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New World Wine Maker Blog - Technical Articles

Botrytis Bunch Rot: Winemaking Implications and Considerations

By Dr. Molly Kelly, Enology Extension Educator, Department of Food Science

In a previous post, Bryan Hed discussed early fruit zone leaf removal and its effects on the development of Botrytis bunch rot and sour rot. This is a good time to review the implications of molds and fruit rots on wine composition and quality. I will also discuss remedial actions in the winery.

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Here we will focus on the most common bunch rot pathogen of mature berries, Botrytis cinerea. How severe can Botrytis bunch rot be before wine quality is impacted? This will depend on the type of rot as well as winemaking techniques however, even low levels of infection have been shown to negatively impact wine quality. Red wine quality was shown to be affected by as low as a 5% infection rate of B. cinerea. Extended skin contact in red winemaking can increase the effect of bunch rots on the finished wine.  While B. cinerea can be linked with sour rot, it is more commonly associated with other fungi including Aspergillus spp. Sour rot is caused by yeast, acetic acid and other bacterial growth. When acetic acid bacteria, yeast and filamentous fungi are present together, high levels of acetic acid can result. Berries infected with sour rot have a distinct vinegar smell that may be combined with the presence of ethyl acetate. Ethyl acetate is an ester described as smelling like nail polish remover.

Laccases are enzymes produced by fungi. They break down anthocyanins and proanthocyanidins which are important phenolic compounds that contribute to palate structure and wine color. In white wines, some aromatic compounds can be oxidized resulting in the production of earthy aromas.

The largest change in must chemistry as a result of Botrytis growth is seen in amounts of sugars and organic acids. Up to 70 to 90% of tartaric and 50-70% of malic acid can be metabolized by the mold. Resulting changes in the tartaric:malic ratio cause titratable acidity to decrease and pH to increase.

There may also be clarification issues as a result of infection. The fungi produce polysaccharides including β1-3 and β1-6 glucans as well as pectins as a result of the production of enzymes capable of degrading the cell wall. In the presence of alcohol, pectins and glucans aggregate causing filtration difficulties. To mitigate this issue, pectinolytic and glucanase enzymes can be used. When adding enzymes allow at least six hours prior to bentonite additions.

Botrytis cinerea strains differ in the amount of laccase produced. This enzyme can lead to oxidation of aroma/flavor compounds and browning reactions. It can be resistant to sulfur dioxide and not easily removed with fining agents. Bentonite may remove enough laccase to minimize oxidative problems. For varieties where the potential for oxidation is increased, ascorbic acid additions can be added to juice. Since Botrytis uses ammonia nitrogen there is less available for yeast metabolism. Vitamins B1 and B6 are also depleted. Therefore supplementation with nitrogen and a complex nutrient is required. Yeast assimilable nitrogen (YAN) should be measured and adjusted accordingly to avoid stuck fermentations and production of hydrogen sulfide. Also consider inoculating with low nitrogen-dependent yeast and use more than the standard amount of 2 lbs. /1000 gallons.

Wine off-flavors and aromas result from a number of compounds when made from grapes with Botrytis(and other bunch rot organisms). Descriptors include mushroom and earthy odors from compounds such as 1-octen-3-one, 2-heptanol and geosmin. Since fruitiness can be decreased, the use of mutés (unfermented juice) from clean fruit can be added to the base wine to improve aroma. Botrytis also secretes esterases that may hydrolyze fermentation esters. Monoterpenes found in varieties such as Muscat, Riesling and Gewürztraminer can also be diminished.

When Botrytis infection is present, consider the following processing practices in addition to those mentioned above.

