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added section Sulfur Diioxide
* For information on mixed culture starters, see [[Mixed_Cultures#Starters_and_Other_Manufacturer_Tips|Mixed Culture Starters]].
====Sulfur Dioxide====
Sulfur dioxide (SO<sub>2</sub>) in the form of potassium metabisulfite is often used in wine to limit microbiological growth of yeast and bacteria, and oxidation. SO<sub>2</sub> maintains an equilibrium between molecular SO<sub>2</sub>, bisulfite, and sulfite ions. The molecular SO<sub>2</sub> portion interacts with cell walls because it has no charge, while the rest of the SO<sub>2</sub> has less of an impact on yeast and bacteria cells (but can still have some inmpact on bacterial cells, depending on species). Below a pH of 3.5, molecular SO<sub>2</sub> is higher, but above a pH of 3.5 the SO<sub>2</sub> binds more into bisulfite and sulfite ions, rendering it less effective against microbes. SO<sub>2</sub> can also become bound to other compounds such as acetaldehyde, pyruvic acid, anthocyanins, glucose, and glacturonic acid, which also renders it less effective against microbes, although they can degrade and release molecular SO<sub>2</sub> as well <ref name="Wade_2018" />.
''Pediococcus'' and other lactic acid bacteria have a wide range of resistance to SO<sub>2</sub> depending on species and strain. One study by Edwards and Jensen (1992) reported that ''P. parvulus'' was able to thrive in 20 mg/L of free SO<sub>2</sub> (0.39 mg/L molecular SO<sub>2</sub>), indicating that small amounts of SO<sub>2</sub> are not enough to inhibit ''Pediococcus''. Other studies have shown that different strains can be inhibited at concentrations between 100-256 mg/L total SO<sub>2</sub> <ref name="Wade_2018" />.
==="Ropy" or "Sick" Beer===