Gut reaction: Viruses cause disease. Disease is bad. Viruses are bad.
Gut reaction muted by a lot of recent genetics research: Viral DNA seems to be embedded in genomes all over the place. We’re not sure why a lot of it is there, or stayed there, or what it does while its there. Some viruses cause disease. Some don’t. Viruses are complex, and we probably don’t know the half of it yet.
A name like “grapevine leafroll-associated virus” gets you thinking about negative consequences. Rolled leaves don’t collect light efficiently, which means that they won’t contribute to the plant’s photosynthetic metabolism efficiently, which means that the plant may be malnourished, grow slowly, and/or not have enough energy to ripen fruit. Rolled leaves are bad. A virus that’s associated with rolled leaves is bad. But the virus is only associated, not causative. Some viruses in this general family of leafroll-associatedness aren’t associated with vine symptoms. And infected vines only show symptoms post-veraison (the stage of ripening at which grapes change color), even though they carry the virus in detectable quantities year-round.
Ergo, a group of vine and wine scientists headquartered in eastern Washington state designed an experiment to ask (published in PLOSOne, and therefore open-access to everyone): do grapes from vines with grapevine leafroll disease, and carrying one of these viruses (GLRaV-3), lag behind their undiseased counterparts throughout ripening, or only when vines show symptoms? Being particularly conscientious*, they also improved on existing studies of grapevine leafroll disease by collecting data for three consecutive years from a commercial vineyard, sampling grapes throughout the season but also harvesting grapes at the typical time and making wine from diseased and undiseased pairs, and subjecting those wines to (limited) chemical and sensory analysis. They also used own-rooted rather than grafted vines, which eliminates some potentially confounding variables.
By: Denise M. Gardner, Enology Extension Associate
Most likely, all of the wines from the 2016 vintage are happily settling away in tank or barrel at this point. After such a busy time, his leaves winemakers with that tricky question, “What do I do now?”
Monitoring Malolactic Fermentation (MLF)
Now is a good time to make sure you are monitoring your malolactic fermentations. Ensure all of your barrels or tanks have been appropriately inoculated, or have started naturally, and get some initial readings on the malic acid concentration.
If you have a spectrophotometer, you can purchase enzymatic kits to measure the concentration of malic acid in your wine over time. Wines with less than 30 mg/100 mL of malic acid are considered “dry” for MLF or MLF-stable for bottling.
However, winemakers can also monitor malic acid degradation through the use of paper chromatography kits. These kits are easy enough for home winemakers to use and can also be applied at the commercial level.
MLF paper chromatogram. This image shows the paper after it has dried, where the spots are pertaining to the acid standards and the acid separation for a wine sample. Wine samples above have not completed MLF due to the fact there is a noticeable dot of malic acid in each sample. Photo by: Denise M. Gardner
Paper chromatography works by separating tartaric, malic, and lactic acids from a wine sample (Figure 1). In addition to blotting small drops of your various wine samples, each paper must also contain 3 standards to show the spots documented by the three acids (tartaric, malic, and lactic). While paper chromatography is not the best at concentrating how much of each acid remains in the wine, you can get an idea when the bulk of malic acid is converted to lactic acid (i.e., MLF is completed) when the malic acid spot associated with the wine samples disappears.
Checking Wines for Off-Flavor Development
It’s also a good time to check wines for hydrogen sulfide (H2S) or sulfur-base off-odor aromas, and volatile acidity (VA), especially for wines that you will want to bottle early in the new year.
Now is also a good time to know what the VA is in your wines, especially those that will be seeing some aging. This is incredibly important to get a baseline value of the VA. That way, if a problem emerges in the future, you will have an indication how much the volatile acidity has increased. Penn State Extension also offers a 2-page fact sheet explaining why knowing volatile acidity is important, provides protocols for its analysis, and how to mediate high VA situations: http://extension.psu.edu/food/enology/wine-production/wine-made-easy-fact-sheets/volatile-acidity-in-wine
If you are having problems identifying these key defects in your wine, don’t forget that the annual “Wine Quality Improvement” Short Course is just around the corner in January.
