Can bacteria grow in soda?

Soda, also known as carbonated soft drinks, are popular beverages consumed around the world. However, there has been some debate around whether bacteria can grow in soda. This article will examine whether bacteria can grow in soda and the factors that impact bacterial growth.

Can bacteria grow in soda?

The short answer is yes, bacteria can grow in soda under certain conditions. However, soda is not an ideal environment for most bacteria to thrive due to its acidity, carbonation, lack of nutrients, and preservatives.


One of the biggest factors inhibiting bacterial growth in soda is acidity. Soda typically has a pH around 2-3, which creates a highly acidic environment. Most bacteria grow best around a neutral pH of 6-7 and cannot tolerate highly acidic conditions. The low pH of soda will prevent the growth of many common foodborne pathogens like Salmonella, E. coli, and Listeria.


The carbon dioxide bubbles in soda also make it difficult for bacteria to grow. The carbonation reduces the amount of oxygen available for bacteria that need oxygen for growth. Additionally, the bubbles can physically disrupt bacterial cells. However, once the soda goes flat and the carbonation is lost, the soda provides a more hospitable environment for bacteria.

Lack of nutrients

For bacteria to replicate and grow, they need nutrients to build cells and sustain life processes. Soda lacks many of the nutrients bacteria need like proteins, fats, and vitamins. Simple sugars like glucose or fructose are the primary energy sources in soda. While some bacteria can survive on sugars alone, many need a more nutritious environment. The lack of nutrients limits the growth of many bacteria species.


Some sodas contain chemical preservatives like potassium benzoate or sodium benzoate. These preservatives work to inhibit bacterial growth and extend the shelf life of soda by slowing spoilage. While not all sodas contain preservatives, those that do create additional barriers for bacteria.

What bacteria can grow in soda?

While soda presents challenges, there are some bacteria that can survive and even thrive under soda’s harsh conditions.


Acetobacter is a genus of acetic acid bacteria that can tolerate acidic environments. Acetobacter grows on ethanol and ferments ethanol into acetic acid. This bacteria is used in the production of vinegar which also contains acetic acid. Acetobacter can survive in soda, converting any residual sugars into acetic acid and further lowering the pH.

Lactic acid bacteria

Lactic acid bacteria like Lactobacillus and Leuconostoc can also tolerate acidic conditions. They produce lactic acid as a byproduct of sugar fermentation. Many lactic acid bacteria are used in food fermentations like yogurt, cheese, and pickles because they can grow in acidic environments. While inhibited by the acidity of soda, some strains of lactic acid bacteria can survive.

Aspergillus niger

Aspergillus niger is a species of fungus that grows well in acidic conditions. It is used industrially to produce citric acid and gluconic acid. Spores of A. niger can survive in soda, although the fungus likely will not actively grow. If provided a more nutritious medium, A. niger can thrive at a low pH.

Acidophilic bacteria

There are some bacteria adapted to highly acidic conditions classified as acidophiles. These bacteria can grow at pH as low as 1-2. Common genera of acidophilic bacteria include Acidithiobacillus, Acidiphilium, and Ferroplasma. Because they naturally live in acidic environments, acidophilic bacteria may be able to survive and grow slowly in soda’s acidic conditions.

What factors promote bacterial growth in soda?

While soda itself inhibits microbial growth, there are some conditions that can enable bacteria to grow.

Oxygen availability

Once soda goes flat and loses carbonation, the oxygen levels increase. This provides more favorable conditions for aerobic bacteria that require oxygen. Flat soda presents less physical disruption and chemical inhibition compared to carbonated soda.

Sugar content

Bacteria need an energy source, so sodas with higher sugar content provide more nutrients to fuel bacterial growth. Diet sodas or those with artificial sweeteners inhibitory to bacteria will have less potential for growth. Complex sugars may enrich soda for bacteria compared to simple fructose and glucose sugars.


Warmer temperatures accelerate chemical reactions and speed up microbial growth. Storing soda refrigerated at cooler temperatures will slow the growth of any contaminating bacteria. However, room temperature or warm soda stored at higher temperatures enable faster bacterial replication.


If bacteria are introduced through backflow or unclean conditions, they can gain a foothold in soda. Common routes of contamination include unwashed glasses, dirty soda fountain nozzles, or fruit flies carrying acetic acid bacteria. When contamination occurs, bacteria have access to the nutrients and conditions needed to establish growth.

Real-world examples of bacteria growth in soda

There are some documented cases of bacterial growth occurring in sodas when the right contamination occurs.

