In the first part of this article about the use of SO₂, we reviewed the functions that SO₂ performs in the process of making wine and looked at the use of SO₂ before and after fermentation. This article covers ways to monitor and adjust SO₂ during aging and bottling.

The use of SO₂ during aging
The two most important pieces of lab equipment for SO₂ management are a pH meter and some method of determining SO₂. There are two primary groupings of equipment for the analysis of SO₂: wet chemistry and instrumental, and there are two analytical procedures to determine the SO₂ content as well.

HIGHLIGHTS
 

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• A winemaker in New York’s Finger Lakes region explains how to measure and adjust SO₂ in wine during aging as well as before and after bottling.
   
• Methods discussed include the Ripper method with wet chemistry and mini-titrator, the aeration-oxidation method, reflectance spectroscopy and upscale titrators.
   
• The article explains how to correlate molecular SO₂ and free SO₂ and discusses their important relationship to pH.

Probably the most common analytical procedure used in small wineries to determine SO₂ is the Ripper method. The wet chemistry method uses simple glassware and relatively common reagents. The procedure can be found in any winery lab manual. Its greatest shortcoming is in finding an endpoint in red wines. In dark red wines, the wet chemistry method is virtually useless. It also suffers from interference from other naturally occurring wine components (phenols) and additives (ascorbic acid, copper), so your reading will be artificially high.

The instrumental analysis for the Ripper methodology is performed by a mini-titrator. Using simple laboratory equipment, you add the same reagents as the Ripper method, but instead of determining the endpoint with your eye, the onboard calculator measures the endpoint determined by an ORP electrode (Oxidation/Reduction Probe). This considerably reduces the endpoint confusion of the manual method. The cost is several hundred dollars.

Another way to analyze for SO₂ is to use reflectance spectroscopy and various test strips. The initial cash outlay is higher than wet chemistry but about the same cost as the mini-titrator on a per-test basis. The equipment can be used for a wide variety of analysis including malate. The biggest limitation is obtaining fresh analytical strips and the need for refrigeration to store them.

Aeration-oxidation is another affordable wet chemistry alternative. Unlike Ripper, it avoids the interference of ascorbic acid, copper and phenols. Since the SO₂ is volatilized out of the wine sample into a clear trapping solution, titration endpoints are easy to see even with red wines. Cost per test is minimal and takes about 15 minutes. The A/O method is the gold standard for SO₂ analysis. Whether you like this method or can tolerate the Ripper method with the mini-titrator, you are still tied to your procedure or your instrument for 15 minutes per test.

A slightly more expensive option is a more upscale titrator, which will perform many other titration-based analyses. These can save hours per day if you are running numerous tests, and they can directly transfer the data to a PC. These titrators sell for between $5,000 and $10,000.

After the reagents are added, the instrument determines the endpoint and then records the data in a data file. These instruments can also run the titration portion of the A/O method. In the long term they may reduce the cost per analysis due to lower reagent costs.

Another popular tool for SO₂ determination is a small, single-use vacuum tube that utilizes a reverse titration. The instruments are fast, handy and easy to use, but they lack precision and are of marginal utility for red wines. Whichever laboratory procedure you select, you will learn to factor its particular idiosyncrasies into your operation.

Focus on free SO₂
Analyses will provide you with free SO₂ in parts per million (ppm), which is the same as milligrams per liter (mg/L). With the same apparatus you may also determine bound SO₂. For now, free SO₂ is the more important number, since this is the form of SO₂ that is still available to act as an antioxidant and antimicrobial agent. The degree to which a given level of SO₂ will protect a wine is highly dependent on its pH, because really only a small portion of the free SO₂ does most of the heavy lifting.

In reality, free SO₂ in wine exists in three forms: molecular SO₂, bisulfite and sulfite. Of these three species of SO₂, molecular SO₂ is the most potent antimicrobial agent, and its existence is favored by a lower pH. Therefore, the lower a wine’s pH, the less free SO₂ will be required. It is commonly accepted that a level of 0.8 ppm molecular SO₂ offers ample protection from oxidation and most yeast and bacteria. Many winemakers attempt to maintain a minimum of 0.8 ppm molecular SO₂ throughout the aging period for white wines.

