Salt in fermentation and calculating brines

One might come across the “importance of calculating salt brines accurately” when fermenting vegetables. It is important to some extent, but there is quite a bit of flexibility allowed when calculating a successful brine.

So it is this contradiction that I came across quite often that led me to delve deeper into salt brines and how to calculate them. In reality, there are some assumptions made when making a brine that automatically make it less accurate - and this is ok!

Why salt is used in fermentation and preservation

Salt, specifically sodium chloride (NaCl), plays a vital role in fermentation and food preservation. In lacto-fermentation, salt gives lactic acid bacteria (LAB) a critical advantage by creating an environment that favors their growth while inhibiting less salt-tolerant, unwanted bacteria. As LAB thrive, they produce lactic acid, which further suppresses harmful microbes, ensuring the fermentation remains stable and safe.

In the making of sauerkraut, salt draws water out of the cabbage through osmosis, creating a brine which is ideal for lactic acid bacteria to flourish. This process not only enables fermentation but also helps maintain the cabbage‘s crisp texture.

Beyond supporting beneficial bacteria, salt is also used in higher concentrations as a standalone preservative. By drawing out water from food, it creates a dry, inhospitable environment for bacterial growth. This method is especially effective for preserving foods like meat, which lack the carbohydrates necessary for LAB-driven fermentation.

Finally, the amount of salt used can influence the speed of fermentation. Higher salt concentrations slow the fermentation process by reducing microbial activity, while lower concentrations speed it up. Adjusting salt levels can also fine-tune the final flavour and texture of fermented foods to suit personal preferences.

Type of salt

There are many different types of salt that can be used in fermentation, which vary in crystal size as well as chemical composition.

Unrefined salts, such as Himalayan rock salt or sea salt, often contain trace minerals alongside the sodium chloride (NaCl) crystals, which can slightly affect the flavour or appearance of the final product. In contrast, some salts are refined and may include additives like anti-caking agents or iodine. While there are claims that these additives might inhibit the fermentation process, studies have largely shown they do not significantly impact fermentation.¹

When measuring salt, crystal size becomes important. If measuring by mass, the size of the crystals doesn’t matter, as a gram is always a gram. However, when measuring by volume, a teaspoon of fine-grain salt will contain more salt than a teaspoon of large rock salt due to the differences in crystal density and how tightly they pack together. For precise results, especially in fermentation, weighing the salt is generally recommended.

Many ways to determine the amount of salt

When fermenting vegetables, a commonly quoted salt concentration is 2%. This serves as a useful starting point, but what this 2% refers to can vary depending on the method and the ingredients used. Below are some of the common approaches to calculating the amount of salt for fermentation.

Vegetable-only calculation

For shredded vegetables like cabbage (e.g., when making sauerkraut), the recommended salt amount is often 2% of the vegetable’s mass. The salt is mixed directly with the vegetable, and through osmosis, water is drawn out of the vegetable to create the brine. In this method, all the liquid used for fermentation comes directly from the vegetable.

Example Calculation:
If fermenting 750 grams of cabbage:
0.02 x 750g = 15g salt.

Some recipes suggest massaging the cabbage until a significant amount of liquid, such as a cup (250mL), is released. This would result in an initial brine concentration of 6% (15g / 250mL = 0.06g/mL). Over time, as more water is drawn from the cabbage, the brine concentration will dilute but should remain above 2%. This method relies on the natural water content of the vegetable to achieve the correct balance.

Vegetable + water calculation

When fermenting larger pieces of vegetables, like cucumbers, it’s common to add brine to cover them instead of relying solely on osmosis. In this method, some people calculate the salt as 2% of the water alone, while others include the mass of the vegetable in the calculation.

When you make a brine, water will get pulled out of the vegetable due to the osmostic potential, just like when making sauerkraut. So by including the mass of the water in the vegetable one takes into consideration this extra water that will become part of the brine. Not all of the water gets pulled out of a vegetable however, so by including the vegetable‘s water, the final brine concentration will be higher than calculated.

Example Calculation:
If fermenting 750mL of water with 750g of cucumber:
0.02 x (750g water + 750g cucumber) = 30g salt.

