This post explores the science behind Listeria, its survival mechanisms and its impact on cheese safety.

What is Listeria monocytogenes?

Listeria monocytogenes is a Gram-positive, rod-shaped bacterium found in soil, water and animal faeces. Unlike many bacteria, it can grow at refrigeration temperatures. This makes it a major concern in chilled foods.

Listeria has several survival advantages:

• It thrives at 0–45°C, making refrigeration ineffective in stopping its growth.
• It tolerates high salt concentrations, allowing survival in processed foods.
• It forms biofilms, making it difficult to remove from food surfaces.
• It can cross the intestinal, blood-brain, and placental barriers, leading to severe infections.

How does L. monocytogenes cause illness?

Listeriosis occurs when contaminated food is ingested. The bacteria invade the intestinal cells and spread through the bloodstream. Severe infections affect the brain, leading to meningitis or encephalitis.

1. Ingestion and survival in the gut: Listeria enters the body through contaminated food. It survives stomach acid and bile, allowing it to reach the intestines.
2. Cell invasion: Listeria binds to intestinal cells using proteins like Internalin A and B. It tricks the cells into engulfing it through a process called phagocytosis.
3. Escape from the immune system: Inside the cell, Listeria escapes the phagosome by producing Listeriolysin O, a toxin that breaks down the surrounding membrane. This allows it to enter the cytoplasm, where it can move freely.
4. Spreading between cells: Listeria uses a protein called ActA to hijack the host’s actin filaments. This forms “actin tails,” propelling the bacteria into neighbouring cells. It spreads without exposing itself to the immune system.
5. Crossing protective barriers: Listeria can cross critical barriers in the body: Blood-brain barrier (Causes meningitis or encephalitis) and placental barrier (Can infect the foetus, leading to miscarriage or stillbirth).

Symptoms of listeriosis

Listeriosis symptoms vary depending on the person’s immune response. Common symptoms include:

• Mild cases: Fever, muscle aches, nausea and diarrhoea.
• Severe cases: Sepsis, meningitis and neurological complications.
• In pregnancy: Mild flu-like symptoms but severe risks to the unborn baby, including miscarriage or stillbirth.

Why is Listeria in cheese a problem?

Cheese provides an ideal environment for Listeria growth. It contains moisture, nutrients and sometimes lacks preservatives.

Which cheeses are high-risk?

Certain cheeses carry a higher risk of Listeria contamination:

1. Soft cheeses made with unpasteurised milk – Brie, Camembert, Queso Fresco and some blue cheeses.
2. Washed-rind cheeses – Reblochon, Taleggio and Munster, which have high moisture levels.
3. Raw milk cheeses – Any cheese made from unpasteurised milk can carry Listeria if contaminated.

Why is soft cheese more dangerous?

Soft cheeses contain more moisture than hard cheeses. This allows Listeria to grow more easily. Hard cheeses, like Cheddar or Parmesan, have lower moisture and a more acidic environment, reducing bacterial survival.

Listeria & Pasteurisation

Pasteurisation is a key defence against Listeria in dairy products.

How pasteurisation works

Heat treatment kills bacteria by damaging their cellular structures. In dairy processing, milk is heated to at least 72°C for 15 seconds (high-temperature, short-time pasteurisation). This eliminates Listeria, Salmonella and E. coli.

Is pasteurised cheese always safe?

While pasteurisation destroys bacteria in milk, contamination can occur later. If cheese is handled in unsanitary conditions, Listeria can still grow. Factory hygiene plays a crucial role in preventing outbreaks.

How does Listeria survive in cheese factories?

Listeria adapts well to food production environments. Once it contaminates a facility, it becomes difficult to remove.

Common sources of contamination

• Drains and floors – Biofilms allow bacteria to persist despite cleaning.
• Processing equipment – Listeria can survive on slicing machines and packaging lines.
• Raw ingredients – Unpasteurised milk or contaminated cultures introduce bacteria.
• Employee handling – Poor hygiene can spread Listeria to finished products.

Biofilms: a hidden threat

Listeria forms biofilms, which are protective layers of bacteria. These attach to surfaces and resist cleaning chemicals. Once a biofilm develops, it continuously sheds bacteria, contaminating food.

Notable Listeria outbreaks in cheese

Several outbreaks have been linked to contaminated cheese. These incidents highlight the risks of improper food handling.

• Listeria in soft cheeses (2019, USA): In 2019, an outbreak in the USA linked to soft cheeses caused 24 illnesses and two deaths. Investigators traced the bacteria to unsanitary cheese-making conditions.
• Listeriosis in France (2022): A 2022 outbreak in France was linked to raw milk Brie. Authorities recalled multiple batches after detecting high levels of Listeria.

How to prevent Listeria contamination in cheese

Both consumers and manufacturers play a role in reducing Listeria risks.

For cheesemakers

• Strict hygiene protocols – Regular cleaning and sanitising prevent contamination.
• Regular testing – Routine Listeria testing ensures safety in production.
• Pasteurisation enforcement – Using pasteurised milk reduces bacterial risks.

