Yeast contamination in cheese is a frequent issue, especially among artisanal and small-scale producers. It can lead to “blowing” or gas production within cheese, affecting texture, flavour and safety. This blog will explore how yeast contamination occurs, its consequences, and effective prevention strategies.
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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.
| Aspect | Spongy Coliform | Yeast Contamination | Late Blowing |
|---|---|---|---|
| Cause | Caused by coliform bacteria (E. coli, Enterobacter). | Caused by certain yeast species (Candida, Kluyveromyces). | Caused by Clostridium tyrobutyricum, which ferments lactate. |
| Fermentation Type | Ferments lactose, producing gas and acids. | Ferments residual lactose or other sugars, producing gas. | Ferments lactate, producing butyric acid and gas. |
| Gas Formation | Produces small, irregular, sponge-like holes. | Produces gas, but holes may vary in size or distribution. | Produces large, round or cracked openings. |
| Timing | Occurs early, during or shortly after fermentation. | Occurs early but may also affect the initial ripening stages. | Happens late, typically during cheese ageing. |
| Flavour & Aroma | Produces sour, rancid or faecal smells and flavours. | Can produce alcoholic, fruity or yeasty flavours and odours. | Creates a strong butyric acid aroma (rancid, cheesy smell). |
| Cheese Types Affected | Both soft and hard cheeses. | Primarily high-moisture, fresh or soft cheeses. | Primarily impacts aged, hard cheeses like Gouda or Parmesan. |
| Prevention | Ensure proper milk quality, pasteurisation and hygiene. | Control yeast levels, proper sanitation, and balanced starter cultures. | Use silage-free feed, add lysozyme and reduce spore contamination. |
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:
- Isolate affected batches to prevent cross-contamination.
- Conduct microbial testing to confirm yeast presence.
- 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.
Case study: yeast contamination in semi-hard cheeses
Semi-hard cheeses, such as Gouda and Edam, are particularly vulnerable to yeast contamination due to their production and ageing conditions. These cheeses undergo a brining process that, if not meticulously controlled, can become a hotspot for yeast growth.
Let’s delve deeper into a real-world example of how yeast contamination affected a cheesemaker and how they resolved the issue.
The problem: Unwanted gas formation in Gouda
A small-scale artisan cheesemaker in the Netherlands began noticing defects in their Gouda cheeses. Swelling and cracking appeared during the ripening phase, leading to concerns about quality and safety. The cheese exhibited irregular holes, along with a slight yeasty smell and an alcoholic tang. These defects rendered the cheese unsellable, resulting in financial losses and wasted time.
Identifying the root cause
The cheesemaker suspected yeast contamination but wasn’t sure of the source. To identify the issue, they:
- Tested the milk: Initial testing showed no significant contamination, ruling out the raw milk as the source.
- Checked the brine tank: Microbial testing of the brine revealed elevated levels of Debaryomyces hansenii, a yeast species known for its high salt tolerance.
- Monitored the ripening environment: Air sampling and surface swabs from the aging room also showed traces of yeast contamination.
The findings confirmed that the brining process was the primary entry point for the yeast, and the warm, humid conditions during aging allowed it to flourish.
How the cheesemaker addressed the issue
Armed with this information, the cheesemaker implemented a series of measures to control the contamination and prevent recurrence. Here’s what they did:
1. Sanitising the brine tank
The brine tank was found to have biofilm build-up, which provided a perfect habitat for yeast. To resolve this, they:
- Drained and discarded the contaminated brine.
- Deep-cleaned the tank using specialised food-grade sanitising agents.
- Introduced a filtration system for the brine to remove microbial contaminants during future use.
2. Adjusting brine management
The cheesemaker realised they had been reusing brine too frequently without sufficient filtration or replacement. Moving forward, they:
- Replaced the brine more frequently to reduce microbial load.
- Monitored brine quality through routine microbial testing.
- Lowered the brine temperature slightly to discourage yeast growth.
3. Improving ripening room hygiene
Since the yeast had spread to the ripening room, the cheesemaker took steps to clean and sanitise it thoroughly. They:
- Scrubbed all surfaces, including walls, shelves, and tools, using antifungal cleaning agents.
- Improved air circulation and installed a HEPA filter system to reduce airborne yeast spores.
- Began using UV light treatments in the ripening room as an additional safeguard.
4. Monitoring and testing
To ensure the issue didn’t recur, the cheesemaker set up a monitoring schedule. Regular testing included:
- Swabs from brine tanks, tools, and aging racks.
- Air sampling in the production and ripening areas.
- Testing finished cheeses for microbial contaminants before sale.
Results of the intervention
These efforts paid off within months. The cheesemaker noticed:
- A marked decrease in yeast-related defects, including blowing and swelling.
- Improved consistency in the texture and flavour of their Gouda.
- Lower financial losses, as fewer batches needed to be discarded.
They also received positive feedback from customers about the improved quality of their cheeses.
Lessons learned from the case
This case highlights several key lessons for cheesemakers:
- Brine tanks are critical control points: Regular cleaning and microbial testing are essential.
- Yeast thrives in warm, moist environments: Controlling temperature and humidity is vital during ripening.
- Proactive monitoring prevents bigger issues: Early detection of yeast activity can save time and money.
By prioritising these measures, the cheesemaker turned a significant challenge into an opportunity to refine their processes and improve their product.
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.
Cheese lover. Scientist. Created a website and a Youtube channel about cheese science because he could not find answers to his questions online.
