GUIDE TO FERMENTATION (2025)

GUIDE TO FERMENTATION
In collaboration with ALMA International School of Italian Cuisine

Origins and history of fermentation

From unplanned and untimely “immediate” need to advantageous, “sought after” result; from “spontaneous” process to “knowingly managed” method: this has been the development undergone by fermentation over time.

Italy is no exception: there are numerous products and preparations which, from the Alps to Sicily, are based on skilful application of this technique. The specialities span from Sauerkraut and brovada in the north-east area to formaggio di fossa along the ridge of the Tuscan-Umbrian-Emilian Apennines while in the south, conciato romano gives way to the colatura di alici of Cetara (Salerno) and the strong ricotta cheese of Apulia.

Fermented foods can be preserved for a longer time and this was extremely important for the rural civilisations for which the urgent need to overcome the limitations posed by the seasonal nature of foods and the lack of efficient refrigeration systems were ever present concerns. Thus the success of this production/preservation system, now considered to be an innovative and eccentric habit inspired by Northern European culinary traditions, also has roots in the gastronomic culture of the Mediterranean area.

Making fermented foods at home

Fermented foods are easier to make than one would imagine. The bacteria and yeasts present help to make these products healthier and less prone to attack by competing bacteria, which can sometimes be pathogenic to humans.

The transformation produced by the fermentation process induces significant beneficial effects on intestinal health as well as improving the nutritional, sensory aspects and shelf-life of the product.

More specifically, the activity of the micro-organisms can make the foods more digestible , as occurs in yoghurt and kefir, and increase the amounts of vitamins and minerals available, a particularly interesting feature of fermented vegetables.

Similarly, the development of micro-organisms generates a whole series of aromatic molecules, which are responsible for the changes in colour, texture, fragrance of the products and enables them to keep for a longer period of time.

This is why certain of the fermented foods we eat are known as “probiotic”, thanks to their high content of live micro-organisms that contribute towards the health of our intestinal microflora.

What is fermentation?

Fermentation can be described as a chemical-physical transformation of the substrate in question by micro-organisms such as bacteria, yeasts or moulds.

In scientific terms, fermentation is a process through which micro-organisms convert sugars into another metabolite, in the absence of oxygen.

Spontaneous fermentation

A selective environment, hospitable only to the required micro-organisms, must be created if the desired fermentation is to be obtained. The key players in these fermentation processes are lactobacilli , which are often assisted by other bacteria, such as those of the acetic species, or by certain fungi, like yeasts. Raw foods, especially vegetables, are extremely rich in these micro-organisms and it is relatively simple to turn them into excellent home-made preserves.

Unlike preserves pickled in vinegar, where the acidic fragrance of the vinegar tends to dominate, fermented vegetables always become more acidic but retain a rather more neutral fragrance. In contrast to acetic acid, lactic acid is much less pungent, thus the vegetables remain crunchy and juicy.

Utensils

Common kitchen utensils, such as chopping boards, knives, a mandoline slicer, wooden spoons or pestles and glass jars with a hermetic seal are all that is needed for perfect fermentation.

Since glass is an inert material and transparent, it turns out to be the best option for this sort of preparation. It also makes it very easy to keep a check on the fermentation process. Food-grade plastic mesh discs , glass or ceramic weights or bags (with no writing on them!) filled with drinking water can be used to ensure the food remains fully submerged in the brine.

Gas (mainly carbon dioxide) is produced during fermentation. For the process to proceed correctly, off gassing is required by opening the jar lids twice a day during the first few days until the production of gas is almost at an end and off-gassing can cease.

The gases released during the production of fermented vegetables to not create high pressures, so there is no risk of the jars breaking. In contrast, fermented beverages or fruit with a high sugar content give off high quantities of CO2 during the production process and failure to off-gas could cause the jars to explode.

Cleaning

The utensils and work surfaces must be perfectly clean. The jars must be washed and dried . They can also be sprayed with 95% ethyl alcohol diluted in water (70% alcohol and 30% water). There is no need to boil the jars for fermentation since proliferation of the pathogenic bacteria that remain in the containers is inhibited by the products of fermentation.

The vegetables or other foods used must be washed under running water, without using products (such as sodium bicarbonate) that could weaken the good bacteria that live in them.

Foods that can be fermented

Any food can be fermented so long as it is fresh , thus in an excellent state of conservation. It is advisable to start with vegetables , which can all be fermented, as can flowers and pulses. They are already naturally rich in good bacteria besides being less likely to be contaminated by the pathogenic sort. Use of old or poorly conserved foods increases the risk of unpleasant odours or tastes forming due to proliferation of undesired bacteria.