For consumers

• Choose pasteurised cheese – This is crucial for pregnant women and the immunocompromised.
• Store cheese correctly – Keep cheese below 5°C and consume it before expiration.
• Practise good hygiene – Wash hands and use clean utensils when handling cheese.

Can hard cheese contain Listeria?

Hard cheese can contain Listeria monocytogenes, but the risk is much lower than in soft cheeses. The low moisture content, high salt levels and acidic environment of hard cheeses, such as Cheddar and Parmesan, make it difficult for Listeria to grow.

Additionally, the long ageing process further reduces the bacteria’s chances of survival. However, Listeria is a resilient pathogen that can still persist under certain conditions.

Cross-contamination is a key risk factor, as Listeria can spread from contaminated surfaces, utensils or infected soft cheeses stored nearby. Improper storage, such as keeping cheese at unsafe temperatures, may also allow bacteria to survive.

Hard cheeses made from raw milk pose a slightly higher risk, though regulations often require ageing for at least 60 days to reduce bacterial contamination. While rare, food safety inspections have occasionally detected Listeria in aged cheeses, showing that even hard cheese is not entirely risk-free.

Does freezing kill Listeria?

Freezing does not kill Listeria monocytogenes; it merely halts its growth. The bacterium can survive freezing temperatures and resume activity upon thawing. Studies have shown that Listeria experiences greater injury and death when frozen and stored at -18°C compared to -198°C.

However, some bacteria survive and can repair themselves under suitable conditions. Therefore, freezing should not be relied upon to eliminate Listeria in contaminated foods.

Does cooking reduce the risk of contamination with Listeria?

Yes, cooking does kill Listeria monocytogenes, but only at sufficiently high temperatures. The bacteria die when food is heated to at least 74°C (165°F). Proper cooking ensures food safety by eliminating Listeria from contaminated products.

Key temperature guidelines

• Pasteurisation (72°C for 15 seconds) – Effectively kills Listeria in dairy products.
• 74°C (165°F) or higher – Listeria is destroyed.
• Reheating leftovers – Should also reach 74°C (165°F) to ensure safety.

Future strategies to combat Listeria

Scientists continue to research ways to control Listeria in cheese production.

Bacteriophage therapy

Bacteriophages are viruses that infect bacteria. Some phages specifically target Listeria, offering a natural method for control.

Natural antimicrobials

Cheesemakers are testing plant extracts and bacterial cultures that inhibit Listeria growth. These include:

• Nisin – A natural antimicrobial found in some cheese cultures.
• Rosemary extract – Contains compounds that slow bacterial growth.
• Lactic acid bacteria – Compete with Listeria in cheese environments.

Improved detection methods

Faster DNA-based tests help cheesemakers detect Listeria before products reach the market. This reduces outbreaks and recalls.

Conclusion

Listeria monocytogenes poses a serious risk to cheese safety. Its ability to survive cold temperatures and form biofilms makes it difficult to control. Soft cheeses, unpasteurised varieties, and poor hygiene increase the risk.

Preventing Listeria requires strict hygiene, pasteurisation and safe handling practices. Scientists are developing new methods to combat this persistent bacterium. Consumers should choose pasteurised cheese, store it properly, and handle it safely.

By understanding the science behind Listeria, we can reduce its risks and enjoy cheese safely.

References

1. Listeria monocytogenes Cross-Contamination of Cheese: Risk Assessment and Control Strategies: This study discusses the prevalence of Listeria monocytogenes in cheese and highlights the risk of cross-contamination during cheese production. PMC
2. Outbreak Investigation of Listeria monocytogenes: Brie and Camembert Soft Cheese Products: The FDA investigated a multistate outbreak linked to Brie and Camembert cheeses, underscoring the potential for contamination in soft cheeses. U.S. Food and Drug Administration
3. Listeria Outbreak Linked to Queso Fresco and Cotija Cheese: The CDC provides details on an outbreak associated with Queso Fresco and Cotija cheeses, emphasizing the risks even in pasteurized products. CDC
4. Listeriosis Caused by Persistence of Listeria monocytogenes in Cheese Production Environment: This article examines a listeriosis outbreak traced to persistent environmental contamination in a cheese dairy, highlighting the challenges of eradicating Listeria once established. PMC
5. Listeria monocytogenes in Cheese Products: The Centre for Food Safety discusses the risk of L. monocytogenes in cheese products and provides guidelines for prevention. FEHD

Sabine Lefèvre

Sabine is the creative force behind Cheese Scientist. She is a sustainable living advocate, a climate change protestor and is pro-choice. And, most relevantly, she is also a lactose intolerant cheese lover.

The post Listeria monocytogenes & Cheese: Why Contamination Happens appeared first on Cheese Scientist.

A Lidl cheese recall has left shoppers worried, as a flavoured version of a widely enjoyed cheese has been withdrawn over safety concerns. The discovery of dangerous bacteria has prompted urgent warnings, with vulnerable groups at particular risk.

The affected batch has a best-before date of 11 May 2025. Customers are advised not to consume the product. Instead, they should return it to any Lidl store for a full refund, with or without a receipt.

