Potassium metabisulfite (PMBS or KMBS) or potassium pyrosulfite, is a derivative of sulfur dioxide that can be used as a preservative, antioxidant, stabilizer and bleaching agent in food with the European food additive number E224. It is commonly used in the production of wine, cider, juice, and beer brewing.
The following are the brief three steps manufacturing processes:
Treat the solution of potassium hydroxide and/or potassium carbonate with sulfur dioxide gases to obtain potassium sulfite. Here is the chemical reaction equation: K2CO3 + SO2 + H2O = K2SO3 + H2CO3
Still pass sulfur dioxide into the above aqueous solution until saturation is reached. In this process, potassium sulfite is converted to potassium bisulfite. Reaction equation: K2SO3 + SO2 + H2O = 2KHSO3
Finally, potassium metabisulfite (K2S2O5) is produced by cooling down the reaction mixture, and following filtration or centrifugation process. Reaction equation: 2KHSO3 = K2S2O5+H2O
|Potassium disulphite, Potassium pentaoxo disulphate
Colourless or white crystalline powder/granular.
Soluble in water (250 g/L 0°C and 44.9g/100ml 20°C); insoluble in ethanol. Gradually oxidized to potassium sulfate (K2SO4) and release sulfur dioxide (SO2) gas in air.
It produces bisulfite (HSO3−) when dissolved in water and the aqueous solution is acidic with the PH 3.4-4.5 (1% aqueous solution).
The following is the interaction equation with water: K2S2O5 + H2O –> 2 KHSO3
Food grade potassium metabisulfite acts as a preservative, antioxidant and bleaching agent in food, especially in acidic foods, such as wine, juice, cider, beer, and other fermented beverages. It can also be used as an equipment sanitizer for fermenting vessels and wine bottles. It is sometimes used as a chloramine removal in tap water.
How Potassium metabisulfite works?
It releases SO2 when dissolved water, and it is the SO2 that plays the role of preservation, antioxidation and bleaching, thereby prolonging food shelf life.
Other sulfites with the similar functions are sodium metabisulfite, sulfur dioxide (E220), sodium sulfite (E221), sodium bisulfite (E222), calcium sulfite (E226), calcium bisulfite (E227), and potassium bisulfite (E228).
The Mechanism behind it
The positive valence of the sulfur atom in sulfur dioxide is 4, while the maximum valence of which is 6.
This means sulfur dioxide is a reducing agent that can react with oxidizing agents to increase the positive valence of the sulfur atom from 4 to 6.
SO2 is a food additive with multiple functions.
Potassium metabisulfite has been used for the manufacturing of wine worldwide. You may have noticed an ingredient named sulfur dioxide in the label of wine, but different with many other ingredients, it is not commonly added to wine directly, but produced by the adding of sodium metabisulfite or potassium metabisulfite.
SO2 or its derivatives (sulfite, metabisulfite, bisulfite) play a role of preservation, sterilization, and stability in the processes grape picking, brewing, storage and bottling.
Potassium metabisulfite is a solid, when added to wine, it releases sulfur dioxide. The following is the mechanism it reacts with water in wine.
- K2S2O5 + H2O <===> 2K+ + 2(HSO3)-
- HSO3- + H+ <===> H2O + SO2
Sulfur dioxide mainly exists in two forms in wine: free state and combined state. Only free sulfur dioxide has bactericidal and antioxidant power.
Combined SO2 is mainly composed of two acid radical ions: HSO3- (bisulfite) and SO32- (sulfite). Free SO2 is dissolved SO2 or sulfurous acid (H2SO3), which has an unpleasant “sulfur” smell.
Purposes of SO2 in wine making
The part of the wine in contact with air is most prone to be oxidized and spoilage. Therefore, increasing the content of free sulfur dioxide in the wine surface is the key to ensuring the storage of wine.
SO2 can inhibit the growth of harmful microorganisms (e.g. wild yeast, lactic acid bacteria, acetic acid bacteria), and so prevent these microorganisms from affecting the quality of wine.