The following environmental parameters can be altered to create selective environments:

pH

pH , or rather potential hydrogen, is extremely important for safe fermentation as it influences the growth of bacteria in the fermented foods. pH measures the acidity or basicity of a product on the basis of a scale of values from 0 to 14: the lower the value, the higher acidity of the product will be.

As in preserves, the pH values of fermented products are closely linked to the amount of salt in the product.

This is why the pH value of fermented products must be no higher than 4.3 in relation to 2% of salt.

The sensitivity of bacteria to salt creates a selective environment that favours the progress of lactic bacteria which, by withstanding and feeding on the brine, control the stability of the pH in a natural way and foster the development of fermentation aromas. The pH can be checked using litmus paper, which can be purchased at a pharmacy, or using more specific instruments, such as pH meters.

Oxygen

To be able to maintain the right amounts of salt in lactic fermentation , it is important to ensure that oxygen is only present in a small quantity or entirely absent. This result is rather simple to obtain using the brine technique since the foods remain submerged in liquid; if the dry salting method is employed, it is necessary to consider the amount of water in the food, which will have to remain submerged in its own liquids. If necessary, an environment with a higher oxygen residue can be maintained by increasing the percentage of salt.

Salinity

When used in certain percentages, sodium chloride (kitchen salt) creates the ideal conditions for certain lactobacilli and yeasts and is inhospitable to undesired micro-organisms.

Non-iodised salt is best as it is low in the iodine that could inhibit the positive bacterial flora. Initially, it is advisable to use at least an overall 2% of salt to prevent undesired micro-organisms from proliferating even though the production method of certain foods (e.g. sauerkraut) requires less than 1% of salt in order to obtain the desired result.

It is essential to follow specific recipes to the letter when preparing fermented products.

Salting can be “dry”, by adding salt to the foods so that they release sufficient liquid to remain fully submerged.

Another method could be to submerge the ingredients in brine, a solution of water and salt in the required concentration. Chlorine-free water must be used in this case, as chlorine is a powerful bactericide. This can be achieved in two ways: the first by filtering the water through a carbon filter and the second by boiling the water and leaving it to cool.

The quantity of salt used for dry salting must not exceed 20% of the weight of the vegetables themselves (e.g. capers) while in brine, the salt must amount to no more than 10% of the weight of the water (e.g. olives). If the vegetables are bitter at their origin, throw away the water produced after adding the salt or change the brine every day to dilute the bitter part.

Temperature

Temperature has a major influence on fermentation, as it encourages certain bacterial strains over others. In lactic fermentation , the bacteria prefer temperatures between 4 and 28°C for vegetables while they function at around 40°C in the production of yoghurt .

Room temperature is ideal when making fermented products at home . If the temperature exceeds 30°C, it is preferable to store the jars in a refrigerator to prevent the vegetables from becoming soft or, once fermentation has begun, to transfer the jars to a refrigerator to complete the process in a cold environment.

Sunlight

Ultraviolet rays tend to alter certain molecules present in food and trigger oxidative processes, so it is advisable to keep the jars away from direct light sources.

Sugars

The sugar naturally present in food is usually sufficient to ensure that fermentation occurs. If different results are required, further sugars can be added to assist the proliferation of bacteria and obtain different degrees of acidity. This will lead to an increase in the production of gas during the initial days of fermentation, so it will be important to remember to release the gas from the containers at least once a day.

Containers able to withstand high pressures must be used for certain types of fermented foods containing larger quantities of sugars, especially beverages. Failing this, they could be liable to explode.

How long does fermentation take?

The time it takes for foods to ferment varies considerably and depends on the temperature and amount of salt. Thus it is very difficult to give precise indications about production times unless temperature control instruments are used, such as a yoghurt maker or a proving cabinet.

It is, however, possible to establish when fermented products are ready for use the moment in which the production of gas stops or slows down. The best way to be certain is to measure the pH.

At the beginning, it is advisable to carefully follow the recipes where all the steps and times for safe and successful fermentation are explained.

The fermentation process can be slowed down simply by placing the sealed jars in the refrigerator. This will preserve the foods they contain for months.

What is lactic fermentation?

Lactic fermentation is a type of fermentation mainly brought about by the action of lactic acid bacteria (LAB) , often simply called lactobacilli. This type of fermentation takes place both in milk-based fermented products and plant-based fermented products. However, it is unlikely to occur if large quantities of salt or sugar are present.