In a statement, Lidl said:

“Lidl Ireland and Northern Ireland is recalling the above Deluxe Sriracha Cheddar Cheese Wedge with a Best Before Date of 11/05/25 due to the presence of Listeria monocytogenes.

If you have purchased the above product do not consume it. Instead, please return the product to a Lidl store for a full refund, with or without a receipt.

Lidl wishes to apologise for any inconvenience caused. Any customers with queries or concerns can contact our Customer Services Team.”

Listeria monocytogenes can cause severe flu-like symptoms, including high temperature, muscle aches, chills, nausea and diarrhoea. In extreme cases, it may lead to meningitis. Vulnerable groups, such as the elderly, pregnant women, new-borns, and individuals with weakened immune systems, are particularly at risk.

This recall follows several others in recent weeks. Farmfoods recalled its own-brand cocktail sausage rolls due to undeclared milk, posing a risk to those with allergies. Asda withdrew its George Home Baby Walkers over choking hazards. Various supermarkets, including TK Maxx, Waitrose, WilderKitchen and Aldi, have also recalled products over allergy risks and safety concerns.

Consumers are urged to stay informed about product recalls to ensure their safety. For further information, customers can contact Lidl’s Customer Services Team.

Jonah Kincaid

Cheese lover. Scientist. Created a website and a Youtube channel about cheese science because he could not find answers to his questions online. 

cheesescientist.com

The post Lidl Cheese Recall: Popular Cheese Recalled Due to Listeria appeared first on Cheese Scientist.

SEE ALSO: The essential pieces of equipment that every home cheesemaker needs →

What is yeast contamination?

Yeasts are single-celled fungi found naturally in the environment. They thrive in dairy environments, particularly when conditions favour their growth. In cheesemaking, yeast contamination occurs when undesirable yeasts colonise milk, curd or cheese. These organisms can outcompete beneficial cultures, creating defects in the final product.

What causes yeast contamination in cheese?

There are several ways yeast contamination can occur. First, raw milk can harbour wild yeasts if poorly handled. Second, contamination can arise during processing due to unclean equipment or an unsanitary environment.

Additionally, ingredients like rennet, starter cultures or salt may sometimes introduce yeasts. High humidity and warm temperatures during cheese ripening can also encourage yeast growth. Transitioning from production to storage without strict hygiene protocols amplifies this risk.

Species of yeasts that can contaminate cheese

Several yeast species can interfere with cheesemaking, but some are more commonly encountered due to their adaptability to dairy environments. Here’s an overview of the most common yeasts associated with cheese production, along with their characteristics:

1. Debaryomyces hansenii

• Characteristics: Highly salt-tolerant and able to grow in low moisture environments.
• Where it occurs: Frequently found in brine tanks and on the surface of cheeses.
• Impact: It can be beneficial in small amounts, contributing to rind development in washed rind cheeses. However, in uncontrolled growth, it produces gas and off-flavours.

2. Candida spp. (e.g., Candida lipolytica and Candida parapsilosis)

• Characteristics: These yeasts thrive in environments with high fat content, as they metabolise fats into free fatty acids.
• Where it occurs: Found on rinds, in raw milk and in the cheesemaking environment.
• Impact: High levels can lead to rancid, soapy, or bitter off-flavours. They also contribute to blowing in semi-hard and hard cheeses.

3. Kluyveromyces lactis

• Characteristics: Commonly found in milk and dairy environments. It ferments lactose into ethanol and carbon dioxide.
• Where it occurs: Present in raw milk and sometimes introduced through poor hygiene.
• Impact: It causes early blowing defects, with gas bubbles forming during the early stages of maturation.

4. Saccharomyces cerevisiae

• Characteristics: Often referred to as baker’s or brewer’s yeast, it is a robust fermenter.
• Where it occurs: May be introduced accidentally via the environment or other ingredients.
• Impact: Excessive fermentation of lactose or residual sugars can result in high gas production and early blowing.

5. Yarrowia lipolytica

• Characteristics: A yeast that metabolises lipids and proteins, often found in ripening cheeses.
• Where it occurs: Common on the surface of aged cheeses or in poorly managed environments.
• Impact: Uncontrolled growth produces strong, unpleasant flavours and surface defects.

6. Zygosaccharomyces spp.

• Characteristics: Known for its resilience, it survives in high sugar and low pH environments.
• Where it occurs: Occasionally found in sweetened dairy products and cheesemaking environments.
• Impact: Produces gas and off-flavours, leading to texture issues in semi-hard cheeses.

7. Geotrichum candidum

• Characteristics: Known as a “good yeast”, it is intentionally used in cheesemaking. It contributes to the development of rind and softening of cheese interiors.
• Where it occurs: Found on the surface of soft-ripened cheeses like Brillat-Savarin and Valençay.
• Impact: While beneficial when controlled, cross-contamination can lead to unwanted growth on cheeses not requiring it.

How does yeast contamination affect cheese?

