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New World Wine Maker Blog - Technical Articles

Can yeasts be used to prevent protein haze?

Winemakers add bentonite to prevent protein haze in white wines. Although this treatment reaches its goal, it also leads to volume losses and sometimes a decrease in wine quality. The question is: are there alternatives available?

Protein haze – some background information

The removal of proteins is a key step during the production of white and rosé wines to avoid the possible appearance of a harmless, but unsightly haze. Haze formation is an aesthetic problem that consumers usually regard as a fault (e g microbial spoilage) leading to potential economic losses. Proteins that are responsible for haze formation in wine have been identified as pathogenesis-related proteins of grape origin. The most abundant class of haze-forming proteins are chitinases and thaumatin-like proteins and are continuously produced in the grape berry and even more so in response to pathogen attack. Because of their physical structure and properties, these proteins are very resilient and are not or poorly degraded during the course of fermentation. Over time and upon exposure to warm/hot temperatures during storage for instance, these proteins denature and aggregate into light dispersing particles resulting in what is referred to as ‘haze’.

The mechanisms of haze formation has received much attention from researchers over the last decade. It is complex by nature and depends on several factors, one of the most important being the presence of sulphate. The removal of these proteins is usually achieved via bentonite fining, but several issues including volume loss, aroma stripping and sustainability have been identified with the use of this clay. Several strategies have therefore been investigated over the past few years. One of the most attractive alternatives would consist in degrading these haze-forming proteins with enzymes. This is particularly appealing since enzymatic degradation of proteins (protease activity) would not lead to any of the issues mentioned for bentonite and could have the additional benefit of releasing yeast assimilable nitrogen.

Where does one find enzymes capable of degrading haze-forming proteins? …

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TAKE A BIG WHIFF!

One of the most important parameters for the measurement of quality is the aroma/flavour profile of a wine (1). Up till now, more than 1000 compounds have been identified in grapes and wine. To add to the complexity of the wine matrix, the individual concentrations of these compounds may vary considerably (2). The aroma profile will also be influenced by production processes, be it in the vineyard or cellar and with an infinite number of variations possible in the production process, the final aroma profile of a wine is a complex matter to say the least (3).

There are various influencing factors that play a role in determining wine aroma composition. These include, amongst many more, climatic conditions (e.g. altitude above sea level), viticultural practices (e.g. canopy management) and enological practices, e.g. fermentation conditions, on which this article will focus (1).

Even though some aroma impact compounds exist for some varieties, seldom can the sensory perception of wine aroma be attributed to a single compound (1). The aroma attributes of a specific compound depends not only on its concentration or the specific odour threshold value (lowest concentration at which it can be detected), but also its interaction with other aroma compounds, be it the enhancement (even compounds present below their odour threshold) or suppression of another compound (1). Because of the complexity of the wine matrix it is almost impossible to predict the interaction between aroma compounds, but certain actions, like the selection of a specific yeast strain, could aid in driving the aroma profile to a certain extent (2). This is an important tool as it has been shown that a difference in flavour profile solely as a result of the choice of yeast strain, can be detected not only by trained panels and wine professionals, but more importantly, wine consumers (6). This implies that besides choice of viticultural practices and grape selection, selecting a specific yeast strain (usually Saccharomyces cerevisiae) for fermentation, as well as a bacteria strain for MLF, can greatly modify the aroma, flavour, mouthfeel, colour and chemical complexity of a wine, making this a tool to create a specific flavour profile according to market specifications (3).

The compounds that play a role in wine appearance, aroma, flavour and mouthfeel can be derived from three sources: the grapes, microbial modification during fermentation and then maturation, be it bottle ageing or wood maturation (3).

Grape-derived compounds do not only provide the basic wine structure, but also results in distinct varietal characteristics (3). The main grape-derived aroma compounds belong to the groups of monoterpenes, norisoprenoids and methoxypyrazines. Some examples of these include rose-like geraniol in Chardonnay, spicy eugenol and guaiacol in Gewürztraminer and floral, fruity and berry-like β-damascenone and violet-like β-ionone in Cabernet Sauvignon, Shiraz and Pinotage (6). While monoterpenes and norisoprenoids are very important in Muscat and aroma-rich varieties, fermentation-derived aroma compounds play a larger role in ‘neutral’ cultivars. The following section will focus on yeast-derived fermentation aroma compounds, although MLF also makes a significant contribution towards the final wine aroma profile.

While the main purpose of yeast is to metabolise sugar in order to produce ethanol and carbon dioxide, this microbial culture also produces a myriad other metabolites that, despite being present in small amounts, significantly alters the wine aroma profile and have a significant sensorial impact (3). Yeast strains are able to modify the wine aroma via three mechanisms (3):

1) via the extraction of compounds from solids in the grape must;
2) modification of grape-derived aroma compounds and
3) producing flavour-active metabolites.

