What will happen to any bacterial cells in the salt or sugar solutions

By Bonnie Crowe

Osmosis is the movement of water across a membrane. Salt triggers osmosis by attracting the water and causing it to move toward it, across the membrane. Salt is a solute. When you add water to a solute, it diffuses, spreading out the concentration of salt, creating a solution. If the concentration of salt inside a cell is the same as the concentration of salt outside the cell, the water level will stay the same, creating an isotonic solution. Cells will not gain or lose water if placed in an isotonic solution.

Water in cells moves toward the highest concentration of salt. If there is more salt in a cell than outside it, the water will move through the membrane into the cell, causing it to increase in size, swelling up as the water fills the cell in its imperative to combine with the salt. If a higher concentration of salt is placed outside of the cell membrane, the water will leave the cell to bond with it. The loss of water from this movement causes plant cells to shrink and wilt. This is why salt can kill plants; it leaches the water from the cells. The movement of water to leave an animal cell will also cause those cells to shrink and cause dehydration. This is why a person could die from dehydration if he drinks enough sea water.

Updated July 02, 2019

By Elliot Walsh

Bacteria is a general term that refers to an entire kingdom of microscopic species. It's estimated that there are over one trillion species of microbes on Earth with a great majority of those species thought to be bacteria. The majority of these bacterial species are not harmful to human and don't cause disease. Only about 1 percent of bacteria are believed to cause disease.

Understandably, most people want to avoid bacterial infection. Antibiotics and proper hygiene are the most common ways to avoid and kill harmful bacteria. Not many people know that salt kills bacteria as well. While not all bacteria can be killed with salt, many can be because of its dehydrating effects on the bacterial cells.

Before understanding how salt kills bacteria, you need to understand what osmosis is. Simply put, osmosis is the movement of water across a membrane from areas of high concentration of solutes to low concentration of solutes. This works to maintain an equilibrium of solutes (aka the dissolved molecules) within the water on either side of the membrane.

For example, say you have cells in a water solution where the water contained a higher concentration of sugar than the concentration of sugar dissolved in water found inside the cell. Another way to put this is that the concentration of water molecules is higher inside the cell than outside the cell. In this case, you would see water move from inside the cell (where water concentration is higher) to outside of the cell (where water concentration is lower).

This does two things to reach equilibrium. First, it increases the concentration of water outside the cell and decreases the concentration inside the cell. This movement of water then, in turn, decreases the concentration of sugar outside the cell and increases the concentration of sugar inside the cell.

It's this process of osmosis that makes high concentrations of salt kill bacteria. When there are high salt concentrations outside of a bacterial cell, water from inside the bacteria diffuses out of the cell in order to reach equilibrium and equalize the salt concentration. When bacterial cells lose all of their water like this, it:

• Dehydrates the cell
• Causes the loss of the cell's structure
• Leads to enzyme and protein malfunction 
• Eventually leads to cell death

Simply put: Salt sucks all of the water out of the bacteria, which leads to cell death. However, some bacteria are tolerant of salty conditions. These types of bacteria are called halotolerant.

While the antibacterial properties of salt are helpful for some everyday uses, you shouldn't rely on salt when you have an infection. It's better to use salt as a preventative measure and see a doctor for other treatments if you believe you have a bacterial infection.

Salt water rinse. Creating a salt water rinse to gargle in your mouth can help kill harmful cavity-causing bacteria. The benefits of gargling salt water include directly killing the bacteria as a result of osmosis as described above and temporarily increasing the pH in your mouth. This creates an alkaline environment that most oral bacteria cannot survive in.

Simply mix 1/2 teaspoon of salt in one cup of warm water. Gargle this solution for 30 seconds before spitting it out. Do not swallow.

Corning and brining foods. Corning, also called salt-curing, refers to rubbing salt pellets onto meat in order to prevent bacterial growth. This process requires you to rub salt into meat to get the salt concentration to 20 percent or higher. It you have a one pound slab of beef, for example, you would need to rub 3 ounces of salt onto the surface of the meat.

Brining is similar except it involves creating a salty solution called a brine instead of rubbing salt directly on the foods. To make a brine, you mix salt and water in a ratio of one part salt to five parts water. You then add in your food, usually vegetables and meats, and this will both prevent bacterial growth and kill most bacteria already on the food.

Washing cutting boards and counters. You can also rub salt directly on bacteria-prone surfaces like cutting boards and counters in order to kill bacteria on those surfaces and prevent future growth.

Some methods of preserving food are easy to understand.

For example, it's easy to see that freezing your food, or packing it in salt, would make it inhospitable to the microbes which would otherwise cause it to spoil. You might wonder, however, about jams, jellies, and preserves, all of which are protected from spoiling by a high concentration of sugar. Sugar is one of the most basic foods for all life – bacteria and mold like to eat it just as much as we do.

Sugar works not by poisoning the food-spoiling microbes, but by causing them to literally die of thirst.

