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New World Wine Maker Blog - Technical Articles

Red Wine Aroma & Oxygen Exposure

Published with the permission of NOMACORC. For further information visit

The influence of oxygen on the sensory characteristics of red wine has been long known, since Louis Pasteur observed that, by exposing red wine to air, the astringent character of young red wine was softened, and the bouquet improved. Nowadays, it is generally accepted that a moderate exposure to oxygen can be beneficial for the overall sensory quality of the wines. Insufficient oxygen can prevent softening of mouthfeel characters and favors excessive accumulation of reductive aroma compounds such as hydrogen sulfide and mercaptans, which can be highly detrimental to wine aroma quality. At the same time, excessive exposure to oxygen can cause oxidative spoilage of the wine, with irreversible loss of fruity and floral varietal aromas. Due to this complex response, proper management of oxygen exposure in the winery remains challenging for the modern wine industry, also in consideration to the intrinsic variability of wine characteristics in relation to origin of grapes, vintage, terroir, and winemaking variables.

In consideration of such variability, the main question that winemakers are facing is how much oxygen a wine needs to achieve optimal expression of its sensory potential without showing reduction or oxidation faults. Between production and consumption, wines spend a considerable portion of their life in bottle, particularly in the case of red wine. Therefore, the selection of appropriate packaging solutions is a crucial aspect of wine quality. In particular, as the different closures available on the market have different oxygen barrier properties (a parameter commonly referred to as OTR), selection of closures with the appropriate OTR become a crucial tool for achieving optimal oxygen exposure.

Depending on the materials and technology adopted to produce the closure, OTR can vary to a large extent. Screw cap closures generally offer very low OTRs, due to the minimum amount of oxygen permeating through the liner. Saratin liner have been reported to have an OTR of 0.03 ±0.03mg O2/year (Crochière 2007), while values are slightly higher (but also more inconsistent) in the case of Saranex liners (0.7 ± 0.7mg O2/year) (Crochière 2007). This range of OTR currently available in screw cap liners is therefore relatively limited. Microagglomerated closures are also reported to have relatively low OTR (Lopes et al. 2007), although permeability can vary depending on the producer, and most producers do not give a specific value or have a range of OTR. Therefore, although there is a general tendency to assume similar performances for all closures in this category, some key characteristics can actually vary significantly among different producers. Likewise, the characteristics of synthetic closures can vary depending on the technology of production. Injection molding offers limited possibilities to adjust OTR, while co-extrusion, allows to obtain a broad range of defined OTR, providing winemakers with OTR values that can fit the specific needs of each wine. For example, Nomacorc co-extruded closures offer an OTR as high as 3.5 mg/year (Smart+), down to 1.1 mg/year for the new Select 300. Conversely, natural cork can have extremely variable OTR, due to the intrinsic structural variability of cork itself (Godden et al. 2001). Measures (Lopes et al. 2005 and 2006) under conditions of humidity simulating those occurring in a wine bottle indicate values between 0.05-3.35 mg of oxygen per year even for a very limited sample size. While this variation appears to be extremely large and possibly linked to specific bottling conditions, most authors have reported large variation in OTR for natural closures (Godden et al. 2005, Limmer 2006). At present, it is not possible to put on the market natural cork closures with defined OTR.
Figure 1 shows an example of the potentialities (and difficulties) of closure selection and OTR management to create wines with different sensory profiles. As it can be observed, by exposing the same wine to increasing amounts of oxygen, it is possible to obtain different aroma profiles after 10 months in the bottle. Low oxygen exposure (0.24 mg O2 and 0.57 mg O2 in ten months in 375 mL bottles) resulted in wines exhibiting low intensity of red fruit aromas, and mainly characterized by animal/reduction notes, a feature often observed in Grenache wines matured under reductive conditions. Conversely, increasing oxygen exposure in ten months to 2.4 mg O2 to 3.57 mg O2 allowed better expression of red fruit and caramel aromas, decreasing at the same time the incidence of the animal note. This could be attributed to lower concentration of the aroma compounds involved in the animal notes, allowing expression of fruity aromas.
If we compare this data with the values present in the literature and/or provided by different closure manufacturers, we can deduce that closures such as screw caps allow oxygen exposures in a range close to the profiles obtained at 0.24 and 0.57 mg O2 (blue and red lines), and therefore are more prone to deliver wines with lower fruit expression and higher animal notes. Although these closures can have more consistent OTR than natural corks, they cover a low range of oxygen exposure, not giving to winemakers the option of choosing the degree of oxygen exposure most appropriate for a specific wine. On the other hand, natural cork closures, due to the intrinsic variability of cork composition, can offer a rather broad range of oxygen exposures (even within the same batch of closures), potentially allowing, over a range of bottles of the same wine, either optimal expression of fruit aromas (green line) or dominance of animal notes (blue and red lines). From this point of view, synthetic co-extruded closures (e.g. Nomacorc) closures offer the advantage of a range of different OTR available, allowing winemakers to choose the degree of oxygen exposure most appropriate for their wines.

In spite of its key role in defining the evolution of wine sensory properties during bottle maturation, the value of OTR declared by the manufacturer is not the only key parameter that defines closure performance. Indeed, in order to deliver wines which will evolve in the bottle as the winemaker intended, the consistency of OTR is essential. In practice, once the winemaker has decided the OTR that is most appropriate for his wine(s), this OTR value will have to be rather consistent within single batches as well as across batches, in order to allow uniform aging. Figure 2 shows the variation in OTR values (expressed as %CV over 5 closures of the same batch) of different inner seal closure types, as obtained from measures with fluorescence method (Diéval et al. 2011). In agreement with Brotto et al. (2010), synthetic closures, in particular extruded products, show very low CV%, reflecting the intrinsic nature of the manufacturing process, which allows a high degree of control of production variables. Higher %CV were observed for natural cork-based products, consistent with the relatively unpredictable characteristics of the raw material. The high variations observed were often due to leakage, which in some cases made the measure impossible (labelled as leakage (L) on all closures).
In conclusion, understanding the influence of OTR on wine sensory quality allows to deliver wines which will reflect, upon consumption, the intention of the winemaker. Availability of a range of OTRs values, as well as consistency of those values across different batches, are key factors to succesful implementation of oxygen management strategies focused on closure selection.
Suggested readings

