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Understanding Difficult Malolactic Fermentations

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

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As harvest comes to a close we have planned which wines will be going through malolactic fermentation (MLF). This article provides some information to assist you in dealing with a potentially difficult MLF.

Malolactic fermentation (MLF) is a process of chemical change in wine in which L-malic acid is converted to L-lactic acid and carbon dioxide. This process is normally conducted by lactic acid bacteria (LAB) including Oenococcus oeni, Lactobacillus spp. and Pediococcus spp. O.oeni is the organism typically used to conduct MLF due to its tolerance to low pH, high ethanol and SO2. Most commercial strains are designed to produce favorable flavor profiles.

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Although inoculation with a commercial starter is recommended, MLF may occur spontaneously. The lag phase associated with spontaneous MLF may increase the risk of spoilage organisms as well as the production of volatile acidity. Inoculation with a LAB culture can help avoid these problems by providing the cell population needed to successfully conduct MLF (more than 2×106 cells/mL). The compatibility of yeast and LAB should be taken into account since failed MLF may be due to incompatibility between these two organisms.

The key to a successful MLF is to manage the process and to monitor the progress. Although there has been extensive research on the MLF process, it may still be difficult to initiate at times. The possible causes of difficult MLF have been studied less extensively than those of stuck/sluggish alcoholic fermentation. In this article, factors that may influence the start and successful completion of MLF will be discussed.

The main chemical properties that influence MLF are well known: pH, temperature, ethanol and SO2 concentration. A study by Vaillant et al (1995) investigating the effects of 11 physico-chemical parameters, identified ethanol, pH and SO2 as having the greatest inhibitory effect on the growth of LAB in wine.

pH

Generally, LAB prefer increased pH’s and usually, minimal growth occurs at pH 3.0. Under winemaking conditions, pH’s above 3.2 are advised. The pH will determine the dominant species of LAB in the must or wine.  At a low pH (3.2 to 3.4) O. oeni is the most abundant LAB species, while at higher pH (3.5 to 4.0), Lactobacillus and Pediococcus will out-number Oenococcus.

Temperature

MLF is generally inhibited by low temperatures. Research demonstrates that MLF occurs faster at temperatures of 200 C (68˚F) and above versus 150C (59˚F) and below. In the absence of SO2 the optimum temperature range for MLF is 23-250C (73.4˚F-77˚F) with maximum malic acid conversion taking place at 20-250C (68˚F-77˚F). However, with increasing SO2 levels, these temperatures decrease and 200C (68˚F) may be more acceptable.

Ethanol

LAB are ethanol-sensitive with slow or no growth occurring at approximately 13.5%. Commercial O. oeni strains are preferred starter cultures due to tolerance to ethanol.  The fatty acid composition of the cell membrane of LAB can be impacted by ethanol content.

Sulfur dioxide

LAB may be inhibited by the SO2 produced by yeast during alcoholic fermentation. A total SO2 concentration of more than 50 ppm generally limits LAB growth, especially at lower pH where a larger portion of SO2 is in the antimicrobial form. Generally, it is not recommended to add SO2 after alcoholic fermentation if MLF is desired.

Some of the lesser known factors impacting MLF are discussed below.

Fatty Acids

MLF can be inhibited by medium chain fatty acids (octanoic and decanoic acids) produced by yeast. It is difficult to finish MLF when octanoic acid content is over 25 mg/L and/or decanoic acid is over 5 mg/L. Bacterial strains that tolerate high concentrations of octanoic and decanoic acids may be important in successful MLF. It is important to check your supplier regarding strain specifications. Yeast hulls may be added before the bacteria are inoculated (0.2g/L) to bind fatty acids. Yeast hulls may also supply unsaturated fatty acids, amino acids and assist with CO2 release.

Fungicide residues

Some fungicide and pesticide residues may negatively impact malolactic bacteria. Residues of systemic pesticides used in humid years to control botrytis can be most detrimental. Care should be taken in harvest years with high incidence of botrytis. Winegrowers should be familiar with sprays used on incoming fruit and also adhere to pre-harvest intervals.

Lees compaction

Lees found at the bottom of a tank can become compacted due to hydrostatic pressure, resulting in yeast, bacteria and nutrients being confined to the point that they cannot function properly. Larger tank sizes may contribute to increased delays in the start of MLF. This inhibition of the start of MLF can be remedied by pumping over either on the day of inoculation or on the second day after inoculation of the bacteria.

Alternatively, contact with yeast lees can have a stimulating effect on MLF. Yeast autolysis releases amino acids and vitamins which may serve as nutrients for LAB. Yeast polysaccharides may also detoxify the medium by adsorbing inhibitory compounds. A general recommendation is to stir lees at least weekly to keep LAB and nutrients in suspension.

Residual lysozyme

Residual levels of lysozyme may impact MLF. Follow the supplier’s recommendations regarding the required time delay between lysozyme additions and the inoculation of the commercial MLF culture. Strains of O. oeni are more sensitive to the effects of lysozyme compared to strains of Lactobacillus or Pediococcus.

Malic acid concentration

Malic acid concentrations vary between grape cultivars and may also differ from year to year in the same grape cultivar. MLF becomes increasingly difficult in wines with levels of malic acid below 0.8g/L. In this case a ML starter culture with high malate permease activity or a short activation protocol is recommended. Check with your supplier to ensure that the chosen strain has these attributes if needed.

Wines with levels above 5 g/L malic acid may start MLF, but may not go to completion. This may be due to inhibition of the bacteria by increasing concentrations of L-lactic acid derived from the MLF itself.

Nutrients

Difficult MLF can result from insufficient nutrients necessary for LAB growth. Since yeast can reduce available nutrients for LAB, time of inoculation is important to avoid competition for nutrients. The addition of nutrients when inoculating for MLF is especially important if the must and wine has low nutrient status or if yeast strains with high nutritional requirements are used. The addition of bacterial nutrients can help ensure a rapid start and successful completion of MLF.