This is because sugar attracts water very well; the more sugar there is in any solution, the more water it tries to draw from its surroundings. This is bad news for any microbe that happens to be inside a jar of jam. High concentrations of sugar will suck the microbe's vital water right through its cell wall, causing it to dehydrate. This process is called "osmosis," and it can be deadly for bacteria and mold.

In order for osmosis to work, the sugar concentration has to be quite high. If any water falls onto the surface of your jam, the sugar concentration at that spot might become low enough to allow mold to grow. That's why it's important to take the back up measure of refrigerating all jams, jellies, and preserves once you've opened them.

Like an oasis in the desert, condensed water dripping from the jar's lid can give a dehydrating microbe the relief it needs.

As mentioned previously, the first major addition of sodium to foods was as salt, which acted to prevent spoilage. Prior to refrigeration, salt was one of the best methods for inhibiting the growth and survival of undesirable microorganisms. Although modern-day advances in food storage and packaging techniques and the speed of transportation have largely diminished this role, salt does remain in widespread use for preventing rapid spoilage (and thus extending product shelf life), creating an inhospitable environment for pathogens, and promoting the growth of desirable micro-organisms in various fermented foods and other products. Other sodium-containing compounds with preservative effects are also used in the food supply.

Salt is effective as a preservative because it reduces the water activity of foods. The water activity of a food is the amount of unbound water available for microbial growth and chemical reactions. Salt’s ability to decrease water activity is thought to be due to the ability of sodium and chloride ions to associate with water molecules (Fennema, 1996; Potter and Hotchkiss, 1995).

Adding salt to foods can also cause microbial cells to undergo osmotic shock, resulting in the loss of water from the cell and thereby causing cell death or retarded growth (Davidson, 2001). It has also been suggested that for some microorganisms, salt may limit oxygen solubility, interfere with cellular enzymes, or force cells to expend energy to exclude sodium ions from the cell, all of which can reduce the rate of growth (Shelef and Seiter, 2005).

Today, few foods are preserved solely by the addition of salt. However, salt remains a commonly used component for creating an environment resistant to spoilage and inhospitable for the survival of pathogenic organisms in foods. Products in the modern food supply are often preserved by multiple hurdles that control microbial growth (Leistner, 2000), increase food safety, and extend product shelf life. Salt, high- or low-temperature processing and storage, pH, redox potential, and other additives are examples of hurdles that can be used for preservation. As shown in Figure 4-1, no single preservation method alone would create a stable product; when combined, however, these methods result in a desirable, stable, and safe product. For example, a food might be protected by a combination of salt, refrigeration, pH, and a chemical preservative.

Multiple-hurdle methods offer the additional benefit of improving other qualities of some foods. For example, hurdle methods can be used to reduce the severity of processing needed, allow for environmentally friendly packaging, improve the nutritional quality of foods (by achieving microbiological safety with less salt, sugar, etc.), and reduce the use of preservatives that are undesirable to some consumers (Leistner and Gould, 2005).

Salt commonly plays a central role in the fermentation of foods. Fermentation is a common process for preserving foods, in which fresh foods are transformed to desirable foods that can be preserved for longer periods of time than their fresh counterparts due to the actions of particular types of microbes (Potter and Hotchkiss, 1995). Products such as pickles, sauerkraut, cheeses, and fermented sausages owe many of their characteristics to the action of lactic acid bacteria. Salt favors the growth of these more salt-tolerant, beneficial organisms while inhibiting the growth of undesirable spoilage bacteria and fungi naturally present in these foods (Doyle et al., 2001). Salt also helps to draw water and sugars out of plant tissues during fermentation of vegetables. This water aids fermentation by filling any air pockets present in fermentation vats, resulting in reduced oxygen conditions that favor growth of lactic acid bacteria. The release of water and sugars also promotes fermentation reactions in the resulting brine, increasing the rate of the fermentation process (Doyle et al., 2001; Potter and Hotchkiss, 1995).

A number of other sodium-containing compounds provide preservative effects in foods. There is a wide variety of these preservatives with various product uses. Preservatives can act to reduce microbial activity and also may, like salt, act as a hurdle to microbial growth and survival. Some additives may also play a role in preserving food quality by reducing undesirable chemical reactions, such as lipid oxidation and enzymatic browning. In some cases, the compounds can have more than one function in a food product, with preservative effects being one of several reasons for use.

A brief listing of common sodium-containing compounds used for food preservation and the foods with which they are associated can be found in Table 4-1.

For many foods, reducing the sodium content of the product should not create food safety or spoilage concerns. Such foods include frozen products, products that are sufficiently thermally processed to kill pathogenic organisms (e.g., canned foods), acidic foods (pH < 3.8), and foods in which water activity remains low when sodium is removed (e.g., foods with low water activity due to high sugar content) (Reddy and Marth, 1991; Stringer and Pin, 2005). For other foods, reducing sodium content has the potential to increase food spoilage rates and the presence of pathogens. For these foods, product reformulation, changes in processing, and changes in handling may be required to ensure that the product has an adequate shelf life and to prevent pathogen growth. Such efforts do incur additional costs and require careful attention to ensure that new formulations and processes are sufficient to ensure product safety. These issues are discussed further in Chapters 6 and 8.