  1. Remove as much rot as possible in the field and sort fruit once it arrives at the winery. Using sorting tables is a great way to improve overall wine quality.
  2. Whole-cluster press whites, using very light pressure, and discard the initial juice.
  3. Harvest fruit cool and process quickly. Sulfur dioxide can be added to harvest bins to inhibit acetic acid bacteria.
  4. Enological tannin additions will bind rot-produced enzymes. They can also bind with protein and decrease the bentonite needed to achieve protein stability. Note: Remember to not add tannins and commercial enzymes at the same time since tannins are known enzyme inhibitors. After an enzyme addition allow six to eight hours before adding tannins.
  5. Minimize oxygen uptake since laccase activity is inhibited in the absence of oxygen. Inert gas can be used at press, during transfers and to gas headspace.
  6. Use a commercial yeast strain that will initiate a rapid fermentation. The resulting carbon dioxide will help to protect against oxidation.
  7. Once fermentation is complete, rack right away. Both Botrytis and laccase settle in the lees.
  8. Phenolic compounds are the main substrate for fungal enzyme activity. Removal of undesirable phenolic compounds can be achieved using protein fining agents (ex: gelatin, casein, isinglass). The synthetic polymer PVPP can also be used in juice or wine to remove oxidized phenolic compounds.
  9. Only cold soak clean fruit. Avoid cold soak and extended maceration on Botrytisinfected fruit as this may encourage fungal and bacterial growth.

As always, it is best to avoid rot-compromised fruit, however, using these practical winemaking tips should help to minimize negative impacts on wine production and quality.


DeMarsay, A. Managing Summer Bunch Rots on Wine Grapes, Maryland Cooperative Extension. Accessed 7 May 2018.

Ribereau-Gayon, P. 1988. Botrytis: Advantages and Disadvantages for Producing Quality Wines. Proceedings of the Second International Cool Climate Viticulture and Oenology Symposium. Auckland, New Zealand, pp. 319-323.

Steel, C., J. Blackman, and L. Schmidtke. 2013. Grapevine Bunch Rots: Impacts on Wine CompositionQuality, and Potential Procedures for the Removal of Wine Faults. J. Agric. Food Chem. 61: 5189-5206.

Zoecklein, B. 2014. Fruit Rot in the Mid-Atlantic Region, On-line Winemaking Certificate Program, Wine Enology Grape Chemistry Group, Virginia Tech. Accessed 16 April 2018.

Zoecklein, B. 2014. Grape Maturity, On-line Winemaking Certificate Program, Wine Enology Grape Chemistry Group, Virginia Tech. Accessed 16 April 2018.

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Making invisible things visible: Do yeast cells stick to cork during bottle aging?

Yeast do not seem to form biofilms on the bottoms of corks when they’re used, rather than metal crown caps, to secure Champagne bottles during their in-bottle secondary fermentation. This, at least, is the conclusion of an article in the current issue of the American Journal of Enology and Viticulture (paywall), in which Burgundy-based authors investigated the question for the sake of understanding whether Champagne producers, some of whom are using cork for their longer-aging wines, risked upsetting in-bottle fermentation dynamics. After a year of bottle fermentation and aging, a few cells apparently got caught in porous crevices of the cork, but systematic growth presaging the tenacity of a biofilm wasn’t happening.

That finding will no doubt interest the odd sparkling wine producer. Much more interesting is the method they used to make “invisible” cells visible and so reach that conclusion.

How do you determine whether a microbial biofilm is growing somewhere?

Before it reaches scrape-it-off-with-a-fingernail thickness, the answer is usually microscopy. But what kind of microscopy? Let’s say you want to see the very earliest stages of what might become a biofilm. You want to visualize individual yeast cells sticking to a rough, craggy surface with lots of crevices for hiding—the cork. Pointing a light microscope at the cork won’t do. The kind that you probably used in school to see cells suspended in liquid (from your cheek, or bacteria from your teeth), and that many small wine and brewery labs use to check for live cells requires that whatever you’re looking at be thin enough for light to pass through. Slicing corks into sections thin enough for light microscopy might destroy or displace any would-be-biofilm-forming yeast cells. Moreover, can you imagine scanning the bottom of cork after cork, continually asking yourself: is that a yeast cell, or a bit of cork shaped like a yeast cell? A digital image analysis program trained to recognize yeast cells might address the latter problem (with some degree of error), but the whole task clearly calls for a more sophisticated technique.

