One of the most important parameters for the measurement of quality is the aroma/flavour profile of a wine (1). Up till now, more than 1000 compounds have been identified in grapes and wine. To add to the complexity of the wine matrix, the individual concentrations of these compounds may vary considerably (2). The aroma profile will also be influenced by production processes, be it in the vineyard or cellar and with an infinite number of variations possible in the production process, the final aroma profile of a wine is a complex matter to say the least (3).
There are various influencing factors that play a role in determining wine aroma composition. These include, amongst many more, climatic conditions (e.g. altitude above sea level), viticultural practices (e.g. canopy management) and enological practices, e.g. fermentation conditions, on which this article will focus (1).
Even though some aroma impact compounds exist for some varieties, seldom can the sensory perception of wine aroma be attributed to a single compound (1). The aroma attributes of a specific compound depends not only on its concentration or the specific odour threshold value (lowest concentration at which it can be detected), but also its interaction with other aroma compounds, be it the enhancement (even compounds present below their odour threshold) or suppression of another compound (1). Because of the complexity of the wine matrix it is almost impossible to predict the interaction between aroma compounds, but certain actions, like the selection of a specific yeast strain, could aid in driving the aroma profile to a certain extent (2). This is an important tool as it has been shown that a difference in flavour profile solely as a result of the choice of yeast strain, can be detected not only by trained panels and wine professionals, but more importantly, wine consumers (6). This implies that besides choice of viticultural practices and grape selection, selecting a specific yeast strain (usually Saccharomyces cerevisiae) for fermentation, as well as a bacteria strain for MLF, can greatly modify the aroma, flavour, mouthfeel, colour and chemical complexity of a wine, making this a tool to create a specific flavour profile according to market specifications (3).
The compounds that play a role in wine appearance, aroma, flavour and mouthfeel can be derived from three sources: the grapes, microbial modification during fermentation and then maturation, be it bottle ageing or wood maturation (3).
Grape-derived compounds do not only provide the basic wine structure, but also results in distinct varietal characteristics (3). The main grape-derived aroma compounds belong to the groups of monoterpenes, norisoprenoids and methoxypyrazines. Some examples of these include rose-like geraniol in Chardonnay, spicy eugenol and guaiacol in Gewürztraminer and floral, fruity and berry-like β-damascenone and violet-like β-ionone in Cabernet Sauvignon, Shiraz and Pinotage (6). While monoterpenes and norisoprenoids are very important in Muscat and aroma-rich varieties, fermentation-derived aroma compounds play a larger role in ‘neutral’ cultivars. The following section will focus on yeast-derived fermentation aroma compounds, although MLF also makes a significant contribution towards the final wine aroma profile.
While the main purpose of yeast is to metabolise sugar in order to produce ethanol and carbon dioxide, this microbial culture also produces a myriad other metabolites that, despite being present in small amounts, significantly alters the wine aroma profile and have a significant sensorial impact (3). Yeast strains are able to modify the wine aroma via three mechanisms (3):
1) via the extraction of compounds from solids in the grape must;
2) modification of grape-derived aroma compounds and
3) producing flavour-active metabolites.
The biosynthetic pathways responsible for aroma production via these mechanisms are influenced by various factors, to name a few (3):
- a) viticultural factors;
- b) composition and pH of grape must;
- c) nature and prevailing temperature of grape must and
- d) technological aspects and vinification methods.
As previously mentioned, the yeast can modify grape-derived aroma compounds for e.g. esters, higher alcohols and lactones in Chenin blanc contributes to varietal aroma; mercapto components formed during fermentation in Sauvignon blanc adds to passion fruit, guava and other tropical aromas and iso-amyl acetate adds to banana aromas in Pinotage (6). The table below also lists some of the most important yeast-derived aroma compounds important in determining the final wine aroma profile that serves as an important quality parameter (5).
