If you are getting started in winemaking, you may notice that nearly any set of instructions involves the addition of both potassium metabisulfite and potassium sorbate. You may then ask... Are these really necessary? What do they do? Will they give me headaches? Like any new winemaker, you may even opt to just skip out on both of these additions, only to later find out that you have a real headache on your hands... exploding bottles and wine that smells like nail polish remover. I wouldn't wish either of these scenerios on anyone, so I am here to explain the purpose of these two additions and when they are necessary as a winemaker.
What is the difference between Potassium Sorbate and Potassium Metabisulfite?
Potassium sorbate acts as a stabilizer. It does not kill yeast, or stop fermentation but it will stop the existing yeast from further multiplying as they usually would in the presence of sugar. Generally this is added right before back-sweetening and bottling in conjunction with potassium metabisulfite. Do not add potassium sorbate during or before fermentation, as it will severely inhibit the natural yeast multiplication process.
Potassium metabisulfite acts as an antioxidant with anti-microbial properties. This serves many purposes along the winemaking journey. In slang, the addition is usually referred to as adding SO2 (sulfur dioxide) because when added to water or wine it produces SO2 gas and SO3 (sulfite). Sulfites have been vilified by the general public ever since wineries were required to write "contains sulfites" on the labels but we'll get to that later. Believe it or not, sulfites are actually produced as a natural byproduct of yeast alcoholic fermentation in quantities up to 100 parts per million (ppm) and used widely in the food industry as a safe means of preserving food.
Will They Give Me a Headache?
While you may get a headache from red wine, the cause is probably not the sulfites. Red wine is extremely complex and includes many compounds that make up the flavors and aromas. Nobody has been able to nail down what causes the "red wine headache" yet. It may stem from anything... Tannin? Oak? Histamine? Yellow Jacket Venom? Copper Sulfate? Common Hangover? Sulfites are generally much higher in white wines and dried fruits than in red wines but neither of those get quite the bad rap. There is a small percentage of the population (<1%) who may experience an allergic respiratory reaction when exposed to sulfites. The compounds created by the bacteria that sulfites suppress generally more scary, such as ethyl acetate, acetaldehyde, and acetic acid. To me these are much worse and maybe more likely to cause a headache than the trace amounts of naturally occurring or added sulfites.
Potassium sorbate does not get such a bad rap, maybe because it is not listed on the wine label. Most commercial wines do not contain potassium sorbate, but it is extensively used in the sausage industry. To my knowledge there has been no evidence or real speculation that potassium sorbate would cause a headache and certainly not a "red wine headache".
Are Potassium Sorbate and Potassium Metabisulfite Really Needed?
The quick answer is that Potassium Sorbate is sometimes needed, and Potassium Metabisulfite is virtually always needed. If you are making a dry wine with little to no residual sugar, then potassium sorbate is generally not needed. If you plan to sweeten a wine via back sweetening or cold crashing and don't have access to expensive sterile filtration equipment, then potassium sorbate is needed to prevent re-fermentation of these sugars.
Before adding potassium sorbate, be sure that the wine is crystal clear and has been racked off the lees. A crystal clear wine generally does not have enough viable yeast cells to ferment actively, but with a little sugar and no potassium sorbate, those few cells will slowly multiply over weeks or months and begin to ferment once again. If fermentation happens inside the bottle, it will first begin to carbonate, next (if you are lucky) it will blow your corks across the room and spew wine everywhere, or worse it can blow your bottles to smithereens. This really can put a dent in your credibility as a winemaker if it happens to a bottle given as a gift.
As for potassium metabisulfite, the scene is a little less graphic but equally devastating. If you choose not to use it, the resulting wine will almost certainly contain aromas of vinegar, nail polish remover and maybe even horse sweat. Many microbes want to metabolize the sugars and acids in fresh grape juice and all but a few will result in off flavors or undrinkable wine. Luckily it is relatively easy to control which microbes you allow to thrive in your wine with a little help from SO2. A fresh batch of grapes from the vineyard will be loaded with microbial activity (good and bad) and many strains of wild yeast. Wine yeast (saccharomyces cerevisiae) has a reasonable tolerance to SO2, while most other microorganisms present do not. A small dose of SO2 (50ppm) at the time of grape crush will help to significantly reduce the populations of harmful microbes and non-wine-friendly yeasts, while giving the wine yeast competitive advantage. Wine yeasts are very competitive and will snuff out many bacteria and bad yeasts by creating an air free, CO2 filled environment high in alcohol while multiplying rapidly.