Brotto, L.; Battidtutta, F.; Tat, L.; Comuzzo, P; Zironi, R. Modified nondestructive colorimetric method to evaluate the variability of oxygen diffusion rate through wine bottle closures. J. Agric. Food Chem., 58, 3567–3572, 2010
 Crochière GK. Measuring oxygen ingress during bottling/storage. Practical Winery & Vineyard, January/February: 74–80. 2007.
Diéval, J-B., Vidal, S., Aagaard, O. Measurement of the oxygen transmission rate of co-extruded wine bottle closures using a luminescence-based technique. Packag. Technol. Sci., 24: 375–385, 2011. 
 Godden, P.; Francis, L.; Field, J.; Gishen, M.; Coulter, A.; Valente, P.; Hoj, P.; Robinson, E Wine bottle closures: physical characteristics and effect on composition and sensory properties of a Semillon wine. 1. Performance up to 20 months post-bottling. Aust. J. Grape Wine Res. 7, 64-105, 2001
Godden, P.; Lattey, K.; Francis, L.; Gishen, M.; Cowey, G.; Holdstock, M.; Robinson, E.; Waters, E.; Skouroumounis, G.; Sefton, M.; Capone, D.; Kwiatkowski, M.; Field, J.; Coulter, A.; D’Costa, N.; Bramley, B. Towards offering wine to the consumer in optimal condition – the wine, the closures and other packaging variables: a review of AWRI research examining the changes that occur in wine after bottling. Wine Ind. J., 20, 20-30, 2005
Limmer, A. The ‘permeability’ of closures. Austr. NZ Grapegr. Winem., 106-111, 2006
 Lopes, P.; Saucier, C.; Glories, Y. Nondestructive colorimetric method to determine the oxygen diffusion rate through closures used in winemaking. J. Agric. Food Chem., 53, 6967–6973, 2005
Lopes, P.; Saucier, C.; Teissedre, P. L.; Glories, Y. Impact of storage position on oxygen ingress through different closures into wine bottles. J. Agric. Food Chem., 54, 6741–6746, 2006
Lopes, P.; Saucier, C.; Teissedre, P. L.; Glories, Y. Main routes of oxygen ingress through different closures into wine bottles. J. Agric. Food Chem., 55, 5167–5170, 2007

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Winemaking Secrets to Share with your Consumers

Monthly Wine Writing ChallengeToday, I have decided to write a post related to the “Monthly Wine Writing Challenge” (MWWC) competition (#MWWC23), to address the current theme, “New.”  The idea is to write a piece related to wine that expresses the current theme.

It’s no secret anymore that sulfur dioxide (sulfites) are in every wine, and sniffing a cork at a restaurant may not be socially acceptable (or required).  But what don’t many consumers know about wine production?  So here is my attempt to write something wine production related that could also be of interest to wine consumers, enthusiasts, and tasters: New winemaking “secrets” revealed… and why we use them to make wine.

1. Red wine color is commonly altered… in fact, you are likely used to drinking red wine in which the color has been changed!

In recent years, I have spent some time critiquing wine after wine with brilliant sommeliers and tasting experts from all over the world.  However, I can actually admit that nothing irks me more, as a producer, than when tasters get hung up on wine color.  In fact, unless the wine is an extremely odd color (like bright orange when it should be a pale yellow or somewhat blue instead of a purplish hue) or showing extreme signs of oxidation (e.g., turning brown), I barely even look at color when evaluating wine.

Why?  Because producers can manipulate color – and do – for no other reason than to meet consumer expectations.

I can still remember hosting an Italian speaker for an educational event, and talking about the Nebbiolo craze that is taking wine lovers by storm.  I loved when he turned to me and said, “Nebbiolo.  Not that deep of a red color.  Everyone adds color.  Everyone.”  Definitely gave me a chuckle, especially when you consider the concentration of wine blurbs that go on and on about the deep red intensity associated with his/her favorite Nebbiolo.

I’m not saying that every wine is colorfully manipulated.  But I am saying that it is one extra thing in a winemaker’s toolbox to meet consumer expectations.  Why go to all the trouble?  Previous studies have shown the mind altering effects of food/beverage color (Morrot et al. 2001, Parr et al. 2003, Spence et al. 2010); it is believed that the deeper the red color (of a red wine), the more powerful fruit perception by the consumer.  Thus, consumers tend to buy (and re-buy) those deep, dark red wines.

The color of these wines were not manipulated.  Both wines are produced from the same grapes at the same vineyard location using the same production methods.  However, they are from 2 consecutive, different vintage years and showing natural color variation. The wine on the left is darker with more intensity and exhibiting purplish edges. The wine on the right has a lower color intensity with a brick red hue.  Photo by: Denise M. Gardner, Penn State Extension Enologist

The color of these wines were not manipulated. Both wines are produced from the same grapes at the same vineyard location using the same production methods. However, they are from 2 consecutive, different vintage years and showing natural color variation. The wine on the left is darker with more intensity and exhibiting purplish edges. The wine on the right has a lower color intensity with a brick red hue. Photo by: Denise M. Gardner, Penn State Extension Enologist

In fact, while at a wedding dinner, I likely offended the woman I was sitting next to when she arrogantly harped on the fact that the wine she was drinking appeared watered-down because the color was not dark enough.  “That’s a relatively expensive Pinot Noir from Burgundy,” I corrected her.  “Pinot Noir doesn’t typically have a dark red color.”  Probably not my shiniest of moments, but yet, I felt the need to make the consumer aware that her expectation was completely, well, wrong.

So how does a winemaker do it?

It’s easy to strip out color.  In fact, many fining agents used to stabilize the chemical component of wine can also remove color pigments from a wine.  You can also minimize color by removing the skins from the fermentation vat.  This is why some red varieties can also produce a pale-pink colored rosé.  All of the color pigments are in the skin of grapevines; the pulp (and juice) is typically a clear white color.  To keep a red wine from becoming red, a winemaker can simply opt to remove the skins out of the fermentation vessel.

To add color, though, especially to red wines or those beautifully pink rosés everyone is drawn to, a little blending can go a long way, and is one of the primary purposes of blending wines together.