Detecting yeast contamination early can save a cheese batch. Common signs include:

• Swollen packaging due to early or late blowing
• Excessive bubbling during fermentation
• Strange, alcoholic or yeasty smells
• Slimy rinds or unusual surface growth

Yeast contamination in cheese can cause early blowing or late blowing, depending on their metabolic activity, the stage of production and the cheese type. Let’s break down the types of blowing caused by yeasts:

Early blowing

• When it occurs: During the initial stages of ripening, often within the first few days after cheese formation.
• Caused by: Yeasts like Kluyveromyces lactis or Saccharomyces cerevisiae ferment residual lactose in the cheese curd, producing carbon dioxide (CO₂).
• Cheese types affected: More common in semi-hard and hard cheeses like Gouda, Edam and Cheddar.
• Defects observed:
  • Irregular, large holes or cracks in the cheese structure.
  • Changes in texture, often becoming spongy or crumbly.
  • Unpleasant flavours, including alcoholic or yeasty notes.
  • Unusually moist and sticky rinds
• Why it happens: Inadequate control of lactose levels or contamination during milk handling, brining or early ageing.

Late blowing

• When it occurs: During the later stages of ripening, sometimes weeks or months into the ageing process.
• Caused by: Certain yeast species, such as Candida spp. or Yarrowia lipolytica, breaking down proteins and fats to produce CO₂.
• Cheese types affected: Often affects aged cheeses like Parmesan, Gouda and Emmental.
• Defects observed:
  • Swelling of cheese blocks or wheels.
  • Formation of internal holes or splits in the cheese body.
  • Rancid or soapy off-flavours due to excessive lipid breakdown.
• Why it happens: Poor hygiene in ageing rooms, contaminated brine or environmental conditions that favour yeast growth.

How yeasts differ from bacteria in causing blowing

Unlike bacteria like Clostridium tyrobutyricum, which produce gas through butyric acid fermentation (a common cause of late blowing), yeasts primarily ferment sugars, fats, or proteins. Yeast-induced blowing often leads to milder off-flavours compared to the rancidity associated with bacterial activity.

The table below summarises the differences between the different types of blowing that can affect cheese during production and ageing.

Swipe across if on a mobile device to see the complete table.

What should you do with contaminated cheese?

Despite best efforts, yeast contamination may still occur. Prompt action can minimise losses and prevent recurrence. If you suspect yeast contamination:

1. Isolate affected batches to prevent cross-contamination.
2. Conduct microbial testing to confirm yeast presence.
3. Adjust production protocols to address identified issues.

In some cases, minor yeast activity can be controlled by modifying salting or pH levels. For severe contamination, discarding the batch might be necessary. As always, you should try to compost your unsafe cheeses.

Why you should discard yeast-contaminated cheeses

While many yeasts are harmless or even beneficial in cheesemaking, certain species can pose significant health risks if they contaminate cheese. These risks arise primarily due to the ability of harmful yeasts to produce toxins, cause infections, or enable the growth of other harmful microorganisms. Understanding the potential dangers is crucial for maintaining both cheese quality and consumer safety.

1. Production of harmful metabolites

Certain yeast species can produce unwanted by-products during their metabolic processes, which may be harmful to health:

a) Biogenic amines

• Some yeasts, particularly those involved in protein degradation, can produce biogenic amines like histamine and tyramine.
• High levels of these compounds in cheese can lead to:
  • Allergic reactions, such as skin rashes, headaches or digestive issues.
  • More severe symptoms in individuals with histamine intolerance.
  • In the case of tyramine, there is a life-threatening medication interaction with a class of antidepressants called MAO inhibitors.
• Candida spp. and Yarrowia lipolytica are examples of yeasts that may contribute to biogenic amine production.

b) Mycotoxins

• Though rare, some yeast strains produce mycotoxins, which are toxic secondary metabolites.
• These toxins can be carcinogenic or immunosuppressive, posing long-term health risks if consumed.

2. Opportunistic infections

Certain yeasts, such as Candida albicans, can act as opportunistic pathogens in humans. While typically harmless in healthy individuals, they can cause infections under specific conditions:

a) At-risk populations

• People with weakened immune systems, such as those undergoing chemotherapy, organ transplant recipients or individuals with HIV/AIDS, are more vulnerable.
• In such cases, even low levels of harmful yeast contamination in food may trigger infections.

b) Infections linked to yeast in cheese

• Ingestion of contaminated cheese could potentially lead to gastrointestinal infections or overgrowth of pathogenic yeast species.
• For example, Candida albicans contamination has been associated with oral thrush, vaginal yeast infections, and systemic candidiasis.

3. Spoilage and enabling harmful bacteria

Yeasts that thrive in cheese environments may indirectly endanger health by creating conditions conducive to the growth of harmful bacteria:

a) Spoilage organisms

• Yeasts such as Debaryomyces hansenii or Pichia spp. can spoil cheese by producing CO₂ and ethanol, which lead to off-flavours, odours and structural defects.
• Spoiled cheese may encourage growth of harmful bacteria like Listeria monocytogenes or Clostridium botulinum.

b) pH alteration

• Some yeasts increase the pH of cheese during ripening, reducing its acidity and lowering the barrier to pathogen growth.
• This pH shift can enable pathogenic bacteria to survive and multiply, particularly in soft or fresh cheeses.