When your sample isn’t transparent enough for light microscopy, fluorescence microscopy can be an alternative; instead of visualizing light passing through the sample, fluorescence microscopy relies on whateveritis you’re examining absorbing and then emitting light—fluorescing—which can then be picked up by the microscope’s detector.* Conveniently, cork and other plant matter comes with a built-in fluorescent molecule; lignin, a rigid polymer and major contributor to the stiffness of wood and bark, is easy to see under the microscope …


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Sensory evaluation of mouthfeel in wine

by Renée Crous, Valeria Panzeri & Hélène Nieuwoudt

Sensory panels often avoid the evaluation of mouthfeel sensations in wine, since it implies (for many non-wine experts) complex, and abstract concepts. However, mouthfeel is an important dimension of wine quality, and it is, therefore, necessary that these properties are also included in routinely sensory tests. In our recently completed Winetech funded research project IWBT W13/02, “Rapid descriptive sensory methods for wine evaluation – special focus on further optimisation of rapid methods and streamlining of workflow”, we have developed two useful protocols for the assessment of mouthfeel in wine that can be used by sensory panels in the industry and research. This article describes the protocol that is based on the classic descriptive analysis (DA) method. In line with the objectives of project IWBT W13/02, we also optimised a rapid sensory method, polarised sensory positioning (PSP) to evaluate mouthfeel in wine. The rapid method does not require a trained DA panel and can be completed in shorter sessions. The protocol for the rapid method is discussed in a subsequent article.

What is meant by mouthfeel?

Mouthfeel refers to the sensory perceptions experienced in the mouth when a wine is consumed. Wine judges often describe wine as “full and round with good concentration, and length”, thereby implying that the product has good mouthfeel properties. While these phrases are commonly used in the popular media, people differ in their understanding of what exactly is meant by them. In scientific publications, several terms are grouped under so-called in-mouth sensations; these include fullness, heat, complexity, balance, length and mouthfeel.

Evaluation of mouthfeel by sensory panels

It is a challenging task to train a sensory panel to evaluate wine mouthfeel sensations. DA is one of our most accurate sensory test methods and provides two important outputs. Firstly, all the sensory properties that are perceived in a set of wines are identified and named by a panel of trained tasters. Secondly, panellists also score the intensity of each property on a line scale (ranging from 0 to 100, for example).

Physical standards are used to train a panel for the first task (identification of sensory properties). For example, a fresh lemon can serve as a standard for the lemon character in wine aroma. It is clear that all the panellists must have agreement on the sensations of the lemon character, as well as the particular word that describes the specific character, before the panel can proceed to assess the wines. With abstract concepts, such as length, complexity, and balance, there are no so-called physical standards, and an alternative plan must be made during training of the panel.

Another major challenge is to calibrate panellists to rate the intensities of the mouthfeel sensations on a line scale. It speaks for itself that the line scale must be used consistently by different panellists and on independent sets of wines; otherwise, comparative studies are not possible.

To illustrate these challenges: in this Winetech project, we wanted to evaluate the mouthfeel sensations of old-vine Chenin blanc wines (produced from vines 40 years and older) with the DA method. It is well known among wine experts that the old-vine Chenins have much more complex mouthfeel properties, compared to some younger vine Chenins. For panellists to use the line scale consistently to showcase these differences, they need to have in-depth experience and knowledge of the entire product category. This is seldom the case. Also in dealing with this challenge, adjustments had to be made to the standard DA protocol …



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Catch a whiff…

The total acidity in wine consists of two main components; non-volatile acid (including malic acid and tartaric acid) and volatile acid (VA). Volatile acid comprises of a group of volatile, organic, steam distillable acids. Concentrations mostly vary between 500 to 1000 mg/L, with almost 90% of volatile acidity consisting of acetic acid. The rest is mostly propionic- and hexanoic acid, as well as other fatty acids from yeast and bacterial metabolism, as well as ethyl acetate.