Now is also a good time to clean up any leftover sore spots from the chaotic harvest season:
Clean up places in the cellar that have gotten dirty or have become areas that are accumulating materials that should otherwise be put away.
Manage all of your harvest records. Make sure all of the wines have the basic wine chemistries (e.g., pH, TA, residual sugar, alcohol, free and total SO2, malic acid, and volatile acidity) in the record book. It is easy to forget all of these details as time progresses.
Running basic chemical analysis on your wines and updating records is an essential component of making quality wine. Photo by: Denise M. Gardner
The conversion of malic acid to the weaker lactic acid during malolactic fermentation (MLF) is not the only important change that occurs during this process. Other intermediate and co-products are also formed, which will have a secondary influence on the wine composition. The decrease in the colour is one such product, but the reason for it is unknown.
Wine quality and its sensory description are determined by factors like flavour, taste and mouthfeel, but especially in the case of red wines the colour is also an important factor. This is especially applicable in the case of light coloured red wines like Pinot noir. Much research was consequently done to determine which viticultural and cellar practices will optimise the colour. The colour of red wines is not only determined by the anthocyanin concentration of the wine. After being extracted from the skins, the anthocyanins can react with different compounds in the wine to form more complex colour compounds. They may for example react with tannins to form polymeric pigments. This formation can be accelerated by reactions with acetaldehyde. Stable anthocyanin derivative products can also be formed by reactions with pyruvic acid. All these pigments are more resistant to oxidation and bleaching by sulphur dioxide (SO2), and also tend to increase during wine maturation.
It is known that winemaking factors like fermentation temperature and extended maceration influence the formation of polymeric pigments. Yeast can also influence the colour of red wines by the adsorption of anthocyanins to their cell walls or by the formation of acetaldehyde and pyruvic acid. Yeast strains differ regarding the acetaldehyde concentration which is formed and will consequently have different effects on the colour of red wines. Other micro-organisms can also influence the colour of red wines. It is for example known that Oenococcus oeni decomposes acetaldehyde and pyruvic acid during MLF …
While in the midst of harvest (and all the craziness that comes with it), I thought I’d take a week to remind people about proper cleaning techniques, improving sanitation, and why these two operations are essential for wineries.
I know many of you are ready to close this page now, but WAIT!
I have heard many excuses for short cutting on cleaning over the years. Do any of the following sound familiar?
There is not enough time in the day to properly sanitize.
There are not enough employees to do all the work to properly clean.
Cleaning would take all night to complete properly.
It’s not necessary to clean/sanitize with wine.
The wine will sell anyway.
Cleaning and sanitizing does not actually improve wine quality.
Sanitation is not really important.
Proper cleaning does not increase the price in which the wine can be sold.
If you or any of your employees have used at least one of these statements in the past, you could be suffering from poor cleaning and sanitation practices!
In all seriousness, having good cleaning and sanitation procedures can actually save the winery time and money in the long run.
In the height of harvest, I’m sure this is a tough sell. But let’s consider some of these practical cleaning and sanitation suggestions for small, commercial wineries.
On the same page with cleaning vs. sanitizing
Let’s start with a review of definitions, as it can get very confusing. Below are some general definitions taken from a series of sources (Fugelsang and Edwards 2007, Iland et al. 2007, Iland et al. 2012, Solis et al. 2013) to explain the differences between cleaning, sanitation, and sterilization.
Cleaning – the physical removal of dirt, debris or unwanted material (solid or liquid) from a surface
Sanitizing – a 99.9% (3 log) reduction of microorganisms
Sterilizing – the complete removal or inactivation of microorganisms
The wine industry is primarily focused on cleaning and sanitation protocols, as there are not many sterile practices utilized in winery operations (unless you are one of the lucky few wineries bottling aseptically). Even if processors are using sterile filtration to remove yeast and bacteria from the wine, once the wine exits the filter, it comes in contact with equipment that is only sanitized (hopefully!).