Foods using sodium as a hurdle to retard microbial growth and survival present a reformulation challenge, since changing the sodium content alters the impact (or height) of the water activity hurdle. Changing this single hurdle may impact the safety and quality of the food because other hurdles that are present (pH, temperature, etc.) may work only in combination with the original sodium level. To maintain a safe, good-quality product, reformulation may have to include the introduction of additional hurdles or an increase in the impact of existing hurdles. If such additional measures are not taken during sodium reduction efforts, the remaining products may not be stable. For example, in cured meats, reducing the sodium content (by removing both salt and sodium nitrite) could allow for rapid growth of lactic acid bacteria and action by proteolytic microorganisms, resulting in a product that spoils more rapidly (Roberts and McClure, 1990; Stringer and Pin, 2005). In some foods, pathogen growth, rather than spoilage, may become a concern.

There is speculation that some past salt reduction efforts may not have adequately accounted for the need to adjust additional hurdles to microbial growth. In the United Kingdom, salt reduction efforts in chilled, ready-to-eat foods were cited as one factor that may have contributed to an increase in the incidence of listeriosis from 2001 to 2005 (Advisory Committee on the Microbiological Safety of Food, 2008). Listeriosis is caused by Listeria monocytogenes, which has a high thermal stability and is able to grow and survive at refrigeration temperatures and elevated salt levels (Zaika and Fanelli, 2003). To decrease the risk of listeriosis, a draft report of the United Kingdom’s Advisory Committee on the Microbiological Safety of Food called on the Food Standards Agency to work closely with food manufacturers to ensure that the microbial safety of food products would not decrease with changes in formulation to reduce salt (Advisory Committee on the Microbiological Safety of Food, 2008).

There is also evidence suggesting that reductions in salt might result in greater risk of toxin formation by Clostridium botulinum (the organism responsible for botulism) in certain foods if additional hurdles are not incorporated. This is particularly the case for foods that have not been heated sufficiently to inactivate C. botulinum spores and have little oxygen present. Processed cheese (Glass and Doyle, 2005; Karahadian et al., 1985), meat products (Barbut et al., 1986), and sous vide products (products that are prepared in vacuum-sealed plastic pouches and heated at low temperatures for long times1) have been recognized as having potential for C. botulinum control problems when sodium is reduced (Simpson et al., 1995). For example, decreases in salt content from 1.5 to 1.0 percent by weight greatly reduced the time needed for C. botulinum type A and B spores to produce toxins in sous vide spaghetti and meat sauce products when stored at typical refrigeration temperatures. At salt concentrations at or above 1.5 percent, no toxin production was detected from the inoculated products during the 42-day storage period, while at 1.0 percent salt addition, toxins were produced within 21 days (Simpson et al., 1995). Similarly, turkey frankfurters inoculated with C. botulinum and held at 27°C showed more rapid toxin production when salt content was 2.5 percent than when it was 4.0 percent (Barbut et al., 1986).

In addition to C. botulinum and L. monocytogenes, the growth of other foodborne pathogens may be more rapid in foods with reduced contents of salt and other sodium-containing preservatives. These pathogens include Bacillus cereus, Staphylococcus aureus, Yersinia enterocolitica, Aeromonas hydrophila, Clostridium perfringens, and Arcobacter (D’Sa and Harrison, 2005; Reddy and Marth, 1991; Stringer and Pin, 2005).

While the pathogens described above must be taken into account, product developers and researchers have been able to accomplish sodium reductions even in products such as processed cheese and processed meats (Reddy and Marth, 1991). A number of hurdles can be added or increased when sodium is reduced to ensure that a product’s safety is maintained. Examples of additional hurdles are listed in Table 4-2. This list includes a number of emerging technologies (e.g., high-pressure processing, electron beam irradiation) that may have wider applications in the future.

Compounds, such as potassium chloride (Barbut et al., 1986) and mixtures of potassium lactate and sodium diacetate (Devlieghere et al., 2009), that might be used to replace salt and other sodium-containing preservatives have been shown to be somewhat effective at retarding growth and toxin production by pathogens. The effectiveness of alternative salts relative to sodium chloride seems to vary based on the pathogen of interest (Barbut et al., 1986).

Partially replacing salt with other compounds, such as potassium chloride and calcium chloride, may also be possible in fermented products (Bautista-Gallego et al., 2008; Reddy and Marth, 1991; Yumani et al., 1999). However, these alternatives may be less effective than salt so higher concentrations may be needed in formulations to achieve the same functionality (Bautista-Gallego et al., 2008).

Some predictive models have been developed that may be promising methods of screening new product formulations for their potential to grow pathogenic microorganisms. A large study conducted by Kraft foods (Legan et al., 2004) modeled the impact of salt on the growth of L. monocytogenes and used this modeling technique to establish no-growth formulations of cured meat products that contain lactate and diacetate to prevent growth of L. monocytogenes.