The most common VA concentrations in wine are around 0.4 g/L, with a legal limit of 1.2 g/L (see Table 1 for legal limits). The sensory threshold value in red wine is approximately 0.6 -0.9 g/L. whereas low, almost unnoticeable levels add to aroma complexity. With regards to the sensory attributes of VA; it contributes to the taste intensity of non-volatile acids and tannins, while the perception of VA itself is masked by high concentrations of sugars and alcohol. Acetic acid smells of vinegar, while ethyl acetate smells more like bruised apple and Cutex remover.

Volatile acidity production takes place mainly due to the oxidation of ethanol or the metabolism of acids/sugars. Ethanol is the primary energy source for acetic acid bacteria (AAB). Acetic acid bacteria are microscopic, single-cell organisms with enzymes included in their cell walls. The most common AAB present in wine include Acetobacter aceti, Acetobacter pasteurianus and Gluconobacter oxydans. These organisms are aerobic and need oxygen for survival. Acetic acid bacteria have the ability to oxidise alcohol to acetic acid, which in turn will, via esterification with ethanol, be converted to ethyl acetate. Ethyl acetate possesses a lower sensory threshold value compared to acetic acid and both acetic acid and acetaldehyde (a by-product of ethanol oxidation), are toxic to Saccharomyces cerevisiae and can contribute to sluggish- or stuck fermentations.


Origin and mechanism of oxidation…

During fermentation, the possibility of VA production is increased through the following practices: high risk must, risky winemaking practices and poor management of cellar conditions. Sources of VA after fermentation include cellar practices with specific focus on barrels: the amount of headspace, barrel age, oxidation and sanitary state of the barrels. Most AAB infections will take place in the cellar itself; mainly due to low acids and sulphur dioxide levels, together with oxygen exposure.

There are various sources that can add to the VA concentration in wine; the most conspicuous being:

  1. wild yeast e.g. Brettanomyces, Kloeckera etc. and as a natural by-product of S. cerevisiae

    Acetic acid is produced as an intermediary product of the pyruvate dehydrogenase metabolic pathway. This metabolic pathway is necessary and responsible for the conversion of pyruvate to acetyl-CoA. Last mentioned is imperative for anaerobic processes like lipid biosynthesis. This reaction is catalysed by alcohol dehydrogenase, whereby acetic acid is formed via the oxidation of acetaldehyde (produced from pyruvate during fermentation).

  2. lactic acid bacteria (LAB) during fermentation

    Heterofermentative LAB possess the ability to metabolise glucose (residual sugar), via the phosphoketolase metabolic pathway, and convert it to CO2, ethanol, acetic acid and lactic acid during malolactic fermentation. The first step in the citric acid metabolism produces acetic acid via citrate lyase activity, during which the conversion of citric acid to oxaloacetate, produces acetic acid.

  3. acetic acid bacteria

    Membrane-bound alcohol dehydrogenase oxidises ethanol to acetaldehyde. This intermediary is then oxidised further to acetic acid via membrane-bound aldehyde dehydrogenase.

  4. non-microbial source

    The chemical hydrolysis of wood hemicellulose, as well as the oxidation of gape phenolic compounds can result in the production of VA.

Factors that influence VA production…

  1. Sugar/osmotic pressure. Higher sugar concentrations result in a longer lag phase, which in turn lead to lower viability and growth potential of the yeast cells. Higher sugar concentrations together with low nitrogen levels lead to increased acetic acid concentrations.
  2. Fermentation temperature. Higher temperatures lead to higher VA concentrations.
  3. Yeast strain selection. The ability to produce VA id dependant on the specific yeast strain.
  4. The production of acetate esters e.g. ethyl acetate. This production is dependent on the yeast strain, the presence of indigenous yeast, fermentation temperature and SO2 concentrations.
  5. High initial acetic acid concentration. Rotten grapes, high sugar concentrations, pH and fermentation temperature at the start of fermentation will lead to increased acetic acid concentrations.
  6. A large bacterial population. High temperatures during storage of the wine (> 15°C), higher pH levels and lower alcohol and free SO2 concentrations, as well as poor cellar hygiene, will favour the survival of a bacterial population. 