Additionally, wine bottles or packages are not sterile when being filled. Even new bottles can contain yeast or bacteria that can potentially contaminate a finished wine. Hopefully, proper sulfur dioxide levels should keep this microorganisms at bay.
For all of these reasons, as the wine has the opportunity to come in contact with existing microflora on processing equipment, wine is bottled in a sanitized environment.
Remember proper sanitation is primarily having good cleaning protocols. Cleaning should always precede sanitation. Failure to physically remove all of the debris from equipment, results in an inability to properly conduct sanitation procedures.
There are several different detergents (cleaners) and sanitizers that wineries can use effectively. Example sanitizers include quarternary ammonium compounds (QUATS), peroxyacetic acid, chlorine dioxide, hot water, and steam. Additionally, wineries can find use in an acidulated (citric acid) sulfur dioxide mixture. However, all sanitizers should be selected specifically for the job at hand (Iland et al., 2012) with consideration towards the microbes that one is trying to avoid.
Most commercial wineries can really focus on improving cleaning practices to provide a step in the right direction towards improving quality and sanitation practices inside the winery.
While this is usually one of the places winemakers feel most complacent about, I would argue that this can be one of the most important places to take care in your cleaning and sanitation practices.
There is a lot of effort that goes into the growing season in order to adequately ripen wine grapes for many sensory nuances. Additionally, the vineyard is the source of many microorganisms that enter the crush pad and cellar. [For those that use mechanical harvesters, do not forget cleaning and sanitation of this vital piece of equipment (Pregler 2011).] Giving the grapes a clean surface to encounter upon entering the winery ensures that all of that hard work is truly appreciated and preserved from the start of fermentation.
Without proper cleaning and sanitation practices, you are likely increasing the microbial populations of your wine before it even gets a chance to ferment. Think about it. After crushing/destemming a lot of rotting Pinot Grigio, Pinot Noir, or botrysized Riesling, how many people spray down the equipment (lightly) and move onto crushing the next lot of fruit even if the second lot is cleaner than the first? Sometimes, the order of grape crushing cannot be avoided. But how it is handled upon receiving can be altered. If this is the case for your winery, and you are avoiding good cleaning and sanitation steps in between lots of fruit, you are cross contaminating your juice with, not only yeast and bacteria present in the rotted fruit, but also residual enzymes, proteins, and other by products that can alter wine chemistry in the clean fruit that follows. Think about the potential production problems this can cause later on down the road: laccase browning, acetic acid development, off-flavor development, etc. If such problems arise, it can cause labor and financial investment at a later time.
Residual foodstuffs (e.g., old grape skins, rice hulls, pulp) can contribute to off flavors within the finished wine. Recent research has shown that there is potential for aromatically-intense varieties (i.e., Niagara, Concord, or Noiret) to leach their flavor compounds into more neutral varieties through absorption and diffusion of equipment-based plastic components that come in contact with the juice and wine (Smith 2014). It is also possible for alien material (i.e., green matter, old rice hulls, and stuck fruit) to contribute to flavors in the final product that may be undesirable or challenging to fix.
Remember that rice hulls are a pressing aid primarily used for A) hard-to-press varieties to increase yield or B) bulk operations in which pressing time is of the essence. Previous studies, such as the one found here, have shown a detriment in flavor and quality of wines pressed with rice hulls for certain varieties. Additionally, rice hulls can be difficult to remove from the wine press and create potential microbial infection sites for later grapes/juice/wine. It is recommended that the use of rice hulls be on aromatically intense or difficult-to-press varieties (e.g., many native varieties). Use of rice hulls in grapes that have a lot of rot will not only help increase yield of the fruit, but also increase extraction and retention of rot byproducts, which can contribute to off-flavor development.
Proper cleaning can help maintain your equipment longer. Over time, plant material can slowly degrade equipment. Doing a little scrubbing and properly sanitizing repeatedly can help keep your equipment in relatively good condition. Additionally, the longer debris is left on equipment, the harder it is to remove.