Research demonstrates that the longer it takes to initiate MLF, there is a greater risk for Brettanomyces growth. Some inoculate during alcoholic fermentation (AF) to avoid this problem. Co-inoculation involves adding malolactic starter 24 hours after AF starts. By controlling microbial populations, the growth of spoilage organisms such as Brettanomyces may be inhibited.

Note that inorganic nitrogen (diammonium phosphate) cannot be used by LAB. Check with your supplier for the optimum nutrient product for your particular MLF needs.

Oxygen

Malolactic bacteria are sensitive to excessive amounts of oxygen. The bacteria should not be exposed to large amounts of oxygen after AF is complete. Micro-oxygenation may have a positive impact on the completion of MLF. This impact may be due to the gentle stirring associated with micro-oxygenation that keeps LAB and nutrients in suspension rather than the exposure to oxygen itself.

Tannins

Some red grape cultivars may have difficulty completing a successful MLF. Some varieties that may experience increased MLF problems include Merlot, Tannat and Zinfandel. This may be related to certain grape tannins negatively impacting the growth and survival of LAB.

Polyphenols can have either stimulatory or inhibitory effects on the growth of wine LAB. This effect depends on the type and concentration of polyphenols as well as on the LAB strain. The tannin fraction of wine tends to complex with other compounds, minimizing their inhibitory effects on MLF. However, in wines that contain a large amount of condensed tannins only, LAB are increasingly inhibited.

MLF nutrients containing polysaccharides have been shown to minimize this effect. This may be due to interactions between the polysaccharides and tannins.

Conclusions

MLF difficulties are usually due to a combination of factors. A stuck or sluggish MLF is usually not the result of one factor alone. It is important, therefore, to both understand and manage the MLF process at each step of the winemaking process. Proper measurement of the process is also vital to be aware when MLF is not proceeding as desired.

 

References

Bousbouras, G.E. & Kunkee, R.E., 1971. Effect of pH on malolactic fermentation in wine. Am. J. Enol. Vitic. 22, 121-126.

Britz, T.J. & Tracey, R.P., 1990. The combination effect of pH, SO2, ethanol and temperature on the growth of Leuconostoc oenos. J. Appl. Bacteriol. 68, 23-3 1.

Costello, P.J., Morrison, R.H., Lee, R.H. & Fleet, G.H., 1983. Numbers and species of lactic acid bacteria in wines during vinification. Food Technol. Aust. 35, 14-18.

Davis, C.R., Wibowo, D., Eschenbruch, R., Lee, T.H. & Fleet, G.H., 1985. Practical implications of malolactic fermentation: a review. Am. J. Enol. Vitic. 36, 290-301.

Henick-Kling, T. & Park, Y.H., 1994. Considerations for the use of yeast and bacterial starter cultures: SO2 and timing of inoculation. Am. J. Enol. Vitic. 45, 464-469.

Henick-Kling, T., 1995. Control of malo-lactic fermentation in wine: energetics, flavour modification and methods of starter culture preparation. J. Appl. Bacteriol. Symp. (suppl) 79, 29S-37S.

Henschke, P.A., 1993. An overview of malolactic fermentation research. Wine Ind. J. 2, 69-79.

Ingram, L.O. & Butke, T.M., 1984. Effects of alcohols on micro-organisms. Adv. Microbiol. Physiol. 25, 254-290.

Krieger, 5., 1993. The use of active dry malolactic starter cultures. Austral. New Zealand Wine md. J. 8, 56-62.

Kreiger-Weber, S. and P. Loubser. 2010. Malolactic fermentation in wine. In Winemaking Problems Solved. C.E. Butzke (ed), pp. 88-89.Woodhead Publishing Limited, Cambridge, UK.

Kreiger-Weber, S., A. Silvano and P. Loubser. 2015. Environmental factors affecting malolactic fermentation. In Malolactic Fermentation-Importance of Wine Lactic Acid Bacteria. In Winemaking. R. Morenzoni and K. Specht (eds), pp.131-145. Lallemand Inc., Montreal, Canada.

Kunkee, R.E., 1967. Malo-lactic fermentation. Adv. Appl. Microbiol. 9, 235-279.

Lafon-Lafourcade, S., Carre, E. & Ribereau-Gayon, P., 1983. Occurrence of lactic acid bacteria during the different stages of vinification and conservation of wines. Appl. Environ. Microbiol. 46, 874-880.

Lonvaud-Funel, A. 2001. Interactions between lactic acid bacteria of wine and phenolic compounds. Nutritional aspects II, synergy between yeast and bacteria, Lallemand Technical Meeting, Perugia, Italy.

Loubser, P.A. 2004. Familiarise yourself with malolactic fermentation. Wynboer Technical Yearbook (a Wineland publication). 5:32-33.

Loubser, P., 2005. Bacterial nutrition – essential for successful malolactic fermentation. Wynboer technical yearbook 2005/2006, pp.95-96.

Malherbe, S., F.F. Bauer and M. du Toit. 2007. Understanding problem fermentations-a review. S. Afr. J. Enol. Vitic. 28(2):169-186. Nel, H.A., Moes, C.J. & Dicks, L.M.T., 2001. Sluggish/stuck malolactic fermentation in Chardonnay: possible causes. Wineland Magazine, Wynboer vol. 144, July, pp.1 13-115.

Nielsen, J.C., Pilatte, E. & Prahl, C., 1996. Maitrise de la fermentation malolactique par l’ensemencement direct du yin. Revue Francaise d’Oenologie 160, 12-15.

Nygaard, M. & Prahl, C., 1996. Compatibility between strains of Saccharomyces cerevisiae and Leuconostoc oenos as an important factor for successful malolactic fermentation. Proc. 4 0, Int. Symp. Cool Climate Vitic. Enol., Rochester, NY.

Renouf, V. and M.L. Murat. 2008. L’utilisation de levains malolactiques pour une meilleure maitrise du risqué Brettanomyces. Rev Enol. 126:11-15.