Rosé (pink) bubbly is usually pink because some red wine produced from the red grapes of Pinot Noir or Pinot Meunier is blended into the cuveé.  This may also be the case with your favorite White Zinfandel.  However, winemakers may also lean towards very small additions of Mega Purple and/or Ultra Red – two grape [juice] concentrates used to add color to wines.

Mega Purple, grape juice concentration.  Photo found at:

Mega Purple, grape juice concentration. Photo found at:

While both of these products are made from grapes, most winemakers will barely admit to their use.  This is, again, likely due to consumer perception and the winemakers’ (and wines’) reputations.

It can be a snobby world out there tasting wines… but don’t be disappointed.  Variation in wine color is a natural process, which we saw in the above image: vintage variation during the growing season and altered processing decisions all influence wine color.  This can confuse consumers who expect to see consistency in a naturally variable product year after year.  The reality is most of your favorite fermented beverages have adjusted color, including your favorite whiskey.  So don’t be too hard on your favorite wine brand.

2. Your <$20 bottle of wine is probably not aged in an oak barrel.

Oak barrels are expensive.  That’s really the bottom line.  The average price for French oak falls around $1,200 per barrel, and for American oak, about $900 per barrel.  Prices for barrels are forest, age, char, and brand specific, but the point is that they cost a lot of money for wineries.

Barrel use is an expensive part of wine production.  This is a barrel room, from Barboursville Vineyards (VA) shows the elegance and love of barrels used for premier winemaking.  Photo by: Denise M. Gardner, Penn State Extension Enologist

Barrel use is an expensive part of wine production. This is a barrel room, from Barboursville Vineyards (VA) shows the elegance and love of barrels used for premier winemaking. Photo by: Denise M. Gardner, Penn State Extension Enologist

American wine consumers love wines under the $30 price point – and I wouldn’t be surprised if many consumers are looking for those $10 – $15 deals.  At such a price point, the cost of oak far outweighs the return a winery can make on that bottle.

So how does the oak get in the wine?

The answer is: oak alternatives.  Products like oak powder, oak chips, and oak staves can all provide oaky nuances to the wine your drinking.  In fact, oak alternative technology is rather spiffy.  Most oak alternative products can be classified by toasting levels and flavor contributions (i.e., vanilla vs. toasted oak vs. spice) so that a winery can select an oak alternative product specific to its flavor profile.  For example, a Chardonnay that sees some oak chips may not be appealing with an added spice flavor from the oak, but a little bit of vanilla and warm caramel flavor could go a long way.  Thus, the winemaker can make the appropriate oak selection.

The reality is oak alternatives are less expensive to produce and require less contact with the wine in order to turn over ready-to-drink products faster to a consumer.  These are important considerations when the wine may only cost $7 per 750-mL bottle.

Oak alternatives tend to generate higher intensities of oak flavors to a wine, and, therefore, the art of blending is typically utilized to “tone down” the oak intensity.  In my conversations with oak companies, many stated that wineries should start their bench trials by blending 50% of the unoaked wine in with 50% of the oaked wine.  This is most reminiscent of what a wine will taste like as if it had come from a barrel.  From there, a winemaker can decide whether to increase the oak intensity (blend in more of the oaked wine) or decrease the oak intensity (blend in more of the unoaked wine).

3. That wine does not taste like cinnamon spice because the winemaker poured cinnamon into it… and other fallacies about wine flavor.

Every year someone asks me how the winemaker adds ___ flavor to the wine.  As a producer, this will surely give you a smile.  Thoughts of adding one flavorant (i.e. extract) after another like a perfume chemist come to mind, but typically, wine writers/experts/tasters refer to nuance flavors that are associated with one of three components:

  • The fruit
  • Fermentation
  • Aging (i.e., influence from oak)

With the exception of formula wines (e.g., chocolate wine), wines gain their flavor from those three components.  Chemists, flavorists, sensory scientists, microbiologists, viticulturists, and food scientists have spent decades documenting nuanced flavors associated with varietials (i.e., Chardonnay, Riesling, Cabernet Sauvignon, Vidal Blanc), region (i.e., Bordeaux, Napa Valley, Finger Lakes), and during processing (i.e., fermentation vs. aging).  These flavors are often referred to as wine “descriptors:” terms that we use to describe the wine.  This can range anywhere from your basic aromas (odors) and flavors through various taste components (like sourness and sweetness).

But the more important question is… how did those flavors get in there?

Well, many flavors are generated in the fruit (the grapes!) and exist in either an “active” form that one could smell from the vine [seriously… walk through a Riesling vineyard during harvest; it’s magical] or an “inactive” form that will be taken in and changed by the yeast or other microflora associated with fermentation and wine processing.  Those “active” flavors or aromas help define “varietal character” – the terminology used to describe a wine variety.  For example, common varietal character descriptors associated with Riesling are: pears, pineapple, hazelnut, citrus (lemon juice and lemon zest), slate, and fresh cut flowers.  Does this mean that all of these items are mixed into the Riesling fermentation vat to extract such flavors?  This picture would suggest so…

The aroma and flavor descriptions associated with a wine do not mean that a winemaker adds those specific fruits or substances to the wine tank.  Photo from:

The aroma and flavor descriptions associated with a wine do not mean that a winemaker adds those specific fruits or substances to the wine tank. Photo from:

…but no; these things are not mixed into the fermentation tank to extra them.  However, the Riesling fruit contains many of the same odor-active components that are also found in those other items, which is why they may be expressed in the Riesling’s wine aroma.

Some of the contribution of aromas and flavor may also come from the yeast itself.  Some yeasts have specific, consistent nuanced flavor contributions to the finished wine (hmm… Brettanomyces comes to mind).  But typical Saccharomyces yeast also acts as a tool to express those “inactive” flavors found in the fruit and convert them into a form that is “active.”  These aromas and flavors also contribute to the wine’s varietal character.

Active fermentation.  This vessel actually contains 2 different yeast strains to encourage different aromas and flavors that are not typically expressed if only 1 yeast strain was used.  Photo by: Denise M. Gardner

Active fermentation. This vessel actually contains 2 different yeast strains to encourage different aromas and flavors that are not typically expressed if only 1 yeast strain was used. Photo by: Denise M. Gardner

A study has shown that, sometimes, flavors are contributed by other fallen plant material that gets stuck in a maturing grape cluster (Capone, Jeffrey, and Sefton 2012).  That typical eucalyptus flavor in your west coast Cabernet Sauvignon may actually be from fallen eucalyptus leaves that have gotten stuck inside a grape cluster.  During processing, the eucalyptus leaf gets macerated and its oils extracted into the wine itself.  Wa la!  Eucalyptus flavor in your wine!