Figure 1: Preparing a small solution of acidulate sulfur dioxide to sanitize processing equipment before crushing/destemming and pressing operations. Photo by: Denise M. Gardner
Properly maintaining harvest equipment also leads a good example for all of the other equipment in the winery.
Tanks, Barrels and Bottles
These are places in the cellar where it can get easy to take short cuts as opposed to properly cleaning or sanitizing equipment.
These are places in the cellar where it can get easy to take short cuts as opposed to properly cleaning or sanitizing equipment.
Remember that tartrate build up in tanks and barrels can make it difficult to properly sanitize the covered portion of the tank/barrel. Make sure to first dissolve large tartrate deposits with hot water before going through a cleaning and sanitation cycle. Without dissolving tartrates, the equipment is not going to get properly cleaned or sanitized.
When getting ready to fill a tank, remember to run a sanitizer through the tankfirst to minimize microbial populations on the interior surfaces that come in contact with the wine. This helps ensure varietal flavor nuance and minimizes the risk for spoilage. [Note: Some sanitizers are no-rinse sanitizers and do not require a rinse after the sanitation chemical is applied. Other sanitizers may require a rinse following application. Always check the directions pertaining to your sanitizer carefully before use to ensure it is being used properly for best efficacy, and always use proper protective clothing when handling sanitizer agents.]
Minimize harboring sites for insects and microbes within the cellar are a practice that can be done at the end of every shift. During harvest, one big problem I see is dripping, dried juice or wine on the exterior of tanks or fermentation bins. While this doesn’t seem like a big deal, it’s an attractive site for fruit flies, which also makes them attractive deposits for spoilage yeast and bacteria. The objective of removing these places of dried juice/wine is to minimize insect infestation in the winery and avoid potential contamination of clean wines.
Barrels need cleaned prior to sanitation regimes like other pieces of equipment. Many barrel cleaning systems are automatic and can be an efficient way to clean the interior of barrels.
Barrels are porous and have a lot of grooves inside of them, which can make it difficult to properly clean and sanitize. It is important to note that due to the nature of the barrel, it cannot be sanitized in a way that a stainless steel tank can be sanitized. However, there are many different cleaning and sanitation options for barrels out there, some of which are explored in this Appellation Cornell newsletter from 2013. This study evaluated natural barrel microflora (yeast, including Zygosaccharomyces and Brettanomyces) before and after a sanitation regime was conducted.
Sulfur wicks are a good way to treat the interior surface of the barrel, but this practice does not penetrate into the interior of the wooden staves (Iland et al. 2007). Also, ensure that the wick is not submerged below any left over water at the bottom of the barrel, as it may extinguish the wick (Iland et al. 2007). Make sure the bung is tightly sealed for best efficacy of a sulfur wick (Rieger 2015).
Bottling lines are not immune to cleaning. In the food industry, it is commonly noted that most contamination comes from the environment in which the food is processed. This can happen in wine processing, as well. Dust on the bottling line can harbor yeast and bacteria that can be disturbed or moved into the air during large movements, like when bottling a finished wine. Keeping the bottling line clean is a good way to help minimize contamination during bottling operations.
Small Steps That a Commercial Winery Can Take to Improve Cleaning and Sanitation
Being a smaller or boutique sized winery can definitely have its advantages in the cleaning and sanitation world. It’s easy to get creative in terms of improving efficiency, use of, and efficacy of cleaning and sanitation practices. Below are some practical solutions for wineries struggling to incorporate cleaning and sanitation practices in the winery.
Use brushes, like Perfex brushes, to properly scrub equipment during cleaning operations. These are especially helpful when getting that pesky debris off of processing equipment.
Color code brushes or cleaning materials to emphasize their use and make it easier on your employees. By keeping the necessary supplies handy and easy to use, efficiency is likely to improve, which can actually help improve the quality of cleaning operations. Typically, white brushes are reserved for food-contact surfaces (the part of the equipment that actually comes in touch with food) during sanitation steps. Yellow brushes can be used for environmental cleaning (non-food-contact surfaces like the exterior of tanks). Other colors can be purchased for additional specific purposes: detergent only, sanitizer only, etc. Keep the brushes handy during all processing operations.