Renouf, V., S. La Guerche, V. Moine and M. Murat. 2009. Techniques for dealing with awkward malolactic fermentations. Wineland Magazine. pp. 82-85.

Vaillant, H., Formisyn, P. & Gerbaux, V., 1995. Malolactic fermentation of wine: study of the influence of some physico-chemical factors by experimental design assays. J. Appl. Bacteriol. 79, 640-650.

Wibowo, D., Eschenbruch, R., Davis, CR., Fleet, G.H. & Lee, T.H., 1985. Occurrence and growth of lactic acid bacteria in wine: a review. Am. J. Enol. Vitic. 36, 301-313.

Zoecklein, B. 2011. Fermentation considerations for the 2011 season. Enology Notes #159. As found on the Wine/Enology Grape Chemistry website

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Highlights from my Australian sabbatical leave​

By Kathy Kelley, Professor of Horticultural Marketing and Business Managment 

I just returned from a six-month sabbatical leave in Australia where I visited many wineries and tasting rooms and talked with various industry members.  I have included a map of Australia’s wine regions for your reference.

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I spent a majority of my time in Adelaide which is surrounded by over 200 tasting rooms situated in the Barossa Valley (known for Shiraz), Clare Valley (Riesling), and more than a dozen other wine regions (see map below).

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South Australia has not yet been impacted by phylloxera (http://bit.ly/2J79Xep); however, several growers and winemakers indicated that they do expect the pest to impact their vineyards at some point.  Currently, they post signs asking consumers not to walk through the vineyards and politely ask those who do to kindly leave the production area.  A few indicated that they are considering other measures (such as fencing) to protect vines near their tasting room, some of which were planted in the mid-1800s.

 

 

 

The Cube, McLaren Vale

The d’Arenberg Cube is a multi-story, Rubik’s Cube-like building (Rubik’s Cubes that look like the building are can be purchased for $10 AUS/$7.40 US).  The building includes a restaurant, a 360-degree video room where visitors can watch an artistic representation of the brand’s various wine labels, and a space for fee-based wine blending sessions.  While the Osborn family has had a presence in the Australian wine industry since 1912, the Cube opened in 2017.

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There is also a sensory room where visitors can squeeze a handpump and smell what they might expect in a glass of wine and an art gallery.  Visitors can download an app that provides additional information about each room and display.

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The tasting room is on the top floor where visitors can taste the wines (included in the $10 AUS/$7.40 US entrance fee) while looking out over the valley.  Visitors can choose from over 70 wines, including The Cenosilicaphobic (which means a fear of an empty glass) Cat (https://www.darenberg.com.au/the-experience/cellar-door/).  Apparently, there was a cat on site that had a bit of a problem with alcohol.

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Sidewood Estate, Adelaide Hills

Sidewood Estate is a winery and cidery located in the Adelaide Hills (https://sidewood.com.au).  The tasting room has an intimate space for couples and small groups to taste their wines while large groups are served in another space a short distance away.  Having two separate spaces provides a nice quiet area for couples/small groups who want to interact with staff and another where large groups don’t have to worry about being loud.

In addition, all guest can buy golf balls and practice their swing.  If they succeed in hitting a ball onto the small green located in the middle of the pond or get a hole-in-one – they can win a prize.  Not only does the driving range give nonwine drinkers something to do while they wait for their wine drinking friends, it also keeps visitors on site longer which then encourages them to purchase additional food and drinks.

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Hahndorf Hill Winery, Adelaide Hills

Hahndorf Hill Winery focuses on cool-climate varieties (due to the cool temperatures at night) and Austrian varieties, especially Gruner Veltliner.  Several years ago, they began propagating cuttings they imported from Austria, evaluated them, and now share the cuttings with other vineyards in the region.  They make four different styles of Gruner Veltliner wines: a classic style, a fruit-driven style, a “more opulent style,” and a late harvest style (https://www.hahndorfhillwinery.com.au/Gruner-Veltliner). The winery has a Gruner-focused blog called “The GRU Files” (https://www.thegrufiles.com.au) and one of the owners, Larry Jacobs, is called Australia’s “Grandfather of Gruner” (https://www.thegrufiles.com.au).

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Terrior, several wine regions

While the staff did not overly focus on terroir, several tasting rooms did display soil samples, profiles, and maps where their vineyards are located.  Below are some examples of the various ways they displayed these items.

Yalumba Family Vignerons c. 1984. A map of their vineyards and corresponding soil samples are displayed at the tasting bar.  Yalumba, located in Barossa Valley, “is one of only four wineries around the world to have its own cooperage” (https://www.yalumba.com)

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Chateau Tanunda is “home to some of the earliest plantings of vines in Barossa Valley” with some planted in the 1840s (https://www.yalumba.com).  Staff refer to soil samples and explain how production in Alluvial Clay Loam soil can differ from production in Deep Sand.

 

 

Pooley Wines, established in 1985 and located in Tasmania, is the state’s first certified environmentally sustainable vineyard (for more information about the program: http://bit.ly/2NResYy).  A fairly unique display shows the soil profiles for two vineyards: a) Sandy Loams over Sandstone in which they grow Syrah, Cabernet Sauvignon, Merlot, Reisling, Chardonnay, and Pinot Noir and b) Dolerite, black crackling clays, limestone over sandstone, in which they grow Chardonnay, Pinot Noir, Pinot Grigio, and Riesling (http://www.pooleywines.com.au/the-vineyards).

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Bleasdale, Langhorn Creek

In addition to seeing several vineyards that were planted in the mid to late-18oos, it was also incredible to see the various artifacts that wineries had kept from this period.  Bleasdale was established by Frank Potts in 1850 (https://www.bleasdale.com.au). Mr. Potts came to South Australia in 1836 and established the first winery in Langhorne Creek in the late 1850s.  As you can see in the images below, he was a skilled craftsman and built machinery that he then used to make wooden plugs for wine corks and vats, and also made his own vats and lever presses.

