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Yeast
The selection of yeast used for cider production is critical to the quality of the final product. As with other fermented beverages, like wine and beer, the strain of yeast used to carry out the alcoholic fermentation also converts precursor molecules into the odorants found in the final product. In general, two broad categories of yeast are used for cider making: commercially developed strains and wild, or autochthonous, strains. In either case, the species tend to be either Saccharomyces cerevisiae or Saccharomyces bayanus. Commercial strains are available for purchase from numerous distributors, and their characteristics are typically outlined in manuals from the companies. Selection for fermentation may be based on a yeast's ability to ferment at particular sugar concentrations, temperatures, or pH. Some producers may also select for yeasts that produce killer factors, allowing them to out compete other yeast in the juice, or they may select yeast that contribute mouthfeel or specific aromas to the cider.

"Wild fermentations" occur when autochthonous yeast from the orchard or cidery are allowed to ferment the juice. This is the traditional method used for cider making, and many producers feel that the strains unique to their cidery contribute a sense of terroir to their product. These native yeast can be incredibly diverse, and may include standard Saccharomyces species, as well as species from the genera Candida, Pichia, Hanseniaspora and Metschnikowia. Typically, the native yeast take up residence in the cidery, and can be important to the unique flavor of the product. Although it was once believed that the native yeast carrying out these spontaneous fermentations also came from the orchard itself, research has shown that the microbes cultured from apples in the orchard do not align with the microbes found during the various stages of fermentation, suggesting that the sole source of native yeast is the cidery. Allowing wild fermentation to occur may be considered risky, however, because it does not allow the cider maker to control which yeast carry out the fermentation, and because it does not necessarily allow consistency from batch to batch. In order to find a middle ground, some researchers have instead suggested culturing native yeast from fruits, testing their properties for alcoholic fermentation, then selecting particularly good strains from the cultured batch.

Aside from carrying out the primary fermentation of cider, yeast may also play other roles in cider production. The production of sparkling cider requires a second round of fermentation, and a second yeast selection. The yeast used for the secondary fermentation in sparkling cider production serve the same purpose as the yeast used in the Champagne method of sparkling wine production: to generate carbonation and distinct aromas with a fermentation that occurs in the bottle. The yeast are selected based on critical properties, such as tolerance to high pressure, low temperature, and high ethanol concentration, as well as an ability to flocculate, which allows for riddling to remove the yeast when the fermentation has finished. Some researchers have also suggested that non-Saccharomyces yeasts could be used to release additional flavor or mouthfeel compounds, as they may contain enzymes, such as β-glucosidase, β-xylosidase, or polygalacturonase, which Saccharomyces yeast may not produce.

Not all yeast associated with cider production are necessary for fermentation; many are considered spoilage microbes and can be a significant source of off odors in the finished product. Brettanomyces species produce volatile phenols, especially 4-ethyl phenol, which impart a distinct aroma called "Bretty", typically described as "barnyard", "horsey", or "bandaid". While these aromas would be considered spoilage odors in wines, many cider producers and consumers do not consider them a fault. Yeast species like Hanseniaspora uvarum, Metschnikowia pulcherrima,Saccharomyces uvarum, Zygosaccharomyces cidri, Candida pomicole, and Pichia membranifaciens have also been found to produce enzymes linked to generation of spoilage odors.

Swallow-tailed Gull
Check these sources for type specimen info?
 * https://www.biodiversitylibrary.org/item/48713#page/349/mode/1up
 * https://archive.org/stream/bulletinofbritis43zoollond/bulletinofbritis43zoollond_djvu.txt
 * https://archive.org/details/bulletinofbritis43zoollond