoculation) and given at least 3 hours of settling time before inoculation with ICV-GRE yeast


Figure 2: Pre-Fermentation tartaric acid addition (2 g/L, 4 g/L, and 6 g/L) trials to 2015 Merlot must. This image shows the wines during malolactic fermentation. Photo by: Denise M. Gardner

Figure 2: Pre-Fermentation tartaric acid addition (2 g/L, 4 g/L, and 6 g/L) trials to 2015 Merlot must. This image shows the wines during malolactic fermentation. Photo by: Denise M. Gardner

Table 2: 2015 Pennsylvania Cabernet Sauvignon must chemistries in 2015; pH and titratable acidity (TA) were adjusted pre-fermentation (i.e., pre-inoculation) and given at least 3 hours of settling time before inoculation with ICV-GRE yeast


The following table (Table 3) shows the differences in pH and TA for each pre-fermentation tartaric acid addition treatment following primary fermentation and MLF for our Merlot wines in the 2015 vintage year.

Table 3: 2015 Merlot wine chemistries (pH, TA, volatile acidity, and alcohol concentration) post-primary fermentation and post-MLF (fermentation trials were not conducted in replicate)


Trends were similar in the 2015 Cabernet Sauvignon wines, as shown inTable 4.

Table 4: 2015 Cabernet Sauvignon wine chemistries (pH, TA, volatile acidity, and alcohol concentration) post-primary fermentation and post-MLF (fermentation trials were not conducted in replicate)


While we do not quite have an explanation for the rise in TA from post-primary fermentation to post-MLF in the 5 g/L tartaric acid addition treatment in the Cabernet Sauvignon wine, we did note that post-bottling, most of the TA’s slightly decreased across all treatments in both Merlot and Cabernet Sauvignon wines.  A decrease in TA would reduce the perception of sourness even further.  This decrease was likely due to better removal of dissolved carbon dioxide within the wines due to the fact the wines had been moved (i.e., racked, transferred and bottled) more routinely prior to bottling.

The treatments within a varietal were also different sensorially, although this was not quantified.  For example, in the Merlot, the first difference noted was the color.   The Merlot wine that had been treated with 6 g/L tartaric acid had the most vibrant and red-hued color.  The Merlot with a 2 g/L tartaric acid addition had a stronger purple-blue hue.  We did not quantify these differences analytically.  In terms of taste, the 6 g/L tartaric acid treatment had more noticeable and perceptible sourness, but many that tasted the wine agreed that it could be manipulated with some deacidification trials.  The 2 g/L tartaric acid addition treatment tasted flat, had burnt rubber-like flavors and was relatively unappealing.  It did not represent a typical flavor profile associated with Merlot.  The 4 g/L and 6 g/L tartaric acid addition treatments had more noticeable red fruit flavors and less earthy characters.

What should you do in the winery if you think you have high pH wines as a result of high potassium concentrations in your grapes?

  1. Find out if potassium concentrations may be a culprit. Now is the time to find out what you are dealing with.  In a previous blog post, Michela recommended getting petiole samples to determine vine nutrition.  However, you can also test the fruit (must, juice) and the wine for potassium concentrations as well.  We recommend sending your samples to an ISO accredited lab to confirm potassium concentrations in those wines that you believe may be suspect.
  2. Make tartaric acid additions pre-fermentation (pre-inoculation).With very high potassium concentrations, a 4 – 6 g/L addition of tartaric acid pre-fermentation may not be a detriment to wine quality.  However, it is best to know the concentration of potassium you are dealing with before adding up to 6 g/L of tartaric acid pre-fermentation as this can have obvious effects on the wine’s taste and flavor as a finished wine (i.e., make the wine thin or overly sour).
  3. If you are unwilling to test to the potassium concentration, but have a wine with frequent high pH problems during production, use a 4 g/L tartaric acid addition pre-fermentation instead of 2 g/L. The 4 g/L tartaric acid addition rate is a relative “good guess” zone.  Depending on the potassium concentration in your wine, this will either work or it will not work.  If you refer to Tables 3 and 4, we can see that the 4 g/L addition rate was not a bad choice for the Merlot as it resulted in an ideal pH (3.63) and a workable TA (5.96 g/L), but for the Cabernet Sauvignon, the wine resulted in a high pH (>3.70) and a high TA (>6.00 g/L).  This high pH, high TA situation can make the wine both difficult to manage for stability reasons (e.g., making applicable sulfur dioxide additions) while retaining a relatively sour taste.
  4. White wines can also suffer from high potassium. While the content of this blog post has focused on red wines and the effects of color stability and flavor associated with higher pH’s and high potassium concentrations, white wines can also be affected by high potassium concentrations.  In most instances, high potassium can relate to a high pH in the finished wine, which makes the white wine difficult to stabilize or add proper sulfur dioxide additions in order to minimize microbial risk.  Also, many of these wines have low TA’s, giving the white wine a fat, round, or flat mouthfeel (dependent on the variety).  Stylistically, this may not be undesirable, but it is a sensory component that winemakers should be aware of that may occur in these chemical situations.
  5. Alter your pH and sourness post-malolactic fermentation. If the wine tastes too sour for your preference, the time to de-acidify is post-MLF with these wines.  By that time, the color pigments will be fully extracted from the red skins and the flavors will be as optimal as they can be for the variety.  For our Merlot wines, we made additions using (ironically) potassium carbonate, but calcium carbonate can also be used to de-acidify wines.  I usually recommend Patrick Iland’s book for practical information on how to make de-acidification trials in wine. WSU also provides appropriate options and instructions for deacidifying wine.



Mpelasoka, B.S., D.P. Schachtman, M.T. Treeby, and M.R. Thomas. (2003) A review of potassium nutrition in grapevines with special emphasis on berry accumulation. Australian Journal of Grape and Wine Research. 9:154-168.