Aging, especially oak aging, can alter and/or add to the flavor of wine.  When oak is charred, as is the case with most wine barrels, the heating process creates a series of “active” flavors that can be extracted by the wine when it comes in contact with the charred wood.  The type/source of the wood (e.g., American vs. French) and the degree of char affects the flavors that are extracted by the wine.  Common wood-associated flavors include: vanilla, coconut, spice, toasted wood, toasted marshmellow, smokiness, nutty, butterscotch, toast, toasty, and charcoal.

4. Winemaking is not romantic. In fact, sometimes it is pretty gross.

We hear it all of the time, “Oh, it must be so romantic to look out at the vineyard and crush the grapes into wine.”  Hard work: yes.  Romantic: no.  When it comes down to it, growing grapes is equivalent to any other mode of farming, and making wine is nothing more than producing a food (wine) from raw materials (grapes).

Every year, I have to remind students to be aware of all of the insects they may encounter while crushing incoming grapes to prepare them for fermentation: yellow jackets, spiders – including black widows, stink bugs, lady bugs, and probably several other little crawlers.  It’s always a bit humorous to know they were not prepared for this part of grape processing.  It’s the part that’s never covered thoroughly in the text books or during wine appreciation classes.

Wine grapes going into a crusher destemmer. Photo by: Denise M. Gardner, Penn State Extension Enologist

Wine grapes going into a crusher destemmer. Photo by: Denise M. Gardner, Penn State Extension Enologist

Luckily, our grapes are hand-harvested and thoroughly sorted before they reach our doors.  Although a yellow jacket sting or spider bite is not desirable, it beats some of the other critters that are routinely collected into wine fermentation bins.  Things like rats, mice, birds, and snakes.  Oh yes… snakes.

Machine harvesting, especially, has the potential to take anything in the canopy, the vegetative portion of the grapevine.  Anyone that has followed a machine harvester has witnessed the devastation.  Let me tell you, it’s not romantic in the slightest.

But these are a part of the toils of agriculture.  It’s difficult to be out in a vineyard regularly without noticing the fragile ecosystem that exists in front of you.

Additionally, wine is one of the few products where it is found somewhat acceptable to have human skin touch the product during production.  When you think about, this is actually fairly disgusting.  Many food products require strict sanitation procedures to avoid any potential risk of pathogenic illness that may be carried in the food.  But wine, well… wine appears to be an exception.  Luckily, there have not been any incidences associated with pathogenic microorganisms affecting consumers through wine consumption.  In fact, for a large part of history, alcohol was considered a safer product compared to water, which harbored multiple disease-causing microorganisms.

There’s a lot of hands that touch the wine as it is processed.  While the students processing wine are using hair nets (to keep hair from falling into the wine), gloves and specific clothing are not being utilized.  However, sanitation techniques are utilized to maintain wine quality.  Photo by: Denise M. Gardner

There’s a lot of hands that touch the wine as it is processed. While the students processing wine are using hair nets (to keep hair from falling into the wine), gloves and specific clothing are not being utilized. However, sanitation techniques are utilized to maintain wine quality. Photo by: Denise M. Gardner

While the general lack of sterility in a wine processing facility may not be out of the norm, the use of sanitation is still widely encouraged to retain freshness, fruitiness, and general quality of the wine.  That’s why it’s important to watch winemakers sanitize a wine thief before dunking it into a barrel of wine.  Such practices help avoid contamination of spoilage microorganisms which could spoil the wine… and your fun tasting experience.



Capone, D.L., D.W. Jeffrey, and M.A. Sefton. 2012. Vineyard and fermentation studies to elucidate the origin of 1,8-cineole in Australian red wine. J. Agric. Food Chem. 60:2281-2287.

Morrot, G., F. Brochet, and D. Dubourdieu. 2001. The color of odors. Brain and Language 79:309-320.

Parr, W.V., K.G. White, and D.A. Heatherbell. 2003. The nose knows: Influence of color on perception of wine aroma. J. Wine Res. 14(2-3):79-101.

Spence, C., C.A. Levitan, M.U. Shankar, and M. Zampini. 2010. Does food color influence taste and flavor perception in Humans? Chem. Percept. 3:68-84.

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Update: Wine Proteins & Stability

Proteins represent a significant portion of wine’s total nitrogen content. In juice/must, proteins usually represent less than 10% of total nitrogen content. In wine, levels are far higher and reach up to 40%. There are several viticultural and enological factors effecting wine protein content.

In the Vineyard

Proteins synthesized during berry development account for approximately half of total wine protein. After veraison, protein synthesis in grapes occurs at a similar rate as sugar level increase. Higher protein levels are associated with:

  • Grapes sourced from warmer growing regions.
  • Grapes grown at lower crop levels.
  • Grapes harvest at higher maturity.
  • Grapes harvested mechanically.

In the Winery

Winemakers tend to give more thought to wine protein levels during maturation of wines than earlier in the process. It is always important to consider down-the-line effects of any winemaking activity. Indeed, pre-fermentation processes have a larger impact on protein levels in the resultant wine than many winemakers realize.

  • Whole cluster pressing will have lower protein uptake than grapes that are destemmed first. The logic behind this is grape stems act in limiting protein diffusion.
  • Skin contact prior to pressing will typically increase protein concentration in the juice, though uptake and length of contact is a variety-specific relationship.
  • Solids separation removes large amounts of juice nitrogen content, including proteins. The amount is dependent on the type of solids separation (settling vs. floatation) and the use of fining agents. For example, bentonite may remove 50% of total nitrogen content.

A small amount of protein is produced by yeast during fermentation, but this tends to not effect overall wine protein significantly. Post-fermentation processes have negative and positive effects regarding wine protein levels

  • Extended lees contact and lees stirring increases wine proteins. This primarily due to yeast autolysis, the process of cell breakdown and destruction by its own enzymes.
  • Maturation in oak barrels or tanks commonly decreases wine proteins, which will react with oak phenols and precipitate out of solution.
  • Fining with benotonite after fermentation will remove large amount of nitrogen composed of over half of the protein.
  • Fortification typically produces significant lees precipitation, containing a large quantity of proteinaceous lees.

