How Is Apple Cider Vinegar Made?
Apple cider vinegar is made by fermenting the sugars that naturally occur in apples. Organic apple cider vinegar is produced from apples that go through very restricted treatment in the orchard. Juices from the apple as well as crushed and cut up pulp matter from these apples is mixed with the juices, and set aside to ferment.
Traditional, home-made apple cider vinegar created in a process carried out by a native microbiota present on the fruit. This process is natural and spontaneous, but can take quite a while to produce the final vinegar product. In contrast, industrial scale production of apple cider vinegar uses submerged bioreactors, which supply the bacteria with a constant inflow of oxygen and enable an efficient and speedy production process.
During the first stage of the process, the sugars in apple juices and pulp matter are fermented by yeasts in to cider. In a second and separate bioprocess, the ethanol in this cider is used as a substrate by acetic acid bacteria to create acetic acid. Acetic acid is a major feature of apple cider vinegar and constitutes around 5-6% of the final mixture. In both organic and conventional apple cider vinegars, the oxidation of ethanol to acetic acid is initiated by native microbiota that originates from the surface of apples, which survive the alcohol fermentation process.  
By the end of this process, there is often the presence of a fine, brown powder which is the remains of the original apple pulp. This matter is called ‘the mother’, as it is the source of origin of the vinegar mixture. In both conventional and industrial production of apple cider vinegars, many of these bacteria and yeast species survive the full process, and are therefore present in the final product. Organic, unfiltered, unpasteurised forms of apple cider vinegar that retain the ‘mother’ have a more diverse bacteria profile than industrial and more highly processed forms. 
Which Bacteria Species Are In Apple Cider Vinegar?
The most abundant bacteria strains in unfiltered apple cider vinegar are acetic acid bacteria strains primarily composed of Acetobacter and Komagataeibacter species. There are also significant numbers of lactic acid bacteria species present, comprised mostly of Lactobacillus and Oenococcus species. Some of the bacteria species identified and isolated from unfiltered apple cider vinegar include Acetobacter pasteurianus, Acetobacter pomorum, Komagataeibacter saccharivorans, Komagataeibacter oboediens, and the yeasts Candida ethanolica, Pichia membranifaciens, and Saccharomycodes ludwigii.   
The exact constitution and volume of bacteria species in apple cider vinegar is subject to variation from many factors including the type of apples used, and the length of fermentation. Unfiltered and unpasteurised apple cider vinegar that still contains the ‘mother’ has a cloudy appearance and a sandy brown coloured sediment that settles to the bottom. This sediment contains many bacteria and yeasts that may not be present in more processed forms. Indeed, filtered and processed forms of apple cider vinegar may contain significantly lower quantities of probiotic bacteria in comparison to the unpasteurised equivalent.
Most people are not generally on first name terms with many of these bacteria species. In order to get a better understanding of the bacterial composition of apple cider vinegar, we need to take a closer look at some of these bacteria species.
Acetobacter are Gram-negative bacteria that are commonly found in spoiled and decaying fruits, vegetables, sour juices, and vinegars. In the presence of oxygen, Acetobacter species oxidize sugars and alcohols in to products such as acetic acid. In vinegars where the percentage of acetic acid is low and less than 6% of the total mixture, the predominant species are strains of Acetobacter. 
Acetobacter pasteurianus, Acetobacter pomorum, Acetobacter oboediens, and Acetobacter ghanens are some of the species of this genus that have been isolated from apple cider vinegar.
Acetobacter have no spores, and they cannot move by their own volition. Acetobacter species easily form a membrane on the surface of liquid mediums. The presence of Acetobacter strains in a medium is hinted at by increased concentrations of Acetic acid, Ethyl Acetate, and D-Lactate. The acetaldehyde and various polyphenols produced by these bacteria also can produce a milky, colloidal precipitate in the mixture.  
Acetobacter mainly oxidise ethanol produced by yeasts, but can also oxidise the lactic acid present in a mixture into acetic acid. Members of the Acetobacter genus also demonstrate the ability to oxidise lactate and acetate into carbon dioxide and water.  
This ability to neutralise the forms of lactic acid and lactate may help to balance the final mixture, as lactic acid bacteria are also present in apple cider vinegar.
Lactobacillus are Gram-positive bacteria commonly found in fermenting plant matter and dairy products. Members of these genus are very tolerant of environments of high acidity, and this allows them to survive through the fermentation process. This factor also allows them to survive the harsh environment of the stomach to arrive in the small bowel alive, and thus impact change in the gastrointestinal tract.
Whilst Lactobacillus species normally occupy a relatively low proportion of the intestinal microbiota, they are a regular component of fermented food products, including Saurkraut, Kefir, Kombucha, Probiotic Yoghurts, and apple cider vinegar. 