Major aroma impact compounds produced and modified by yeast during fermentation
- produce 0.2-0.7 g/L acetic acid during fermentation
- ethanol: influence volatility of other aroma compounds
- higher alcohols: positive or negative effect on wine aroma
- involves degradation of amino acids
- acetaldehyde: 10-75 mg/L produced (bruised apple; oxidation)
- diacetyl: small amount (0.2-0.3 mg/L) produced by yeast (butter )
- off-odours: medicinal, barnyard
- vinyl-phenols: stabilise colour in red wine
- Brettanomyces: ethyl-phenol (negative sensory impact)
- influence fruity and floral aromas
- dependant on: yeast strain, fermentation temp., precursors
- acetate esters: ethyl acetate (fruity); iso-amyl acetate (banana, pear); 2-phenylethyl acetate (honey, rose, flower)
- ethyl esters: ethyl hexanoate and ethyl octanoate (apple
|Volatile Sulphur Compounds
- low sensory threshold (generally negative to wine quality)
- positive: thiols (grape-derived compounds modified by yeast)
- guava, passion fruit, grapefruit, gooseberry (Sauvignon blanc)
- release and modification is yeast strain dependant
- grape-derived: aromatic (free) and non-aromatic glucose-bound
- free form: fruity and floral
- yeast release bound form via β-glucosidase activity; add to aroma
It has also been shown that chemical changes that occur as a result of ageing, either bottle or wood, may also alter the wine composition and quality (1). During the ageing period, compounds are extracted from wood (oak lactones) and these add to aroma complexity. Certain compounds are also transformed and/or liberated from bound forms, which mean they can then play a role in the aroma perception of the wine.
Due to the fierce competition in the wine industry, wine producers are being forced to investigate and understand consumer preferences and expectations and produce wine accordingly. This has become a market-driven industry whereby winemakers are challenged with responding to consumer sentiments and preferences (3). One of the tools in a winemaker’s arsenal that is available to address this challenge is the selection of the microbial populations that will be responsible for fermentation. Therefore the yeast and bacteria strain(s) can be seen as a flavour-impact tool to produce a certain style of wine. This will only be possible with an understanding of the impact aroma compounds and the role the selection of the correct yeast and bacteria can play in the production and or modification of these compounds. This is the reason for the extensive and careful research that goes into the development of all Anchor yeast and bacteria cultures. This way we ensure not only optimal fermentation, but also optimal contributions to the final aroma profile.
So take a big whiff…
1. Wine aroma-important aspect of wine quality. www.newworldwinemaker.com
2. Sensory perception. www.newworldwinemaker.com
3. Swiegers J.H., Bartowsky E.J., Henschke P.A. & Pretorius I.S., 2005. Yeast and bacterial modulation of wine aroma and flavour. The Australian Journal of Grape and Wine Research, 11, 139-173.
4. The complete A-G understanding to waking up your wine. www.newworldwinemaker.com
5. The impact of yeast on the sensory quality of wine. www.newworldwinemaker.com
6: Cordente A.G., Curtin C.D., Varela C. & Pretorius I.S., 2012. Flavour-active wine yeasts. Applied Microbiology and Biotechnology. DOI 10.1007/s00253-012-4370-z
Q. When and where were you born ?
“I was born on 10th January 1967 in Wellington and grew up on a wine farm.”
Q. Where did you study and what qualifications do you have ?
“I went to Elsenberg Agricultural College and did cellar technology. I did various marketing and management courses through the University of Stellenbosch. I also did all the official wine judging certificates which enabled me to judge on all the various competitions.”
Q. Do you have any varieties you prefer to work with ?
He answers as if that was a dumb question ! “Obviously Chenin Blanc and Pinotage.”
Q. Have you been influenced by any particular winemaker or region ?
“When I first started working I was an assistant winemaker to the legendary Dassie Smith of Rooiberg Wines. He taught me discipline, the importance of hygiene in the cellar and to pay attention to detail.” After a little thought “ Ja, Dassie was a great and very positive influence.”
Q. What would you consider your greatest achievement as a winemaker ?
“The first ABSA Top Ten Pinotage Competition when my pinotage under the Swartland label was one of the top ten. Then when I started my own brand, Blake Family Wines, our maiden vintage, Blake’s Amethyst 2011 was selected as one of the ABSA Perold Top Five Cape Blend winners.”
Q. What “secrets” have you “developed” that make your wines different to others ?
“ Not really a secret, but attention to detail, hard work and a passion is the golden key.”
Q. How important is modern winemaking equipment in your winemaking ?
“It is very important, but it is not the only important thing. Things well away from equipment such as Terroir, good winemaking practices and attention to detail are all more important than equipment.”
Q. You have recently made some interesting changes to your career ?
“Yes, after 19 vintages at Swartland I started my own brand of Blake Family Wines. Then in November 2013 I left Swartland and took up an interesting challenge as CEO of Klawer Wines. I still do my own brand.