As sulfites are exposed to oxygen and oxygen reactive compounds in wine, they will bind to these molecules and lose effectiveness as an anti-microbial or anti-oxidant. Periodically the levels will need replenished to maintain effectiveness against bacteria like acetobacter which is carried on fruit flies and is extremely hard to hide from. Wines affected by the wrath of acetobacter can have strong aromas of sherry, acetone, and/or vinegar which are virtually impossible to remove without industrial vacuum distillation equipment.
How and When do I Add Potassium Metabisulfite and Potassium Sorbate to My Wine?
Adding potassium metabisulfite:
If you are making wine from a kit, then you are going to want to follow the instructions as to when and how much potassium metabisulfite to add. Many kits will have already been sulfited or sterilized, so the instructions are a good place to start. If you are making a wine from a Juice bucket, it may have already been sulfited at the time of pressing. Consult with the supplier, or perform a sulfite test before adding more SO2 prior to fermentation. If you are making wine from fresh grapes or fruit, here are some general guidelines to use if you do not have the ability to test your free SO2 levels:
1. While Crushing and Destemming: Add 50ppm of SO2 to suppress bad yeasts and bacteria
2. After Alcoholic Fermentation Has Completed: Add 0-10ppm SO2 if you are planning to perform a Malolactic Fermentation. If you are not planning to perform Malolactic Fermentation, move on to 3.
3. After Malolactic Fermentation and Prior to Aging: Add 50-75ppm SO2. This is your "big add". Beyond this, you will just want to assure that your levels do not drop below about 25ppm free SO2.
4. Each time you rack: Add about 25ppm SO2
5. At bottling: add about 35 or 40ppm of SO2
*1/4 tsp of potassium metabisulfite to 6 gallons is about 50ppm
*ppm to milligrams of Potassium Metabisulfite: (desired ppm)/.57 x liters of wine
*Alternatively potassium metabisulfate can be added in the form of crushed campden tablets.
Note, these guidelines are extremely rough. If you have the ability to measure both pH and free SO2, then you can make much more precise additions. Ideally you are trying to keep molecular SO2 high enough to hold off bacteria and prevent oxidation. Low pH wines require significantly less SO2 to be effective vs as higher pH wine. High alcohol and tannin can reduce the need for as much SO2. It is really a case by case situation, but it is a lot safer to add a little extra SO2 than to skimp on it. If you added an extremely high dose, you might smell a faint burnt match smell on the wine. A little air contact will eliminate this, or in extreme cases, a few mL of 3% hydrogen peroxide can rapidly decrease the free SO2 in a wine.
Adding Potassium Sorbate:
Potassium Sorbate is a lot easier to figure out, since it is only added just before bottling, and only if you are going to leave a little sugar or add sugar to the wine. Add 1/2 tsp of potassium sorbate per gallon just prior to sweetening, or after cold crashing a fermentation. Make sure that the wine is extremely clear and has been racked off of the lees to permit stirring without churning in any settled out yeast. Always add SO2 or verify the free SO2 levels are above 35ppm when adding potassium sorbate (>50ppm if your pH is 3.5 or higher). The reason for this is that you want to assure that there will not be any further malolactic fermentation and ML bacteria are sensitive to SO2. If malolactic fermentation occurred AFTER adding potassium sorbate, an unpleasant geranium smell will occur as a byproduct of the ML bacteria metabolizing sorbic acid. Not cool.
Summary
Both potassium metabisulfite and potassium sorbate serve important roles in winemaking. Getting a handle on when and how to use them can really make the difference in the quality of a homemade wine. For more winemaking info, visit my youtube channel and please post any comments or questions below.
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