Protein Solubility

The solubility of wine proteins is highly dependent on the ionic strength of the partiularly protein and the wine’s alcohol concentration, temperature, and the pH.

An interesting relationship exists between the isoelectric point of proteins and wine pH. The isoelectric point is where positive and negative charges are equal: proteins have a negative charge when pH is above the isoelectric point, and vice versa. Typical wine pH is very close to its proteins’ isoelectric point, when proteins are least soluble. This relationship makes removing unstable proteins tricky, requiring the correct type and rates of fining agents.

Protein Stability

Winemakers are primarily concerned with proteins in regards to wine stability, which is still largely undetermined. A long list of factors, from grape variety and climate to protein molecular size and interactions with other wine components, effect the exact type and concentration of proteins in wine.

The phenomenon known as protein haze occurs when soluble proteins precipitate in bottled wines. Protein haze makes the wine appear cloudy or highly turbid, considered a defect by most producers and consumers. It is likely composed of several compounds: soluble proteins, polysaccharides, insoluble protein-polyphenol complexes, and metal-protein complexes (proteins act as nuclei for soluble iron, copper, etc.).

Wines with high phenol concentrations will rarely have issues with protein haze since phenols will react and remove sufficient amounts of protein to make the wine stable. This is the reason that protein levels are far more of a concern in white grape varieties, since most red varieties have sufficient phenol levels to stabilize proteins. Protein levels and color instability is highly correlated in red varieties such as Pinot Noir. Due to phenol content, oak maturation also increases protein stability in wines compared to those held in stainless steel vessels.

As the largest source of wine protein, grapes are also the largest source of protein instability. It is believed that proteins originating from yeast do not pose issues with stability.

Evaluating Protein Stability

Evaluation of protein stability should only be conducted after all other winemaking procedures have been completed. In other words, just prior to bottling. Any change to the balance of temperature, pH, and alcohol content from processes such as acidification, malolactic fermentation, fortification, and cold stabilization can lead to precipitation of wine protein complexes.

Protein stability evaluation is not an exact science, and thus involves predictive techniques. These can include heat testing, heat-and-cold testing, and bentonite testing. Most winemakers err on the side of caution, resulting in wines that will be over-fined to ensure stability in their finished product.

The following is a method common to many wineries that I have used with good success.

  • Filter sample through a sterile filter (0.45 µm). If the sample is still cloudy (i.e. from tank sitting on lees), you may need to centrifuge it prior to filtering.
  • Fill one test tube with filtered sample as a control.
  • Fill a second test tube and heat to 80° C (180° F) for two hours (many wineries like to do so for six hours instead, while others heat at lower temperature for up to 24 hours; I believe two hours at this temperature sufficiently precipitate proteins).
  • After sample is heated, allow it to return to room temperature. Giving the sample several hours or overnight is advisable to allow precipitation; if this is to be done, refrigerate both the control and heated sample but make sure to allow both to return to room temperature prior to reading.
  • Compare the two samples. Ideally, this should be completed with a turbidity meter (nephelometer); protein stable samples are deemed those with an NTU <1.0. Visual comparison with a bright light can be completed in lieu of a turbidity meter but my not adequately assess stability.

If a haze does appear, the wine should be fined to remove excess proteins. There are several different types of bentonite available today, and most wineries have their favorite. A bentonite fining trial should be conducted by preparing samples at varying addition rates (addition rates will vary depending on varietal, location, type of bentonite, etc.).

Then, complete the heat stability test again. I often find that bentonite fining in a controlled environment like this results in over-fined wine in the cellar, so I advise choosing a slightly lower rate than determined in the trial. Of course, heat stability should be re-tested once bentonite fining is completed.

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Brettanomyces – the yeast lurking in your wine

Who am I?

Most winemakers know the feared five-letter word “Brett” all too well. Brettanomycesbruxellensis is a famous red wine spoilage yeast, responsible for financial losses within the wine industry yearly. Contaminated wine can usually not be readily commercialised and ridding contaminated wineries of this yeast is a difficult task. B. bruxellensis produces various spoilage compounds that not only affect the aroma profile of the wine, but also the appearance of the wine, often resulting in a colour loss and haze. These compounds include acetic acid (Scheffers, 1961 & Freer, 2002) and fatty acids (Rozès et al., 1992; Malfeito-Ferreira et al., 1997 & Licker et al., 1998). However, B. bruxellensis is best known for the production of volatile phenolic compounds that are generally considered to have a negative impact on the organoleptic properties of the wine. Although the metabolic pathway leading to the production of these volatile phenols has been elucidated more than 20 years ago, the enzymes catalysing the 2-step reaction have only been identified recently, following the sequencing of B. bruxellensis’ genome (Curtin et al., 2012a; Piškur et al., 2012 & Crauwels et al., 2014). Concurrently, research has allowed significant advances in our global understanding of B. bruxellensis, especially concerning this yeast’s peculiar ability to survive and develop in a matrix as harsh as wine. This article provides an overview of these recent research findings.

My genetic make-up

The first genome sequencing was attempted in 2007. However, only a partial sequence (40% of whole genome) could be assembled (Woolfit et al., 2007). The first whole genome sequence was actually released in 2012 (Curtin et al., 2012a), i.e. 16 years after that of Saccharomyces cerevisiae. The strain whose genome was sequenced had been isolated from an Australian wine. A few months later, the whole genome sequence of the initially partially sequenced French B. bruxellensis strain, was also made publicly available (Piškur et al., 2012). Finally, the genome of one more strain, this time isolated from beer, was sequenced in 2014 (Crauwels et al., 2014). Strain variability, complexities and unusual characteristics of this yeast were evident form the former two sequenced genomes, with chromosome numbers ranging from 4 to 9 (depending on the strain) in comparison to the 16 found in all strains of S. cerevisiae. In addition, unlike what was originally assumed that B. bruxellensis was a haploid organism, it was demonstrated to rather exhibit an intricate ploidy with one strain being diploid and the other triploid. Moreover, in a recent study, the genome sequences of the three strains were compared in order to explore the genome plasticity and diversity among the different isolates. It was reported that the beer strain had a significantly altered genome sequence in comparison to the two wine strains. The study also revealed that the genomes of the two wine isolates, even though very different, were more similar to one another than compared to the beer isolate. In particular, 20 genes present in both wine strains are absent in the beer strain. These genes could possibly confer a specific adaption to living in wine (Crauwels et al., 2014).