Fruit juice contamination

In 2015, there was an outbreak of Salmonella Poona linked to contaminated cucumbers used in fruit mix for soda fountain beverages. The acidic conditions did not prevent Salmonella growth once introduced via unclean cucumbers. This caused widespread gastrointestinal illness across multiple states.

Biofilm build up

Acetic acid bacteria can establish biofilms on soda fountain nozzles over time when not properly cleaned. As the biofilm accumulates, it inoculates each pour of soda with bacteria that can convert sugars into cellulose and acetic acid. This causes soda to develop slimy textures and vinegar notes.

Metal contamination

A study published in the International Journal of Food Microbiology found that sodas contaminated with iron could support microbial growth and metabolism. Iron increased microbial populations in soda over 12 days compared to sodas with no iron added. The researchers identified bacteria from genera Pseudomonas, Enterobacter, and Bacillus growing in iron-contaminated soda.

Study Conditions Bacterial Growth
Soda with No Iron Added Very Minimal Growth
Soda Contaminated with 0.25% Iron Moderate Bacterial Growth
Soda Contaminated with 0.5% Iron Significant Bacterial Growth

Can mold grow in soda?

Like bacteria, most fungi struggle to grow in the acidic, nutrient-limited environment of soda. However, mold growth in soda is possible under certain circumstances.

Common soda molds

– Penicillium – A genus of mold that can tolerate acidic conditions. Penicillium is used to produce cheese, ferment sausages, and make antibiotics like penicillin. It can form fuzzy blue-green colonies in soda.

– Aspergillus – Aspergillus niger is the most acid-tolerant species and may form sparse black filaments in soda over time. Other Aspergillus species struggle to grow below a pH of 4.

– Mucor – Has moderate acid-tolerance and may grow in soda as a fuzzy gray mold. Rhizopus is another related genus.

– Fusarium – A fungal genus that can withstand somewhat acidic conditions down to pH levels around 3-4. May slowly grow in soda.

Requirements for mold growth

For mold to establish growth in soda, oxygen, moisture, time, and other nutrients are required:

– Oxygen – Mold needs aerobic conditions, so flat, uncapped soda supports more growth.

– Moisture – Soda provides an aqueous environment ideal for fungal growth.

– Time – Mold needs days to weeks to establish good growth in soda. Refrigeration slows growth.

– Nutrients – Additives like fruit, sugar, juices, etc. provide nutrients mold needs to proliferate.

Preventing mold

To limit the potential for mold growth, soda should be:

– Stored refrigerated
– Consumed promptly before going flat
– Keep containers closed when not in use
– Avoid introducing additives/fruits that enrich soda
– Clean soda dispensers and taps regularly

With proper handling and refrigeration, the growth of mold and bacteria in sodas can be minimized. However, adding fruits or leaving soda unrefrigerated increases risks.

Effects of bacterial growth in soda

Bacterial growth in soda can lead to changes in taste, texture, and appearance. There are also potential health risks if pathogenic bacteria grow.

Changes in taste and texture

Bacteria like acetic acid bacteria metabolize sugars into acids like acetic and gluconic acids. This gives soda a vinegar-like taste. Cellulose production by bacteria creates thick, ropy textures. Off-flavors, acidity, and sliminess are common sensory indicators of microbial growth.

Gas production

As bacteria ferment sugars, they can produce gas byproducts like hydrogen, methane, and carbon dioxide. This can cause closed soda containers to bloat and burst. The release of trapped gas results in a sudden loss of carbonation and messy spills.

Health risks

While rare, the growth of pathogenic bacteria like Salmonella or E. coli in contaminated soda can cause serious gastrointestinal illness if consumed. Bacteria may reach high populations before changes are perceptible. Immuno-compromised individuals are especially susceptible.

Changes in appearance

Microbial growth often coincides with visual changes. Bubbles clinging to sides of a glass, cloudiness, and particulate matter suspended in soda may indicate microbial growth. Slimy stringy textures on surfaces like caps and nozzles can also develop with bacteria present.

Ways to prevent bacterial growth in soda

There are several strategies to help limit bacterial growth in soda:

Refrigerate after opening

Keeping opened sodas chilled in the refrigerator slows microbial growth. The cooler temperatures restrict the metabolism and replication of any bacteria present.

Minimize air exposure

Leaving soda uncapped allows oxygen to dissolve back in, which bacteria need for growth. Keeping sodas capped and bottled helps maintain carbonation and limits air access.

Drink soda promptly

Consuming sodas soon after dispensing provides less time for any contaminating bacteria to multiply to dangerous levels. Bacteria levels remain low immediately after opening.

Avoid adding extra ingredients

Ingredients like fruit slices or juices enrich the soda, providing nutrients to support more extensive bacteria growth. Keep soda in its original form.

