Yeasts in Food: Role and Impact on Food Products

Yeasts are among the oldest microorganisms used in food production. These single-celled fungi have been quietly shaping what we eat and drink for thousands of years – from the bread on your table to the beer in your glass. But yeasts are not always helpful. Under certain conditions, these same organisms can spoil foods, and a few species even pose health risks. Understanding yeasts in food is essential for anyone studying food science, food safety, or meat science.

What are yeasts? 🔗

Yeasts are unicellular fungi that belong to the kingdom Fungi. Unlike their multicellular relatives such as mushrooms and moulds, yeasts exist as individual cells. They are typically oval or spherical, measuring about 3-5 micrometres in diameter – much larger than most bacteria, but still invisible to the naked eye.

Most yeasts reproduce asexually through a process called budding, where a small daughter cell forms on the surface of the parent cell and eventually breaks away. Some species can also reproduce by binary fission (simple cell division). According to the FAO, there are roughly 500 known species of yeasts and yeast-like fungi, though only a handful are commonly associated with food production.

Yeasts are widely distributed in nature – found in orchards, vineyards, soil, air, and even in the intestinal tracts of animals. Some yeasts can produce pigments (green, yellow, or black), and others are capable of synthesising B-group vitamins, making them nutritionally significant as well.

How yeasts work: the basics of fermentation 🔗

The most important biochemical activity of yeasts, from a food perspective, is fermentation. Yeasts thrive on sugars and starches, and when they metabolise simple sugars like glucose and fructose, they produce two key byproducts: ethyl alcohol (ethanol) and carbon dioxide (CO₂).

This reaction can be summarised as:

Glucose → Ethyl Alcohol + Carbon Dioxide

This process, known as alcoholic fermentation, occurs under anaerobic (oxygen-free) conditions. When oxygen is abundant, yeasts use it for growth and energy production through aerobic respiration. But in the absence of oxygen, they switch to fermentation. This fundamental metabolic flexibility is what makes yeasts so useful in food production.

As documented in research published by PMC, Louis Pasteur was the first scientist to demonstrate in the 1850s and 1860s that fermentation was carried out by living cells, laying the foundation for modern food microbiology.

Beneficial roles of yeasts in food production 🔗

Yeasts are indispensable in the food industry. Their fermentation activity is responsible for the production of a wide range of everyday foods and beverages.

Bread making 🔗

In baking, Saccharomyces cerevisiae (commonly called baker’s yeast) is the key organism. When added to dough, the yeast ferments the sugars present in flour, producing carbon dioxide gas. This gas gets trapped in the gluten network of the dough, causing it to rise and giving bread its soft, airy texture. The small amount of alcohol produced during this process evaporates during baking.

Baking yeast strains are specifically cultivated to be aggressive and fast-acting, designed to carbonate dough as quickly as possible. Sourdough bread, on the other hand, relies on natural or spontaneous fermentation, where wild yeasts present in the environment work alongside lactic acid bacteria to create its distinctive tangy flavour.

Beer and wine production 🔗

Alcoholic beverage production is perhaps the oldest and largest application of yeast fermentation. In winemaking, yeasts convert the sugars in grape juice (must) into ethanol and CO₂. Saccharomyces cerevisiae dominates wine fermentation because it can tolerate alcohol concentrations up to 15% or more – far higher than other yeast species, which typically die off at 5-8% alcohol.

Beer production involves two main yeast species. Saccharomyces cerevisiae is used for top-fermenting ales, while Saccharomyces pastorianus is preferred for bottom-fermenting lagers. During fermentation, these yeasts produce not only ethanol and CO₂ but also hundreds of secondary metabolites that shape the aroma and taste of the final product. As explained by Explore Yeast, yeasts are responsible for approximately 80% of the aromatic compounds detectable in wine.

Other fermented foods 🔗

Beyond bread and alcohol, yeasts play roles in the production of many other fermented products. These include cheese, yogurt, soy sauce, kefir, kombucha, and even chocolate and coffee. In many of these foods, yeasts work in combination with lactic acid bacteria or moulds to develop flavour, texture, and nutritional value. According to a review in Springer, yeasts also help reduce anti-nutritional factors and enhance health-promoting properties in fermented food products.