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The image below shows a red gum lever press that was built by Frank Potts’ sons in 1892.   It is the second press that was built on the property, the first one was built by Mr. Potts in the 1860s and had just one basket.  The design is based on basket presses Mr. Potts saw in Portugal.

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According to a sign at the winery: “Both presses were build of red gum, with the density of the wood meaning the levers would not need to be pushed down to provide mechanical advantage.”   The sign also stated that “the two presses stood side-by-side for around 20 years until the first press was deconstructed circa 1910-1915.”

Now, a little of what I saw in the marketplace.

A cider & wine concoction

While it has been on the market for a bit in Australia and New Zealand, Jacob’s Creek (Australia’s largest wine brand) released an alcoholic beverage that is a combination of white grape and apple juice called Pip & Seed.  Flavor profiles include: fruity (“exploding with the flavour of fresh, sweet apples and pears”), crisp (“bright floral aroma and fresh, crunchy apples on the palate), and sweet (“sweet taste sensation bursts with apple and pear aromas while sweeter, juicier apples party on the palate”) (http://www.jacobscreek.com/au/pip-and-seed).  At the time of this posting, the price for one 500mL bottle was $3.88 US.

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Is your wine at the correct temperature to drink?

Taylors Wines, a third generation wine business located in Clare Valley, South Australia, has taken the guesswork out of knowing when a wine is at the optimum temperature for drinking.  I found this bottle of Taylors Estate 2015 Cabernet Sauvignon ($14.42 US dollars) in a wine shop – and though it was mixed in with several other brands, the bottle neck tag attracted my attention.

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The neck tag instructs the purchaser to compare a glass of the red wine at room temperature and at the optimum temperature, per the temperature sensor on the label on the back of the bottle.

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Below, I’ve included an image of the temperature sensor printed on the back label.  According to their winemaker, this wine’s optimal drinking temperature is between 16 and 18C (60.8 to 64.4F), which correlates to the “raspberry” color section on the scale.  In the top-right portion of the image, you can see that the current temperature indicator is “lilac” which is in the range considered “too warm.”

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These are just a few of the winery tasting rooms and products that I saw in Australia.  There are many other wineries in these regions and others that provide visitors with an amazing experience and fabulous wine.  I will share more observations in future blog posts.

 

 

 

 

 

 

 

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 08.49.50Here 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.

  • 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.
  • Whole-cluster press whites, using very light pressure, and discard the initial juice.
  • Harvest fruit cool and process quickly. Sulfur dioxide can be added to harvest bins to inhibit acetic acid bacteria.
  • 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.
  • 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.
  • Use a commercial yeast strain that will initiate a rapid fermentation. The resulting carbon dioxide will help to protect against oxidation.
  • Once fermentation is complete, rack right away. Both Botrytis and laccase settle in the lees.
  • 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.
  • 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 Composition, Quality, 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.

 

 

 

Incorporating Microbiology Techniques in the Winery

By Molly Kelly

There are a number of spoilage microorganisms and yeasts that we are concerned with as winemakers. Two of the most common spoilage yeasts include Kloeckera apiculata and Brettanomyces bruxellensis. The most common form of yeast spoilage is due to Brettanomyces bruxellensis. Although mature grapes may harbor this spoilage yeast, the bigger problem can occur when winery equipment is infected due to poor sanitation practices. This yeast produces volatile phenols and acetic acid. Examples of wine flaws include aromas described as “medicinal” in white wines and “leather” or “horse sweat” in red wines. Other aromas descriptors include barnyard, wet dog, tar, tobacco, creosote, plastic and band aids.

The two major groups of wine spoilage bacteria can be placed in either the acetic acid bacteria (AAB) group or the lactic acid bacteria (LAB) group. The AAB includes the genera Acetobacter and Gluconobacter. Both have aerobic (requiring oxygen) metabolisms and thus their growth generally occurs on wine surfaces as a translucent film that tends to separate into a patchy appearance. In contrast, the LAB require low oxygen conditions for growth (i.e. they are microaerophilic to facultative anaerobic micro-organisms). The LAB includes the genera LactobacillusPediococcus and Oenococcus.

During fermentation the presence of such microbes may be indicated by a spontaneous or sluggish fermentation, or a spontaneous malolactic fermentation (MLF); or the presence of ethyl acetate, volatile acidity (VA) or other off-odors.

Winery Microbiology Laboratory

Because of these possible faults arising due to the presence of spoilage organisms, some wineries have incorporated sanitation monitoring and microbiological techniques into their production practices. Some considerations when planning a winery microbiology laboratory are: space considerations, availability of trained staff to perform testing, willingness to maintain adequate record-keeping, equipment costs as well as the cost of consumables.

Brett phase contrast

Brettanomyces spp. using phase contrast                                                                    Photo by David Hornack

Equipment/Microbiological Methods

Microscope

A microscope capable of 1000x magnification is needed to view bacteria and yeast. These can cost anywhere from $1000-$3000 or more but bargains can be found on used microscopes. A phase-contrast microscope requires no staining of slides due to enhanced differences in refractive index between the microorganisms and surrounding medium. This feature also allows for rapid detection and response. The staff in the microbiology lab should have training in the proper use of a microscope as well as identification of microorganisms.

In addition to identifying spoilage organisms, a microscope can be used to monitor yeast populations. By using a simple methylene blue stain, yeast viability can be determined.

Culture plate

Bacterial culture media is available for the growth of spoilage organisms for identification. This requires additional equipment including an incubator. This also requires further training in sterile technique and organism identification techniques. Several types of culture media exist for the detection of the organism of interest. For example, media used to plate for Brettanomyces contains chloramphenicol (200 mg/L) to prevent bacterial growth while others may contain cyclohexamide to prevent Saccharomyces growth. Common media used in culturing juice, wine and environmental samples include WL and WL-differential agar.

