Changes

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 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>

''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>.

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 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 sulfur containing compounds to already sulfurous cider may not be the best decision, however, so you would have to balance the potential flavour contribution with the stability of the cider.