Conditions that favour yeast growth 🔗

Understanding the environmental conditions that support or inhibit yeast growth is critical for both promoting beneficial fermentation and preventing spoilage.

Temperature 🔗

Yeasts are active across a broad temperature range – from 0°C to 50°C. However, the optimum growth temperature for most food-related species is between 20°C and 30°C. Many yeasts can still grow slowly at refrigeration temperatures, which is why cold storage alone does not always prevent yeast-related spoilage.

pH and acidity 🔗

Unlike most bacteria, yeasts are remarkably acid-tolerant. They can grow at pH levels as low as 4.0 to 4.5, and some species thrive even at pH 2.0. This acid tolerance is one reason why yeasts are the primary spoilage organisms in acidic foods such as fruit juices, pickles, and fermented products.

Water activity and sugar tolerance 🔗

Normal yeasts require a minimum water activity (aw) of about 0.85 – lower than what most bacteria need (above 0.95). This means yeasts can grow in intermediate-moisture foods where bacteria cannot. Yeasts are also fairly tolerant of high sugar concentrations, growing well in solutions with up to 40% sugar. A special group called osmophilic yeasts can survive even in sugar concentrations of 65-70%, though they grow very slowly under such conditions. As the FAO notes, species like Zygosaccharomyces rouxii can also tolerate high salt concentrations, making them relevant in salt-based fermentations.

Yeasts as food spoilage organisms 🔗

The same metabolic capabilities that make yeasts valuable in fermentation can also make them agents of spoilage when they appear uninvited in the wrong foods.

Which foods are vulnerable? 🔗

Yeasts have a particular affinity for high-sugar foods. Honey, jams, jellies, fruit juices, syrups, and dried fruits are all prime targets for yeast spoilage. When yeasts contaminate these products, they ferment the available sugars, producing alcohol, CO₂, and off-flavours that make the food unpalatable.

A brief published by the Food Research Institute at the University of Wisconsin identifies four main groups of spoilage yeasts. Among them, Zygosaccharomyces species are the most significant – they tolerate high sugar and high salt concentrations and are the usual spoilage organisms in honey, dried fruit, jams, and soy sauce. These yeasts grow slowly, producing off-odours and CO₂ that can cause food containers to swell and even burst.

Other notable spoilage yeasts include Debaryomyces hansenii, which can grow in salt concentrations as high as 24%, and certain Saccharomyces strains that spoil wines and alcoholic beverages by creating turbidity, gassiness, and unpleasant flavours.

Signs of yeast spoilage 🔗

Yeast spoilage typically shows several telltale signs: a slightly alcoholic or “winey” smell, visible surface films, cloudiness in liquids, or white to cream-coloured growth on food surfaces. In sealed containers, the CO₂ produced by fermenting yeasts can cause swelling or bulging of packaging – a clear warning sign that the product has been compromised.

Honey spoilage is a common example. While pure, low-moisture honey has natural antimicrobial properties, honey with higher moisture content can support the growth of osmotolerant yeasts, leading to fermentation that produces a sour, alcoholic taste.

Pathogenic yeasts and food safety 🔗

Compared to bacteria, pathogenic yeasts are far less common as causes of foodborne illness. For the general population, most yeasts found in food do not cause serious infections. However, certain species are recognised as opportunistic pathogens, particularly dangerous for people with weakened immune systems.

Candida species 🔗

Candida albicans is the most well-known pathogenic yeast. It is part of the normal human gut and mouth flora, and about 80% of people will never experience any harmful symptoms from it. However, in immunocompromised individuals – such as those with HIV/AIDS, cancer patients, or people on immunosuppressive drugs – C. albicans can cause opportunistic oral, intestinal, and genital infections. In severe cases, it can lead to life-threatening systemic candidiasis. Other pathogenic Candida species include C. tropicalis, C. krusei, C. glabrata, and C. parapsilosis, as documented by ScienceDirect research.