Acetobacter and yeast culture

Acetobacter spp. and yeast on WL agar
Photo by Molly Kelly

Membrane Filter MethodScreenshot 2018-02-27 13.16.26

The membrane filter method can be used to isolate small numbers of microbes from a liquid sample. A sterile cellulose nitrate membrane (0.45 microns for bacteria, 0.65-8 microns for yeasts) is placed on a vacuum flask and filtered. Using sterile technique, the membrane is placed on the culture plate and monitored for growth. This method could be used to check bottle sterility.

Environmental Monitoring

The swab test method is used for semi-quantitative analysis. Moist sterile cotton swabs are used to monitor dry areas (moistened with sterile saline or water). Dry swabs can be used to test moist areas. The swabs can then be used to inoculate the proper agar medium, depending on the organism of interest. Agar plates can also be used to detect airborne organisms at critical winery locations. Plates are left open for 30 minutes to 2 hours and then incubated. Airborne organisms that settle on the plate will grow and can be further identified.

Bioluminescence

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Monitoring systems exist that utilize bioluminescence technology to measure adenosine triphosphate (ATP). ATP is found in all plant, animal and microbial cells and is the prime energy currency that fuels metabolic processes. It is therefore possible to detect and measure biological matter that should not be present if proper sanitation practices are followed. One system by Hygiena uses an enzyme found in fireflies (luciferase). In the presence of ATP, an oxidation reaction occurs that results in light formation that is directly proportional to the amount of ATP present. Results are numeric and expressed as relative light units (RLU).

Cellar Hygiene

It should be stressed that cellar hygiene is critical in maintaining wine integrity and quality. Poor wine quality is usually due to poor sanitation practices. Areas of spoilage organism build-up include: the vineyard, second-hand barrels, imported bulk wine and areas of the winery that are difficult to reach.

Outsourcing

There are commercial enology laboratories that provide all of the microbiological services discussed here. For more information please contact Molly Kelly at mxk1171@psu.edu.

References

Crowe, A. August 2007. Avoiding Stuck Ferments. Wine Business Monthly

Just, E. and H. Regnery. 2008. Microbiology and wine preventive care and monitoring in the wine industry. Sartorius Stedim Biotech.

Margalit, Y. 1996. Winery Technology and Operations. The Wine Appreciation Guild, San Francisco.

Ritchie, G. 2006. Stuck Fermentations. Fundamentals of Wine Chemistry and Microbiology. Napa Valley College.

Specht, G. Sept/Oct 2003. Overcoming Stuck and Sluggish Fermentations. Practical Winery and Vineyard Journal.

Van de Water, L. Sept/Oct 2009. Monitoring microbes during fermentation. Practical Winery and Vineyard Journal.

Zoecklein, B., Fugelsang, K.C., Gump, B.H. and Nury, F.S. 1999. Wine Analysis and Production, Kluwer Academic/Plenum Publishers, New York.

Zoecklein, B. 2002. Enology Notes #65, Enology-Grape Chemistry Group, Virginia Tech.

Welcome, Dr. Molly Kelly, our new Enology Extension Educator

 

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Dr. Molly Kelly, Penn State Enology Extension Specialist

Please join us in welcoming Dr. Molly Kelly as our new Penn State Enology Extension Educator.  In this role, Molly will support the technical needs of the Pennsylvania wine industry and lead educational programming focusing on wine quality.  She has lead workshops, including winery sanitation, filtration, microscopy, wine analysis and berry sensory analysis. Molly’s research has focused on the effect of nitrogen and sulfur applications on Petit Manseng wine aroma and flavor. Her current research includes a pre-harvest, on-the-vine dehydration study in collaboration with Virginia Tech University.

 

Prior to this position, Molly was the Enology Extension Specialist at Virginia Polytechnic Institute and State University in Blacksburg, Virginia. She also held the position of enology instructor at Surry Community College in Dobson, N.C., where she developed the enology curriculum and managed all aspects of the college’s 1,000-case bonded winery. Under her direction, Surry produced numerous international award-winning wines. Prior to her position at Surry, she was a biodefense team microbiologist with the New York State Department of Health.

Molly can be reached by phone (814-865-6840) or email (mxk1171@psu.edu).  Her address is Department of Food Science, 327 Erickson Food Science Building, University Park, PA 16802.

Winemaking in Austria: An introduction and comments from an 8th generation winemaker

By Dr. Helene Hopfer

Since starting to work at Penn State last year, I am excited about all these local wines made of Austrian varieties. As a native Austrian, Grüner Veltliner, Zeigelt, and Blaufränkisch (called Lemburger in Germany and Kékfrankos in Hungary) and in particular, the (even) lesser known Rotgipfler, Zierfandler and St. Laurent, are near and dear to my heart (and my palate).

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The more I learn about viticulture in Pennsylvania, the more similarities I discover: Similar to Pennsylvania, Austrian growers worry about late spring frosts, fungal pressure and fruit rot, wet summers, and damaging hail events [1]. So it is only fitting to provide some details and insight to Austrian winegrowing and winemaking through this blog post.

Located in the heart of Middle Europe, the Austrian climate is influenced by a continental Pannonian climate from the East, a moderate Atlantic climate from the West, cooler air from the north and an Illyrian Mediterranean climate from the South. Over the past decades, the number of very hot and dry summers is increasing, leading to more interest in irrigation systems, as on average the annual average temperature in Austrian wine growing areas increased between 0.3 to 1˚C since 1990 [2].

 

Austrian wine growers also see a move towards larger operations: Similar to other wine regions in Europe, the average vineyard area per producer is increasing, from 1.28 ha / 3.16 acres in 1987 to 3.22 ha / 7.96 acres per producer in 2015. Many very small producers who often run their operations besides full-time jobs are now selling grapes or leasing their vineyards to larger wineries. A similar trend is true for wineries [2].

 

Different to other countries where widely known varieties like Chardonnay or Pinot noir make up the majority of plantings, the most commonly planted grape varieties in Austria are the indigenous Grüner Veltiner (nearly 50% of all whites) and Zweigelt (42% of all reds), followed by the white Welschriesling and the red Blaufränkisch (Lemburger).  Another interesting fact is that over 80% of all planted vines are 10 years or older, with 30% of all vines being more than 30+ years old [2].