4. Allergic reactions

Some individuals may develop allergic reactions to certain yeast species present in cheese. Symptoms of yeast-related allergies can include:

• Skin irritation, such as rashes or eczema.
• Respiratory issues, including asthma or sinus congestion.
• Digestive disturbances, such as bloating or diarrhoea.

Conclusion

Yeast contamination is a complex challenge for cheesemakers, but it can be managed effectively. By understanding its causes and effects, producers can take proactive measures to maintain cheese quality. Through hygiene, monitoring, and testing, yeast-related problems like blowing can be minimised.

Cheesemakers who prioritise prevention will find greater success and consistency in their craft.

Jonah Kincaid

Cheese lover. Scientist. Created a website and a Youtube channel about cheese science because he could not find answers to his questions online. 

cheesescientist.com

The post Yeast Contamination In Cheese (Why It Happens & How To Fix) appeared first on Cheese Scientist.

SEE ALSO: The must-have pieces of equipment for home cheesemaking →

What is spongy coliform?

Spongy coliform is a defect in cheese caused by coliform bacteria such as Escherichia coli and Enterobacter. These bacteria ferment lactose, producing gas and acids that disrupt the cheese structure.

The defect is classified as a type of early blowing, occurring during or shortly after fermentation. It is characterised by small, irregular holes and an unpleasant flavour or odour. Spongy coliform can affect both soft and hard cheeses, making it a concern for a wide range of cheesemakers.

Another type of early blowing can be due to yeast contamination. And the most common type of late blowing is due to Clostridia bacteria. We’ll dive into the differences a bit further down.

What are coliform bacteria?

Coliform bacteria are Gram-negative, rod-shaped microorganisms that can contaminate cheese during production. They are naturally present in soil, water and the intestines of warm-blooded animals. While many coliforms are harmless, their presence in cheese often indicates poor hygiene or contamination during processing.

Coliforms in cheese are generally categorised into two groups:

1. Total coliforms

• These are all coliform bacteria commonly found in the environment.
• Examples include Enterobacter, Klebsiella and non-pathogenic strains of Escherichia coli (E. coli).
• While not all are harmful, their presence in cheese signals lapses in sanitation.

2. Faecal coliforms

• A subset of coliforms originating from the intestines of animals.
• Includes strains like E. coli, often used to detect faecal contamination.
• Some strains, particularly pathogenic ones like E. coli O157:H7, pose serious health risks if present in cheese.

Understanding these bacteria helps cheesemakers address contamination risks and maintain the highest standards of safety and quality.

Causes of spongy coliform

Spongy coliform typically arises from contamination during cheesemaking. Common causes include poor milk quality, insufficient pasteurisation and unsanitary equipment.

1. Poor milk hygiene: Raw milk often contains coliform bacteria, especially if it comes from animals kept in unsanitary conditions. Contaminated milking equipment can worsen the problem. Even small amounts of coliform bacteria in milk can multiply during cheesemaking.
2. Improper pasteurisation: Pasteurisation kills most bacteria, but it must be done correctly. If the milk is not heated to the right temperature, coliforms may survive. These surviving bacteria can thrive during the cheesemaking process.
3. Dirty cheesemaking equipment: Cheesemaking tools and surfaces must be thoroughly cleaned and sanitised. Contaminated equipment can introduce coliforms to the milk or curd. Bacteria on utensils or vats can spread quickly, affecting entire batches of cheese.

How to identify spongy coliform defects

Cheesemakers should look for certain signs to identify spongy coliform. These include:

• Irregular, sponge-like holes in the cheese body.
• Sour and acidic notes
• Faecal or barnyard smells
• Rancid or soapy flavours
• Bitter aftertaste

Testing milk and curds can also help detect coliform bacteria early. Laboratory tests can measure coliform counts to assess milk quality.

Spongy coliform vs yeast contamination vs late blowing

Cheese defects like spongy coliform, early blowing caused by yeast and late blowing are common challenges for cheesemakers. Each defect results from different microbial activity, impacting texture, flavour and overall cheese quality. Understanding the causes, timing, and prevention strategies is crucial to maintaining high production standards.

This guide compares these three defects, highlighting their key differences to help cheesemakers identify and address them effectively.

Swipe across if on a mobile device to see the complete table.

How spongy coliform creates a sponge-like texture

The sponge-like texture in cheese caused by coliform bacteria is primarily the result of gas production during fermentation. This process is driven by the bacteria’s metabolism and their interaction with the cheese’s components. Here’s how it works:

Coliform bacteria metabolism

• Coliform bacteria, such as Escherichia coli or Enterobacter, thrive in nutrient-rich environments like cheese curds.
• These bacteria ferment lactose (milk sugar) and other available carbohydrates.
• The by-products of this fermentation include:
  • Carbon dioxide (CO₂): A gas that forms bubbles within the cheese matrix.
  • Organic acids (e.g., acetic and lactic acids): These can further alter cheese texture and flavour.

Formation of gas pockets

• As CO₂ is produced, it becomes trapped in the dense protein structure of the curd.
• The gas cannot escape easily, leading to the formation of irregular holes or bubbles.
• Unlike the controlled eye formation in Swiss cheese, these gas pockets are uneven and chaotic, resulting in the “spongy” appearance.