Such foods have become increasingly popular in the modern age due to evidence that ingestion of Lactobacillus species can improve many gastrointestinal related issues, including Helicobacter pylori infection, diarrhoea, and inflammatory bowel disease.  
Whilst Lactobacillus species are widely consumed as a probiotic, there is an often forgotten aspect of these bacteria that mean they may not suit everyone. As lactic acid bacteria, Lactobacillus species are able to ferment carbohydrates, and lactic acid is produced as a byproduct of this process. Whilst all Lactobacillus species produce lactic acid in this way, many are also known to produce a further compound called D-Lactate. 
D-Lactate is neurotoxic. Whilst some D-Lactate is produced naturally by our everyday biological processes, high levels of some lactic acid bacteria can produce excess levels of D-Lactate in the blood stream. This situation is called hyperlactaemia, and can produce symptoms such as confusion, dizziness, headaches, increased aggression, brain fog, and memory impairment until the D-Lactate is cleared from the body. Healthy individuals are usually able to quickly clear D-Lactate from their systems, and so may not even notice such effects.  
Individuals with Small Intestinal Bacterial Overgrowth (SIBO), impaired renal function, or shortened bowel are much more at risk of hyperlactaemia from high amounts of Lactobacillus species present in many fermented products including apple cider vinegar with mother. A microbiome that predominantly consists of bacteria species that do not produce lactic acid or D-Lactate should be encouraged for individuals that experience unlpeasant symptoms from lactic acid bacteria.  
Komagataeibacter is a novel genus of Gram-negative aerobic bacteria that produce acetic acid during fermentation. Data suggests that in vinegars with a high percentage of acetic acid of 6% of higher, the predominant bacteria species found in the final product tend to be Komagataeibacter. 
Among the various species of this genus, Komagataeibacter europaeus, Komagataeibacter oboediens, and Komagataeibacter intermedius have all been found in organic apple cider vinegar.
Komagataeibacter species are known to be able to produce cellulose nanofibres, which can produce a cloudiness and viscosity in a liquid medium. These cellulose nanofibres may have a wide variety of uses, including material for filters, high gas barrier packaging material, electronic devices, foods, medicine, and cosmetics. 
Oenococcus is a genus of Gram-positive bacteria species that are well adapted to environments of high acidity. They are non-motile, meaning they cannot move of their own volition. Oenococcus are lactic acid bacterium, meaning they produce lactic acid as they ferment carbohydrates. This species also produces D-Lactate during this process. As we mentioned for Lactobacillus, warnings must therefore apply for those with mitochondrial diseases, impaired renal function, or shortened bowel, who may experience unpleasant side effects from the metabolites produced by these bacteria in the gastrointestinal tract.
Lactic acid bacteria species, including Oenococcus oeni can produce a compound called Diacetyl. It can be produced as a metabolite of citric acid when all of the malic acid in a mixture has been consumed. In winemaking, low concentrations of Diacetyl in a mixture can introduce positive nutty or caramel characters. At higher levels above 5 mg/L, it creates an intense buttery or butterscotch flavour, which is generally considered a flaw. 
Which Yeast Species In Apple Cider Vinegar?
As well as containing bacteria, unpasteurised apple cider vinegar also contains many species of yeasts. During the first fermentation stage during vinegar production, the sugars in apple juices and pulp matter are fermented by yeasts in to cider. But how are yeasts different to bacteria?
Whilst both bacteria and yeasts are single celled organisms, yeasts are actually single celled fungi. As fungi, yeasts are eukaryotes, where as bacteria are classified as prokaryotes.
Eukaryotes have a complex cell structure that more closely resembles our own cells compared to the simple cell structure of bacteria. For example, whilst being a single cell, yeasts have membrane-bound organelles, such as a nucleus, mitochondria, vacuoles, and Golgi apparatus. As prokaryotes, bacteria have and no membrane-bound organelles and a have much simpler cellular structure.
Let’s now take a closer look at a few of the yeast species that have been confirmed and isolated from apple cider vinegar in studies. It should be noted however, that there is very little data about how these species behave in the human gastrointestinal tract.
Candida ethanolica is spoilage yeast that is found in apple cider vinegar. It is also found in wines and sour rot-damaged grapes that are sources of wine spoilage yeasts. As a spoilage yeast, Candida ethanolica releases enzymes that assist with the degradation and solubilisation of pulp matter, allowing penetration into the fermenting mass. This in turn, allows acetic acid bacteria to grow and flourish.  