Q. Of the future ?
“ I think wine prices will increase drastically over the next few years as there will be a shortage of quality wine. I intend to be in the quality wine business. !” After some thought he adds “Marinda, my wife, and I are very proud of the success of our wines and they are named after gems, Tourmaline and Amethyst, and we intend to treat them as such.”
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.
Here we will focus on the most common bunch rot pathogen of mature berries, Botrytis cinerea. How severe can Botrytis bunch rot be before wine quality is impacted? This will depend on the type of rot as well as winemaking techniques however, even low levels of infection have been shown to negatively impact wine quality. Red wine quality was shown to be affected by as low as a 5% infection rate of B. cinerea. Extended skin contact in red winemaking can increase the effect of bunch rots on the finished wine. While B. cinerea can be linked with sour rot, it is more commonly associated with other fungi including Aspergillus spp. Sour rot is caused by yeast, acetic acid and other bacterial growth. When acetic acid bacteria, yeast and filamentous fungi are present together, high levels of acetic acid can result. Berries infected with sour rot have a distinct vinegar smell that may be combined with the presence of ethyl acetate. Ethyl acetate is an ester described as smelling like nail polish remover.
Laccases are enzymes produced by fungi. They break down anthocyanins and proanthocyanidins which are important phenolic compounds that contribute to palate structure and wine color. In white wines, some aromatic compounds can be oxidized resulting in the production of earthy aromas.
The largest change in must chemistry as a result of Botrytis growth is seen in amounts of sugars and organic acids. Up to 70 to 90% of tartaric and 50-70% of malic acid can be metabolized by the mold. Resulting changes in the tartaric:malic ratio cause titratable acidity to decrease and pH to increase.
There may also be clarification issues as a result of infection. The fungi produce polysaccharides including β1-3 and β1-6 glucans as well as pectins as a result of the production of enzymes capable of degrading the cell wall. In the presence of alcohol, pectins and glucans aggregate causing filtration difficulties. To mitigate this issue, pectinolytic and glucanase enzymes can be used. When adding enzymes allow at least six hours prior to bentonite additions.
Botrytis cinerea strains differ in the amount of laccase produced. This enzyme can lead to oxidation of aroma/flavor compounds and browning reactions. It can be resistant to sulfur dioxide and not easily removed with fining agents. Bentonite may remove enough laccase to minimize oxidative problems. For varieties where the potential for oxidation is increased, ascorbic acid additions can be added to juice. Since Botrytis uses ammonia nitrogen there is less available for yeast metabolism. Vitamins B1 and B6 are also depleted. Therefore supplementation with nitrogen and a complex nutrient is required. Yeast assimilable nitrogen (YAN) should be measured and adjusted accordingly to avoid stuck fermentations and production of hydrogen sulfide. Also consider inoculating with low nitrogen-dependent yeast and use more than the standard amount of 2 lbs. /1000 gallons.
Wine off-flavors and aromas result from a number of compounds when made from grapes with Botrytis(and other bunch rot organisms). Descriptors include mushroom and earthy odors from compounds such as 1-octen-3-one, 2-heptanol and geosmin. Since fruitiness can be decreased, the use of mutés (unfermented juice) from clean fruit can be added to the base wine to improve aroma. Botrytis also secretes esterases that may hydrolyze fermentation esters. Monoterpenes found in varieties such as Muscat, Riesling and Gewürztraminer can also be diminished.
When Botrytis infection is present, consider the following processing practices in addition to those mentioned above.
- 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.
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.
Any wine lover, wine maker and wine connoisseur worth their weight can tell you one of the largest problems with the South African wine industry is the total lack of local market interest. The current South African wine drinkers are predominately rugby moms who drink iced Chardonnay and while watching Jan play his Saturday morning match against Paarl or tragically basic, young teeny boppers who drink “rosé all day”. The rest of the population sticks to “branners and coke”, beer or ciders.
As a student I have found myself confined to the four Bs – Bartinney, Bramptons, Balboa and Bohos. Now the first three are what we need in South Africa if we want to encourage a new generation of informed, interested and adventurous wine drinkers. The latter – not so much. Robertson box wine may be a successful product but I am a firm believer that wine should not be drunk from a juice carton and served with a beer glass of ice.