Figure 2: Perfex Brushes that are great for cleaning and minimize bacteria retention.
Consider keeping your cleaning and sanitation system on wheels. While in Oregon, I found it clever how larger wineries kept their fittings on mobile units to aid in availability, cleaning, and organization (Figure 3). While this concept may be helpful to some wineries, I think it can also be applied to cleaning materials. Keeping cleaning materials isolated to a mobile until allows for quick use and organization throughout the entire production facility and minimizes needless travel time to walk back and forth towards where supplies may be kept. Examples, below, for how to improve mobility of your cleaning supplies are given in Figure 4.
Figure 3: Mobile unit for holding cellar fittings is a great idea for easy organization, cleaning, storage, and use of fittings throughout a cellar. (Looking at mobile stand from the top) Photo by: Denise M. Gardner
Figure 4: Utility carts like this plastic one from School Outfitters or the metal one from Grainger can be easy additions to hold necessary cleaning supplies like citric acid/sulfur dioxide and pH strips, as well as hang spray bottles or hold gloves for cleaning. Carts can be easily moved and stored in the cellar for convenience.
You do not need to use fancy (or expensive!) cleaners or sanitizers all of the time in the winery. For quick clean ups, use warm water mixed with potassium carbonate to get stuck or sticky material off of equipment. Use with caution as it can get slippery!
Follow a potassium carbonate rinse with a warm water rinse to remove the solution from equipment and environmental surfaces.
Acidulated sulfur dioxide (Figure 5) can act as a quick sanitizer as well, and is easy to make up and use in the winery. Plus, citric acid, sulfur dioxide, and water are found in wine and will not have an effect on wine quality or flavor.
Figure 5: Keeping acidulated sulfur dioxide handy can be a quick sanitation solution during processing days. Photo by: Dr. Rob Crassweller
Finally, I always recommend wineries keep a supply of 70% ethanol in a spray bottle handy for quick cleaning solutions. Ethanol can be used to clean up small spills, quickly rinse sampling valves before and after sampling, or act as an exterior sanitizer towards things like wine thieves, sampling pipettes, and lab benches where one is running analysis. This is an easy chemical to keep on a mobile cart or scattered throughout the winery. However, be sure to purchase food grade ethanol from a chemical supplier and dilute down to ~70% with non-chlorinated water.
Cleaning up at the end of a processing day makes the start up for the next processing day a lot easier. If the equipment is clean to start, then all you have to do is run a quick sanitizer through the equipment before the start of processing operations.
Use hot water to rinse your equipment and make sure your hose has good pressure. Cold water is definitely energy efficient, however, hot water can help remove a lot of debris quicker and make any potential scrubbing easier. Be cautious of the metal on equipment heating up with use of hot water. Also, increasing hose pressure can help dislodge any debris from equipment, which can save time during cleaning operations.
On large processing days (those days when 3 or 4 varieties are being crushed at the winery), designate the day to processing and wait until the next day to complete other operations that can be delayed. Now, some flexibility needs to be made for things like punch downs or pump overs. However, teamwork is key: punch down time can be reduced if there is more than one punch down tool available for employees to use. Juice analysis (pH, TA, Brix, and YAN) is time sensitive, because if the juice starts going through spontaneous fermentation, the results of these chemical indices will change. However, obtaining all of the juice samples from all lots of incoming fruit before starting analysis can save your employees time and avoid splitting up duties during a processing day. With 3 employees, one person could run analyses while the remaining 2 finish cleaning up at the end of a processing day. Reserve racking or moving wines for days when a little less is going on in the cellar unless it is absolutely necessary to open up space in tanks for incoming fruit.
Minimize barrel-to-barrel or tank-to-tank contamination by having small sanitation vessels/buckets (filled with sanitizer) handy and isolated for cleaning/sanitation use. Use a bucket filled with acidulated sulfur dioxide solution to submerge (and fill) your wine thief in prior and after each barrel sample. For smaller samples, consider using one-time-use or disposable pipettes (Figure 6). If you have a 70% ethanol solution in a spray bottle, the metal fittings at the end of hoses can be quickly sprayed in between barrels when transferring barreled wine into a tank or transferring wine from a tank into barrels to help minimize cross contamination (Illand et al. 2007).