Impact on protein structure

• The activity of coliform bacteria can also weaken the protein network in cheese.
• Enzymes secreted by the bacteria may break down casein, the primary milk protein.
• This degradation contributes to a softer, less cohesive texture, making the cheese feel sponge-like.

Environmental factors that exacerbate the issue

Several conditions in the cheesemaking process can amplify the effects of coliform bacteria:

• Temperature: Warm temperatures during early stages encourage rapid bacterial growth and gas production.
• pH Levels: Coliform bacteria thrive in slightly alkaline environments, which can occur if proper acidification doesn’t take place.
• Moisture Content: High moisture levels provide an ideal medium for bacterial activity, increasing the risk of spongy defects.

Chemical reactions in cheese

• Over time, the texture becomes more irregular as the curd stretches and collapses around the gas pockets.
• The acids produced by coliform bacteria can disrupt the delicate balance of calcium and phosphate in cheese.
• This imbalance weakens the curd structure, allowing gas to accumulate more freely.

How unpleasant smells and flavours arise from coliform contamination

When coliform bacteria contaminate cheese, they produce a range of metabolic by-products during fermentation. These compounds are responsible for the unpleasant smells and flavours associated with spongy coliform defects. Here’s a breakdown of how this occurs:

1. Fermentation of lactose

Coliform bacteria ferment lactose, the primary sugar in milk, producing:

• Acetic acid: Contributes sour, vinegary flavours.
• Lactic acid: Adds an overly tangy or sour taste if produced in excess.
• Carbon dioxide (CO₂) and hydrogen gas: While these gases affect texture, they can also carry volatile compounds that intensify odours.

2. Production of volatile compounds

During fermentation, coliform bacteria generate volatile organic compounds (VOCs), which contribute to odour and flavour. Examples include:

• Diacetyl: Often associated with buttery flavours, but in excess can taste rancid or harsh.
• Ammonia: Produced during protein breakdown, giving a sharp, unpleasant smell.
• Sulfur compounds: Result in rotten egg or sulphurous odours.

3. Protein breakdown (proteolysis)

Coliform bacteria release enzymes that degrade milk proteins like casein. This process creates:

• Amines (e.g., putrescine and cadaverine): Responsible for strong, faecal or rotting odours.
• Peptides and amino acids: While these are normal in cheese ripening, their breakdown by coliforms produces bitter or astringent flavours.

4. Fat breakdown (lipolysis)

Coliform bacteria can also break down fats in the cheese, producing:

• Free fatty acids: These contribute to rancid or soapy flavours.
• Ketones: Compounds like methyl ketones can add strong, unpleasant aromas.

5. Environmental influence

• Moisture and Temperature: Warm, humid conditions accelerate bacterial metabolism, intensifying the production of odorous and flavour-altering compounds.
• pH Levels: High pH environments, often resulting from coliform contamination, enhance the activity of enzymes that produce these unpleasant by-products.

Preventing spongy coliform in cheese

Preventing spongy coliform requires strict attention to hygiene and cheesemaking techniques. Cheesemakers should follow these steps to minimise risk:

• Source high-quality milk: Start with milk from trusted suppliers. Farmers should ensure animals are kept in clean environments. Milking equipment must also be regularly cleaned and sanitised.
• Use proper pasteurisation techniques: Milk must be pasteurised at the correct temperature and time. This process eliminates most bacteria, including coliforms. Ultra-pasteurisation can be an option for some cheese types.
• Maintain rigorous sanitation: Cheesemaking equipment, tools, and surfaces must be cleaned and sanitised after each use. Steam or chemical sanitisers can effectively kill bacteria. Regular testing of equipment can identify contamination risks.
• Use reliable starter cultures: Starter cultures play a key role in cheese fermentation. They help outcompete undesirable bacteria like coliforms. Choose cultures that are well-suited to the specific cheese being made.
• Monitor pH levels carefully: Maintaining the correct pH during cheesemaking is critical. Coliform bacteria thrive in higher pH conditions. Acidification of the milk and curd can help prevent their growth.

What to do if spongy coliform occurs

Spongy coliform can be devastating for a cheesemaker, but taking prompt and effective action can mitigate the damage. Here’s how to handle an outbreak of spongy coliform:

1. Identify the affected cheese: Examine your cheese for signs of spongy coliform. Look for small, irregular holes and check for sour or rancid smells.
2. Dispose of the affected cheese: Unfortunately, cheese with spongy coliform cannot be salvaged. Discard the affected batches responsibly to prevent cross-contamination. Consider composting the cheese for an eco-friendly disposal.
3. Trace the contamination source: Investigate where the contamination might have occurred. Test raw and pasteurised milk for coliform bacteria levels. Review cleaning and sanitisation procedures for equipment and facilities. And verify the integrity and effectiveness of the starter cultures used.
4. Clean and sanitise thoroughly: Coliform bacteria can persist in cheesemaking environments, so deep cleaning is essential.
5. Test subsequent batches: Before resuming production, test the next few batches for signs of coliform bacteria. Conduct microbial testing on milk, curds, and early-stage cheese. And monitor for abnormal gas formation or pH changes during fermentation.
6. Implement stricter controls: Take this opportunity to improve practices to reduce the risk of recurrence: use higher-grade milk from trusted sources, adjust pasteurisation parameters to ensure coliforms are eradicated and increase the frequency of hygiene checks and microbial testing.
7. Document the incident: Maintain detailed records of the incident, including test results, affected batches, and corrective actions taken. This documentation can help track patterns, demonstrate compliance, and avoid regulatory issues.