Preventative measures…

  1. 1.     before fermentation:
  • monitor sugar and pH in the vineyard
  • do not mechanically harvest grapes that could be a potential risk
  • maintain sanitary conditions in the cellar e.g. equipment
  • use healthy grapes (avoid overripe)
  • do not excessively clarify the must, but a degree of clarification will reduce the indigenous microbial population


  1. 2.     during fermentation:
  • do acid adjustments if necessary to maintain low pH
  • maintain protective SO2 concentrations
  • use low VA-producing yeast strain
  • use sufficient nutrients during alcoholic fermentation
  • ensure fermentation is complete (no residual sugar / temperature fluctuations / re-inoculation)
  • reduce exposure to oxygen, but keep in mind that oxygen is necessary for alcoholic fermentation, as well as colour stabilising tannin reactions in red wine, so a degree of oxygen is required


  1. 3.     after fermentation:
  • inhibit malolactic fermentation with lysozyme if necessary
  • remove wine from yeast lees
  • adjust free SO2 levels to 40 ppm
  • ensure that wine is being stored in full containers
  • ensure sufficient sanitary state of barrels
  • correct usage of barrels
  • regular top up of barrels
  • bottling practices are important e.g. membrane filtration



As mentioned above, there are a variety of preventative measures, but all these techniques are irrelevant if a winemaker sits with a high final VA concentration in his wine. Correctional options include blends, reverse-osmosis and nano-filtration.


1. How to diffuse a volatile situation. Zoecklein et al. 2005.

2. The origins of acetic acid in wine. M. Lambrechts.

3. Volatile acidity in wine. R. Gawel.

4. Current vineyard and cellar events. Sources of volatile acid formation in wine and potential control measurements. C. Theron.

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Creating Hybrid Wines True to Style

Over the years, I’ve noticed that it’s easy for wineries to fall subject to – what I like to call – the “ice cream syndrome.” In this case, one varietal wine is made for each wine grape variety brought into the cellar. Soon, a wine list in a tasting room can feel a bit like a list of ice cream flavors on an ice cream shop menu:

  1. Chardonnay
  2. Grüner Veltliner
  3. La Crescent
  4. Cayuga
  5. Cayuga Reserve
  6. Moscato
  7. Pinot Noir
  8. Cabernet Sauvignon
  9. Cabernet Franc
  10. Cabernet Franc Reserve
  11. Chambourcin
  12. Noiret
  13. Chancellor
  14. Concord

The list can go on and on.

Listing the variety name on a wine label has its benefits. Many fruit wines, obviously, would benefit from a name that reflects the fruit the wine is made from. Additionally, American consumers tend to identify with many wine grape variety names on a wine bottle. This is especially true when names are well-known like Chardonnay, Cabernet Sauvignon, Moscato, etc.

However, what about French American Hybrid wine grape varieties? In some of my previous travels, I heard local grape growers and winery owners reject the integration of more hybrid wine grapes because they found them difficult to sell to consumers. There is lots of reasons that may contribute to this including

  • Unfamiliarity with the name of the variety grape/varietal wine,
  • Prestige leading to other wine selections,
  • Worry to try new things,
  • Dislike for another winery’s wine with the same varietal wine, or
  • Poor wine production for that wine grape

Nonetheless, hybrid wine grape varieties are often needed in wine regions outside of the primary western wine-producing regions in the U.S. Through my travels, I’ve found consumer acceptance of these varieties varies from state-to-state and region-to-region.

What is Wine Style?

From a wine sensory perception, many hybrid grape varieties produce wines of similar style when they are produced with routine processing techniques. A wine style often describes the wine’s color, mouthfeel and aromatic composition. Most wines can be grouped into a few select wine styles regardless of where the wine is produced. Looking at wine style pulls away from classifications that focus on wine grape variety and provides a broader perspective in looking at your wine portfolio.

For example, in the list above, when grouped by varietal name, there are 14 different wines. When grouped by color, however, there are two groups (the first six are “white” wines and the remaining eight are “red” wines). When we start to look at a wine portfolio, or tasting list, by groupings or classifications, we can better identify where there are redundancies in production. This practice can help improve wine production efficiency, allocate gaps in the portfolio, and contribute to winery branding techniques.