Figure 6: Serological or disposable pipettes are a great way to avoid cross contamination when smaller samples are needed. Photos from BioVentures.
Check to see how clean your equipment is with quick testing strips like Pro-Clean Protein Residual testing strip by Hygiena. These testing strips are a good indicator on how well your cellar crew is cleaning equipment. The problem with protein test strips, like the one shown, is that it will detect all organic matter (Iland et al., 2007). It does not represent live or viable microorganisms; there are rapid tests available that may be more representative of microorganism populations.
The video below indicates the ease in which these are to use:
Other options include luminometers like Hygiena’s SystemSURE Plus or 3M Clean-Trace (Rieger 2015), which are also non-specific, but can indicate the cleanliness of a contact surface that is swabbed properly.
While cleaning and sanitation may seem arduous, most wine quality problems I encounter – including funky off-flavors that are challenging to identify, presence of VA, large quantities of wine affected by cork taint, and lack of varietal character – could be primarily avoided with more routine and better cleaning operations. Improving cleaning and sanitation operations can be a step in the right direction for wineries to improve quality associated with their business.
Smith, JC. 2014. Investigating the Inadvertent Transfer of Vitis labrusca Associated Odors to Vitis vinifera Wines. Retrieved from Electronic Theses and Dissertations for Graduate School: Penn State: https://etda.libraries.psu.edu/catalog/23501.
Once grapes have arrived at the winery, they are processed in preparation for fermentation. White grape varieties are typically fermented without skins, while red varieties are typically fermented with skins. Either way, a winemaker needs to ensure that the yeast have a happy environment for a successful fermentation and there are several components to keep in mind.
is often considered the most important pre-fermentation characteristic in wine production. Alcohol is produced by the conversion of sugar by yeast during fermentation (1° brix equates to approximately 0.55% v/v alcohol), so typical brix levels prior to fermentation vary greatly depending on stylistic choices made by the winemaker (of course, harvest conditions can lead to some crazy brix levels; way too low to way too high). Every country has its own laws regarding whether water can be added (to decrease sugar levels) or sugar can be added (to increase sugar levels) prior to fermentation, which gives winemakers a little bit of room to play depending on where the wine is being made and where the wine is being sold. Fermenting wine to dryness (less than 2 g/L residual sugar, which is particularly desired for still wine production) is often difficult when there is too much sugar; many yeasts are subject to sugar toxicity levels and alcohol toxicity levels, which is why many commercial yeast producers offer high-brix yeast (they can start ferment with lots of sugar and finish ferment with lots of alcohol).
pH and Acidity
The balance between pH and acidity level is an important indicator of ripeness prior to harvest, used in conjunction with brix levels and sensory evaluation, to determine the best picking dates. Once in the winery, pH and acidity play a major role in winemaking.
Titratable acidity, also referred to as total acidity, is the combined measurement of all acids in a juice/must/wine presented as grams per liter (g/L). There are several different acids present in grapes and wine, but tartaric and malic acids are found in the highest concentrations. Different grape varieties are predisposed to have higher concentrations of one or the other, though environmental factors play a major role. Usually, titratable acidity levels fall between 6-10 g/L prior to fermentation, largely dependent on things such as grape variety and wine style. A winemaker will usually desire higher acid levels for wines destined for malolactic fermentation and even more for those earmarked for sparkling wines.
Most winemakers would likely argue pH carries more importance than titratable acidity. Why? pH has a major effect on microorganisms present. Yeasts require juice/must within a certain pH range to perform non-stressful fermentation. This range is dependent on the specific strain, but usually is somewhere between 3.1 and 3.8 pH. A major issue for winemakers are undesirable microorganisms, many of which can also thrive in this pH range.