As it is, the only time the average student is exposed to a “Savvy B” instead of a “Sauvignon Plonk” is if they’ve ventured out to the surrounding wine farms, usually only for the pre-dance fashionable photos, because no one can afford the übers fees for a weekly wine tour. Luckily for wine students this occurs more often due to enthusiasm from all your class mates to go for a sneaky tasting after every minor accomplishment. Yay! We finished a prac report let’s go wine tasting. Yay! We attended all out classes today let’s go wine tasting.
Ironically, Stellenbosch as the wine capital of South Africa has a serious deficiency of wine-drinking platforms for students, particularly those who don’t confine their drinking times to 10am until 5pm as most wine farms do. University is the place of innovations, where trends are born and die and sadly, the trend of wine drinking starts and stops at “Tassies” and “Four cousins”, not to say these aren’t well-respected wines in their own right, but let’s be honest, any wine snob would be horrified to find themselves with a glass of tassies while unwinding after a horrible test.
I have been fortunate enough to travel many places within and outside of South Africa. The wine lists littered across the rest of South Africa are depressing and it’s no small wonder that wine is not the hot accessory we need it to be. In international metropolitan areas, like Barcelona and London wine is the answer to everything. Hot new wine bars open up more often than a cellar intern stress-cries during harvest (which in my case was at least once a day). And this is where South Africa is seriously lagging behind. With so much to offer to the new untapped hipster, trend-setter consumers we seriously fall short.
Part of South African wine charm is the diversity (if you like clichés we could say rainbow nation of wine). We have historic Constantia wines to the reputable Stellenbosch powerhouses and now we also enjoy the yuppie, alternative Swartland surprises.
Wine is seen as snobbish and elusive by the majority of the population. But, that is something so easy to change. Bring out the screw caps, crown caps and orange wines to the young population. We’re all equally lost, alone and confused when it comes to the apricot-bomb Viogniers or orange Semillon. It’s the great equaliser that we’ve been waiting for and together students can become the pioneers in the alternative wine movement. Once wine can be viewed as trendy and hip then the young adults will flood the market and perhaps we will have a local wine consumption to be proud of.
So why is it so difficult to get these wines to break into the new market of the young trend setters, ubiquitous across campuses? Why are there no wine bars populating every nook and cranny of South Africa’s CBDs? Bring on the hipster wine bars where Malbec comes in a mason jar and we get deconstructed Pinotage tastings. Every youth with their vintage clothing, vinyl records and “Rocking the Daisies” wristbands from 2017, 2016 and 2015 will show up to experience the alternative and innovative wines on offer.
Yeast do not seem to form biofilms on the bottoms of corks when they’re used, rather than metal crown caps, to secure Champagne bottles during their in-bottle secondary fermentation. This, at least, is the conclusion of an article in the current issue of the American Journal of Enology and Viticulture (paywall), in which Burgundy-based authors investigated the question for the sake of understanding whether Champagne producers, some of whom are using cork for their longer-aging wines, risked upsetting in-bottle fermentation dynamics. After a year of bottle fermentation and aging, a few cells apparently got caught in porous crevices of the cork, but systematic growth presaging the tenacity of a biofilm wasn’t happening.
That finding will no doubt interest the odd sparkling wine producer. Much more interesting is the method they used to make “invisible” cells visible and so reach that conclusion.
How do you determine whether a microbial biofilm is growing somewhere?
Before it reaches scrape-it-off-with-a-fingernail thickness, the answer is usually microscopy. But what kind of microscopy? Let’s say you want to see the very earliest stages of what might become a biofilm. You want to visualize individual yeast cells sticking to a rough, craggy surface with lots of crevices for hiding—the cork. Pointing a light microscope at the cork won’t do. The kind that you probably used in school to see cells suspended in liquid (from your cheek, or bacteria from your teeth), and that many small wine and brewery labs use to check for live cells requires that whatever you’re looking at be thin enough for light to pass through. Slicing corks into sections thin enough for light microscopy might destroy or displace any would-be-biofilm-forming yeast cells. Moreover, can you imagine scanning the bottom of cork after cork, continually asking yourself: is that a yeast cell, or a bit of cork shaped like a yeast cell? A digital image analysis program trained to recognize yeast cells might address the latter problem (with some degree of error), but the whole task clearly calls for a more sophisticated technique.