The biosynthetic pathways responsible for aroma production via these mechanisms are influenced by various factors, to name a few (3):

  1. a) viticultural factors;
  2. b) composition and pH of grape must;
  3. c) nature and prevailing temperature of grape must and
  4. d) technological aspects and vinification methods.

As previously mentioned, the yeast can modify grape-derived aroma compounds for e.g. esters, higher alcohols and lactones in Chenin blanc contributes to varietal aroma; mercapto components formed during fermentation in Sauvignon blanc adds to passion fruit, guava and other tropical aromas and iso-amyl acetate adds to banana aromas in Pinotage (6). The table below also lists some of the most important yeast-derived aroma compounds important in determining the final wine aroma profile that serves as an important quality parameter (5).

Major aroma impact compounds produced and modified by yeast during fermentation

Volatile Acids
  • produce 0.2-0.7 g/L acetic acid during fermentation
Alcohols
  • ethanol: influence volatility of other aroma compounds
  • higher alcohols: positive or negative effect on wine aroma
  • involves degradation of amino acids
Carbonyl Compounds
  • acetaldehyde: 10-75 mg/L produced (bruised apple; oxidation)
  • diacetyl: small amount (0.2-0.3 mg/L) produced by yeast (butter )
Volatile Phenols
  • off-odours: medicinal, barnyard
  • vinyl-phenols: stabilise colour in red wine
  • Brettanomyces: ethyl-phenol (negative sensory impact)
Esters
  • influence fruity and floral aromas
  • dependant on: yeast strain, fermentation temp., precursors
  • acetate esters: ethyl acetate (fruity); iso-amyl acetate (banana, pear); 2-phenylethyl acetate (honey, rose, flower)
  • ethyl esters: ethyl hexanoate and ethyl octanoate (apple
Volatile Sulphur Compounds
  • low sensory threshold (generally negative to wine quality)
  • positive: thiols (grape-derived compounds modified by yeast)
  • guava, passion fruit, grapefruit, gooseberry (Sauvignon blanc)
  • release and modification is yeast strain dependant
Monoterpenes
  • grape-derived: aromatic (free) and non-aromatic glucose-bound
  • free form: fruity and floral
  • yeast release bound form via β-glucosidase activity; add to aroma

 

It has also been shown that chemical changes that occur as a result of ageing, either bottle or wood, may also alter the wine composition and quality (1). During the ageing period, compounds are extracted from wood (oak lactones) and these add to aroma complexity. Certain compounds are also transformed and/or liberated from bound forms, which mean they can then play a role in the aroma perception of the wine.

Due to the fierce competition in the wine industry, wine producers are being forced to investigate and understand consumer preferences and expectations and produce wine accordingly. This has become a market-driven industry whereby winemakers are challenged with responding to consumer sentiments and preferences (3). One of the tools in a winemaker’s arsenal that is available to address this challenge is the selection of the microbial populations that will be responsible for fermentation. Therefore the yeast and bacteria strain(s) can be seen as a flavour-impact tool to produce a certain style of wine. This will only be possible with an understanding of the impact aroma compounds and the role the selection of the correct yeast and bacteria can play in the production and or modification of these compounds. This is the reason for the extensive and careful research that goes into the development of all Anchor yeast and bacteria cultures. This way we ensure not only optimal fermentation, but also optimal contributions to the final aroma profile.

So take a big whiff…

References:
1. Wine aroma-important aspect of wine quality. www.newworldwinemaker.com
2. Sensory perception. www.newworldwinemaker.com
3. Swiegers J.H., Bartowsky E.J., Henschke P.A. & Pretorius I.S., 2005. Yeast and bacterial modulation of wine aroma and flavour. The Australian Journal of Grape and Wine Research, 11, 139-173.
4. The complete A-G understanding to waking up your wine. www.newworldwinemaker.com
5. The impact of yeast on the sensory quality of wine. www.newworldwinemaker.com
6: Cordente A.G., Curtin C.D., Varela C. & Pretorius I.S., 2012. Flavour-active wine yeasts. Applied Microbiology and Biotechnology. DOI 10.1007/s00253-012-4370-z

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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.

Screenshot 2018-06-01 08.49.50
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.

References

DeMarsay, A. Managing Summer Bunch Rots on Wine Grapes, Maryland Cooperative Extension.http://extension.umd.edu/sites/extension.umd.edu/files/_docs/programs/viticulture/ManagingSummerBunchRots.pdf. 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. http://www.vtwines.info/. Accessed 16 April 2018.

Zoecklein, B. 2014. Grape Maturity, On-line Winemaking Certificate Program, Wine Enology Grape Chemistry Group, Virginia Tech.http://www.vtwines.info/. 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

 

Conclusions…

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.

Refrences…

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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