Acetic Acid: How Cider Becomes Vinegar

Can hard cider go bad? You know, can cider spoil? I often see posts about someone who found an old bottle of cider they forgot and the question often asked is whether it’s safe to drink. The answer is usually, yes, it’s safe to drink. That is because hard cider won’t really spoil, it simply evolves. However, that evolution can include the development of undesirable flavors including the dreaded acetic acid or what would be called apple cider vinegar. In a very small amount, acetic acid can enhance the aromatic bouquet of a cider and its flavor. However, it is a type of organic acid you try to avoid when fermenting. So how realistic is it that a cider will become vinegar if you unintentionally age it. To answer that question, we need to understand how acetic acid is formed during the cider making process. There are two main times when acetic acid is likely to be created.

  • During Alcohol Fermentation
  • During Maturation

The main reason there are two periods when acetic acid is generated is because each period is enabled by different organism: yeast and acetic acid bacteria. Let’s explore what happens during these times in more detail to understand how acetic acid is created and how much of a risk it actually poses to your fermentation process.

During Alcohol Fermentation

During alcohol fermentation, simple hexose sugars (mainly glucose and fructose) are broken down into ethanol and carbon dioxide (CO2) through a variety of pathways. This involves numerous steps creating various compounds like glucose-6-phosphate, 2-keto-3-deoxygenated-6-phosphogluconic acid, pyruvic acid, and acetaldehyde to name a few. The alcohol fermentation process is generally an anaerobic one, which means it occurs in the absence of oxygen. As often noted, acetic acid is usually associated with oxygen rich environments. So, how does acetic acid form in the anaerobic environment of alcohol fermentation? The answer is it usually doesn’t or it usually doesn’t in large quantities. That is because what creates acetic acid during alcohol fermentation isn’t the normal acetic acid bacteria, acetobacter, but yeast. This is one of the reasons Saccharomyces cerevisiae is the standard yeast used for fermenting alcoholic beverages. Generally, it produces low levels of acetic acid. However, yeast have pathways that can directly breakdown glucose into acetic acid. As shown below in the pathway map, glucose is converted into pyruvate, which is converted into acetaldehyde and ultimately, acetic acid.

Acetic Acid Pathway - During Alcohol Fermentation
Acetic Acid Pathway – During Alcohol Fermentation

As I have found in my research on non-Saccharomyces cerevisiae yeast, production of acetic acid is one of the key sensory elements assessed. The findings are that some yeast can produce significant amounts. Again, this is a process that directly converts sugar into acetic acid. It will not turn your apple juice directly into vinegar but it can make your cider taste more like a shrub (i.e. a vinegary alcoholic drink) than a cider without exposure to excessive oxygen. One of the worse offenders that I have found is Metschnikowia pulcherrima(2). It can produce as much as 10 times the amount of acetic acid as some other non-Saccharomyces yeasts like Hanseniaspora uvarum. It also had the lowest nitrogen use and produces some of the highest levels of glycerol. It’s a great example of how the yeast you use can have a large impact on the sensory characteristics of your cider. It’s important to remember that not all strains within a given family perform the same but there is usually a tendency for those strains to perform similarly given their common genetic makeup. It also highlights why it can be important not to generalize too much and to better understand the science behind your fermentations. While most hard cider made with Saccharomyces cerevisiae yeast will not have excessive acetic acid production, using a wild/natural microflora will probably result in higher levels and if a strain that does produce significant levels dominates, you might end up with a quite sour cider. On the other hand, you might also end up with a more flavorful and sweeter cider because of the volatile compounds and glycerol produced by those same non-Saccharomyces strains. Ultimately, to avoid acetic acid formation during alcohol fermentation, you have to understand your yeast and select yeasts strains that produce lower levels of acetic acid. However, the level of acetic acid formed during alcohol fermentation is generally significantly less than what can be formed during maturation.

During Maturation

When we think of creating acetic acid or vinegar, we normally think of the post alcohol fermentation process. With cider or other alcoholic beverages, acetic acid would form during what we consider maturation or aging. This is the time when lactic acid bacteria along with what might be considered spoilage organism like acetic acid bacteria and Brettanomyces yeasts can get involved in the evolution of your hard cider aromas and flavors. If you were intentionally making vinegar, you wouldn’t probably call this maturation, you would call it acetic acid fermentation. It’s a process you would encourage rather than try to protect against. However, the chemical process that occurs is the same whether it’s something you want or not. In this case, the ethanol you just created with your yeast through alcohol fermentation is the starting point. In some regards, creating vinegar is the natural progression in the fermentation cycle where sugars are converted to ethanol and ethanol is converted to acetic acid, which is a natural preservative. Basically, ethanol is broken down into acetaldehyde, which in turn is hydrated by water and then broken down into acetic acid(1). The following pathway map provides a basic overview of this process.

Acetic Acid Pathway - During Maturation
Acetic Acid Pathway – During Maturation

The key point to make for acetic acid fermentation is that it is aerobic. That simply means it requires oxygen for this pathway to be realized. Also of note is that it takes a significant amount of oxygen or a significant amount of time with access to oxygen. Unlike the alcohol fermentation process, which can create acetic acid without oxygen, oxygen is central to acetic acid formation during maturation. This is why you often hear the advice of avoiding oxygen after fermentation is complete. Again, the rule of absolute can fall apart if you get down to the scientific nuances of oxygen exposure during maturation. I have read a lot of research on micro-oxygenation and how it can actually improve wine and cider sensory characteristics. Barrel aging for example is partially about micro-oxygenation and working with the chemical aspects of the wood. However, as a general rule of thumb, it’s good to avoid excessive exposure to oxygen post-fermentation. There are more faults that can be created like film yeast, oxidation of various compounds, acetic acid fermentation, and even some spoilage yeasts like Brettanomyces. Purposely exposing your cider to oxygen in order to seek a specific chemical reaction is different from leaving too much headspace in a bucket and finding you have a thick layer of flor on top. However, that brings us back to the original question of whether hard cider can spoil.

Hard cider can definitely change into vinegar but if I stored some sliced cucumbers or cabbage in it, I know that I would enjoy those on a sandwich. It’s hard to say it’s spoiled but it’s definitely evolved. As a side note, if you want to turn a cider into vinegar, oxygen is your friend and you should aerate it to speed that transformation. However, for the cider makers, try to avoid yeast that produce higher amounts of acetic acid and avoid excessive exposure to oxygen post-fermentation. Even if your cider won’t spoil or readily turn into vinegar, your flavor profile will most likely be better if you avoid the production of acetic acid when possible.

Hopefully, this helps put the formation of acetic acid and vinegar in context and gives you some ideas of why you might find that sour bite immediately after fermentation versus months later if you aged it where it could be exposed to oxygen. As always, I hope it helps you make better cider. If you want more tips, recipes, or ideas that might expand your cider journey, follow me.

(1) H.P. Vasantha Rupasinghe and associates, Chapter 3 – Chemistry of Fruit Wines, Editor(s): Maria R. Kosseva, V.K. Joshi, P.S. Panesar, Science and Technology of Fruit Wine Production, Academic Press, 2017, Pages 105-176

(2) H. Roca-Mesa and associates, Nitrogen Preferences during Alcoholic Fermentation of Different Non-Saccharomyces Yeasts of Oenological Interest, Microorganisms 2020, 8, 157


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