Fungi in Dairy Products: Roles of Yeasts and Moulds

Fungi are everywhere in the dairy world – sometimes working in your favour, sometimes against it. When you bite into a wedge of Roquefort or sip a glass of kefir, you are experiencing the direct result of fungal activity. But the same group of organisms that gives blue cheese its distinctive bite can also silently contaminate a batch of yoghurt or produce toxins in poorly stored milk products. Understanding how yeasts and moulds function in dairy production – and when they become a problem – is essential knowledge for anyone involved in milk processing or food safety.

What are fungi in the context of dairy microbiology? 🔗

Fungi are eukaryotic organisms – meaning their cells contain a defined nucleus and membrane-bound organelles, unlike bacteria which are prokaryotic. In dairy microbiology, fungi are divided into two main groups: yeasts, which are unicellular, and moulds, which are multicellular and grow as branching filamentous networks. Both groups are capable of fermenting or degrading dairy components, and both can either benefit or harm a dairy product depending on the species involved and the conditions under which they grow.

Yeasts in dairy products 🔗

Yeasts are single-celled fungi that reproduce primarily through budding. They are prevalent in natural environments and are common components of numerous traditional fermented foods, either working alone or as part of a mixed microbial population. In dairy products, yeasts can be introduced deliberately through starter cultures or arrive as environmental contaminants. Their key metabolic capability is fermentation – converting sugars, particularly lactose, into alcohol, carbon dioxide, and a range of flavour compounds.

Desirable yeast fermentation 🔗

Not all yeast activity in dairy is unwanted. In products like kefir, specific yeast strains work alongside lactic acid bacteria to produce the characteristic tangy flavour and slight effervescence that defines the product. Kluyveromyces marxianus is one of the most important dairy yeasts, partly because of its ability to ferment lactose efficiently. It is consistently detected in certain kefir cultures and can be acquired from unpasteurised milk, and its long history of safe use has earned it GRAS (Generally Recognised As Safe) status.

Beyond kefir, yeasts play a significant role in cheese ripening. The ability of Kluyveromyces species to metabolise milk constituents – lactose, proteins, and fat – makes them very important in cheese ripening and fermented milk products, where they contribute to maturation and aroma formation. Specifically, K. marxianus affects cheese ripening through its proteolytic and lipolytic activities, as well as the formation of volatile aroma compounds. These activities break down milk proteins and fats into smaller molecules – free amino acids and fatty acids – that contribute directly to flavour development. The yeast also plays a structural role: the production of CO₂ from lactose fermentation generates small holes in the cheese curd, helping the growth of aerobic Penicillium during the ripening process of blue-veined cheeses.

Other commercially relevant yeasts in dairy include Debaryomyces hansenii, which dominates the surface smear of certain ripened cheeses like Danish Danbo, and Yarrowia lipolytica, known for its pronounced lipolytic and proteolytic activity that contributes significantly to blue cheese maturation. Saccharomyces unisporus and Kluyveromyces marxianus are used as commercial starter cultures in the production of milky kefir, while Galactomyces geotrichum (formerly Geotrichum candidum) serves as a starter for mould-ripened cheeses such as Camembert.

Undesirable yeast activity 🔗

When yeasts grow where they are not wanted, they can cause significant spoilage. In products like yoghurt, sour cream, and soft cheeses, uncontrolled yeast growth leads to gas production, off-flavours, and textural defects. Yeasts can also cause pink discoloration, brown patches, and deacidification of the cheese surface when they catabolize amino acids to produce ammonia. A particularly problematic characteristic is that some yeast species can grow slowly even at refrigeration temperatures, making cold storage alone an unreliable control measure.

Moulds in dairy products 🔗

Moulds are multicellular fungi that grow as branching networks of thread-like structures called hyphae, which collectively form a visible mass called a mycelium. This filamentous structure gives moulds the capacity to produce powerful enzyme systems – particularly proteases (which break down proteins) and lipases (which break down fats). These proteolytic and lipolytic activities are precisely why certain moulds are deliberately introduced into specific dairy products, while also being the reason that uncontrolled mould growth causes spoilage.

Beneficial moulds: the foundation of mould-ripened cheese 🔗

The use of moulds in cheese making is one of the oldest and most deliberate applications of microbiology in food production. Two species stand out in particular: Penicillium camemberti and Penicillium roqueforti.

Penicillium camemberti is responsible for the characteristic white, bloomy rind of soft cheeses like Camembert and Brie. It grows on the surface of the cheese, where its proteolytic and lipolytic enzymes gradually soften the texture from the outside in, developing the creamy interior and mild, earthy flavour these cheeses are known for.

