Safety In Home Fermentation: The Facts


The factors that make fermentation safe are salt, acidity, oxygen availability, bacterial competition, basic hygiene and common sense. These will collectively ensure that C. botulinum and other bacteria associated with food poisoning, including Salmonella sp., E. coli sp., Staphylococcus aureus, Listeria monocytogenes and moulds, are not able to grow.


Salt is an essential addition in vegetable fermentation. It has a long history of use in preservation, because of its antibacterial and dehydrating properties. To understand why, it is important to have some knowledge about the physical concepts relating to the microbial requirement of water for growth.

Water activity is a little-known concept, but it underpins the safety of fermentation. The moisture content of a food is simply the water it contains, which can be calculated by working out the difference between fresh and dehydrated samples. For example, the moisture content of fresh red peppers is about 93%, while in dried peppers it is about 6%. In the 1950s, the concept of water activity (aw) was introduced by William James Scott to describe the ‘boundness’ of water in food. He defined it as being present in two ways:

  • Chemically bound water is bound so tightly that it cannot be utilized by microbes, enzymes, and so on.
  • Free water is bound through weak bonds. It can be utilized by microbes and can exchange with the environment.

Calculating water activity is tricky. Generally speaking, pure water has an aw of 1, while honey, which is both viscous and complex, has an aw of about 0.6. When salt is added to normal water, the sodium and the chloride interact with the water molecules making them less ‘free’ and the water activity is lowered – also, there will be other impurities that will have an additional effect. It is sufficient to know that adding 2% salt to a fermentation system is enough to make the aw too low for pathogens. Fortunately, the LAB are more tolerant. Some non-pathogenic yeasts can survive in honey at much lower water activities.


As with all other ingredients, there are choices to be made when it comes to salt. Ordinary refined table salt is easily available and cheap, but it is not ideal. It has a stronger taste than artisan varieties, which can taint a ferment. This may be due to the anti-caking agents used, usually sodium or potassium ferrocyanide; generally, it will contain about 99% salt and 1% additives. However, if it is the only type available, you can still go ahead with it. Iodized salt should not be used, as the presence of iodine could inhibit the growth of beneficial bacteria; again, if it is all you have, it is worth a try.

Other varieties of salt that are more gentle on the palate include Maldon sea salt (98% salt); Himalayan salt, a type of mined rock salt from the Punjab in Pakistan (96% salt); and Fleur de sel (97% salt), which comes from a thin layer that forms in salt marshes.

Broadly speaking, even though salts are not pure (including table salt, which has additives instead of additional mineral content) and contain about 97–99% salt, it is not necessary to compensate for this when adding salt to a ferment. It is unlikely that domestic scales would be accurate enough to be able to do so anyway.

When following a recipe that talks in terms of teaspoons or tablespoons of salt, it is important to remember that, because of the grain size, a tablespoon of table salt will weigh more than a tablespoon of Maldon salt with its large crystals. Approximate comparisons are shown here, but you will need to weigh out the quantities to be sure:

  • 1 tablespoon Maldon salt 11g;
  • 1 tablespoon of table salt 15g;
  • 1 tablespoon of rock salt 13g.

Maldon sea salt crystals dissolve easily and are ideal for fermentation.

Osmosis and Osmotic Pressure

The salt that is lowering the aw is also causing osmotic stress for the bacterial cells in a fermentation system. Osmosis is the movement of water through a semipermeable membrane from a less concentrated solution (for example, salt or sugar) into a more concentrated one. It is a passive process, which happens naturally. Both bacteria and plant cells are affected by it; both contain water, and both will respond to external osmotic pressure. One of three things can happen:

  • If the concentration of salts/sugars is the same both inside and outside the cell, it will be in isotonic balance with the solution.
  • If the concentration of salts/sugars outside is lower, then water will move from the environment into the bacterium, and it will swell up.
  • If the concentration of salts/sugars outside is higher, then water will come out of the cytoplasm through the bacterial membrane to try to dilute the solution, shrinking the cells.

