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Unlike beer, cider does not sour due to microbial activity, but instead undergoes [[Cider#Malolactic_Fermentation|Malolactic Fermentation]], which convert malic acid to lactic acid. Cider can, however, get funky with the presence of alternative microbes in either primary or secondary fermentation.

'''Malolactic fermentation (MLF)''' is a process through which malic acid is converted to lactic acid by lactic acid bacteria (LAB). The main impact of MLF on cider is likely to be seen in de-acidification, as malic acid is a stronger acid than lactic acid<ref>https://winemakermag.com/493-mastering-malolactic-fermentation-tips-from-the-pros</ref>, and the conversion will decrease increase pH and change the perception of acidity. The process can create other compounds and change the flavor or aroma of the cider; notably , MLF can produce diacetyl well above the taste threshold and other compounds that may not be above the taste or aroma threshold, but together may increase perceived complexity.<ref>http://byo.com/hops/item/1448-taking-control-of-malolactic-fermentation</ref>. For natural MLF, which is caused by lactic acid bacteria present on the fruit, MLF begins after primary fermentation. Yeast cells undergo autolysis and release nitrogen-rich compounds, polysaccharides, and fatty acids, which can serve as food sources for LAB <ref name="Wade_2018">[https://onlinelibrary.wiley.com/doi/full/10.1111/ajgw.12366 Role of Pediococcus in winemaking. M.E. Wade, M.T. Strickland, J.P. Osborn, C.G. Edwards. 2018. DOI: https://doi.org/10.1111/ajgw.12366.]</ref>.

''Oenococcus oeni'' is considered the primary MLF bacteria in wine because it can withstand the harsh conditions of wine. Other lactic acid bacteria besides ''O. oeni'' also have some malolactic fermentation capabilities. ''[[Pediococcus]]'', [[Nonconventional_Yeasts_and_Bacteria#Leuconostoc|''Leuconostoc'']] and ''[[Lactobacillus]]'' species are often present at the beginning of MLF, but generally die off in wine towards the end of MLF. These genre genera are generally seen as spoilage species that create off-flavors in wine and cider making, specifically [[Pediococcus#.22Ropy.22_or_.22Sick.22_Beer|exopolysaccharide (ropy)]] production and [https://en.wikipedia.org/wiki/Biogenic_amine#Determination_of_biogenic_amines_in_wines biogenic amines]. Despite this general rule, some strains of ''P. damnosus'' have been found to positively affect Albariño and Caiño white wines due to their ability to perform MLF, and dominated over inoculations of ''O. oeni''. The strains used were ones that did not produce exopolysaccharides or biogenic amines, and were determined to have a positive influence on the white wine <ref>[http://onlinelibrary.wiley.com/doi/10.1111/jam.12392/full Effect of malolactic fermentation by Pediococcus damnosus on the composition and sensory profile of Albariño and Caiño white wines. M. Juega, A. Costantini, F. Bonello, M.-C. Cravero, A.J. Martinez-Rodriguez, A.V. Carraschosa, E. Garcia-Moruno. 2013.]</ref>.  In the presence of citric acid, which is the case of wine and perry, ''O. oeni'' has been shown to produce diacetyl and acetic acid during MLF <ref>[http://wineserver.ucdavis.edu/industry/enology/methods_and_techniques/reagents/citric_acid.html "Citric Acid". Viticulture & Enology website. 01/03/2017. Retrieved 10/02/2017.]</ref>. Tariq Ahmed from Revel Cider Co. attributes a "Flanders red-like" character to his perries made from Bartlett pear due to the acetic acid production from citric acid and sorbitol in the pears <ref>[https://www.facebook.com/groups/MilkTheFunk/permalink/1838663342828538/ Tariq Ahmed on the role of citric acid in perries. Milk The Funk Facebook group. 10/02/2017.]</ref>.

Spontaneous fermentation of apple juice to cider is very easy and can be done without any more effort than buying fresh -pressed, unpasteurized, and untreated (raw) apple juice and then forgetting about it for a few weeks or even months. While not available everywhere, if you live in a region with apple orchards it is very easy to find in the fall. Usually sold directly by the orchards, or available at a cider mill, the fresh -pressed juice will already contain all the microorganisms needed to ferment out the sugars.

In the US your fresh -pressed juice will most likely come in a plastic milk jug style container. You have two options to ferment this juice:

# At room temperature - this would proceed much like any normal fermentation, and due to the unknown nature of the microbes contained within, it is likely to really dry out.

The cold fermentation option has some pitfalls, however, so your results may vary. Much like a lager, it is possible that you might need a diacetyl rest, requiring you to warm the cider up for a short period of time. This can kick off fermentation by microorganisms that were previously dormant, which is likely desired to reduce the diacetyl content anyway. The other major issue faced by cold fermented ciders is sulfur (H₂S) production. Due to the colder temperatures, the sulfur will off -gas less, again requiring warming of the cider to encourage off -gassing. If the cider tastes good otherwise, further fermentation can be halted with the use of sulfites. Adding more sulfites (SO²) will also reduce the (H₂S) sulfur containing compounds to already sulfurous cider may not be (see [https://brocku.ca/ccovi/wp-content/uploads/sites/125/Karl-Kaiser-Controlling-reductive-red-wine-aroma-2010-.pdf "Controlling Reductive Wine Aromas" by Dr. Karl J. Kaiser]). A study by Wicklund et al. (2020) compared several variables of 2 spontaneously fermented ciders and 6 ciders fermented with Mangrove Jack's M02 (''S. bayanus''). As far as the spontaneously fermented ciders versus the ciders that were fermented with the M02 yeast, the difference in volatile esters was insignificant. However, one of the best decisionspontaneously fermented ciders scored higher in fruitiness, howevercomplexity, so you would have to balance and aftertaste than all of the potential flavour contribution other ciders except one M02-pitched cider that was dry-hopped with stability Amarillo hops. The spontaneously fermented ciders also had less succinic acid than the M02-fermented ciders. The greatest differences between the ciders ass far as esters go was determined by the different blends of different varieties of cider apples, with the Gravenstein and Summerred varieties scoring highest for fruitiness and complexity <ref>[https://www.iseki-food-ejournal.com/ojs/index.php/e-journal/article/view/713 Various factors affect product properties in apple ciderproduction. Trude Wicklund, Elizabeth R. Skottheim, Siv F. Remberg. 2020. DOI: https://doi.org/10.7455/ijfs/9.SI.2020.a7.]</ref>.

"Keeving" is the traditional process of Western England and France for naturally carbonating a sweet cider. * [http://www.cider.org.uk/keeving.html "Keeving" from The Wittenham Hill Cider Pages.] * Video demonstration (To donot using PME; see the Wittenham Hill Cider Pages above for the full instructions)::<youtube width="300" height="200">h3j-iQRnaes</youtube> ===Using Hops===* [https://www.facebook.com/groups/MilkTheFunk/permalink/2994066257288235/ MTF thread from Benedikt Koch on using aged hops in a natural cider, inspired by Revel Cider.]