Penicillium roqueforti is used in the production of blue-veined cheeses such as Roquefort, Gorgonzola, and Stilton. These moulds introduce veins of blue or green growth throughout the cheese, lending it its unique pungent taste and appearance. The mould is introduced either by adding spores to the milk before cheese making, or by inoculating the young cheese, and then the cheese is aged under conditions that allow the mould to grow throughout its interior structure.

Several mould species are used for ripening of various types of cheese, enzyme production, and dairy waste management. Their proteolytic and lipolytic activities are not side effects – they are the whole point. The breakdown of casein proteins by mould enzymes releases peptides and free amino acids that are directly responsible for the sharp, complex flavours associated with aged cheeses. Fat breakdown releases short-chain fatty acids that give blue cheeses their characteristic pungency.

Undesirable moulds: spoilage and contamination 🔗

Not all mould presence in dairy is deliberate. A wide range of mould species contaminate dairy products from the environment – from air, processing equipment, packaging materials, and handlers. Up to 100 mould species have been identified as being responsible for dairy product spoilage, including Penicillium and Mucor species. Penicillium is by far the most frequently reported spoilage genus, with around 40 species identified, followed by Aspergillus with 10 species. These genera are isolated from hard and semi-hard cheeses, but also from butter, yoghurt, and various milk types.

Spoilage moulds typically produce visible coloured patches – black, green, or pink – on dairy product surfaces. They also cause off-flavours, softening, and structural breakdown that make products unmarketable. Fungal contamination causes approximately 5-10% of food product losses globally, with significant economic consequences – a figure that underscores why mould control in dairy facilities is a major operational priority.

Mycotoxins: the hidden health hazard 🔗

The most serious concern associated with spoilage moulds is not the visible contamination itself – it is what certain moulds produce as metabolic by-products: mycotoxins. These are toxic secondary metabolites that can persist in dairy products even after the mould that produced them is destroyed by heat or other processing.

Mycotoxins appear in the food chain as a result of mould infection both before and after harvest. Exposure can occur directly by eating contaminated food, or indirectly through animals fed contaminated feed – particularly via milk. The effects range from acute poisoning to long-term health consequences including immune deficiency and cancer. The WHO and FAO, through their joint expert committee JECFA, are responsible for evaluating and regulating mycotoxin risk in foods globally.

The main mycotoxins of concern in dairy include:

• Aflatoxins – produced by Aspergillus flavus and Aspergillus parasiticus, aflatoxins are among the most potent naturally occurring carcinogens. They can reach dairy products indirectly when dairy animals consume contaminated feed; aflatoxin B1 in feed is converted to aflatoxin M1 in milk.
• Ochratoxin A – produced by several Aspergillus and Penicillium species, it is associated with kidney damage and has been detected in various dairy products.
• PR toxin and roquefortine C – produced by Penicillium roqueforti, these mycotoxins are of concern in silage fed to dairy cows and, in some contexts, in improperly produced cheese.

The invisible nature of mycotoxins makes them particularly dangerous. A product can look and smell normal while containing harmful levels of these compounds. It is only in relatively recent times that several mycotoxins have been regarded as a major threat to human and animal health, especially in developing countries. This is why food safety protocols in dairy processing focus heavily on preventing mould contamination at every stage – from farm to packaging – rather than attempting to detect mycotoxins after they have already formed.

Managing fungi in dairy: balancing benefit and risk 🔗

The central challenge in dairy microbiology is that the boundary between beneficial and harmful fungal activity is often the same organism under different conditions. Penicillium roqueforti, used intentionally to make blue cheese, also appears on spoilage lists when it contaminates other products. Kluyveromyces marxianus is a valued starter organism in kefir production but can become a spoilage agent in regular dairy products if it grows unchecked.

Control strategies include strict temperature management, modified atmosphere packaging (MAP), the use of antimould agents such as sorbates, and rigorous application of Good Manufacturing Practices (GMP) and Hazard Analysis Critical Control Points (HACCP) throughout the production chain. In the dairy environment, mould spores become airborne and contaminate products after heating, which means post-processing hygiene – during bottling, packaging, and storage – is just as critical as controlling the production environment itself.

On the beneficial side, commercial cultures of yeast have become increasingly popular for producing fermented milk, cheese, and ethanol from whey and lactose solutions, reflecting a growing scientific understanding of how to harness fungal activity with precision. As detection technologies and starter culture development advance, the ability to deliberately manage both yeast and mould populations in dairy production continues to improve.

What do you think? Given that the same mould species can be both a valued ingredient in artisanal cheese and a serious spoilage organism in other dairy products, how should dairy producers draw the line between intentional and unintentional fungal activity in their facilities? And with mycotoxins being invisible and heat-stable, do you think current dairy safety protocols adequately address the risk they pose to consumers?

References

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