 
The Austrian wine market is very small on a global scale, with just over 45,000 hectares / ~ 112,000 acres of planted and producing vineyards by around 14,000 producers nation-wide [2]. Nevertheless, Austrian wine exports are steadily increasing, particularly into countries outside of the European Union, such as the USA, Canada, and Hongkong, indicating a strong interest in this small wine-producing country. Austrian wines are considered high quality, attributable to one of the strictest wine law in the world, the result of the infamous wine scandal of 1985 [3]. Today, the law regulates enological treatments (e.g., chaptalization, deacidification, and blending), levels and definitions of wine quality (e.g., the “Qualitätswein” designation requires a federal evaluation of chemical and sensory compliance), and viticultural parameters such as maximum permitted yield of 9 tons/ha or 67.5 hL/ha and permitted grape cultivars (currently 36 different varieties) [4].

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One of the leading figures in developing the now well-established Austrian wine law was Johann Stadlmann, then president of the Austrian Wine Growers’ Association. During his 5-year tenure starting in 1985 at the peak of the wine scandal, he made sure that the wine law could be implemented in every winery and ensured strict standards; Johann Stadlmann could be called the father of the Austrian ‘Weinwunder’ (=’wine miracle’), the conversion of Austria as a mass-producing wine country to one with an emphasis on high quality.

 

Weingut Stadlmann – an estate with a very long history

If you ever visit Austria, you most likely fly into Vienna, the country’s capital. Vienna is one of the few cities in the world that also has producing vineyards located within city limits. Just outside of the city limits to the South, lies another important wine region in Austria, the so-called ‘Thermenregion’, named after thermal springs in the region. The region has a long wine history, dating back to the ancient Romans, and later Burgundian monks in the Middle Ages. The region is characterized by hot summers and dry falls, with a continuous breeze that reduces fungal pressure. One of the leading producers within the region is the Weingut Stadlmann, dating back to 1778 and now run by the eighth generation, Bernhard Stadlmann. He is the latest in a line of highly skilled winemakers that combine innovation with a conservative approach. His grandfather, Johann Stadlmann (yes, the same guy of the Austrian wine law), was one of the first ones in Austria to use single vineyard designations on his wine labels. Bernhard’s father, Johann Stadlmann VII, a master in creating wines from varieties only grown in this region, and named ‘winemaker of the year’ in 1994, is known for his careful approach and is now working alongside his son, Bernhard. In 2007, Bernhard started the conversion of the family-owned vineyards to certified organic. The family cultivates some of the best vineyards in Austria, including the single vineyard designations ‘Mandel-Höh’, ‘Tagelsteiner’, ‘Igeln’, and ‘Höfen’, planted with the indigenous varieties Zierfandler and Rotgipfler only grown here in the region. Wines from these vineyards are among the very best Austria can offer!

 

The vineyards cultivated by the Stadlmann family also differ quite dramatically in soil composition: While the ‘Mandel-Höh’ vineyard is highly permeable to water and nutrients, with lots of ‘Muschelkalk’ (limestone soil formed of fossilized mussels shells), is the ‘Taglsteiner’ vineyard characterized by more fertile and heavier ‘Braunerde’ soil, capable of retaining more water.

Screenshot 2017-10-23 10.39.07

The long winemaking history becomes apparent once one steps into the wine cellar, full of large barrels, made of local oak: Some of these barrels have hand-carved fronts, depicting their vineyards and Johann Stadlmann senior. All of these barrels are in use, and part of the Stadlmann philosophy of combining tradition with innovation.

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Another increasing threat is the spotted wing Drosophila, Drosophila suzukii, damaging ripening grape berries from véraison onwards. Bernhard sees some varieties more affected by Drosophila suzukii than others. There is intensive research on pest control, including shielding nets, fly traps, and insecticide strategies, and the Stadlmanns currently run experiments within their organic program: They blow finest rock flour (Kaolin and Dolomite rock) into the leaf canopy and fruit zone to create unfavorable conditions for different insects, including Drosophila, wasps, which pierce sweet berries, earwigs and Asian lady beetle, both leafing residuals causing off-flavors in the wine once they’re crushed.  Drosophila suzukii was first discovered in Austria a few years ago and is also an issue in the US (see also Jody Timer’s blog post).

 
During a recent visit at the Stadlmann estate, I had the chance to chat with Bernhard about the challenges of Austrian winegrowing and winemaking. I was interested in a young winemaker’s perspective, especially as Bernhard has been trained all around the world, including Burgundy, Germany, and California. This year, spring frost in late April threatened vineyards in many winegrowing regions in Austria, requiring the use of straw bales and paraffin torches to produce protective and warming smoke. Luckily, not too much damage was done to the Stadlmann’s vineyards, however, it caused some sleepless nights for Bernhard and his family, and shows also the importance of developing effective spring frost prevention alternatives (see also Michela Centinari’s blog post).

 
We also talked quite a bit about wine quality: while the Austrian ‘Qualitätswein’ designation ensures basic chemical (e.g., ethanol content, titratable and volatile acidity, residual sugar, total and free SO2, malvidin-3-glucoside content (for reds), etc.) and sensory (i.e., wine defects like volatile acidity, Brettanomyces, atypical aging, mousiness and other microbial defects) quality, this only ensures a lower limit of quality. In the recent years, the Austrian governing bodies added another layer of wine quality, based on the Romanic system of regional typicity and origin: The so-called DAC (Districtus Austriae Controllatus) wines are quality wines typical for a region, made from varieties that are best suited for that region. DAC wine producers adhere to viticultural, enological, and marketing standards, with the goal to establish themselves as famous wines of origin (think Chablis, Cote de Nuits, Barolo, Rioja or Vouvray).  As this is a relatively new system for Austria, we will see how successful these DAC regions will be. Their success will also depend on the regional producers, and how stringent they set the criteria for the DAC designation, as they have to walk a fine line between establishing a recognizable regional typical wine without losing individual character that each producer brings to their wines.