The word Candida is often used as online as a shorthand way to refer to the infamous strain Candida albicans, which can be pathogenic and cause havoc in the human digestive system. Candida ethanolica however, has not been linked to digestive disorders. There are around 150 members of the Candida genus, and not all of these species behave in the same way. Candida ethanolica has several differences that make it distinct from other members of the genus.
Unlike other Candida species, Candida ethanolica is believed not to ferment sugars. It is able to assimilate both lactate and glucose, but does not assimilate nitrate. Unlike some members of the genus, Candida ethanolica does not produce urease – an enzyme that catalyses the hydrolysis of urea, in a process that leads to the formation of ammonia and carbon dioxide.
Candida ethanolica successfully survives the harsh bioreactor of submerged vinegar fermentation, and shows the ability to survive in the presence of acetic acid at concentrations of at least up to 30g/L. It is capable of growing at a high temperature of 42° C, with optimal growth occurring at a temperature of around 28° C.
Pichiea membranifaciens is a yeast that is commonly found on various crops and plants and is known to be involved in ethanol fermentation. It is an interesting strain of yeast as it is known to be able to release chemicals that fight various forms of pathogens. This ability to kill other yeasts has led to multiple studies trialing it’s ability to be an organic fungicide. 
Amongst this research, It has been studied for it’s potential to defend grapevines from grey mould disease, caused by the fungus Botrytis cinerea. It has even shown in laboratory settings to inhibit the growth of Staphylococcus aureus, a pathogenic bacteria frequently responsible for cases of food poisoning in humans.  
Pichia membranifaciens can survive the high volume of acetic acid Whilst it exhibits urease activity. The strain does not exhibit amylase, protease, pectinase and cellulase activity.
This yeast also shows the ability to form bioflocculants in water. Flocculants are defined by their ability to cluster dispersed droplets together, much like a particle version of rounding up sheep. The pressence of flocculants in a medium causes fine particulates to clump together into what is called a ‘floc’. The bioflocculants produced by Pichia membranifaciens can remove organic and inorganic pollutants, as well as pathogens from wastewater. This process also produces oxygen, which this has led to Pichia membranifaciens being studied for it’s potential use in water purification. This method could provide an efficient, safe, natural degradation characteristic to flocculate and sediment many solid suspended particles and colloidal particles which are not easily degradable in water. 
Furthermore, Pichia membranifaciens has the capability to ferment sugars into ethanol at a very high capacity. This process could potentially provide an affordable solution to to global energy crisis. 
Saccharomycodes ludwigii is a spoilage yeast that is known for its ability to contaminate fruit juices and fermented beverages such as wines and cider. It has a high tolerance to sulphur dioxide (SO2) and is often referred to as the ‘winemaker’s nightmare’ due to the difficulty in eradicating it from contaminated winemaking environments. This yeast produces acetoin, ethyl acetate, and acetic acid. High levels of acetic acid will upset balance of the subtle flavours of wines, resulting in a much more acrid taste. But whilst wine makers understandably don’t want their wines to taste like vinegar, such a problem does not exist when deliberately setting out to create apple cider vinegar. 
Saccharomycodes ludwigii is capable of fermenting glucose, sucrose and fructose sugars. However, as it lacks the enzymes invertase and maltase, and as a result cannot ferment larger sugars such as maltose, galactose and lactose. This inability to ferment large sugars means that Saccharomycodes ludwigii does not spoil beers, and as such can be used as a non-conventional brewing yeast. It can be used to produce low-alcohol and alcohol-free beers (max. 0.5 Vol.%), due to it’s lower Ethyl Acetate production when compared to similar yeast strains.
Can You Use Apple Cider Vinegar As A Probiotic?
Yes, organic unfiltered and unpasteurised apple cider vinegar contains a multitude of bacteria species, and some of these are able to exert probiotic effects in the gastrointestinal tract. Not all apple cider vinegar is equal, and processed industrial forms may not have significant levels of live bacteria present. Raw, organic forms do show the potential to deliver significant volumes of probiotic bacteria species, especially the lactic acid bacteria species Lactobacillus. These forms of apple cider vinegar also deliver many forms of yeasts, and some of these may also play active roles in the gastrointestinal tract.
It should be noted that the presence of lactic acid bacteria such as lactobacillus and Oenococcus species may make apple cider vinegar an unsuitable probiotic choice for people who do not tolerate D-Lactate well or are prone to lactic acidosis. Whilst the Acetobacter species that are also present in apple cider vinegar can help to neutralise some of the lactic acid bacterias’ metabolites, it is likely that D-Lactate and D-Lactate producing bacteria will still have a significant presence in the final mixture. Individuals who experience unpleasant side effects from D-Lactate or are prone to Lactic Acidosis may be therefore wish to consider a more suitable probiotic option than apple cider vinegar.
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