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 flavor 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.

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

A common salt concentration quoted is 2% so we’ll use this number as an example. What this 2% is of, however, tends to vary.

Vegetable only calculation

When fermenting shredded vegetables like cabbage the amount of salt recommended is often 2% of the weight of the cabbage. By massaging the vegetable with salt, the higher salt concentration outside of the vegetable, compared to inside the vegetable, pulls water out from the vegetable. All the water used in the fermentation is from the vegetable itself.

2% salt:
0.02 x 500g of cabbage = 10g of salt.

Some recipes recommend massaging the cabbage until a cup of water has been released. So, 10g / 250mL = 0.04g/mL = 4% brine.
More water may be released by the cabbage over the fermentation but, even if another 250mL were released, the brine won‘t go below 2%.

Water + vegetable calculation

When fermenting larger pieces of vegetables, like cucumbers, one can add brine to cover the vegetables instead of relying on osmosis. To determine the amount of salt in this situation, some people calculate 2% of the water they add and some people calculate 2% of the water plus the vegetable.

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 that calculated.

2% salt, 750mL water + 750g cucumber:
0.02 x (750g water + 750g cucumber) = 30g of salt.

The cucumbers will not release its entire mass in water so the brine will always be more than 2% and, in this situation, less than 4% (if the cucumber released no water the calculation would be 30g / 750g = 0.04).

Water only calculation

On the other hand, some people don’t include the vegetable‘s own water in the calculations of salt. Their final brine concentrations should be slightly lower due to the dilution and so some increase the salt percentage to account for this.

2% salt, 750mL water:
0.02 x 750g of water = 15g of salt.

The extra water released by the cucumber depends on many factors including the type of vegetable, the amount and preparation of the vegetable and the brine. To avoid worrying about this though, people often start with 3%, 4% or an even higher salt concentration when using this method.

Vegetable + water + salt caculation

A common formula for larger batches for “accuracy“

(Total mass (g) x C_{salt concentration}) / (100 - C_{salt concentration})

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 weight 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

The assumptions made when calculating salt brines

density of everything is the same as water. (ignore salt, vegetable) this means that g/g is actually different to what g/mL would be. A 2% solution is usually written as g/mL. Some add on the salt weight in the total, which makes it more accurate but the density of salt is required to make it truely 2% g/mL (if this is the amount originally stated as being good for safe microbacterial growth)

How these assumptions affect brine concentrations

compare, calculations by g/g and g/mL, when measuring volume of water+veg and mass of water+veg, when assuming water density and small batch. Then compare for large batch. Then compare calcualtions when considering densities.

References