Because they are usually dry, red wines have undergone a complete malic acid fermentation and possess the anti-oxidative properties of their tannins. Therefore they require slightly less—more along the lines of 0.6 ppm molecular SO₂. Also, red wines usually have a higher pH, and maintaining a 0.8 ppm molecular SO₂ would give excessively high total sulfur levels, which can lead to off flavors and excessive color depletion due to sulfite bleaching. In any case, total sulfur levels should be kept below 200 ppm unless the impact of such a level is tested in the lab beforehand.

Making adjustments
To adjust a wine to a given molecular SO₂ level, first determine the wine’s pH and free SO₂, then consult the table “Amount of Free SO₂ Given Molecular SO₂ Level” below. This will indicate the amount of free SO₂ necessary to yield a given 0.8 ppm or 0.6 ppm molecular SO₂.

The trick here is that only about 50%-60% of your addition will become free SO₂, with the remainder going to the bound state. The higher your free SO₂, the greater percentage will likely remain free, and you should adjust accordingly.

For example, if you wish to achieve a 0.8 ppm molecular SO₂ in a wine with a pH of 3.2 that has a free SO₂ of 15 ppm, you determine by consulting the table that the wine requires a free sulfur of 20 ppm. An increase of 5 ppm free SO₂ is required. Knowing that only about 50% of your SO₂ addition will become free, you will add 10 ppm total SO₂. The SO₂ you add will take three to four days to stabilize, at which time you should retest the wine and make any necessary adjustments to achieve your target. Practically speaking, if bottling is a while off, and the wine can stand a little extra SO₂, then err on the high side and save yourself the hassle.

Cellar operations such as racking or filtering reduce the free SO₂ in wine. Storage alone will usually rob a wine of at least 5 ppm per month, so an occasional sulfur check is important in maintaining adequate free SO₂. You may also take into account the storage time until bottling and fluff up your addition to compensate for this anticipated loss.

Bottling precautions
Having adequate SO₂ at bottling is a critical financial issue. If there is too much SO₂, consumers will rightfully reject the wine, or release dates may need to be postponed until levels moderate. The bigger danger is in too little SO₂. Without the protection from adequate SO₂ in the bottle, white wines can die an early death—or even worse, suffer from biological spoilage.

If proper attention is paid to SO₂ maintenance in the cellar, the pre-bottling adjustment will be minimal. Obviously, the minimum threshold will be the molecular SO₂ you’ve attempted to maintain throughout storage. But since bottling is a fairly violent process even under the best of conditions, there will always be some oxygen pickup during the process. Enough SO₂ needs to be present to compensate for this and still leave the wine with a molecular SO₂ at or above your target.

Testing your free SO₂ before and a week after bottling can give you a rough estimate of the amount of SO₂ that will be consumed during your bottling operation. This information can be used to help determine the proper addition for future bottlings. To get started, an additional 8-10 ppm free SO₂ over and above what you need to achieve a specific molecular SO₂ is a pretty safe bet.

Another consideration is time until release of the wine. If the wine is going to market right away, you really don’t want SO₂ to be apparent in the nose, and so the dosage should be just enough to provide the chosen molecular SO₂ and to compensate for the oxygen pickup in your specific bottling operation. A minimum of two weeks bottle aging is a good policy, because this gives time for the SO₂ to bind up any acetaldehyde generated by bottling. This one- to two-week period after bottling when the wine seems a little dumbed down is known as bottle shock.

Inert gas sparging of bottles will reduce the amount of oxygen pickup and reduce the loss of free SO₂. Also, cold wine more readily dissolves oxygen than warm wine. If the wine will be in the warehouse for a year, a little extra SO₂ is a good idea. You can count on wine losing an additional 8-10 ppm SO₂ during the first year of storage under cork. Screwcaps generally show less loss.

Most wineries in the East use potassium metabisulfite (PMBS) as their source of SO₂. It is a stable white powder as long as it is kept dry and sealed in a container. PMBS is about 58% sulfur dioxide by weight, so it takes 1.72 grams of PMBS to yield 1 gram of SO₂. Additions are made by first dissolving PMBS in cool water, then adding it to the wine or must. If you are sensitive to SO₂, it is prudent to wear a respirator while working with it.

SO₂ can be a winemaker’s best friend, but using it to compensate for lack of sanitation, careless handling and general laziness will produce inferior wine with high total SO₂. Good winemaking practices utilize SO₂ but are designed to minimize its necessity. With time and effort you will develop a feel for its usage in the context of your equipment, technique and style.