Cryptococcus neoformans 🔗

Cryptococcus neoformans is another notable pathogenic yeast. According to the Public Health Agency of Canada, this spherical yeast (4-6 µm in diameter) produces a protective polysaccharide capsule that helps it evade the host immune system. It can cause cryptococcosis – a disease that primarily affects the lungs and central nervous system, potentially leading to fatal meningitis if left untreated. Unlike Candida, Cryptococcus infections are typically acquired through inhalation from environmental sources rather than through food.

Emerging concerns 🔗

Research increasingly shows that the line between “safe” and “pathogenic” yeasts is not always clear-cut. Some studies have found that even Saccharomyces cerevisiae – traditionally considered safe (GRAS status) – can act as an opportunistic pathogen in rare cases, particularly in immunosuppressed patients. As noted in a Frontiers in Nutrition study, the status of Saccharomyces has evolved from “generally recognised as safe” to being acknowledged as a potential opportunistic pathogen of low virulence, though the number of clinical cases remains very small.

Controlling yeasts in food 🔗

Effective yeast control in food production relies on a combination of physical, chemical, and environmental strategies.

Heat treatment 🔗

Most yeasts are killed at temperatures between 60°C and 71°C (140-160°F). This makes pasteurisation one of the most effective methods for eliminating yeasts from fruit juices, dairy products, and other susceptible foods. Unlike bacterial spores, yeast cells are relatively heat-sensitive, making thermal processing highly reliable.

Chemical preservatives 🔗

Food manufacturers commonly use preservatives such as potassium sorbate and sodium benzoate to inhibit yeast growth in processed foods. These chemicals interfere with yeast metabolism or cell membrane function, preventing reproduction and spoilage.

Controlling the environment 🔗

Reducing the moisture content of food through dehydration makes it difficult for yeasts to grow. Proper refrigeration slows down yeast metabolism and reproduction. In products like jams and jellies, maintaining high sugar concentrations creates an osmotic environment that limits microbial growth – as explained by the NDSU Extension Service, reducing sugar in jam recipes can allow yeasts and moulds to grow. Proper sealing and packaging that limits oxygen exposure is also important, as many spoilage reactions require air to proceed.

Good manufacturing practices 🔗

In food production facilities, maintaining strict hygiene and sanitation protocols is essential. This includes regular cleaning of equipment, monitoring raw materials for contamination, and controlling storage conditions to minimise yeast proliferation.

Yeasts in meat and meat products 🔗

While yeasts are more commonly associated with plant-based foods and beverages, they also appear in meat systems. Common yeast genera found in fresh and refrigerated meats include Candida, Cryptococcus, Debaryomyces, Rhodotorula, and Trichosporon. Debaryomyces hansenii is frequently the most commonly isolated yeast from meat products, contributing to flavour development in fermented sausages and cured meats where it is deliberately used as a starter culture.

However, uncontrolled yeast growth in fresh meat can cause surface sliminess, off-odours, and discolouration. Since yeasts can grow both aerobically and anaerobically, controlling their contamination in meat is an important priority in food safety and preservation.

What do you think? Given that many of the yeasts found in our food can be both helpful and harmful, how should food producers balance the use of beneficial yeasts while managing the risks of spoilage and pathogenic strains? And with the rise of engineered yeast strains for novel food production, what new challenges might emerge for food safety regulation?

References

1. https://www.fao.org/4/x0560e/x0560e08.htm
2. https://pmc.ncbi.nlm.nih.gov/articles/PMC7466055/
3. https://pmc.ncbi.nlm.nih.gov/articles/PMC10301770/
4. https://www.exploreyeast.com/yeast-and-fermentation/what-is-the-role-of-yeast-in-fermentation/
5. https://link.springer.com/chapter/10.1007/978-981-10-2621-8_4
6. https://fri.wisc.edu/files/Briefs_File/2017-07-18_0857_FRI_Brief_Microbial_Food_Spoilage_7_07.pdf
7. https://www.sciencedirect.com/science/article/abs/pii/S0924224420306968
8. https://www.canada.ca/en/public-health/services/laboratory-biosafety-biosecurity/pathogen-safety-data-sheets-risk-assessment/cryptococcus-neoformans.html
9. https://pmc.ncbi.nlm.nih.gov/articles/PMC6388tried/
10. https://www.ndsu.edu/agriculture/extension/publications/food-preservation-jellies-jams-and-spreads