In this case, if none of the cucumber’s water were released, the brine concentration would be 4% (30g salt / 750g water = 0.04). Since cucumbers do release water, the final brine concentration will fall somewhere between 2% and 4%, depending on the vegetable and conditions.

Water-only calculation

Some people prefer to calculate salt as a percentage of only the added water, excluding the vegetable’s water. This results in a slightly lower final brine concentration, as the vegetable’s water dilutes the solution. To compensate, some recipes suggest starting with a higher salt concentration, such as 3% or 4%.

Example Calculation:
If adding 750mL of water:
0.02 x 750g water = 15g salt.

The actual brine concentration after fermentation will depend on the amount of water released by the vegetable. Factors like the type of vegetable, its preparation, and the brine strength influence how much water is released.

Vegetable + water + salt caculation

This method is often regarded as an accurate way to calculate salt for fermentation because it accounts for the total mass of all components: water, vegetable and salt. Unlike simpler methods, it incorporates the salt‘s mass into the calculation, ensuring the desired brine concentration reflects more closely the actual composition of the solution. Ignoring the salt’s contribution can lead to slight underestimations of the final concentration, especially higher salt percentages. The formula:

Salt (g) = (Vegetable + water mass (g) × C) / (100 - C) ​ ​

Where C = salt concentration (%).

Example Calculation:
If fermenting 750mL of water with 750g of cucumber:
(1500g x 2) / (100 - 2) = 30.6g salt.

Water + salt caculation

This method is similar to the previous method and accounts for the mass of both water and salt components. Again, it incorporates the salt‘s mass into the calculation, ensuring the desired brine concentration reflects more closely the actual composition of the solution. If the vegetable doesn‘t constribute that much water, this method could be more accurate than the “vegetable + water + salt” method. Ignoring the salt’s contribution can lead to slight underestimations of the final concentration, especially higher salt percentages. The formula is:

Salt (g) = (Water mass (g) × C) / (100 - C) ​ ​

Where C = salt concentration (%).

Example Calculation:
If fermenting 750mL of water with 750g of cucumber:
(750g x 2) / (100 - 2) = 15.3g salt.

Units of measurements

The salt is usually measured in grams, which can be converted to teaspoons if need be, while taking into consideration the type of salt (their densities differ). The water can be either measured in cups/mLs (volume) or grams (mass). Conversion between the two is easy because the density of water means 1 mL of water weighs 1 gram.

When people include the measurement of the vegetable in their salt calculation they also use either cups/mLs or grams. Even though the density of vegetable isn‘t the same as water, conversion between mass and volume of the vegetable is also easy. This is because we assume that the vegetable does have the same density as water, since vegetables have a high content of water (cucumbers are ~96% water).² This simplifies calculations, even though it introduces some minor inaccuracies.

Assumptions made when calculating salt brines

When calculating salt concentrations for fermentation, a few key assumptions are often made, which can affect the accuracy of the final brine concentration:

1. Assumption: All densities are the same as water.
   It is common to assume that the density of vegetables, water, and salt is equal to water‘s density (1 g/mL). This simplifies the math but leads to differences between calculations based on grams of salt per gram of total mass (g/g) and grams of salt per milliliter of volume (g/mL). Concentrations are typically expressed in g/mL. Some methods improve accuracy by adding the salt‘s mass to the total volume, but achieving a truly accurate 2% g/mL (for instance), requires considering the salt and vegetable densities too.


2. Assumption: Vegetables release either 100% or none of their water.
   Calculations based on “vegetable only” or “vegetable + water” assume the vegetable releases all its water during fermentation. Conversely, “water only” calculations assume the vegetable releases no water at all. In reality, vegetables release a varying portion of their mass in water during fermentation, and it depends on factors like the vegetable type, vegetable freshness, vegetable storage, preparation method, and brine concentration. This variability means the final brine concentration will often differ from the calculated value.


3. Assumption: Salt‘s mass is negligible
   Many methods ignore the contribution of the salt’s mass to the total mass of the brine. This assumption is reasonable for very small amounts of salt, but it becomes less accurate at higher concentrations (e.g., above 3%) or for larger batches. Including the salt‘s mass in the total ensures a more accurate calculation of the brine concentration.