 

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If you are interested in learning more about Austrian wines, and Bernhard and his family’s wines, they were recently highlighted in a couple of US wine publications, including a great podcast episode on ‘I’ll drink to that!’ and an article in the SOMM journal about Zierfandler. Zierfandler is one of Stadlmann’s signature varieties, indigenous to the region, but tricky to grow, as it requires long and dry ripening periods and has a very thin skin, prone to botrytis. However, when done well (like the Stadlmanns do), it produces extraordinary wines with fruity, floral, and sometimes nutty notes that have a long aging potential. If you are able to get your hand on these Zierfandler wines get them while you can!

Last, a big Thank You to Bernhard Stadlmann for his help with this blog post: He took time out of his super busy harvest schedule to show me around, never getting tired of answering my questions. He also graciously provided all but one of the pictures.

References

[1] Huber K (2017) Durchschnittliche Weinernte 2017 erwartet. LKOnline. Available at (in German): https://noe.lko.at/weinbau+2500++2455141

[2] Austrian Wine Marketing Board (2017) Austrian Wine Statistics Report 2015. Available at (in German): http://www.austrianwine.com/facts-figures/austrian-wine-statistics-report/

[3] New York Times (1985) Austria’s Wine Laws Tightened in Scandal. Available at: http://www.nytimes.com/1985/08/30/world/austria-s-wine-laws-tightened-in-scandal.html

[4] Austrian Wine (2017) Wine Law. Available at: http://www.austrianwine.com/our-wine/wine-law/

Harvest Preparation for Sub-Optimal Fruit: Botrytis

By: Denise M. Gardner

The eastern U.S. growing seasons can be somewhat unpredictable.  Late season rains or untimely hurricane events can be a recipe for disaster for local grape growers (http://www.pawinegrape.com/uploads/PDF%20files/Documents/Viticulture/Harvest/Rain%20at%20Harvest.pdf), and a few have been unprepared for such events in the past.  These weather events can lead to higher incidences of the grey-rot form of Botrytis in addition to other rots, which may also be related to pest damage.  Furthermore, these weather incidences and pest damage can ultimately impact picking decisions for growers and wineries (Osborne, 2017).

It is almost inevitable that wineries need to be prepared for end-of-season weather flops, and plan for the best possible ways to manage or maintain wine quality in light of above-average disease pressure.

One disease that winemakers can prepare for prior to harvest is Botrytis.  For the purpose of this article, we’ll be using the term Botrytis to indicate the grey-mold or grey-rot form of the disease.  Grey-mold, the form of Botrytis more commonly noticed in humid regions or during heavy-precipitation seasons, can ultimately affect wine quality.  Peynaud (1984) has defined 4 ways in which the grey-mold can negatively affect wine quality:

  • Deplete wine color (especially important in red varieties),
  • Increase the risk of premature browning (through oxidative enzymes),
  • Deplete varietal character (through degradation of grape skins), and
  • Contribution to off-flavors developed by the mold’s presence on the fruit.

Botrytis, grey-mold, infection can force winemakers into alternative winemaking techniques in order to retain wine quality. Photo by: Denise M. Gardner

Based on a 1977 study by Loinger et al., guidelines pertaining to wine quality were developed with regards to a visual assessment of Botrytis incidence on incoming fruit:

  • 5-10% Botrytis rot on clusters: noticeable reduction in wine quality; wine quality is still “good” (as opposed to very good with 0% rot on clusters)
  • 20-40% Botrytis rot on clusters: marked reduction in wine quality; wine quality is “low”
  • >80% Botrytis rot on clusters: wine is commercially unacceptable

With a noticeable sensory and chemical difference in Botrytis-infected clusters, it is best for wineries to develop a standard operating procedure (SOP) for assessing rot-infected fruit, as well as how the grapes should be handled and processed during production.  While there is no one correct way to work with the wine, below are some suggestions or options that wineries can integrate when dealing with Botrytis-infected grapes.  For a full list of possibilities, please visit: http://extension.psu.edu/food/enology/wine-production/producing-wine-with-sub-optimal-fruit/fermenting-with-botrytis-101

Pre-Fermentation Sorting

Some wineries will sort through all incoming grape clusters prior to the crushing/destemming process to assess for any cluster damage or presence of unwanted material.  If your operation is not set up with this equipment, sorting can also take place in the vineyard.  Depending on the concentration of disease and on the projected wine style or quality parameter the fruit will go towards, disease portions of clusters can be cut out in the vineyard.  Or diseased fruit can be left in the vineyard to deal with after the harvest is complete.  Sorting out diseased fruit from that of decent quality will reduce the impact of the mold on the wine’s aroma, flavor, and quality.

Limit Contact Time with Skins

Depending on the resource, there are various recommendations for how to handle diseased fruit.  In whites, some recommend whole cluster pressing and tossing the first 10+ gallons, which are rich in Botrytis metabolites (Fugelsang and Edwards, 2007).  Many recommend separating juice press fractions for white and rosé wines, as this will give the vintner more control over the chemical constituents (e.g., phenolics, enzymes, and disease-related off-flavors) in the final wine.

Depending on the desired outcome for a red wine, treating or limiting skin contact with diseased fruit may be ideal post -primary fermentation.  This would include avoiding extended maceration processes.  Due to the fact that the presence of Botrytis on red varieties reduces anthocyanin and phenolic extraction (Razungles, 2010) in addition to the varietal aromatics, excessive skin contact may not be ideal during primary fermentation.  Whole berry fermentations, as opposed to a more aggressive crush and destem process, may help minimize extraction of Botrytis metabolites, which can also contribute to mouthfeel variations or off-flavors.