It is important to remember that the term Lactic Acid Bacteria is without an official classification and that, in point of fact, it is merely a general term used to describe a group of functionally and genetically correlated bacteria, but belonging to different genera.

Lactic bacteria are called in this way because they produce lactic acid and are not necessarily linked to the fermentation of milk.

During the fermentation process, these bacteria mainly produce Lactic Acid, hence the name Lactic Fermentation. Lactic acid is not the only compound produced during the transformation of foods. There is also a group of secondary products that give the ingredients a pungent and aromatic flavour, with a good level of acidity (due, in fact, to the presence of lactic acid and other acids to a lesser degree).

Lactic Bacteria

The bacteria in lactic fermentations can be divided into two groups with different metabolisms:

Homofermentive, which produce almost exclusively lactic acid from the sugars present.

Heterofermented: in this case, 50% of the sugars are metabolised to lactic acid while the remaining percentage is converted into ethanol, acetic acid, citric acid and carbon dioxide.

When fermentation begins, the heterofermented bacteria are often the ones that lead the way. This is also visually evident because fermentation releases lots of gas. Fermentation is less visible when mostly homofermentive bacteria are present.

Yeasts

Yeasts are are members of the the fungus kingdom and have a facultative anaerobic metabolism like LAB. They are able to draw energy from sugars and produce ethyl alcohol, CO2 and aromatic esters.

As is well known, alcohol possesses bactericidal powers and therefore tends to assist by making the environment even more selective.

Depending on the food to be fermented, yeasts may be more or less active in helping lactobacilli in the lactic fermentation of vegetables; this naturally applies to those with a higher sugar content, such as vegetables of the Brassica genus, fruit or starchy tubers. In addition, certain specific products like Kimchi, where part of the fermentation process takes place in a refrigerator, encourage their proliferation.

Starter or stock bacterial cultures

Bacteria and fungi often live in symbiosis in a natural environment. They form stable populations and are useful for various types of fermentation. The only way they can be replicated is to pass them on to others. Certain bacterial cultures, such as milk kefir grains or kombucha stock culture, an be easily purchased online . Besides bacterial cultures, laboratory selected starter cultures with specific bacteria strains able to ensure steadier fermentation results can also be used: those used for the production of yoghurt are an example. Starters can be purchased from pharmacies or online. In other cases, starters or stock cultures are unnecessary since the bacteria are naturally present in the foods.

Possible problems and solutions

Moulds

Different types of moulds exist, some of which are noble and wholesome, while others are toxic. Unfortunately, these latter are hard to distinguish. If moulds develop in fermented products, it is therefore better to throw everything away.

Fruit flies

The odour of fermentation attracts a lot of flies. It is essential to close the jars tightly with their lids or, in the case of certain types of fermentation, with a piece of gauze or linen cloth secured with an elastic band.

If the flies get into the jar and lay eggs, the fermented product must be discarded.

Thick brine

When the fermentation process is lengthy and only a small amount of salt has been used, the brine could thicken and the food inside become soft and not very pleasant. The product must be discarded in this case as well.

Cloudy brine

It is perfectly normal for the brine to become cloudy sometimes, but this should not be considered a problem. In actual fact, this sort of brine is the best kind for use as a starter for other fermented products.

Sediments at the bottom of the jar

These are also perfectly normal , especially if the brine is cloudy and the particles settle on the bottom over time.

Pinkish hue on the vegetables or on the sides of the jar

Bacteria could develop and cause intestinal problems if the vegetables fail to remain completely submerged in the brine. Here again, the fermented product should be thrown away.

The fermented products become discoloured

The colour of the vegetables could fade during fermentation, but this is perfectly normal . This discolouration may also be more or less evident. Fermentation processes, including acidification, are the cause of these changes in colour.

How to store fermented products

Fermented products made with less than 2% of salt (e.g. sauerkraut) must be stored in a refrigerator and can be kept for about 6 months.

Those made with 2% of salt or more can also be kept at room temperature for several months. The greater the amount of salt, the longer the storage period. Obviously, this period can be lengthened still further when the products are stored in a refrigerator.

Products containing a very high quantity of salt (20% and above) can be kept at room temperature for over a year.

THE JARS TO USE FOR FERMENTATION

Fido and Quattro Stagioni jars are suitable for fermentation. Choice of the jar size depends on the quantity of product you want to make. Thus the end consumer must choose jars that suit the volume of product required.