Chris Stamp is president and winemaker at Lakewood Vineyards in Watkins Glen, N.Y. He started his wine career as winemaker at Cayuga Vineyards in the Finger Lakes and in 1986 took a position as enology research and extension associate at the Ohio Agricultural Research Development Center in Wooster, Ohio. When his family opened Lakewood Vineyards winery in 1988, he returned to New York to become the winemaker there.

In the first part of this article about the use of SO₂, we reviewed the functions that SO₂ performs in the process of making wine and looked at the use of SO₂ before and after fermentation. This article covers ways to monitor and adjust SO₂ during aging and bottling.

The use of SO₂ during aging
The two most important pieces of lab equipment for SO₂ management are a pH meter and some method of determining SO₂. There are two primary groupings of equipment for the analysis of SO₂: wet chemistry and instrumental, and there are two analytical procedures to determine the SO₂ content as well.

Probably the most common analytical procedure used in small wineries to determine SO₂ is the Ripper method. The wet chemistry method uses simple glassware and relatively common reagents. The procedure can be found in any winery lab manual. Its greatest shortcoming is in finding an endpoint in red wines. In dark red wines, the wet chemistry method is virtually useless. It also suffers from interference from other naturally occurring wine components (phenols) and additives (ascorbic acid, copper), so your reading will be artificially high.

The instrumental analysis for the Ripper methodology is performed by a mini-titrator. Using simple laboratory equipment, you add the same reagents as the Ripper method, but instead of determining the endpoint with your eye, the onboard calculator measures the endpoint determined by an ORP electrode (Oxidation/Reduction Probe). This considerably reduces the endpoint confusion of the manual method. The cost is several hundred dollars.

Another way to analyze for SO₂ is to use reflectance spectroscopy and various test strips. The initial cash outlay is higher than wet chemistry but about the same cost as the mini-titrator on a per-test basis. The equipment can be used for a wide variety of analysis including malate. The biggest limitation is obtaining fresh analytical strips and the need for refrigeration to store them.

Aeration-oxidation is another affordable wet chemistry alternative. Unlike Ripper, it avoids the interference of ascorbic acid, copper and phenols. Since the SO₂ is volatilized out of the wine sample into a clear trapping solution, titration endpoints are easy to see even with red wines. Cost per test is minimal and takes about 15 minutes. The A/O method is the gold standard for SO₂ analysis. Whether you like this method or can tolerate the Ripper method with the mini-titrator, you are still tied to your procedure or your instrument for 15 minutes per test.

A slightly more expensive option is a more upscale titrator, which will perform many other titration-based analyses. These can save hours per day if you are running numerous tests, and they can directly transfer the data to a PC. These titrators sell for between $5,000 and $10,000.

After the reagents are added, the instrument determines the endpoint and then records the data in a data file. These instruments can also run the titration portion of the A/O method. In the long term they may reduce the cost per analysis due to lower reagent costs.

Another popular tool for SO₂ determination is a small, single-use vacuum tube that utilizes a reverse titration. The instruments are fast, handy and easy to use, but they lack precision and are of marginal utility for red wines. Whichever laboratory procedure you select, you will learn to factor its particular idiosyncrasies into your operation.

Focus on free SO₂
Analyses will provide you with free SO₂ in parts per million (ppm), which is the same as milligrams per liter (mg/L). With the same apparatus you may also determine bound SO₂. For now, free SO₂ is the more important number, since this is the form of SO₂ that is still available to act as an antioxidant and antimicrobial agent. The degree to which a given level of SO₂ will protect a wine is highly dependent on its pH, because really only a small portion of the free SO₂ does most of the heavy lifting.

In reality, free SO₂ in wine exists in three forms: molecular SO₂, bisulfite and sulfite. Of these three species of SO₂, molecular SO₂ is the most potent antimicrobial agent, and its existence is favored by a lower pH. Therefore, the lower a wine’s pH, the less free SO₂ will be required. It is commonly accepted that a level of 0.8 ppm molecular SO₂ offers ample protection from oxidation and most yeast and bacteria. Many winemakers attempt to maintain a minimum of 0.8 ppm molecular SO₂ throughout the aging period for white wines.

Because they are usually dry, red wines have undergone a complete malic acid fermentation and possess the anti-oxidative properties of their tannins. Therefore they require slightly less—more along the lines of 0.6 ppm molecular SO₂. Also, red wines usually have a higher pH, and maintaining a 0.8 ppm molecular SO₂ would give excessively high total sulfur levels, which can lead to off flavors and excessive color depletion due to sulfite bleaching. In any case, total sulfur levels should be kept below 200 ppm unless the impact of such a level is tested in the lab beforehand.