When your sample isn’t transparent enough for light microscopy, fluorescence microscopy can be an alternative; instead of visualizing light passing through the sample, fluorescence microscopy relies on whateveritis you’re examining absorbing and then emitting light—fluorescing—which can then be picked up by the microscope’s detector.* Conveniently, cork and other plant matter comes with a built-in fluorescent molecule; lignin, a rigid polymer and major contributor to the stiffness of wood and bark, is easy to see under the microscope …
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“Meisietjie, Lena is die Wingerd-Aaantie (aunty)”, this was one of the very first sentences I heard when I started my internship this year (2018). While I sit here, taking a hefty sip of chilled Sauvignon blanc, I can’t help but think about all of the unseen and almost unheard of hard work that has been poured into each lingering sip of wine.
Some of my fellow cellar-dwellers have been working in the industry for longer than I have been alive; it very quickly dawned on me that these men and women are fountains of knowledge and quirky stories. A winemaker is only as good as the team supporting them, this is a very important lesson for any young or aspiring winemaker to learn. As far as my winemaking internship goes, I can wholeheartedly say that I have landed with my bum in the butter. In my very first week, I had already been taught (by the cellar team) how to build lines, operate pumps, pressure-rack barrels, set up acid and alcohol trials etc..
Almost two months down the line, I can very clearly see why my colleagues are so eager to get to work in the mornings. Each time the viticulturalist brings in grape samples from the vineyard, one of the cellar workers rushes to find me and tell me that I must come and do ripeness asssesments. Within the same breath, forgetting that I have not yet taken any sugar readings, I either get asked, “Where is the sugar lying? Wanneer gaan ons pars (when are we going to harvest)?” or promptly told, “No, no, they are not ready yet”. I couldn’t help but feel a little envious of the winemakers, to have a team that is this enthusiastic and motivated to start harvesting.
In the vineyard, I am constantly greeted with smiles, and even though I don’t yet know the entire team’s names, they all know my name. Every opportunity I have to work in the vineyard is met with people enthusiastically asking me to work with them, as they show me the do’s and don’ts of the day’s vineyard tasks. I think, as consumers, it is very easy to give the winemakers all of the credit when we taste a phenomenal wine, and it’s equally as easy to forget that no person can make a great wine on their own. It’s definitely a team effort.
As much as the teams are teaching me, I have found that learning and teaching work both ways. While doing some crop dropping (green bunches were removed) in order to allow for the Merlot bunches undergoing veraison to ripen, one of the team members said she could taste something familiar in the berry but couldn’t quite put her finger on it. I was very quick to pop a berry in my mouth, purely out of curiosity, and rapidly replied with, “it’s minty, and tastes a bit like bell-peppers”. Her face lit up as she recognised these flavours and agreed with me, where after she explained to me that it made a lot of sense as the viticulturalists wanted us to do the crop-dropping in order to try and lessen the green characteristics of the berries.
I would like to take my hat off to all of the men and women behind wine and thank them for the countless hours of hard work they have put into each vintage. From working long days in 30-40°C weather, to working 15+ hour shifts during the harvest season, I have not heard a single complaint. I am constantly surprised by the amount of passion and hard work the teams put into every task they’re given and how readily they offer help to anyone who struggles. They hold each other accountable for their work, truly working as a team by making sure everyone is doing their work properly.
When harvest started, I didn’t expect anyone to remember me after my pruning practical on the farm in June, however very few had forgotten. While walking through the vineyards on a very hot day, hanging up containers with biological control agents (parasitic wasp eggs), I bumped into one of the suckering teams. I was immediately recognised and greeted with a smile, while being called over to come and see what the team was doing. I was feeling exhausted, and quite honestly did not want to walk all the way back to the other side of the row I had just hung the control agents in. I somehow mustered up enough motivation to put one foot in front of the other and make the journey. What should have been a quick “hi-and-bye” turned into a 45-minute chat about everything from insects, to love advice and weekend shenanigans. I learned another important lesson that day, sometimes, all someone needs is a smile and a good story or two to motivate them and give them a little bit of encouragement. After talking to some of the team members, I had forgotten all about my exhaustion and immediately felt more energized.
Remembering something a viticulturalist had once said to me about the workers always being loud and chatty in the vineyards, I finally understood why. When you’re enjoying your work and the company, laughing and singing while you get through the day, you begin to look forward to tomorrow. Hard work is made a lot easier when it’s appreciated. So, here’s a big THANK YOU, to all of the winemakers, cellar workers, viticulturalists and vineyard workers for all of the hard work that goes into each divine sip of wine!