These studies have already shed some light on the complexity and huge diversity observed among strains. The availability of the full genome sequences is a significant step forward that will certainly allow for more in-depth investigations to better comprehend morphological and physiological characteristics, as well as specific adaptations associated with B. bruxellensis.


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Technical Information about Pét-Nats

By: Denise M. Gardner

Author’s Note: Current technical information regarding the production of pétillant naturels is limited.  The following information is summarized and detailed from a series of text books and personal discussions with Paul Guyard from Enartis, Daniel Granes from the ICV in Languedoc-Rousillion, whose contact comes courtesy of Gordon Specht from Lallemand, and Michael Jones from Lallemand.  The author would like to thank all contributors for the following information.

The recent interest in sparkling wine production ( has winemakers and sommeliers talking about another trendy bubbly: Pétillant Naturels, or pét-nats when abbreviated.  These bubblies are consumer friendly: less expensive than traditional Méthode Champenoise-produced wine, usually contain an enhanced fruitiness, are meant to be consumed early (i.e., no long term aging required by the consumer), and are currently trendy amongst wine professionals, bloggers and sommeliers.  A quick search online can lead one to a plethora of articles indicating consumer awareness of pét-nats:

Recent food trends indicate that consumers are searching for “more natural” selections (, and pét-nats may appear as a less intrusive winemaking approach in the eyes of consumers.  Pét-nats offer a winery marketing potential, as many are highlight as being made with limited technological influence and following more traditional winemaking practices.

The concept of production is rather simple: start fermentation and bottle before it is finished fermenting to retain some residual carbon dioxide, and likely sugar, in the final product.  However, production requires winemaker attention to ensure final wine quality.  As David Lynch quoted one producer in his Bon Appetit article, pét-nats production can seem like “Russian roulette winemaking” from the production perspective.  Although in French, a detailed diagram displaying the steps of the méthode ancestrale production practices associated with pét-nats (follow the column labeled “méthode rurale”) can be found here:

History of Pét-nats

Pét-nats are believed to be the original source of sparkling wine production in France, preceding Champagne production (Robinson and Harding 2006).  It is believed that wines from naturally-cooler regions in France would undergo primary fermentation until the winter when temperatures would naturally drop and inhibit fermentation.  Winemakers, unaware that the wine was not fully fermented, bottled the young wine and found that it re-fermented in the bottle when the ambient temperatures became warmer.  Some of the first sparkling wines produced have been traced back to Gaillac, located in the southwest part of France, north of Toulouse, and Limoux, located in the higher mountains of the Languedoc-Roussillon region (Robinson and Harding 2006).

The term “pétillant” generally describes a sparkling wine with less retention of carbon dioxide compared to a sparkling wine like Champagne (WSET 2001).    The grape variety traditionally used for pét-nat production in Gaillac and Limoux is mauzac (known locally as blanquette in Limoux), which has a distinguishable “dried-apple-skin” flavor (Robinson et al. 2014).  Today, pét-nat production has exceeded the boundaries of their origins, extending through the Loire and various regions around the world.

The production method associated with of Blanquette de Limoux is often referred to as the méthode ancestrale, or as the méthode gaillacoise in Gaillac (Robinson and Harding 2006).  The methods are quite similar in execution, which consists of one primary fermentation that is started in tank and finished in the bottle.  This results in a cloudy wine, typically with varying concentrations of residual sugar, and retention of carbon dioxide.

Thinking of Giving Pét-Nat Production a Try?

While the production of pét-nats may seem appealing, one of the experts suggested trying to bottle condition a wine before attempting the full méthode ancestrale production technique as it involves a lot of winemaker attention.  This may also be a practical alternative when current production facilities are not equipped for full-range temperature control.   Bottle conditioning is typically used by homebrewers, home cider makers, and home winemakers to get carbon dioxide in bottles.  You can read some of the home production literature here if you are unfamiliar with the process:

It is recommended that you use a low alcohol (≤12% alcohol v/v), low pH (<3.50) wine if you are exploring the bottle conditioning technique.  Add enough sugar to generate 3-4 ATM of pressure, maximum, and bottle with a yeast addition based on the suggestions below.  Bottles should be suitable to retain pressure and sealed with a crown cap.

Bottle conditioning a wine should give you a clear indication regarding the finishing technique and style associated with pét-nats.  It also acts as good practice before committing to pét-nat production.

Safety First

Since pét-nats are sparkling wine products that contain a fair amount of pressure, winemakers and cellar staff should proceed with caution during production.  Use common sense: purchase appropriate bottles made to withstand pressure, double check calculations for sugar-to-pressure conversions, and use protective eye glasses.  Accidents can happen, and it is best to be prepared for any hazard associated with any stage of wine production.  Sanitation is a key point through production, and proper protective clothing should be worn at all times when using sanitizing agents of any kind.

Parameters to Look for in the Fruit

Pét-nat production may be applied to any grape variety, and offers a wide opportunity for winemakers to explore the production of new and unique wine products.  Although there are no variety limitations, production experts caution that grapes should lack vegetal flavors in the berries.

Berry sensory analysis may be useful for winemakers to evaluate grape flavor quality and to help determine picking times.  In general, ripe (non-vegetal) flavors should persist in the berry in order to encourage their development in the final wine.  However, grapes should avoid “overly ripe” flavor characteristics as this may be an indication of higher pH and lower acidity values that may cause complications through the winemaking process.

Grapes are often picked with a potential alcohol of 10 – 12% v/v, and at this concentration of natural sugar, the pH should be lower (<3.50).  The pH of the wine will offer microbial protection to the wine through the méthode ancestrale process and offer some protection to wine quality through vulnerable production steps.