The effects of osmosis on cells, plant, animal and bacterial.

Just as with the aw, LABs have features that make them more tolerant to osmotic stress than most pathogens – another fortunate event that makes fermentation more robust.

Taste and Texture

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There are other reasons for adding salt, the most obvious of which is taste. A mouthful of fermented vegetables should be tangy, crunchy and crisp, and that texture is determined by several factors: the osmotic state of the cell, the pH, and the amount of strengthening pectins of various vegetable cell walls and how they stick together.

After salting cabbage, osmosis reduces the water inside the cells, increasing the amount of plant material relative to the amount of water present. This is thought to be one of the reasons for the crispness, but it is not sufficient explanation, because mere loss of water to plant cells causes a vegetable to go floppy (like that bendy carrot dying in the fridge). Osmotic pressure from salt is thought to be involved – researchers have found that low-salt sauerkraut is soggy.17 Also while osmosis is responsible for drawing water out, various products of fermentation and sodium ions will enter the plant cells via channels in the cell walls. Once inside, some of these may bind with pectins to give greater stability18 or inhibit some cell processes that might lead to softening. pH may also play a role, as the incredibly complicated structure of pectin changes in acidic conditions.

Some might suggest that it is possible to ferment without salt, perhaps for dietary reasons, but it is not advisable. Salt is crucial to safe and successful fermentation. Ferment with salt, and then soak your fermented vegetables to remove some of the salt before you eat them, or resist adding any extra salt to your meal.

The amount of salt can affect the rate of fermentation. If it is too high, at about 10% salt concentration, the osmotic stress will overpower the LAB. The advice in this book is to use 2% pretty consistently – with a couple of exceptions.


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The second parameter for safe fermentation is the production of lactic acid. It allows LAB to thrive while preventing germination and/or proliferation of potential pathogens, which are naturally present in the soil and on vegetables in low numbers, as well as being critical for maintaining your kombucha cultures. pH is a crucial element in the production of fermented foods. Eventually, the LAB produce so much lactic acid and lower the pH to such an extent that they can even inhibit their own growth!

Before fermentation begins, vegetables in salt water will have a pH of about 5–7. This will fall rapidly as the process begins. The holy grail for ‘safe’ fermentation is pH 4.5, the level at which Clostridium botulinum is unable to survive and produce its lethal toxin, and at which another potential pathogen, Listeria monocytogenes, is also inhibited. Almost all of the ferments in this book will reach this level – if not ph 3.7 – within three to four days, and fermentation will be seen to be actively happening, with the production of vast amounts of carbon dioxide. Occasionally, dark green vegetables fermented alone will struggle to reach this pH, so this is not recommended. For a list of vegetables and any likely difficulties with each. Get informed about lactic acid fermentation.


pH, where the p probably stands for potential and the H definitely stands for hydrogen, is a concept developed over 100 years ago by Danish chemist, Søren Peder Lauritz Sørensen, to determine how acidic or basic (alkaline) water-based solutions are. Acids are substances that in water-based solutions can release hydrogen ion H+, while bases release hydroxide OH- ions. If a solution contains more H+ ions it will be acidic, and if it contains more OH- ions it will be basic. If it contains equal numbers of H+/OH- it will be neutral.

Measuring pH with pH paper (left to right): apple cider vinegar in progress, pH 2.5; continuous brew kombucha, pH 4; tap water, pH 7; newly set up kvass, pH 6.

The pH scale operates between 0 and 14, where 0 is extremely acidic, 14 is extremely alkaline and 7 is neutral. The pH scale is logarithmic and works exponentially, so a solution with pH 3 is 10 times more acidic than one with pH 4, 100 times more acidic than pH 5 and 100,000 times more acidic than pH 8.

Now, fermentation was part of human survival for thousands of years before pH was even imagined and many fermenters never check pH, preferring to use their senses of sight, taste and smell. However, it is sometimes useful to know, and if you are fermenting anything unusual, it could be important to ensure that your ferments do reach the magic figure of pH 4.5. If you intend to produce fermented foods commercially it will be essential for you to do so.

