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Potassium Metabisulphite 1kg - 94319

Potassium metabisulphite, is a white crystalline powder with a pungent sulphur odour.

The main use for the chemical is as an antioxidant or chemical sterilant.

It is a disulphite and is chemically very similar to sodium metabisulphite, with which it is sometimes used interchangeably. Potassium metabisulphite is generally preferred out of the two as it does not contribute sodium to the diet.

Potassium metabisulphite is a common wine or must additive, in which it forms sulfur dioxide gas (SO2). This both prevents most wild microorganisms from growing, and it acts as a potent antioxidant, protecting both the colour and delicate flavours of wine.

The typical dosage is 1⁄4 tsp (1.2322304 milliliters) potassium metabisulphite per 5 gallons of must (yielding roughly 75 ppm of SO2) prior to fermentation; then 1⁄2 tsp per five-gallon bucket (150 ppm of SO2) at bottling. Most commercial wineries do not add more than 30ppm at bottling.

Winemaking equipment is sanitized by spraying with a 1% SO2 (2 tsp potassium metabisulphite per L) solution

When dissolved in water this releases sulphur dioxide which has many important properties in wine and beer making:-

It has antiseptic qualities which will stun, if not kill, the bacteria and wild yeasts in or on fruit.

It has antioxidant qualities which help prevent the effects of oxidation in beer, cider or wine.

It destroys the enzyme that causes enzymatic browning in juice, similar to what happens to an apple if it is sliced and exposed to the air.

Without sulphite in some form much wine would be brown or amber in colour, smell oxidized (or have a sherry-like aroma), and would probably be ruined by bacterial spoilage.

Beer makers will also appreciate that sulphite also removes chlorine from water and therefore reduces the chance of beer being tainted with TCP flavours.

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As you can see there are many variables we have to deal with when figuring out how much potassium metabisulfite we need to add to our wines. Now that we understand how sulfites work and where they go let’s take a look at how to calculate the right amount of potassium metabisulfite to add to your wine.

How to Calculate Potassium Metabisulfite Additions

  1. Take a pH reading.

The least expensive way to get a pH reading is with test strips. Test strips are quick and easy, however, may be difficult to interpret. For a small investment of about $40 you could get a digital pH meter which will be more accurate.

  1. Read the recommended SO2 range from the following chart.

Based on your pH from step one determine what your free sulfur dioxide goal is. This is where we want our free sulfur dioxide levels to be after we add the potassium metabisulfite. Any where in the gray range should be sufficient.

Recommended sulfer dioxide levels based on wine pH. This chart is useful for determining how much potassium metabisulfite to add to your wine. Posted with permission from Accuvin.

This chart is included with the Accuvin Free SO2 Test Kit. The folks at Accuvin were kind enough to grant me permission to share this table with you.

The sulfur dioxide levels presented in this chart are in ppm which is equivalent to mg/L. So if your goal is 30ppm that is the same as 30 mg/L.

  1. Measure the free SO2 presently in your wine.

To do this you’ll need a Free SO2 test kit. Accuvin makes a really easy to use and read kit.

There are machines you can use to measure free sulfur dioxide, however, they are much more expensive and require a bit of maintenance to keep them in working order.

  1. Determine how much free SO2 you need to add to your wine.

Subtract the amount of free sulfer dioxide you already have in your wine from your free sulfur dioxide goal from step 2. This tells us how much free sulfur dioxide we need to add.

According to The Wine Maker’s Answer Book only about 57% of potassium metabisulfite powder is free. This will be accounted for in our calculations below.

  1. Calculate Your Potassium Metabisulfite Addition.

The following equation can be used to determine how much potassium metabisulfite you need to add in grams. This formula is based on US gallons of wine and comes from The Wine Maker’s Answer Book by Alison Crowe (affiliate link).

Calculation for potassium metabisulfite additions. http://winemakersacademy.com/wp-content/uploads/2013/06/pm-equation.png

If you don’t have a scale to measure your potassium metabisulfite a generally accepted equivalent is 1/4 tsp = 1.4 grams.

Mix in your potassium metabisulfite by gently stirring your wine. Be careful because the more oxygen and solids introduced to your wine the more SO2 that your adding will become bound and not free to protect your wine.

Keep tightly closed. Store in a cool Place.

May be Harmful by inhalation, ingestion or skin absorption. May cause Irritation

There are a few things you should know about potassium metabisulfite before you use it again. First, the compounds it creates can be hazardous to your health in large quantities.

SO2 is a toxic gas to breath. It can cause breathing difficulties, swelling, rashes, and difficulty swallowing. If you feel any of these go for help.

Be careful not to breath the dust in or gas that is released when dissolving in water. I’d also steer clear of sipping on any samples immediately after adding this to your wine. Give it time to bond with the oxygen.

Potassium metabisulfite is a controlled substance in food and wine preservation. There are strict legal guidelines on concentrations that are allowed in the final product.

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