Focusing on hybrid grape varieties, many retain their acidity through processing and are deficient in a tannic mouthfeel compared to their Vitis vinifera(e.g., Chardonnay, Cabernet Franc) cousins. The reds often exude a bright red, sometimes purple hue. Many of the whites often have somewhat neutral aromatics, bursting of lots of fresh citrus flavors. While there are always exceptions, the similarities among the varieties, compounded with unique wine names that are not common among the wine market, can often lead to consumer confusion.

Improving Hybrid Grape Variety Winemaking

Therefore, I’ve been working with a few of my clients in reviewing their hybrid wine programs by tasting through the finished-wine portfolio. The certifications I’ve received in wine education and tasting exercises that many sommeliers use allow me to identify wines that taste similar in texture and aromatics, or a combination of both. Additionally, in working with the winemaking team, I find we can often group three or more wines into the same style.

This practice allows us to isolate where there may be stylistic gaps and re-evaluate the production focus for a given grape variety or group of varieties.

The same is also true if a “new” wine grape variety falls into the winery’s lap. What can you make of that variety (other than the same-old processing techniques and putting a varietal name on it)? I can provide insight on different wine styles appropriate for the variety, or recommend blending options before grapes are made into wine. This can provide insight and direction for processing, which can ultimately improve cellar efficiency and fill in tasting menu gaps.

The benefit of this practice also allows me to work with clients and introduce them to new wine styles for a given wine variety. This is a very useful practice in getting away from varietal names and creating wines with unique labels that may have beneficial marketing advantages.

Truthfully, we can do this with V. vinifera grape varieties, too!

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Oxygen management during winemaking

By Charl Theron of Wineland Media

The first spontaneous reaction of winemakers, when air or oxygen during winemaking is discussed or mentioned, is its negative association with the oxidation or browning of wines. The correct oxygen control can, however, have various advantages and contribute positively to wine characteristics.

An analysis of faulty wines at the well-known International Wine Challenge in London showed that oxidation or reduction are the two most important sources of faults, which occurred the most in wines. It is the extremes of oxygen exposure, either too much or too little. The controlled exposure to oxygen can, however, prevent both problems. The following six ways of controlled oxygen exposure exist during winemaking:

  1. Hyper oxygenation is the planned browning of juice prior to fermentation by means of a high oxygen addition of 8 to 30 mg/ℓ over a few hours, in order to remove potential browning components from the juice.
  2. Macro oxygenation is the dosing of 8 mg/ℓ oxygen halfway during fermentation to ensure a smooth and complete fermentation. It is often added together with nitrogen yeast nutrients.
  3. First phase of micro oxygenation (MOX): It is applied after fermentation, but before SO2-addition over a period of two to six weeks to stabilise colour, add more body to the wine and improve the longevity of the wine. It is a continuous dosing of oxygen at 1 to 5 mg/ℓ daily and depends on the oxygen appetite of the wine. This is 20 to 100 more than the oxygen supplied by barrels. The dosing units are either expressed as mg/ℓ/month (30 to 150) or mℓ/ℓ/month (20 to 100).
  4. Second phase of micro oxygenation (MOX): It is continuously applied after malolactic fermentation (MLF) and SO2-addition over a period of four to 12 weeks to refine the wine structure, integrate aromatic compounds like pyrazines and oak flavours, soften wood tannins and limit reductive tendencies. Dosages vary from 3 to 12 mg/ℓ/month or 2 to 8 mℓ/ℓ/month.
  5. Third phase of micro oxygenation (MOX): It is continuously introduced after barrel maturation when the wine is one or two years old over a period of two to 12 weeks to harmonise wood tannins and limit reductive flavours before bottling. Typical dosages are 0.4 to 3.0 mg/ℓ/month or 0.25 to 2.0 mℓ/ℓ/month.
  6. Ciqueage: It is named after the noise made by the solenoid, when the remote control is pressed to liberate oxygen. It is the punctual introduction of oxygen during maturation at 1 to 2 mg/ℓ, which is equivalent to the oxygen uptake during rackings.


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