There are different regulations for the types and amount of additives a winemaker can use to adjust pH and acidity. Tartaric acid is the overwhelmingly favorite for increasing acidity and therefore decreasing pH. Potassium carbonate is commonly used for decreasing acid, though it has little effect on pH levels.
Managing oxygen levels in wine is a major topic in the modern wine industry. Research continues to show that slight variations in oxygen throughout the winemaking process can have significant effects on wine quality. Nevertheless, oxygen is often left unmentioned when discussing fermentation, one of the stages its levels are most important.
Yeasts are facultative microorganisms, capable of conducting aerobic and anaerobic respiration depending on environmental conditions. Supplying wine yeast with adequate levels of oxygen during the stationary and growth phases is essential for a successful fermentation. Research shows that yeast propagated aerobically contain a higher proportion of unsaturated fatty acids and significantly more steroids than those anaerobically propagated, translating to higher yeast viability.
nce fermentation is underway, consumption of oxygen and subsequent production of carbon dioxide quickly removes oxygen present in the juice/must. In most circumstances, this is desired (not during yeast propagation activities). Oxygen may be added during fermentation of somered varieties. It is sometimes induced as a method of removing carbon dioxide, which becomes toxic to yeast above 0.2 atm concentration.
Yeast cells require several different micro-nutrients during the growth phase, including nitrogen-containing compounds, vitamins, sterols, and minerals. The extent of nutrient requirement is largely based on the amount of sugar the yeast will need to consume and convert to alcohol. The higher the juice/must brix prior to fermentation, the more nutrients the yeast will require. Yeast-assimilable nitrogen (YAN) is a measurement showing the quantity of ammonium salts (NH4+) and free alpha-amino acids (FAN) that are available in the juice/must for yeast consumption.
Proprietary nutrient products are available from several different wine additive companies. Some are specifically designed for use during yeast rehydration, while others are designed for use during fermentation. Diammonium phosphate (DAP) is additive for increasing nitrogen, though varying opinions on its effectiveness exist. Many winemakers use routine rates for nutrient additions, but this can also lead to ‘too much of a good thing’ situation. Read Yeast Rehydration Nutrients and Fermentation Nutrients for more info.
Most people probably don’t realize that sulfur dioxide is a naturally occurring compound in grapes, but it is a very important additive for most winemakers. Sulfur dioxide is added to help protect juice/must/wine against oxidation and spoilage.
Balancing adequate levels of sulfur dioxide to inhibit undesirable microorganisms while allowing desirable yeast to remain active is essential. Prior to fermentation, a good baseline recommendation for free sulfur dioxide is less than 10 ppm and less than 50 ppm for bound sulfur dioxide. These levels can vary widely depending on factors including yeast strain, juice/must pH, presence of infection in the juice/must, and processing considerations. The winemaker’s stylistic choices also play a part; for instance, sulfur dioxide helps retain fresh fruit characteristics in white wines.
Just like with pH, yeast have an ideal temperature range in which they are most likely to successfully ferment within. Too low or too high of temperature will stress yeast. Since fermentation produces heat, it’s usually a good idea to begin fermentation at a slightly lower temperature than desired.
Winemakers have varied opinions on ideal temperatures dependent on the grape variety, grape quality, and wine style. White grape varieties are typically fermented between 54-63° F (12-17° C), while red varieties will commonly be fermented at higher temperatures upwards of 80° F (26° C).
About the Author
Mike Horton has been a New World Winemaker since 14 May 2012.
How a wine tastes is dependent upon many factors, including (but not limited to) the variety, the vintage, where the grapes are grown (soil, climate, etc), as well as the viticultural and winemaking techniques employed during processing. The compounds responsible for how wine tastes are known as free volatile compounds, as well as aromatic precursors, the latter of which are present at much higher concentrations. Non-volatile sugar-bound conjugates (a.k.a. “glycosidic compounds) have been well studied and have been shown to be released over time during wine aging or by using specific winemaking techniques. Specific glycosidic compounds known to be released over time, thus affecting how a wine develops and tastes, include terpenes, C13 norisoprenoids, benzenic derivatives, volatile phenols, and C6 compounds. All of these glycosidic compounds have low odor thresholds, thus requiring very little to elicit a sensory response.