Somers, T.C. (1975) In search of quality for red wines. Food Technology in Australia. 27:49-56.

Somers, T.C. (1977) A connection between potassium levels in the harvest and relative quality in Australian red wines. Australian Wine, Brewing and Spirit Review. 24:32-34.

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Pinot Grigio with Personality

By Noelle Allen. Noelle is currently working through WSET Level 4 and starting her thesis at the Wine School of Philadelphia, where she also teaches private Wine 101 classes. 

We all know that the same grapes produce very different wines, depending on a number of factors including country of origin, vintage, weather and growing conditions, and production techniques, to name a few. Grape taste spectrums are mind blowing.

Early in my wine career, I generally thought of Pinot Grigio / Pinot Gris (they’re the same grape; ‘Grigio’ is Italian and ‘Gris’ is French and they both mean ‘gray’ — pinot means pinecone — but the two styles can be very different) as maybe zesty and florally, acidic and flinty, but overall, just, really… unexciting. Flat and thin. I had come to that conclusion even before hearing other wine nerds speak dismissively of the little gray pine cone.

And then I was presented with a bottle of Pinot Grigio by a wonderful producer in northeast Italy. I had no idea that I was about to go on a little journey.

The wine tumbled into the glass, and it even sounded good. No glugs or disjointment. It simply swirled around then settled in like a pro. It was a clear and bright lemon yellow. The aroma made itself known almost immediately, and what an amazing combination. Banana, pear, and almond. The inhale was gorgeous.

On the tongue, the wine was full bodied and citrusy, – full bodied, hmmm – and had a long, minerally finish. The weight and balance were absolutely splendid. To put it in the vernacular of my maturity level during those times, the wine was amazeballs. Any notions of flatness or thinness drained away, just like the wine from my glass and eventually the bottle, as my friends and I happily talked and sipped.

Afterward, intrigued, I visited the producer’s website. After digging a bit, and even ambitiously translating Italian into English, I came across the technical description that included a ‘Dry Extract’ count. Specific to this wine, it was 19.8 grams per liter. What is dry extract? And is 19.8 g/l a lot or a little or normal?

Quite simply, dry extracts (DE) are the powdery solids that are left over after taking away the sugar, water, ethanol, and acid from wine, all of which are major components. (To separate out these items, you will likely need a centrifuge, so good luck with that if you try it on your own.) Dry extracts are made up of minerals and trace elements such as potassium, phosphorus, iron, calcium, and magnesium; all of which come from the soil in which the vine grows and affect the biology of the plant and its resulting fruit.

The grapes for this particular wine had grown in loam, a soil type made of silt, clay, and sand, all of which are collections of minerals that produce and capture different quantities of given elements. While some argue that minerals do not have aromas, given that they are inorganic and non-volatile, others say that they do give off definite sensory perceptions, whether through weight, aroma, flavor, texture, or some synergistic


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Managing skin contact during red wine fermentation

by Charl Theron - 

Skin contact is a basic requirement during red winemaking, unless other techniques like thermovinification are used to extract colour from the skins. Different factors like the duration of skin contact, cultivar, fermentation temperature and the ways in which skin contact is applied, play an important role in the result that is eventually obtained.

During red wine fermentation the formed carbon dioxide carries the skins to the surface of the container. This is known as the cap and the management of it will determine the extraction of the colour and tannins from the skins, which will eventually determine the colour and taste of the wine. The correct management of the cap will also influence the potential development of detrimental micro-organisms, determine an equal temperature in the cap and fermenting juice, promote the alcoholic fermentation by the addition of air (oxygen) and prevent the drying out of the skins.

The handling of grapes after destemming plays an important role in the onset of alcoholic fermentation. Vigorous crushing will cause the extraction of more astringent and bitter tannins, because the concentration of insoluble solids is increased by it. Extraction methods causing the damage or breaking of the seeds, must also be prevented. Minimal crushing of the berries prolongs the alcoholic fermentation, decreases phenolic extraction and also increases the fruity flavours of the wine. The volatile flavour compounds in the fermenting juice can also be lost at higher temperatures, because they are removed by the formed carbon dioxide and volatilise more at higher fermentation temperatures. In the case of whole bunch fermentations, some of the flavours are retained in the unbroken berries and only released into the wine after the vigorous fermentation.

The management of skin fermentation during red winemaking differs between red cultivars and techniques like pumpovers, punch-downs or cold soaking are consequently applied differently. In case of Pinot noir, grapes are only destemmed or whole bunch fermentation is used. Although the same principles are sometimes applied with Cabernet Sauvignon, berries are sometimes only broken by loose crusher rollers. The application of cold soaking is also an important principle decision, which needs to be made for red winemaking. It comprises the cooling of crushed grapes or bunches to such a temperature that the onset of alcoholic fermentation is delayed. The initial colour and flavour extraction from the grapes is consequently in an alcohol-free medium and coarse tannins, bitterness or excessive astringency are prevented. Tannins are then eventually only extracted afterwards by the formed alcohol. If spontaneous alcoholic fermentation is also preferred, cold soaking will create an opportunity for such yeasts to initiate the fermentation. In case of Pinot noir, cold soaking is advantageous if it is maintained at 10°C for four to five days. In order to limit spoilage organisms and excessive oxidation in open fermenters during cold soaking, the surface of the fermenters can be covered with a plastic sheet over the cap and a sulphur dioxide solution and dry ice can be used twice daily to exclude air. During cold soaking the cap must not be punched down more than twice daily. In case of Cabernet Sauvignon and Merlot, where colour extraction is usually sufficient, cold soaking is applied for a shorter period …


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