Fruit should also be of sound quality (i.e., with limited disease pressure) to avoid detriment to flavor and overall quality of the wine.  Some diseases may contribute secondary byproducts which could cause fermentation complications.  Therefore, the winemaker is encouraged to use sound fruit.  Cellar hygiene, or proper sanitation techniques, will be essential for quality control purposes through production.  Extra sanitary care should be taken if the winemaker wants to remove the lees from bottles by traditional disgorging techniques (refer to a previous post on Sparkling Wine Production Techniques).  A summary of grape parameters required for pét-nat production is shown in Figure 1.

Figure 1: Grape Specifications Recommended for Pét-Nat Production

Figure 1: Grape Specifications Recommended for Pét-Nat Production

Base Wine Production

The production method associated with pét-nats (Figure 2) is alluded to rather simply in the wine literature: the primary fermentation is started in tank, arrested before primary fermentation is completed, bottled, and finished in the bottle.  The consumer can expect a slightly sweet (i.e., presence of residual sugar), cloudy, lightly bubbled wine (usually below 4 ATM pressure; Amerine et al. 1972).  Winemakers should refer to the TTB for additional tax purposes associated with sparkling wines (

Grapes are crushed/destemmed (if preferred) and pressed.  In France, press cycles and parameters are based on regulation.  Press cycles are set to extract 100 L of juice for every 150 kg of fruit.

Some attention should be given to clarification of the juice, pre-fermentation, in the production process of pét-nats.  It is recommended that juice is clarified to 30 – 80 NTUs with use of centrifugation, flotation or assistance with settling enzymes and/or fining agents.

There is some debate as to whether or not sulfur dioxide should be added to the juice during settling.  In the juice-settling phase, a sulfur addition may help clarify the juice and minimize spoilage yeast and bacteria that could harm the quality of the wine.  However, like with still wine production, sulfur dioxide additions should not be made to excess as too much could hinder primary fermentation.  (Note: For those looking to produce a “more natural” wine, or to appeal to the “no-sulfur-added” market, it would be prudent to skip sulfur dioxide additions at this step.)

Following clarification, the juice should be racked and prepared for inoculation.

Starting Fermentation

Yeast selections (Table 1) should be based on the winemaker’s preference, but there are some tips that have been provided by wine supply companies:

  • Use low-sulfur dioxide-producing yeast strains
  • Select yeasts for secondary aroma potential
  • Supply yeasts with proper hydration and fermentation nutrient additions
  • Use yeasts that grow optimally in cool temperatures, 14-16°C (~57-60°F)
  • If the winemaker is going to remove lees (e., disgorging) before selling the product, and is only going to undergo one fermentation without a second inoculation, choose a yeast strain that is recommended for Méthode Champenoise sparkling wine production

Table 1: Yeast Recommendations from Lallemand and Enartis Vinquiry for Pét-Nat Production (Note: Other suppliers may have additional yeast recommendations. Please consult your regular supplier for further suggestions.)

Table 1: Yeast Recommendations from Lallemand and Enartis Vinquiry for Pét-Nat Production (Note: Other suppliers may have additional yeast recommendations. Please consult your regular supplier for further suggestions.)

Use a hydration nutrient (e.g., GoFerm Protect Evolution, Enartis Ferm Arom Plus) properly at inoculation.  Depending on the winemaker’s preferred techniques or the perceived difficulty of alcoholic fermentation, oxygen additions can be made to activate the fermentation.  Some winemakers choose oxygen ingress through the use of micro-ox, and base dosage rates [of oxygen] on sensorial perceptions.

Use of temperature control is essential for producing pét-nats.  If you need more information and suggestions regarding how to integrate temperature control into your winery operation, please visit this report here:

Fermentation should proceed at 14-16°C (~57-60°F).  At about +/- 3% v/v from the target alcohol, winemakers should chill the wine down to 8°C (~46°F) to hinder the fermentation.  The act of cooling will also clarify the wine and minimize the transfer of lees.  Too much lees transfer will result in a “yeasty” flavored wine, which is not preferred in pét-nat wines.

Finishing the Wine

Once the wine is properly chilled, it will need to be racked to remove most of the lees.  It is not uncommon for winemakers to remove all of the lees by centrifugation or filtration, and later, restart the wine with a fresh culture and hydration nutrient.  From a French winemaking perspective, the addition of nitrogen is usually added at racking in the pump flow (1 L of nitrogen gas for each 20 L of wine).

Winemakers may opt to blend at the racking stage as well.  Blending can help elicit the production of a “house-style” pét-nat, and ensure consistency despite natural vintage year variation.

Malolactic fermentation is optional, and should be inoculated after racking, based on winemaker preference.  For those that are considering malolactic fermentation, it is important to remember that there is a significant quantity of residual sugar in the wine at this stage in the process, which can lead to a series of winemaking problems:

  1. Consider the wine’s pH before undergoing malolactic fermentation. Malolactic bacteria have a higher risk of producing more acetic acid during malolactic fermentation if the wine pH is greater than 3.50.  Great attention and care must be given to a pét-nat undergoing malolactic fermentation with a higher pH to avoid extreme spoilage issues.
  2. Malolactic bacteria require a warmer temperature for growth, which requires the winemaker to increase the temperature of the wine. Therefore, it is suggested that winemakers sterile filter the wine prior to inoculating for malolactic fermentation to avoid primary fermentation from re-starting and completing before the wine is bottled.
  3. The remaining residual sugar puts the wine at risk for other microbial contaminants. Sanitation and monitoring of malolactic fermentation progression is of the utmost importance.

Tartaric acid stabilization, or cold stabilization, can progress at this stage after the wine is racked.  However, it is more common for winemakers to add CMC to avoid crystallization of tartaric acid as opposed to undergoing a cold stabilization process.

At this point, the wine should be prepared to complete primary fermentation.  If the wine were to go to tank and complete fermentation, then the process of completion follows a Partial Fermentation process that is used in the Asti region of Italy to produce Moscato.

To complete the méthode ancestrale technique, the base wine is bottled to complete fermentation.  A second inoculation of yeast is typically required to complete primary fermentation, but it is optional to add more yeast nutrient at inoculation.  Some wineries choose a second edition of a hydration nutrient (prepared during yeast hydration) and a smaller dose of a complex nutrient (e.g., Fermaid K, Nutriferm Advance).  Yeast addition dosage rate is recommended at about 2 million colony forming units (CFU) per mL of living yeast.  Ideally, yeast addition should be less concentrated than a “normal” inoculation to minimize biomass in the bottle and encourage a slow fermentation in the bottle.  Yeast strain should be selected according to winemaker preference (see the above list, Table 1, for suggestions from Lallemand/Scott Labs and Enartis Vinquiry).