SO2 can be used as a reducing agent to scavenge oxygen, hydrogen peroxide and free radicals in wine, therefore slow down wine aging caused by oxidation (e.g. browning, aroma loss, bitterness) and preserve the flavor and color.
Compared with other ingredients in wine, SO2 is most likely to react with oxygen and be oxidized to sulfuric acid or sulfate, thereby inhibiting or delaying the oxidation of other ingredients.
SO2 can also inhibit the activity of polyphenol oxidase & peroxidase and Vitamin C loss, and also Maillard reaction.
In addition, SO2 can combine with carbonyl compounds to improve the taste of wine, such as acetaldehyde, pyruvic acid, 2-hydroxyglutaric acid, etc.
When to add Potassium Bisulfite?
If you are making wine from fresh grapes or fruits, SO2 is needed in almost every process, before fermentation, end of fermentation, during storage & aging, and bottling.
At the crush, it is added before putting yeast, to destroy wild yeasts, molds and bacteria from the must that present on grapes or fruit. SO2 will be lost during fermentation, and will be discharged together with CO2, so it is needed to add at the end of the fermentation.
The similar purposes in storage and bottling, to inhibit spoilage caused by microorganisms, as well as scavenge oxygen.
Another preservative, potassium sorbate, may be added just before bottling. This ingredient cannot be used before or during fermentation, as it will inhibit the natural yeast further multiplying although it cannot kill them.
How much Potassium Bisulfite to use?
The free sulfur dioxide content required during wine making, storage and bottling is closely related to the pH value of the wine.
PH value has an impact on the ratio of three forms of sulfur dioxide, here is the chemical balance formula: H2O + SO2⇌H2SO3⇌H+ + HSO3 –
We can learn that low pH wines need less SO2 compared with higher pH wines.
Potassium metabisulfite can also be used to eliminate free oxygen and to inhibit the growth of wild yeasts and bacteria in beer brewing but not used commonly. The concentration used is around 5%-10% of that in wine.
Juice will easily be contaminated by microorganisms originated from the fruit itself and from the manufacturing facility.
After pressing, treating juice with potassium metabisulfite to prevent juice browning (act as an antioxidant), and as an antimicrobial agent to inhibit the growth of wild yeasts and bacteria.
Yes, potassium metabisulfite almost has no side effects (maybe the allergy caused by sulfur dioxide) and the safety has been approved by the authorities.
It is generally recognized as safe (GRAS) when used as a chemical preservative by the FDA, but cannot be used in meats; in food recognized as a source of vitamin B1; on fresh or raw fruits and vegetables. (2)
Potassium metabisulfite (E224) is listed in Commission Regulation (EU) No 231/2012 as an authorised food additive and categorized in “ additives other than colours and sweeteners” (3).
Its safety was re-evaluated by EFSA in 2016 and EFSA concluded that the current group ADI of 0.7 mg expressed as SO2 equivalent/kg bw per day for E220-E228 was temporary and would be re-evaluated due to the uncertainties and limitations in the database. (4)
Besides its uses in wine, beer, cider, juice, the following food may contain it with the maximum use levels “10-2000 mg/kg” (5):
- Fresh, Peeled, cut and shredded, Frozen, Dried fruit and vegetables
- Jams, jellies and mermelades
- Dry biscuits
Sodium metabisulfite vs potassium metabisulfite?
The latter is preferred as it does not add sodium. The theoretical yield of SO2 (%) of potassium metabisulfite is 57.6%, the percentage in sodium metabisulfite is 67.4%.
See the detailed comparison in the article of sodium metabisulfite.
Is it Vegan?
Yes, it is vegan as the raw materials and the manufacturing process without the use of animal matter or products derived from animal origin.
Now you may have a knowledge of the antioxidant and preservative – Potassium metabisulfite (E224), from the following aspects:
- Production process
- Uses in wine, beer, juice
What kinds of food labels have you found this additive in? Or if you have any questions or remarks about this additive, feel free to let me know in the comments.