By acting decisively and addressing the root cause, cheesemakers can recover from spongy coliform outbreaks. More importantly, these steps help ensure the problem doesn’t happen again.

Conclusion

Spongy coliform is a challenging defect that can harm both cheese quality and a cheesemaker’s resolve. However, it can be prevented with proper milk handling, sanitation and pasteurisation. By understanding the causes and implementing strict hygiene standards, cheesemakers can produce high-quality, defect-free cheeses.

Cheesemaking is both an art and a science. With knowledge and care, you can ensure that every wheel of cheese meets the highest standards.

Jonah Kincaid

Cheese lover. Scientist. Created a website and a Youtube channel about cheese science because he could not find answers to his questions online. 

cheesescientist.com

The post Spongy Coliform Defect In Cheese (Causes & Prevention) appeared first on Cheese Scientist.

SEE ALSO: The most important pieces of equipment you need to start making your own cheese →

What is late blowing?

Late blowing occurs when gas-producing bacteria multiply in cheese during ageing. This gas causes the cheese to swell, crack or develop an irregular texture. The problem often arises weeks or months after the cheese has been made, hence the term “late blowing.”

Causes of late blowing

Late blowing is primarily caused by Clostridium tyrobutyricum, a spore-forming bacterium found in soil, silage or contaminated milk. These bacteria thrive in anaerobic environments, converting lactate into butyric acid and gas, which creates the characteristic defects.

Key causes of this contamination include:

1. Poor milk hygiene: Contaminated raw milk or milk from cows exposed to silage can introduce Clostridium spores.
2. Improper sanitation: Unclean equipment can harbour bacteria that contribute to late blowing.
3. Inadequate starter culture: Weak or insufficient starter cultures may fail to outcompete unwanted bacteria.
4. High moisture content: Excess moisture in cheese creates an ideal environment for Clostridium growth.
5. Improper salt levels: Salt inhibits bacterial growth. Insufficient salting can allow harmful bacteria to thrive.

Other clostridium species causing late blowing

While Clostridium tyrobutyricum is the most common culprit of late blowing, other Clostridium species can also contaminate cheese and cause similar issues. These include:

1. Clostridium butyricum: Similar to C. tyrobutyricum, this species ferments lactate to produce butyric acid, carbon dioxide and hydrogen gas. It thrives in anaerobic conditions and is often linked to silage-fed cows.
2. Clostridium sporogenes: This bacterium can cause gas formation in cheese but is less common. It produces spores that survive pasteurisation and grow in low-oxygen environments during ageing.
3. Clostridium beijerinckii: Although rare in cheesemaking, it can cause gas-related defects in dairy products. Like other species, it thrives in high-moisture and low-salt conditions.
4. Clostridium perfringens: While primarily a pathogen associated with foodborne illness, C. perfringens spores can occasionally contaminate milk. It’s not a typical cause of late blowing but may produce gas and spoilage in improperly handled cheese.

Is Clostridium botulinum a risk in cheesemaking?

Clostridium botulinum is a serious foodborne pathogen known for producing botulinum toxin, one of the most potent toxins. While it is more commonly associated with improperly canned or preserved foods, it can pose a risk in cheesemaking under specific conditions.

Conditions for Clostridium botulinum growth

For C. botulinum to grow and produce toxins, it requires:

• Anaerobic environments: Like other Clostridium species, C. botulinum thrives in low-oxygen conditions, such as vacuum-sealed or waxed cheese.
• Moisture: High-moisture cheeses are more susceptible because water activity facilitates bacterial growth.
• Low acidity: C. botulinum struggles to grow in acidic environments (pH below 4.6). Most cheeses fall within a safe pH range after proper acidification.
• Inadequate salt levels: Salt inhibits bacterial growth, but insufficient salting can allow C. botulinum to proliferate.

Is C. botulinum a common concern in cheese?

Clostridium botulinum contamination is rare in cheese because:

• Proper cheesemaking lowers pH to levels that inhibit C. botulinum growth.
• Most cheeses are salted adequately, creating an inhospitable environment for the bacteria.
• Pasteurisation kills vegetative cells, though spores can survive and grow later if conditions allow.

Preventive measures for all clostridium species

Since several Clostridium species can lead to late blowing, it’s vital to adopt broad preventive strategies:

1. Improve milk quality: Use clean, high-quality milk from cows not exposed to silage.
2. Sterilise equipment: Proper sanitation helps minimise all forms of bacterial contamination.
3. Add lysozyme: This enzyme effectively inhibits multiple Clostridium species.
4. Monitor salt and moisture: Low salt and high moisture create ideal conditions for anaerobic bacteria.

