Cider is an alcoholic beverage made from apples and, depending on country or region of origin, ranges from a clear, amber colour, to turbid brown-green. It is usually highly carbonated. The powerhouses of cider production in Europe are Ireland and Britain, France, Slovenia and Spain. While many might regard cider as the poor man of the fermentation world, lacking the subtleties associated with beer formulation such as choice of malts to include, which, when and how much hops to use, the microbiology of the traditional cider fermentation rivals only that of wine in its complexity. Indeed, many of the yeasts involved in the conversion of apple must (a fancy word for juice) into cider are also present in the context of winemaking, the source for these yeasts being the skins of apple and grape respectively.
Some of the yeasts in question are Brettanomyces anomalus, B. bruxellensis, Dekkera polymorphus, Hanseniaspora uvarum, Metschnikowia pulcherrima; Hansenula anomola, H. fermentas, H. guilliermondii, Saccharomycodes ludwigii and Saccharomyces cerevisiae (the yeast behind beer and wine’s primary fermentation). As with many of the fermentations where non-Saccharomyces yeast form an important component of the microflora, the fermentation of a traditionally brewed cider is sequential, with one trophic group of organisms kicking things off, altering the must chemically so that their own survival becomes compromised. Another layer then takes over the fermentation, using the metabolites produced by the first group as energy and nutrient sources, until they themselves are replaced.
In a study of the role of indigenous yeasts in traditional Irish cider, Morrissey et al. (2004) divided the fermentation in three broad phases. During the first phase, the fruit yeast phase, species that were shown to arise from the apples’ surface dominated the fermentation. Among these was H. uvarum, which accounted for 80% of the yeast cells during the fermentation’s short lag phase and which reached counts of six million per millilitre. As fermentation proceeded and other species took over, H. uvarum was no longer detectable after twelve days. S. cerevisiae became the dominant yeast during the second fermentation phase. This phase witnessed a rise in levels of ethanol, and numbers of S. cerevisiae reaching eight million cells per millilitre. Among the possible reasons attributed to the overgrowth of S. cerevisiae at the expense of other organisms were: S. cerevisiae‘s excretion of ethanol; oxygen depletion in the must; increasing levels of must carbon dioxide; S. cerevisiae‘s faster growth rate and outcompeting of other yeasts for nutrients and sugars; and the flocculation (clumping together) of non-Saccharomyces species. During the third maturation phase, Dekkera/Brettanomyces species came to replace others, accounting for 90% of the yeast by day twenty-two of the fermentation. These species were shown to enter the must from installations in the press house (termed “resident yeast” by the authors), and to a lesser extent the surfaces of the apples, and contribute greatly to the organoleptic quality (taste and flavour) of the finished product. This is why there are commonalities in the flavour profiles of traditional ciders and Belgian lambic beer, where Brettanomyces also play an important role. Dekkera/Brettanomyces species are also encountered at this stage in French cider.
This traditional form of cider making incorporates neither temperature control nor deliberate inoculation. The organisms entering the must all come from the apples and what could be considered to be the “conditioned” environment of the cider house. Temperatures vary widely during the fermentation, determining as well as being determined by the particular yeasts dominating the fermentation at any point of time. Because of the loose control the traditional cider brewer exerts over the process, quality control is compromised, with large batch-batch variation. Additionally, seasonal variations in the microflora present on apples (as well as seasonal variations in ambient temperature) lend an extra layer of chance to the process. So, just as in the case of winemaking, we could talk about a good vintage of a particular brand of traditionally made cider.
Morrissey W.F., Davenport B., Querol A., Dobson A.D.W. (2004) ‘The role of indigenous yeasts in traditional Irish cider fermentations’, Journal of Applied Microbiology, 97,647–655.