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New World Wine Maker Blog - Technical Articles

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 (http://bit.ly/SparklingWineTechniques) 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 (http://fortune.com/2015/05/21/the-war-on-big-food/), 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: http://www.wine-and-bubbles.com/maj/phototheque/photos/schema/ShemaEtTexteCv4_3.jpg

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 (http://www.ttb.gov/wine/wine_regs.shtml).

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: http://bit.ly/LowTempFerm.

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.

References

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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The production of lower alcohol wines

by Charl Theron

During the 1980’s and 1990’s some wine critics propagated dark coloured, full-bodied, heavy red wines. This was exemplified by tannic wines with relatively high alcohol content. The use of phenolic ripeness as criterion for harvesting contributed to this phenomenon. These wines were usually medal winners, but were not necessarily favourites of public consumers. Over the last few years pressure is internationally applied to produce wines with lower alcohol. Traditional Bordeaux and Burgundy wines with alcohol levels between 12.5 to 13.5% have consequently become popular again.

The alcohol content of Californian Napa Cabernet Sauvignon wines increased from an average of 13.2% in 1981 to an average of 15.2% in 2013. A research survey by the Californian wine giant, E&J Gallo, found that persons between 25 to 45 years are interested to experiment with wine and other alcoholic beverages by adding fruit, sparkling water and ice to it in order to develop wine based cocktails.

In 2007 market research by the Californian cellar, Francis Ford Coppola, in the United Kingdom found that 28% of the respondents were concerned about the alcohol content of the wines they buy. The same research found in 2013 that the percentage of concerned respondents increased to 40%. This confirms the international concern about wines with relatively high alcohol content. The most important reasons for the concern are health, social responsibility and the taste of the wine, but countries like the United Kingdom (UK), New Zealand and Scandinavian countries adjusted their tax structures according to the alcohol content of the wines and also restricted the marketing and promotion of high alcohol wines.

The New Zealand government initiated partnerships with 17 wine cellars to research the production of lower alcohol wines without the addition of water or using alcohol reduction technology. The research program will last for 7 years and NZ$ 13 million was budgeted for it. Both viticulture and cellar procedures will be addressed. The challenges in the vineyard will be to produce grapes with lower sugar concentrations, without having natural higher fixed acid concentrations. Sites that naturally produce lower acid grapes must be identified and viticulture practices like lower leaf-grape ratios, irrigation, fertilisation, yield and Botrytis options will be investigated. Water addition prior to alcoholic fermentation or to wine is not permitted in New Zealand and alcohol reduction technology like reverse osmosis and spinning cone are seen as too expensive for low alcohol wines. Cellar practices like the harvest time, pressing, the termination of alcoholic fermentation to ensure balanced wines, yeast selection and temperature control will consequently be investigated. As a result of the cool climate of New Zealand, the production of typical cultivar wines with the necessary balance and an alcohol content of 9% is definitely possible.

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Who’s Your Daddy?: Kerner

The subject of today’s Who’s Your Daddy post is a grape that I’ve never heard of before. As you are probably well aware, there are only really a handful of grape varieties that make up the vast majority of wines that are sold in stores, whereas in reality, there are hundreds of different varieties that have been and currently are made into wines all over the globe.

As a reminder, diversity is important not only for a little variety in your life, but more importantly for the overall health and sustainability of the wine industry as a whole, particularly in this time of climate change. Here is just one example of that diversity:

Without further ado, the focus of this “Who’s Your Daddy?” post is the Kerner grape variety (Vitis vinifera).

Brief History

The origins of Kerner are not too hard to find, considering its creation was relatively well documented compared to many other thousands of wine grapes whose origins are unknown.

Kerner, a white wine grape, was created in the greenhouse in 1929 by August Herold in Lauffen, Württemberg, Germany. The name “Kerner” was assigned to the grape in honor of a German physician and poet named Justinus Kerner. This particular poet was selected due to his works on wine (and if I can ever get my hands on some of this I’ll update this post at that time!).

The plant stayed in the laboratory/greenhouse setting for quite some time, but by 1969 was granted vartietal protection and given approval for commercial production. Finally, in 1993, it was given DOC status by the Italian Demoninazion di Origine Controllata.

Most of the plantings of Kerner are currently all over Germany, with greater concentrations planted in the Pfalz and Rheinhessen regions. The acreage of Kerner is somewhat uncertain; with some sites referencing 8,000 hectares while others referencing only 3,700 hectares. I’m not certain of the date published, but probably the figure I would trust most would be from the Wines of Germany website itself, which states that there are currently about 3,500 hectares of Kerner planted in Germany. In addition to Germany, Kerner is also found in Italy, Austria, Switzerland, England, and Japan …

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Acetaldehyde in wine

by Francois van Jaarsveld and Francois October

Acetaldehyde (ethanal; C2H4O) is a low molecular weight, flavour compound found in a wide variety of aromatic foods and beverages that have, prior to their final stage of production, undergone a degree of fermentation (McCloskey & Mahaney, 1981; Jackowetz et al., 2010). Acetaldehyde has been known to be a product of alcoholic fermentation by yeasts for almost a hundred years, but its presence in wine was not confirmed until 1984 by Dittrich and Barth. It is one of the most important aldehydes (carbonyl compounds) and constitutes more than 90% of the total aldehyde content in wine. Aldehydes, together with a large number of other volatile compounds, are responsible for wine aroma (Liu & Pilone, 2000).

Production

Acetaldehyde is primarily a product of yeast metabolism of sugars during the first stages of alcoholic fermentation. It is the last precursor in yeast fermentation before ethanol is formed, and is produced when pyruvate, the end product of glycolysis, is converted by the enzyme, alcohol dehydrogenase (ADH), to acetaldehyde. Conversely, a secondary source of acetaldehyde production in red wine, which usually occurs after ageing, is oxidation (exposure to air/oxygen) of ethanol, once again facilitated by the enzyme, alcohol dehydrogenase (Jackowetz et al., 2010).

Temperature and acetaldehyde production levels

Controversy still persists regarding the influence of fermentation temperature on acetaldehyde production levels. It was previously reported that acetaldehyde concentration levels, relative to 12, 18 and 24°C, increased significantly at a fermentation temperature of 30°C, which was in direct contrast to reports by Amerine and Ough in 1964 that fermentation temperature does not affect the final aldehyde content. However, it was recently found that cooler fermentation temperatures, in a strictly oxygen-regulated environment, actually led to higher acetaldehyde levels, which could be as a result of a reduced reutilisation of acetaldehyde by the yeasts during the last stages of fermentation (Jackowetz et al., 2010).

Production levels and stage of fermentation

Production levels of acetaldehyde during the early stages of fermentation, differ widely from the final acetaldehyde concentration in wine (Cheraiti et al., 2010) due to reutilisation by the yeast cells (Jackowetz et al., 2010; Li & Mira de Orduña, 2010), as well as degradation by bacteria (Jussier et al., 2006) during the last stages of fermentation …

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