There are two simple ways of determining pH in a home kitchen: with yellow universal indicator paper, or a pH meter. When using paper, simply tear off a piece, dip it into the ferment and compare the resulting colour to the chart provided. Be aware that, when making kimchi, which is bright red, the true reading may be altered slightly.

An electric pH meter makes life very easy (with buffers and storage solution).

Battery-operated pH meters are available for less than £10. They may not be the most durable on the market but are worth the investment. They need to be calibrated before use, following the instructions and using pre-made buffer solutions at known pHs, so that the readings are accurate.

Concentration of Acid

Acidity may also be considered in terms of the concentration of the acid, which is thought to be a better guide than pH to judge the endpoint for kombucha or vinegar. pH measures only the free H+ ions in the brew, but measuring concentration takes into account all of them – and they all contribute to the taste of the finished product! A 5% acetic acid concentration means that 5ml of 100ml are acetic acid.

If you are planning to use home-made vinegar for preserving foods, you will need to ensure that it has a concentration of at least 5%, so that it will still have the power to preserve. Acid concentration can be calculated by using an acid titration test kit, but amateur-level ones are tricky to use.

Anaerobic Environment

It may be necessary to create an anaerobic environment, but not always. The exclusion of air is crucial for vegetable fermentation, but it is less important for milk and water kefir (depending upon the desired end result), although it is recommended. Kombucha, on the other hand, does require oxygen.

The purpose of excluding oxygen is two-fold: first, to provide an environment that will favour the growth of LAB, which will produce carbon dioxide and displace further oxygen in the system, and second, to prevent the growth of almost all moulds.

Bacterial Competition

In their ideal mesophilic, anaerobic environment, LAB will be happily and rapidly increasing in numbers. This has several effects on less favourable organisms that might be present:

  • It limits nutrient availability.
  • LAB can produce hydrogen peroxide and other antibiotic compounds to inhibit other organisms.
  • They can ‘crowd out’ any unwanted microbes that are not part of the establishing ecosystem/existing SCOBY (symbiotic culture of bacteria and yeasts).


I am always keen to lower barriers to entry for fermentation. Do you need filtered water? Probably not. Do you need organic veg/fruit/sugar? Not so much. Do you need sterile jars? Not really. Do you need specialist equipment? Although it will help, it is not essential. However, one area where corners should not be cut is basic hygiene: specific elements of this are covered in each chapter but there are some rules that generally apply.

Whether fermenting vegetables or straining milk kefir, you should always ensure that your kitchen work surface has been washed down with an antibacterial cleaner and a clean cloth. Hands must be clean and free from possible contamination by raw animal, pet or grubby-vegetable matter. Remove all pets from the work surface (someone once asked me if the strange bacterial colonies growing on their kombucha SCOBY could have been caused by their cat licking it!). Work smartly to minimize the chance of uninvited air-borne microbes joining the party.

Equipment does not need to be sterile (free from all microbial contamination), but it does need to be ‘dishwasher clean’ – that is to say, thoroughly washed, rinsed and dried at a hot temperature. If you do not have a dishwasher, wash everything in hot soapy water, rinse thoroughly to remove soap residues and dry off either in a low oven or with a freshly washed tea towel. If you have had any contamination problems, pop a Milton tablet in a bowl of water and leave your equipment to soak before washing well. This is suitable for plastic too.

The exception to sterilization is when you are making fruit syrups: these contain no LABs to keep opportunistic moulds at bay, and should be either microwave sterilized (rinsed in warm water, then microwaved for 1–2 minutes on high power until the water has evaporated) or baked in the oven for 15 minutes at 150°C.

Tea towels and dishcloths are often a source of opportunistic pathogens. A daily change is a good idea – put them in with the washing.