Making adjustments
To adjust a wine to a given molecular SO₂ level, first determine the wine’s pH and free SO₂, then consult the table “Amount of Free SO₂ Given Molecular SO₂ Level” below. This will indicate the amount of free SO₂ necessary to yield a given 0.8 ppm or 0.6 ppm molecular SO₂.

The trick here is that only about 50%-60% of your addition will become free SO₂, with the remainder going to the bound state. The higher your free SO₂, the greater percentage will likely remain free, and you should adjust accordingly.

For example, if you wish to achieve a 0.8 ppm molecular SO₂ in a wine with a pH of 3.2 that has a free SO₂ of 15 ppm, you determine by consulting the table that the wine requires a free sulfur of 20 ppm. An increase of 5 ppm free SO₂ is required. Knowing that only about 50% of your SO₂ addition will become free, you will add 10 ppm total SO₂. The SO₂ you add will take three to four days to stabilize, at which time you should retest the wine and make any necessary adjustments to achieve your target. Practically speaking, if bottling is a while off, and the wine can stand a little extra SO₂, then err on the high side and save yourself the hassle.

Cellar operations such as racking or filtering reduce the free SO₂ in wine. Storage alone will usually rob a wine of at least 5 ppm per month, so an occasional sulfur check is important in maintaining adequate free SO₂. You may also take into account the storage time until bottling and fluff up your addition to compensate for this anticipated loss.

Bottling precautions
Having adequate SO₂ at bottling is a critical financial issue. If there is too much SO₂, consumers will rightfully reject the wine, or release dates may need to be postponed until levels moderate. The bigger danger is in too little SO₂. Without the protection from adequate SO₂ in the bottle, white wines can die an early death—or even worse, suffer from biological spoilage.

If proper attention is paid to SO₂ maintenance in the cellar, the pre-bottling adjustment will be minimal. Obviously, the minimum threshold will be the molecular SO₂ you’ve attempted to maintain throughout storage. But since bottling is a fairly violent process even under the best of conditions, there will always be some oxygen pickup during the process. Enough SO₂ needs to be present to compensate for this and still leave the wine with a molecular SO₂ at or above your target.

Testing your free SO₂ before and a week after bottling can give you a rough estimate of the amount of SO₂ that will be consumed during your bottling operation. This information can be used to help determine the proper addition for future bottlings. To get started, an additional 8-10 ppm free SO₂ over and above what you need to achieve a specific molecular SO₂ is a pretty safe bet.

Another consideration is time until release of the wine. If the wine is going to market right away, you really don’t want SO₂ to be apparent in the nose, and so the dosage should be just enough to provide the chosen molecular SO₂ and to compensate for the oxygen pickup in your specific bottling operation. A minimum of two weeks bottle aging is a good policy, because this gives time for the SO₂ to bind up any acetaldehyde generated by bottling. This one- to two-week period after bottling when the wine seems a little dumbed down is known as bottle shock.

Inert gas sparging of bottles will reduce the amount of oxygen pickup and reduce the loss of free SO₂. Also, cold wine more readily dissolves oxygen than warm wine. If the wine will be in the warehouse for a year, a little extra SO₂ is a good idea. You can count on wine losing an additional 8-10 ppm SO₂ during the first year of storage under cork. Screwcaps generally show less loss.

Most wineries in the East use potassium metabisulfite (PMBS) as their source of SO₂. It is a stable white powder as long as it is kept dry and sealed in a container. PMBS is about 58% sulfur dioxide by weight, so it takes 1.72 grams of PMBS to yield 1 gram of SO₂. Additions are made by first dissolving PMBS in cool water, then adding it to the wine or must. If you are sensitive to SO₂, it is prudent to wear a respirator while working with it.

SO₂ can be a winemaker’s best friend, but using it to compensate for lack of sanitation, careless handling and general laziness will produce inferior wine with high total SO₂. Good winemaking practices utilize SO₂ but are designed to minimize its necessity. With time and effort you will develop a feel for its usage in the context of your equipment, technique and style.

Chris Stamp is president and winemaker at Lakewood Vineyards in Watkins Glen, N.Y. He started his wine career as winemaker at Cayuga Vineyards in the Finger Lakes and in 1986 took a position as enology research and extension associate at the Ohio Agricultural Research Development Center in Wooster, Ohio. When his family opened Lakewood Vineyards winery in 1988, he returned to New York to become the winemaker there.