Tannin additions pre-fermentation may also be good considerations to compensate for phenolic losses associated with Botrytis infection.  Pre-fermentation and post-fermentation additions may help rebuild the wine’s structure or provide constituents for color stabilization.

Flash pasteurization (i.e., flash détente) has been previously recommended for Botrysized fruit to inactive the laccase enzyme associated with Botrytis, enhance color stability in reds, as well as improve the aromatics and flavors associated with the final wine.  Wines that undergo a thermovinification step tend to extract more anthocyanins and phenolics compared to traditionally fermented wines (Razungles, 2010).  Additionally, this heat step helps to inactivate laccase, which can contribute to early browning or oxidation of young wines.  However, commercial producers may not find this technological application easily accessible.

Therefore, in addition to minimizing skin contact time, winemakers will want to reduce contact time with the gross lees, and may also remove the wine from fine lees associated with the mold-infected fruit quickly.  The integration and use of clean, fresh lees, however, is still encouraged.  Removing the lees associated with mold-infected fruit can help reduce additional contact time with rot metabolites that have settled out with the lees.  This inhibits further integration of those metabolites into the wine.

Inoculate with a Commercial Yeast Strain

The presence of rot is one incidence in which processing techniques (e.g., cold soak) that encourage native microflora to dominate the fermentation are probably not desired.  Things like cold soak and native ferments allow ample opportunity for the mold to progress and contribute to the wine’s flavor.

Fruit that has rot or microflora issues is best inoculated with commercial yeast and malolactic bacteria strains to outcompete the native microflora (including those microorganisms that contribute to the rot), and to give the fermentation its best chance at completing the fermentation cleanly.  Remember that proper yeast nutrition is important to support the yeasts’ growth and to reduce the risk of hydrogen sulfide development.  For more information on determining the starting nitrogen concentrations (YAN) and how to properly treat your fermentation with added nutrients, please refer to:

Penn State Extension’s Wine Made Easy Fact Sheet: Nutrient Management During Fermentation

With high Botrytis concentrations, a more robust yeast strain may be preferred in order to quickly get through primary fermentation.  A quicker fermentation may simplify the aromatics associated with the wine, but it will also ensure little opportunity for additional spoilage.  Saccharomyces bayanus strains are often selected as more robust yeast strains.

Use of commercial yeast strains can be a valuable tool when dealing with disease-infected fruit. Photo by: Denise M. Gardner

Use of Sulfur Dioxide

Sulfur dioxide additions at crush will be determined based on the style of wine in which you are producing (e.g., white, rosé, red, etc.), but in general, the use of sulfur dioxide can help inhibit further spoilage of your product and retain antioxidant capacity.  Sulfur dioxide additions in the juice stage will help minimize early browning, but primarily inactivate PPO.

In general, botrysized wines tend to require more sulfur dioxide as Botrytis metabolites bind with free sulfur dioxide (Goode, 2014).  This is true even when processing wines with the noble rot version of Botrytis.

When primary fermentation, and malolactic fermentation (dependent on style), is complete it is a good idea to ensure that the wine has an adequate free sulfur dioxide content in order to retain its antimicrobial protection.

Fining

Some fining agents may also be applicable in the juice stage.  For example, some producers find it helpful to fine juice with bentonite in order to reduce protein content, as well as help minimize rot-associated off-flavors or partially reduce laccase concentrations.

PVPP can be added to the juice to reduce potential browning pigments or their precursor forms (Van de Water, 1985).

In both of these scenarios, neither bentonite or PVPP is specific for rot-related constituents, but each could be helpful to avoid potential challenges later on in the production process.

The presence of Botrytis can also contribute glucans to the must/wine, which can cause filterability problems for heavily-infected wines.  In this situation, many suppliers have beta-glucanase enzymes that can be applied either to the juice, wine, or both, to help breakdown the glucans and enhance ease of filterability.

A Word about Laccase

Both polyphenol oxidase (PPO) and laccase can cause early browning in grapes and wine.  However, PPO is inhibited by the alcohol content that is developed during primary fermentation.  Laccase, however, is not inhibited by the presence of alcohol, and can only be inactivated by a pasteurization step, heated to at least 60°C (140°F) (Wilker, 2010).

Grapes tend to be higher in laccase concentration when infected with Botrytis, and, thus, wines produced from grapes that had a high incidence rate of Botrytis can develop a brown hue post-primary fermentation.  This oxidative activity can occur even in young wines.

If you are concerned about the prevalence of laccase in diseased-fruit, wineries can submit wine samples to a wine lab for a laccase test.  Or, if you own a copy of “Monitoring the Winemaking Process from Grapes to Wine: Techniques and Concepts” by Patrick Iland et al., pg. 90 and 94 have 2 laccase test protocols that outline how wineries can assess oxidation by laccase.  The results of these test will indicate if extreme treatments are required during production to avoid the rapid and early oxidation caused by laccase.

 

Additional Resources:

 

Literature Cited:

Goode, J. 2014. The Science of Wine: From Vine to Glass. (2nd Ed.) University of California Press: Berkley, California. 216 pg.

Fugelsang, K.C. and C.G. Edwards. 2007. Wine Microbiology: Practical Applications and Proceedings. (2nd Ed.) Springer: New York, NY. 393 pg.

Loinger, C., S. Cohen, N. Dror, and M.J. Berlinger. 1977. Effect of grape cluster rot on wine quality. AJEV. 28(4): 196-199.

Peynaud, E. 1984. Knowing and Making Wine. Wiley-Interscience: New York, NY. 391 pg.

Razungles, A. 2010. Extraction technologies and wine quality. In Managing Wine Quality, Vol. 2 Oenology and Wine Quality. Andrew G. Reynolds, Ed. Woodhead Publishing: Philadelphia, PA. 651 pg.

Van de Water, L. 1985. Fining Agents for Use in Wine. The Wine Lab.

Wilker, K.L. 2010. How should I treat a must from white grapes containing laccase? In Winemaking Problems Solved. CRC Press: Boca Raton, Florida. 398 pg.