Clean dispensers and taps

Soda fountains and taps should be cleaned and sanitized regularly to remove bacterial biofilms. This prevents cross-contamination of fresh pours.

Should soda be considered a risky beverage?

While bacterial growth in soda is possible, especially when contaminated or handled improperly, soda alone is not an inherently risky beverage.

Low inherent risk

When properly stored after opening and consumed promptly, soda by itself poses a very low microbial risk. The acidic pH, lack of nutrients, carbonation, and sometimes preservatives make soda an unlikely place for extensive bacterial growth.

Potential risks from mishandling

The main risks come from improper handling like leaving soda unrefrigerated for prolonged periods, introducing contaminating ingredients, or dirty dispensing equipment. When soda is managed appropriately, risks stay low.

Susceptible populations

Immunocompromised individuals and those with highly acidic stomachs may be at slightly higher risk for infection. However, for healthy adults and children, consuming soda poses very minimal inherent risk on its own.

Safer than nutritious beverages

Unpasteurized milk, juice, and nutritious drinks support extensive microbial growth and typically carry higher risks than sodas. In healthy adults, any bacteria inadvertently consumed in soda will likely pass through the digestive system without issue.

Can bacteria grow in diet soda?

Diet soda provides a harsher environment for bacterial growth compared to regular soda. However, bacterial growth is still possible under certain conditions.

Higher acidity

Diet sodas tend to be more acidic with lower pH levels. The increased acidity makes survival more difficult for bacteria. Common diet soda pH values are 2-3.

Artificial sweeteners

Rather than sugar, diet sodas are sweetened with non-nutritive artificial sweeteners like aspartame, saccharin, sucralose, or acesulfame potassium. Many bacteria cannot metabolize these synthetic sweeteners.

Lack of nutrients

With no sugars and carbohydrates, diet soda offers very limited nutrients to sustain bacterial growth. The nutrient limitation slows replication and prevents extensive growth.

Potential for growth

Given these inhibitory factors, bacterial growth in diet soda is extremely unlikely under normal conditions. However, some bacterial spores or fungi may persist in the harsh environment. Additionally, if contaminated, oxygenation or added ingredients could enable limited growth.

Can soda go bad if unrefrigerated?

Soda can eventually go bad if left unrefrigerated for an extended period as bacteria start growing. However, the timeline depends on the specific conditions.

Lasts longer than other beverages

Unpasteurized fruit juices and dairy beverages support extensive microbial growth within hours if left unrefrigerated. Soda’s acidic pH and lack of nutrients make it far more resistant to going bad.

Factors impacting shelf life

– Carbonation level – Flat soda enables more microbial growth
– Sugar content – More sugars allow increased bacterial metabolism
– Container seal – Open containers have more airborne contamination

– Storage temperature – Heat accelerates chemical reactions and bacterial growth

Signs soda has gone bad

– Change in carbonation, pressure, hissing sound when opening
– Off odors, vinegar-like smell
– Mold growth visible
– Fizzing, bubbling, or cloudiness
– Slime, grit, filmy texture
– Changes in taste – sour, bitter, metallic, salty

With refrigeration after opening, regular soda typically lasts 1-2 weeks before going bad from bacterial growth.

Does soda kill bacteria in the body?

There are two key considerations when determining if soda kills bacteria in the body:

Acidity of soda

The acidic pH of soda around 2-3 is highly inhospitable to most bacteria species. However, the stomach lining naturally secretes hydrochloric acid creating an even more acidic environment in the stomach around pH 1-2.

Buffer capacity of saliva

Before reaching the stomach, soda passes through saliva in the mouth which contains bicarbonate buffers. This immediately neutralizes some of the acidity. The short contact time and buffering make it unlikely the soda acidity directly kills oral bacteria.

Evidence for bacteria inhibition

Most research has found little to no antibacterial effect of soda in the body. While the acidity can impact enamel, once buffered and diluted, it does not significantly impact oral or gut microbiota. Any inhibition of bacteria would likely rely on preservatives like sodium benzoate rather than just pH.


In conclusion, while soda is not an ideal environment for extensive bacterial growth due to its acidity, carbonation, and lack of nutrients, growth can still occur under certain conditions. Bacteria contamination introduced through backflow, unclean equipment, or added ingredients like fruit can initiate microbial growth. Flat, uncapped, and unrefrigerated soda stored at warm temperatures provides the most permissible conditions for growth to occur. However, when handled properly, the risks of bacterial growth remain low in soda. Healthy adults and children face minimal risks consuming soda, although immunocompromised individuals should exercise caution. Preventing contamination and promptly refrigerating opened sodas can help minimize microbial growth potential in soda.

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