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Questions and Answers

What causes SLO (sulfur like off-odors) and how can I prevent them?

Sulfur compounds can produce aromas perceived both through smelling as well as in-mouth aromas. Descriptors include rotten egg, onion garlic, burnt rubber etc. Some of the chemical compounds implicated in sulfur odors include hydrogen sulfide, ethanethiol, methanethiol, dimethyl-sulfide and dimethyl disulfide. All are produced at different concentrations in wine by Saccharomyces.

The yeasts, their nutrition and fermentation temperature management have a direct impact on the occurrence of sulfur compound production, both during and after fermentation. During yeast population growth phase (first third of fermentation), undesirable sulfur compounds can be dealt with by adding complex yeast nutrients. Organic nitrogen will rebalance the nitrogen-sulfur metabolism, helping to avoid further sulfur production. Polyunsaturated fatty acids and sterols from the complex yeast nutrient will improve the cell membrane physiology and diminish the yeast stress and the resulting sulfur compound production.

The addition of oxygen at the same time (4-6 mg/L in white juice and 6-10 mg/L in red must) will improve the effect due to yeast sterol synthesis and better physiology of the membrane as well as by acting directly on some sulfur compounds, stripping or oxidizing them to less odor-active forms.

After the second half of fermentation, if sulfur compounds are detected, it is recommended to add specific inactivated yeasts due to the sorption capacity of the yeast cell wall structure on sulfur compounds. At this time of fermentation, in white and rose’ juices, as long as there is more than 30 g/L of sugar, a short 6-12 hour and small 0.5-2.0 mg/L/month addition of oxygen may help. As soon as alcoholic fermentation is over, it is recommended to rack the wine off the lees trying to eliminate a part of the inactive yeast that had adsorbed sulfur compounds during the active fermentation.

Another source of sulfur compounds is residual sulfur sprays. If any (as little as 2 ppb) remains on grapes, hydrogen sulfide may form with the wine ferments, often with other forms such as disulfides and mercaptans forming as well.  Post-fermentation, hydrogen sulfide can be formed in barrel-aged wines from interaction with debris from sulfur wicks or rings burnt in the barrel.

Rauhut D, Kürbel H, Ellwanger S, Lӧhnertz O and Groβmann M (1999), Influence of yeast strain, assimilable nitrogen, fermentation temperature and sulfur residues on the occurrence of volatile sulfur compounds during and after fermentation, Proceedings, Oenologie 99, 6e Symposium International d’Oenologie, Bordeaux/France, 10-12 June, 305-308.

Reynolds, A , ed. (2010), Managing Wine Quality, vol.2 Oenology and wine quality, Woodhead Publishing Ltd, Philadelphia, 21-23.

What is the best way to measure YAN?

Yeast Assimilable Nitrogen (YAN) can be measured a number of different ways, that all have benefits and drawbacks.

Formol Titration

A home winemaker or a small winery can use the Formol Titration, which uses Formaldehyde to react with nitrogen compounds.  The solution is titrated back to the starting pH and the nitrogen level can be calculated.  The Fromol Titration has the benefit of measuring almost all the amino acids and ammonia, with the drawback of measuring proline, which is not used by the yeast as a nitrogen source in fermentation, and not measuring the sidechain of arginine, which can be a significant source of YAN.  Also formaldehyde is toxic and not all of the formaldehyde is used during the reaction, so the practitioner must dispose of the hazardous formaldehyde waste. 

NOPA / Ammonia Probe

A better equipped laboratory can measure YAN using the spectophotmetric method known as NOPA.  Nitrogen by o-phthaldialdehyde (NOPA) reaction measures  the nitrogen contribution from free amino acids.  Because ammonia is not measured by the NOPA reaction, an ion-selective probe (much like a pH probe) that is sensitive to ammonia is used to quantify get an accurate measure of the ammonia contribution to YAN.  The NOPA test does not measure the contribution from proline, so the YAN is not overstated.  It does not measure the side chain of arginine which can be a significant portion of the YAN.  The equipment cost is high because this test requires a spectrophotometer and analytical pipettes, along with an ammonia probe which has a very short useful life and must be calibrated hourly.

NOPA / Ammonia / Urea / Arginine

Our laboratory uses a combination of 4 tests to measure all the significant components of YAN. The NOPA test is used alongside enzymatic tests for ammonia, urea, and arginine.  This test is the most comprehensive measure of YAN.  It can be done quickly and accurately on many sample simultaneously with very little waste.  The drawbacks are that the test kits are expensive and have limited shelf life and the equipment price can be very high.

There are trade-offs between all the methods.  The winemaker will have to decide which balance of precision, accuracy, and cost best suits their needs.