By taking these precautions, you can reduce the risk of late blowing caused by any Clostridium species.

Can yeast cause late blowing?

While yeast is not the primary cause of late blowing, certain yeast strains can contribute to similar defects in cheese. Yeast contamination during the cheesemaking or ageing process can lead to gas production, resulting in swelling, cracks or other undesirable changes.

How yeast contributes to late blowing:

• Gas production: Some yeast strains, such as Candida or Saccharomyces species, ferment lactose or residual sugars in cheese, producing carbon dioxide and other gases. This can mimic the effects of Clostridium species, particularly in cheeses with high moisture content.
• Altered microbial balance: Yeast growth can disrupt the balance of the starter culture, weakening the ability of beneficial bacteria to dominate. This may indirectly encourage Clostridium or other harmful bacteria to proliferate.

Distinguishing the contaminants that can cause blowing in cheese

Early and late blowing defects can both cause gas formation and irregular holes in cheese, but their causes and characteristics differ. The most common cause of early blowing is coliform contamination which can cause a defect called spongy coliform.

Let’s take a look at the main differences between late blowing, yeast contamination and spongy coliform.

Swipe across if on a mobile device to see the complete table.

Cheeses most commonly affected by late blowing

Late blowing is most frequently observed in hard and semi-hard cheeses. These styles undergo prolonged ageing, providing an ideal environment for gas-producing bacteria like Clostridium tyrobutyricum. Here are some examples of cheeses commonly affected by late blowing:

• Gouda: Gouda is one of the cheeses most associated with late blowing. Its moderate moisture content and long aging process create conditions that can allow Clostridium spores to thrive. This is why lysozyme is often added during Gouda production to prevent the defect.
• Edam: Similar to Gouda, Edam has a semi-hard texture and ages in a low-oxygen environment, especially when waxed. If the cheese is not adequately salted or the milk is contaminated, late blowing can occur.
• Parmesan: Despite its lower moisture content, Parmesan is still susceptible to late blowing due to its extended aging period. Any contamination in the milk or during production can result in unwanted gas production months into ageing.
• Emmental and Swiss-style cheeses: Swiss-style cheeses like Emmental are naturally prone to gas formation due to their propionic acid bacteria, which create desirable holes (or “eyes”). However, contamination with Clostridium can lead to excessive or uneven gas formation, causing defects rather than the controlled eye development typical of these cheeses.
• Cheddar: While less common, Cheddar can also experience late blowing if hygiene during production or ageing conditions are inadequate. The defect is more likely in Cheddars aged for extended periods.

How to handle affected cheese

Cheese affected by late blowing is often unsafe to eat due to the presence of harmful bacteria, such as Clostridium tyrobutyricum. While it may not always cause illness, it is best to err on the side of caution and discard the affected cheese.

Instead of throwing the cheese in the trash, consider composting it. Here’s how:

1. Break it into smaller pieces: This speeds up decomposition.
2. Mix with other compostable materials: Combine with brown materials like leaves or cardboard to balance the nitrogen content.
3. Monitor your compost pile: Avoid adding too much cheese to prevent an unpleasant smell or attracting pests.

By composting, you can reduce waste while giving back to the environment.

Learning from late blowing

Late blowing is frustrating, but it’s also a valuable learning opportunity for home cheesemakers. By carefully reviewing your process, you can identify what went wrong and make adjustments for future success. Here’s how you can turn a setback into a lesson:

1. Keep detailed records

Maintain a cheesemaking journal to document each step of the process, including:

• Milk source and type (e.g., raw, pasteurised).
• Cleaning and sanitation methods.
• Starter culture used and its quantity.
• Temperatures, pH levels, and pressing conditions.
• Salting method and amount.

Comparing records from successful and unsuccessful batches can help pinpoint potential causes.

2. Examine your milk source

Raw milk is more likely to contain Clostridium spores, especially if cows are fed silage. If you used raw milk, consider switching to pasteurised milk or ensuring stricter hygiene practices at the source.

3. Assess your sanitation

Late blowing often stems from contamination. Evaluate whether your cleaning methods were thorough enough. Sterilising all equipment and working in a clean environment can significantly reduce bacterial contamination.

4. Evaluate your starter culture

Weak or insufficient starter cultures might allow harmful bacteria to outcompete the beneficial ones. Next time, consider using a stronger culture or adjusting the quantity.

5. Analyse your ageing conditions

Improper humidity or temperature levels can contribute to late blowing. Double-check your ageing setup to ensure it aligns with the cheese type’s requirements.

6. Experiment with preventive additives

If you frequently make cheeses prone to late blowing, consider adding lysozyme or increasing salt levels to inhibit Clostridium spores.

Conclusion

Late blowing doesn’t have to be the end of your cheesemaking journey. By analysing your process and making informed changes, you’ll become a more skilled cheesemaker—and your next batch will be even better.

Mistakes are part of the learning curve, and every issue resolved is a step toward mastery.

Jonah Kincaid

Cheese lover. Scientist. Created a website and a Youtube channel about cheese science because he could not find answers to his questions online. 

cheesescientist.com

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