Other assumptions could include: a uniform dissolution and distribution of salt (stirring, packing, and container shape can affect this), no salt absorption by vegetables and having a sealed container (evaporation can concentrate the brine).

How these assumptions affect brine concentrations

To illustrate how the assumptions in brine calculations affect the final concentration, we’ll compare the 2% salt calculation across all methods. For consistency, we will use 750g of cucumber for the small batch, 25kg of cucumber for the large batch, assume 50% cucumber mass in water is released in shredded form, 20% cucumber mass in water is released in whole/cut large form, 750mL water added (where applicable) in the small batch, 25kg water added in the large batch, and a target of 2% salt (with a density of 1.2g/mL) for all methods.

Method 1 - Vegetable-only (shredded) calculation:

Small batch: 0.02 × 750g = 15g salt

Assumed brine %:
Concentration (g/g) = 15g / 750g = 0.02g/g = 2%
Concentration (g/mL) = 15g / 750mL = 0.02g/mL = 2%

More accurate brine %:
Concentration (g/g) = 15g / (750g x 0.5 + 15g) = 0.038g/g = 3.8%
Concentration (g/mL) = 15g / (750mL x 0.5 + 15g x (1mL/1.2g) ) = 0.039g/mL = 3.9%

Large batch: 0.02 × 25,000g = 500g salt

Assumed brine %:
Concentration (g/g) = 500g / 25,000g = 0.02g/g = 2%
Concentration (g/mL) = 500g / 25,000mL = 0.02g/mL = 2%

More accurate brine %:
Concentration (g/g) = 500g / (25,000g x 0.5 + 500g) = 0.038g/g =3.8%
Concentration (g/mL) = 500g / (25,000mL x 0.5 + 500g x (1mL/1.2g) ) = 0.039g/mL = 3.9%

Method 2 - Vegetable (whole/cut large) + water calculation:

Small batch: 0.02 × (750g cucumber + 750g water) = 30g salt

Assumed brine %:
Concentration (g/g) = 30g / 1500g = 0.02g/g = 2%
Concentration (g/mL) = 30g / 1500mL = 0.02g/mL = 2%

More accurate brine %:
Concentration (g/g) = 30g / (750g x 0.2 + 750g + 30g) = 0.032g/g = 3.2%
Concentration (g/mL) = 30g / (750mL x 0.2 + 750mL + 30g x (1mL/1.2g) ) = 0.032g/mL = 3.2%

Large batch: 0.02 × (25,000g cucumber + 25,000g water) = 1,000g salt

Assumed brine %:
Concentration (g/g) = 1,000g / 50,000g = 0.02g/g = 2%
Concentration (g/mL) = 1,000g / 50,000mL = 0.02g/mL = 2%

More accurate brine %:
Concentration (g/g) = 1,000g / (25,000g x 0.2 + 25,000g + 1,000g) = 0.032g/g = 3.2%
Concentration (g/mL) = 1,000g / (25,000mL x 0.2 + 25,000mL + 1,000g x (1mL/1.2g) ) = 0.032g/mL = 3.2%

Method 3 - Water-only calculation:

Small batch: 0.02 × 750g water = 15g salt

Assumed brine %:
Concentration (g/g) = 15g / 750g = 0.02g/g = 2%
Concentration (g/mL) = 15g / 750mL = 0.02g/mL = 2%

More accurate brine %:
Concentration (g/g) = 15g / (750g x 0.2 + 750g + 15g) = 0.016g/g = 1.6%
Concentration (g/mL) = 15g / (750mL x 0.2 + 750mL + 15g x (1mL/1.2g) ) = 0.016g/mL = 1.6%

Large batch: 0.02 × 25,000g water = 500g salt

Assumed brine %:
Concentration (g/g) = 500g / 25,000g = 0.02g/g = 2%
Concentration (g/mL) = 500g / 25,000mL = 0.02g/mL = 2%

More accurate brine %:
Concentration (g/g) = 500g / (25,000g x 0.2 + 25,000g + 500g) = 0.016g/g = 1.6%
Concentration (g/mL) = 500g / (25,000mL x 0.2 + 25,000mL + 500g x (1mL/1.2g) ) = 0.016g/mL = 1.6%

Method 4 - Vegetable (whole/cut large) + water + salt calculation:

Small batch: 2 × (750g cucumber + 750g water) / (100 - 2) = 30.6g salt

Assumed brine %:
Concentration (g/g) = 30.6g / (1500g + 30.6g) = 0.02g/g = 2%
Concentration (g/mL) = 30.6g / (1500mL + 30.6mL) = 0.02g/mL = 2%

More accurate brine %:
Concentration (g/g) = 30.6g / (750g x 0.2 + 750g + 30.6g) = 0.033g/g = 3.3%
Concentration (g/mL) = 30.6g / (750mL x 0.2 + 750mL + 30.6g x (1mL/1.2g) ) = 0.033g/mL = 3.3%

Large batch: 2 × (25,000g cucumber + 25,000g water) / (100 - 2) = 1020.41g salt

Assumed brine %:
Concentration (g/g) = 1020.41g / (50,000g + 1020.41g) = 0.02g/g = 2%
Concentration (g/mL) = 1020.41g / (50,000mL + 1020.41mL) = 0.02g/mL = 2%

More accurate brine %:
Concentration (g/g) = 1020.41g / (25,000g x 0.2 + 25,000g + 1020.41g) = 0.033g/g = 3.3%
Concentration (g/mL) = 1020.41g / (25,000mL x 0.2 + 25,000mL + 1020.41g x (1mL/1.2g) ) = 0.033g/mL = 3.3%

Method 5 - Water + salt calculation:

Small batch: 2 × 750g water / (100 - 2) = 15.3g salt

Assumed brine %:
Concentration (g/g) = 15.3g / (750g + 15.3g) = 0.02g/g = 2%
Concentration (g/mL) = 15.3g / (750mL + 15.3mL) = 0.02g/mL = 2%

More accurate brine %:
Concentration (g/g) = 15.3g / (750g x 0.2 + 750g + 15.3g) = 0.017g/g = 1.7%
Concentration (g/mL) = 15.3g / (750mL x 0.2 + 750mL + 15.3g x (1mL/1.2g) ) = 0.017g/mL = 1.7%

Large batch: 2 × 25,000g water / (100 - 2) = 510.20g salt

Assumed brine %:
Concentration (g/g) = 510.20g / (25,000g + 510.20g) = 0.02g/g = 2%
Concentration (g/mL) = 510.20g / (25,000mL + 510.20mL) = 0.02g/mL = 2%

More accurate brine %:
Concentration (g/g) = 510.20g / (25,000g x 0.2 + 25,000g + 510.20g) = 0.017g/g = 1.7%
Concentration (g/mL) = 510.20g / (25,000mL x 0.2 + 25,000mL + 510.20g x (1mL/1.2g) ) = 0.017g/mL = 1.7%

Accurate, but unnecessary, calculations

When calculating the amount of salt for fermentation, the assumptions we make about vegetable water release and the densities of salt and vegetables influence the difference between the assumed brine concentration and the real-world brine concentration.

Among the various methods, the water + vegetable + salt formula is often claimed to be the most accurate. However, it heavily relies on the assumption that vegetables release 100% of their water and if the vegetable doesn‘t release much water it becomes less accurate than other methods. If the vegetable releases less than half its mass in water, the water + salt method can be more accurate. In reality, vegetables release only a portion of their water and the portion can vary. If not accounted for correctly, this variability can lead to over-diluted or over-concentrated brines. Since vegetable water release can be unpredictable, it remains a key variable that can be accepted as an assumption.

Another factor to consider is the difference between g/g and g/mL brine concentrations when incorporating salt density. While this technically affects accuracy, the differences are very small and generally negligible in most fermentation settings.

The common claim that the water + vegetable + salt method is better for large batches isn’t supported by calculations. Including the salt mass in calculations does improve accuracy but only becomes increasingly relevant when using high salt concentrations, not larger batch sizes.

Finally, it’s worth emphasizing that while there are variations in calculation methods, their effects are relatively minor. Whether you use a water-only method, a water + vegetable method or incorporate the salt‘s mass, all of them can ultimately work for fermentation. The small differences in brine concentration rarely impact the safety or success of the process. So, while precise calculations might appeal to some, fermentation remains a forgiving technique where nearly all methods can yield successful and delicious results.

References