While wine aromatics have been extensively studied, it is not well known exactly how compounds responsible for aromatic character in wines interact with the physiological make-up of the human mouth. In addition to environmental and chemical sources, it is possible that the perception of different wine aromas can be altered by physiological factors like mouth temperature, salivacomposition, or the oral microbial community present in each individuals’ mouths. Studies focusing on onions, bell peppers, and grapes found that the microbial community in the human mouth hydrolyzed odorless compounds into their corresponding volatile aromatic compounds, giving reason to believe something similar could potentially happen with wine. Perhaps the microbiota living in the human mouth can hydrolyze these odorless precursors and convert them into their corresponding aromatic compounds, just like it’s been shown with other foods.
A 2015 study in the journal Food Chemistry aimed to evaluate whether or not human oral microbiota can convert odorless aromatic precursor compounds in wine into their corresponding aromatic glycosidic compounds. The results could potentially have a profound impact on our understanding of how we taste and evaluate wines.
Experiment 1: In vitro
For the in vitro experiments, three microbes commonly found in the human mouth were cultured on sterile growth media. Different concentrations of grape extract were added to the microbes, with bacteria growth measured after 24-48 hours, depending upon the specific microbe. From these growth measurements, inhibition of growth was also calculated.
Experiment 2: Ex vivo:
For the ex vivo experiments, fresh saliva was collected from three volunteers (ages 28-31). Prior to collection, the volunteers had not taken any antibiotics or other medications, and were non-smokers. Volunteers did not consume any food or beverages within two hours of the saliva collection time.
The saliva from each volunteer was divided up into four different treatments: 1) fresh saliva in an aerobic culture, 2) fresh saliva in an anaerobic culture, 3) sterilized saliva (pasteurized), and 4) non-enzymatic saliva (heated). Microbe counts were measured after 24-48 hours at 37oC.
Grape extract (comparable to 40g of grapes) was added to the saliva cultures and bacterial growth was monitored. Glycoside hydrolysis by the microbes was measured by monitoring and analyzing the volatile compounds released after four time periods (0hr, 2hr, 24hr, and 72hr).
To test the ability of human oral microbiota to hydrolyze glycosidic compounds in general, a standard solution of octyl-β-D-glucopyranoside was cultured with the saliva samples and monitored over time.
No human oral microbiota was found in the sterile or non-enzymatic saliva treatments (as expected).
Adding octyl-β-D-glucopyranoside to human saliva resulted in hydrolysis and the release of the volatile compound aglycone.
None of the oral microbes were inhibited by the glycosidic extract.
Every oral microbe tested was able to hydrolyze the glycosidic compounds in the grape extract, resulting in the release of terpenes, benzenic derivatives, and C6 alcohols.
Note: Many of these compounds can be associated with various aromatic characteristics in wines (e.g. terpenes can produce flowery or citrus aromas and certain benzenic compounds such as β-phenylethanol can produce rose aromas.)
While some aromatic compounds were found as a result of the oral microbes hydrolyzing the glycosidic compounds in the grape extract, other common compounds were not present (i.e. C-13 norisoprenoids, vanillins, and volatile phenols).
The ability to hydrolyze and the resulting aromatic compounds produced from this hydrolysis depended upon the type of oral microbe present.
A. naeslundii was the producer of the most linalool and its oxides, which is associated with floral notes in grapes and wine.
E. faecalis, A. naeslundii, and S. mutans produced the most aroma-causing aglycones.
G. adiascens, V. dispar, and F. nucleatum produced the fewest aroma-causing aglycones.
NOTE: Some of these bacteria have difficulty growing in culture media, so it isn’t clear whether their lack of glycosidic hydrolysis is real or a function of being unable to grow under the experimental conditions.
There were significant differences between the three volunteers in terms of the species make up of their mouths. No person had the same species in the same amounts.
While the microbial counts were statistically the same for each volunteer, the aromatic aglycones produced were significantly different, which is likely due to the different species present in each individual’s mouth.