  • Common Sense
  • Physical Safety

There is always a slight risk of things exploding, especially as glass is commonly used. Fermentation is a live continuous process, sometimes producing (except vinegar) copious quantities of carbon dioxide, which can potentially cause a hazard. Here are some pointers on how to avoid disaster:

  • For kombucha and water kefir, always use either clip-top brewer’s or screw-top glass bottles that are suitable for carbonated drinks.They can be purchased online. Never use square clip-top bottles as the edges weaken the structure. Wider-necked ‘milk’ bottles seem to suffer from less pressure build-up.
  • Use one plastic ‘guide’ bottle for secondary fermentation to tell you when your batch is ready (the bottle will go hard, reflecting what is happening in the glass bottles).
  • Consider storing your kombucha or water kefir at room temperature in an icebox or cupboard during secondary fermentation, in order to contain any breakages.
  • The same goes for vegetable ferments in closed systems, although these tend to crack at the base rather than explode. You can replace the metal catch with a rubber band wound round a few times, which has a bit more give.
  • Open bottles of water kefir and kombucha over the sink, making sure they are pointing away from your face.

Using Your Senses

The final way to ensure you are fermenting safely is to use your senses – sight, sound, smell, taste, touch, and gut feeling – to guide you. When dealing with natural systems, as you will be here, no troubleshooting guide will be able to predict exactly what might happen.

If your fermentation looks different from what is described or shown, take a moment to work through some options. Were the conditions you fermented under correct in terms of salt content, fermentation vessel, temperature, and oxygen exclusion?

Your sense of smell is crucial too – although you might want to wait a few seconds before sniffing your sauerkraut for the first time, to allow the sulphur to dissipate! If vegetable ferments smell cheesy, something has gone awry and butyric acid has probably been produced. If your milk kefir smells of gone-off milk, then perhaps the base product was on the turn and Paenibacilli in it have outgrown the kefir microbes. If the smell of any of your ferments turns your stomach, do not taste them; there is no need. They should smell vinegary and intensely of vegetable, or yoghurty and wholesome. Read more about wild fermentation.

If in doubt, you can use a pH meter or indicator paper to establish whether a safe pH of 4.5 or less has been reached before sampling a little.

Measuring Sugar/ Alcohol Content

Using a hydrometer is a very easy way to determine the sugar content of a water kefir or kombucha. This is an inexpensive piece of kit comprising a glass weighted tube, designed to measure the specific gravity or relative density of liquids, based on buoyancy, and a plastic tube. A sugar solution will have a higher density than water. Take a reading with your hydrometer before you begin fermentation, and then take another after fermentation. When taking the second reading, the hydrometer will sink much deeper into the liquid. This indicates that it is now low in density, because the sugar has been utilized by the microbes.

Before fermentation with a 3.5% sugar solution (A), and after (B), where the difference corresponds to a 0.5% concentration of sugar in the finished water kefir.

Testing the alcohol content of continuous brew kombucha (none detected) and 5-day-old water kefir made with 3.5% sugar (0.08%). The yellow colour of the lemon ginger and turmeric has slightly affected the reading.

By using the tables that accompany the device, you will be able to tell how much sugar has been used up, and therefore calculate the sugar content of your water kefir. In a pure system, for example, when making wine using just a single known species of yeast, sugar usage will predict how much alcohol has been produced. In kefir and kombucha, however, things are much more complicated. Different organisms are utilizing that sugar, but they are not all converting it to alcohol, so it is not a reliable indication. There are test strips available online that can detect 0.3% alcohol. If there is more than 0.3% alcohol, you will have to dilute the ferment by a known amount and try again.

A refractometer can also be used for working out the level of sugar concentration. Instead of measuring the density of the solution, this will measure its refraction, which changes with the dissolved solids (sugar). It will be more expensive than a hydrometer, but it can be more accurate and easier to read. Sometimes, the sugar content is described in Brix or bx. This simply means 1% sugar. Just as with a hydrometer, the measurement is complicated by other dissolved components, including acetic acid, glucluoronic acid and ethanol.

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