How do I interpret heat stability results from the Enology Services Lab?

There is no single test that is a perfect marker for Protein Heat Stability.  The laboratory uses a combination of 3 tests to gauge stability of a wine.

Control Test

The wine is 0.45 micron filtered and stored at 20C for 24 hours.  The turbidity is then measured.  If the turbidity is below 2.0 Nephel Transmittance Units (NTU) , the wine is considered stable, pending results from the other two tests.

Hot Box

The wine is 0.45 micron filtered and stored at 60C for 24 hours.  The turbidity is then measured.  If the turbidity is below 2.0 Nephel Transmittance Units (NTU) , the wine is considered stable, pending results from the other two tests.  The heat used in this test has two purposes, to drive the degradation of heat-instable proteins and to use heat to mimick long term storage, even if the wine is stored at a proper temperature.

TCA Test

The wine is 0.45 micron filtered.  A 30 mL aliquot of the filtered sample is treated with 3 mL of 55% trichloroacetic acid (TCA). After sufficient mixing the solution is placed in a boiling water bath for two minutes.  The sample is then allowed to cool to room temperature and the turbidity is measured.  If the turbidity is below 10.0 Nephel Transmittance Units (NTU) , the wine is considered stable, pending results from the other two tests.  The TCA test is also a worst-case scenario test.  The combination of boiling temperatures with extremely harsh acid will make virtually 100% of the proteins fall out of solution and flocculate. Trichloroacetic acid (TCA) should not be confused with the wine fault 2,4,6-Trichloroanisole (TCA) otherwise known as cork taint.

A wine is considered stable only if it passes all three tests.

The NTU measurement is a measure of turbidity/clarity in a liquid. 

For comparison general comparison without equipment, a turbidity of:

  • 2.0 NTU is what can be seen in a clear glass bottle as slightly hazy
  • 10.0 NTU is what can be seen in a clear glass as slightly hazy

Use of a laser pointer or other concentrated light source will help identify hazy wines.

The Protein Heat Stability Test results can be used to estimate additions for bentonite fining, but these estimations are also based on wine varietal, pH, alcohol, TA, and RS values.

My sterile filter keeps clogging even when I prefilter with a plate and frame (1.0 um). What is causing this?

We filter to remove particulates, fining agents, colloids, as well as yeast and bacteria. All of these particles are easily deformed and excess pressure can cause a coating that retards filtration. Types of filtration include depth and absolute filtration. In depth filtration, the wine moves through a torturous path with filtration occurring on the surface and in the interior matrix. Examples include plate and frame filters and disc filters. With absolute filtration (membrane filtration), the filtration occurs at the surface. Standard size pores trap all particles larger than the pore size. An example is a 0.45 micron membrane filter (“sterile”). These filters clog easily and not all particles with diameters less than the pore size flow through. Rather, they may be retained in the pore passage, blocking flow. After a wine is prefiltered, it is important to perform a membrane filtration within no more than 48 hours.

The majority of wine particles are deformable. That is, they are elastic in nature and will spread over a surface area with applied pressure, blocking filtrations. These include yeast, bacteria, colloids (including fining agents such as gelatin), and protein/phenol/polysaccharide complexes. To deal with these particles, one can rack, fine and use enzymes to prevent filtration problems.

Wines that do not clarify naturally should be tested for the presence of pectins and/or glucans. These are found in high concentration in concentrates and are also associated with microbial activity. Glucans can result from the growth of Botrytis on grapes or be the result of spoilage by lactic acid bacteria.  Pectins and glucans can be tested for using acidulated ethanol precipitation. The use of pectolytic enzymes and/or glucanases can prevent filtration problems.

It is also important to test the integrity of membrane filters using the bubble point or pressure hold integrity test. This should be performed before, during and after a run.


Zoecklein, B. Filtration. Wine/Enology and Grape Chemistry Group, Virginia Tech. 2014.

Mansfield, A.K. Cellar Dweller. Cornell Extension Enology Lab, February 2010. Accessed Feb 29.

Bisson, L. Post Fermentation Processing. Accessed Feb 20 2014.

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Virginia Cooperative Extension is an educational outreach program of Virginia's land-grant universities: Virginia Tech and Virginia State University. Extension agents bring research-based solutions from Virginia's land-grant institutions to Virginians. The Enology Extension program offers a variety of services to both new and existing wineries including workshops, on-site visits, winemaker roundtables and newsletters.

Molly Kelly
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(540) 315-6920

Please contact Amanda Stewart if you have questions regarding Enology Research, Undergraduate, or Graduate education in Enology & Fermentation.

Amanda Stewart
Assistant Professor
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1230 Washington Street
(540) 231-0868