#FEMSmicroBlog: Which microbes are in your cider?


On today’s World Cider Day, we’re not only celebrating a tangy apple drink; we’re also highlighting the microbes that are essential for producing cider. Thanks to their flavourous and multifaceted fermentation products, cider gets its unique and mood-boosting taste. Sarah Wettstadt discusses which microbes are found in cider and are responsible for producing this sweet and tangy refreshment. #MicrobiologyIsEverywhere


About cider and its production

Cider is a fermented alcoholic beverage, ranging from 1.2–8% alcohol, made from apple juice. It generally varies in sweetness and alcohol content depending on the type of apples used. Many regional variations of fermented apple juices are known from fermenting Lebanese apples to the so-called Apple chicha of Andean Patagonia. Yet, in Western Europe, almost every country has their own version of the alcoholic apple beverage, ranging from Irish and English Ciders and French Cidre, Spanish Sidra to German Apfelwein.

Traditionally, the process of making cider involves three stages. After crushing the apples, they are pressed to extract the juice. Finally, the extracted juice undergoes fermentation involving both alcoholic and malolactic fermentation.

The main fermentation is a generally slow process, requiring at least 2–3 weeks. Maturation then takes place in wooden casks for several months at 3 °C–12 °C. Depending on the sweetness desired, the cider is bottled after a certain amount of time.


Fungal communities in cider

Naturally present on apples, yeasts play important roles in fermenting the apple juice. Due to the changing conditions during fermentation, the composition of yeasts changes throughout the process.

The initial ‘fruit yeast’ phase is dominated by Hanseniaspora uvarum and Kloeckera apiculata yeasts. In the ‘fermentation phase’, Saccharomyces yeasts like Saccharomyces bayanus and Saccharomyces cerevisiae take over due to their high ethanol tolerance. Some studies also found the two species Candida sake and Pichia fermentans to be common in cut apples suggesting they would contribute to the initial fermentation phase. The final ‘maturation phase’ is dominated by Brettanomyces/Dekkera yeasts, which could even be traced back to the original fruits.

Other yeast species contribute differently to the final product. For instance, Schizosaccharomyces pombe influences polysaccharide production, which reduces acidity and enhances aroma complexity. Depending on the temperature, Hanseniaspora vineae enhances flavour and sensory complexity by producing aromatic acetate esters.


Bacterial strains in cider

Certain bacterial strains were found to be present during the cider production process, starting from the apple flowers up to the final cider product. Fermenting bacteria of the Lactobacillaceae, Leuconostocaceae and Acetobacteraceae families have been identified and seem to be crucial for the process.

Mainly prevalent are lactic acid bacteria Lactobacillus brevis and Oenococcus oeni. These are very tolerant to the low pH, thus the high acidity, and the presence of alcohol. Yet, they also efficiently ferment organic acids.

In addition to the species, also acetic acid bacteria such as Gluconobacter sp. may play a key role in cider production. This bacterium thrives in high-sugar, low-pH beverages, and oxidises glucose to gluconic acid.


Microbial activities during cider production

To produce cider, two types of fermentations occur: Alcoholic fermentation is typically carried out by yeast strains and involves the conversion of sugars into ethanol and carbon dioxide. Malolactic fermentation by lactic acid bacteria then transforms malic acid into lactic acid.

Malic acid, naturally present in apples, has a strong, earthy, and sharp taste. Hence, its conversion into lactic acid, which tastes softer and is less acidic, significantly influences the final taste of the cider. Since this process is slower than alcoholic fermentation, cider maturation can take up to several months.

Yet, secondary metabolites made by both yeasts and bacteria give cider its unique qualities. Esters, such as ethyl acetate, isoamyl acetate, or aromatic acetate esters, provide mainly fruity and floral notes, while higher alcohols give background flavours. On the other hand, phenolic compounds provide either bitter or unpleasant aromatic notes, significantly impacting the overall taste of the cider.

Different combinations of microbes impacting cider quality.
Different combinations of microbes impact cider quality. From Ye et al. (2014).

Since several factors influence the microbial diversity in cider production, these also impact the quality of the final product. For example, apple varieties, climate conditions, and soil types can alter the composition and abundance of the microbiota present on the apple surface. Also, the process of cider production and temperature profile during fermentation and maturation greatly determine the microbial diversity and metabolic activities.

When looking into the impact of microbes on cider production, I was surprised that scientific research is not as extensive as research on other fermented beverages. Why not submit your next paper on cider fermentation processes to FEMS Yeast Research as there clearly seems to be a gap in our knowledge?

With several cider varieties existing around the globe, knowledge of this traditional drink keeps the tradition alive. Also, understanding the microbial cider communities can help cider producers control their diversities and, therefore, the taste, aroma, and quality of the final product. I’d cheer for that!


About the author of this blog

Dr Sarah Wettstadt is a microbiologist-turned science writer and communicator publishing for professional associations and life science organisations. She runs the blogs BacterialWorld to share the diverse and colourful activities of microbes and bacteria and Sunny Scientist to support researchers in their busy scientific days. As science communication manager for the Scientific Panel on Responsible Plant Nutrition, blog post commissioner for the FEMSmicroBlog and author of the colouring book “Coloured Bacteria from A to Z“, Sarah writes about microbiology and plant nutrition research for expert and non-expert audiences. To coach and mentor scientists in science communications, she co-founded the Partnership business SciComm Society. Before her science communication career, Sarah did a PhD at Imperial College London, UK, and a postdoc at the CSIC in Granada, Spain. In her non-scicomm time, she enjoys the sunny beaches in Spain playing beach volleyball, or she travels the world.

About this blog section

The section #MicrobiologyIsEverywhere highlights the global relevance of microbiology. The section acknowledges that microbiology knows no borders, as well as the fact that microbiologists are everywhere and our FEMS network extends well beyond Europe. This blog entry type accepts contributions from excellent blogs translated into English. Regional stories with global relevance are welcomed. National or international events sponsored, organised or connected to FEMS are also covered.

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The #FEMSmicroBlog welcomes external bloggers, writers and SciComm enthusiasts. Get in touch if you want to share your idea for a blog entry with us!

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