Méthode ancestrale does not involve a sugar addition at the second inoculation.  However, a sugar addition to manipulate the final desired concentration of pressure in the bottle is an option for winemakers at the second inoculation of yeast.

Bottle selection is important, and needs to be of high enough quality to retain the internal pressure left over from fermentation.  If the expected pressure is above 4 ATM, ensure that you are using the correct bottles to retain pressure.  Yeast selection should also be altered if the final preferred pressure is greater than 4 ATMs.

Although a slight detour from the méthode ancestrale process, it is possible to remove the lees after fermentation has completed in the bottle.  If the winemaker would like to riddle and disgorge the yeast at the completion of primary fermentation in the bottle, a riddling agent (e.g., Adjuvant MC by Enartis Vinquiry) may be desired.

After the base wine is re-inoculated in the bottle, bottle fermentation should progress in a temperature controlled space, optimally set at 13-15°C (~55-59°F).  For retention of residual sugar, chill the room to 0-2°C (32-36°F) to arrest fermentation in the bottle.  [Note: When the wine is warmed up, it may continue to ferment in the bottle.]

With the minimal yeast population, minimal nutrient availability, increase pressure in the bottle, and low fermentation temperature, fermentation will progress slowly and may stop with residual sugar naturally as all of these factors put stress on the yeast.  It may take several months until an appropriate amount of pressure has built up in the bottle.

Figure 1: Flow Diagram Representing General Production of Pétillant Naturel Sparkling Wines (Méthode Ancestrale)

Figure 2: Flow Diagram Representing General Production of Pétillant Naturel Sparkling Wines (Méthode Ancestrale)

Potential Disgorgement

Some winemakers choose to sell a product that is clearer than traditional pét-nats and disgorge the yeast lees using similar techniques that were previously discussed pertaining to the traditional method, Méthode Champenoise, way of making sparkling wine.  Here, the lees are collected, riddled, and disgorged.  If the wine was fermented to dryness, a sugar addition with a dose sulfur dioxide can minimize risk for re-fermentation when the bottle is in the hands of consumers.  Furthermore, disgorgement allows a winemaker to make sensory alterations to the wine with a dosage addition.  Sensory adjustments can be made using Arabic gums, inactivated yeast/polysaccharide products, or tannins that have been added to the dosage.

Traditionally, pét-nats are sealed with a crown cap.

Final Production Thoughts

Large producers of bottle conditioned cider may opt to flash bottle pasteurize hard ciders that retain some residual sugar.  Flash bottle pasteurization will inactive the yeast and ensure an extra line of protection to ensure that fermentation does not continue to progress once the consumer has purchased the product.

However, part of the fun associated with pét-nats is not truly knowing the end residual sugar!

Note: Do NOT add potassium sorbate to the wine at any stage if you are trying to make a pét-nat.  Potassium sorbate will inhibit the yeast from fermenting through any stage of this process.

Familiarizing Yourself with Pét-Nats

Like with any wine style, it is ideal to have a sensory library of what quality pét-nats taste like using examples from the commercial market.  The practice of tasting multiple examples of a specific wine style creates a benchmark library in the mind of the winemaker, which aids in making processing decisions in relation to an end-goal for the final product.  It also helps define “quality” for that wine style.

While I have not embarked on an exploration to understand pét-nat quality, the following wines have been suggested in the above-mentioned articles or from individuals that have enjoyed pét-nats in today’s market.  I highly suggest that any winemaker aiming to produce pét-nats, obtain various examples to evaluate 1) their individual preference of the product, 2) the potential consumer preference of the product, and 3) the quality parameters that the winemaker will aim for during production of a pét-nat style wine.


Amerine, M.A., H.W. Berg, and W.V. Cruess. 1972. Technology of Wine Making, Third Edition.  The AVI Publishing Company: Westport, Connecticut.

Granes, Daniel. 2015. Personal Discussion.

Guyard, Paul. 2015. Personal Discussion.

Jones, Michael. 2015. Personal Discussion.

Robinson, J. and J. Harding. 2006. The Oxford Companion to Wine. ISBN: 978-0198609902

Robinson, J., J. Harding, and J. Vouillamoz. 2014. Wine Grapes.

Wine and Spirit Education Trust (WSET). 2011. Wine and Spirits: Understanding Style and Quality. ISBN: 978-1 905819 15 7.

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What Causes Dry Mouth After Drinking Red Wine?: Tannic Acid Effects on Saliva Production

As many of you probably know from experience, sometimes when you drink a red wine you notice that your mouth gets very dry. This is usually attributed to the tannin levels in the wine—the “bigger” the tannins, the more it seems moisture is wicked away from your mouth and you’re left with something akin to the Sahara happening on your tongue.

So, what is really going on here? Is it the tannins? Why do they make your mouth feel so dry after sipping?

A study published in January in the Open Journal of Stomatology aimed to address a very similar question. In essence, what is the effect of tannic acid in different beverages on glandular function in the mouths of mice?

Quick Background

Before launching into the study and the results, it is important to get a primer on what has been done so far in the world of tannic acid and secretory glandular function so far.

First, the salivary glands in the mouth are basically made up of two different types of parts: those that produce a sort of “preliminary saliva”, and those that absorb salt, and add potassium and bicarbonate to create the final hypotonic saliva. Having this hypotonic property allows the flavors of the food to better pass through the saliva into the taste buds so we can actually taste what it is we are eating or drinking.

It is during the transport of fluids as well as salt, potassium, and bicarbonate that problems with salivary secretions can arise. If something is preventing these processes from occurring, one could be left with excess saliva or alternatively dry mouth.

It is thought that tannic acid (TA) might mucks with this process thus often leaving the feeling of dry mouth after drinking some red wines. Specifically, TA might inhibit the calcium-activated transport channels that allow for diffusion of the necessary compounds needed to create the final saliva, resulting in decreased saliva production and observed dry mouth.


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