Yeasts: Useful and Harmful Roles in Food Processing

Yeasts are among the most fascinating microorganisms in the food world. These tiny, single-celled fungi have been shaping what we eat and drink for thousands of years – from the bread on our table to the beer in our glass. But yeasts are not always our allies. In the wrong setting, they can spoil fruit juices, dairy products, and syrups just as effectively as they can ferment dough or grape must. Understanding their biology, their types, and the environmental conditions that govern their growth is essential for anyone working in or studying food science.

What exactly are yeasts? 🔗

Yeasts are unicellular fungi – meaning each yeast organism consists of just a single cell. They are eukaryotic, which means their cells contain a defined nucleus and membrane-bound organelles, much like human cells. Most yeast cells are oval or round in shape and typically measure between 3 and 10 micrometres in diameter, far too small to see with the naked eye.

The defining feature of yeasts is their primary mode of reproduction: budding. During budding, a small outgrowth (the “bud”) forms on the parent cell. The nucleus divides, one copy migrates into the bud, and the bud eventually pinches off to become a new, independent cell. Under favourable conditions, some yeast species can double their population in as little as 90 minutes. In certain species, the buds do not fully detach and instead remain connected in chains, forming structures called pseudohyphae (false hyphae). This ability to shift between single-celled and chain-like forms is one reason yeasts are so adaptable.

True yeasts vs. false yeasts (pseudo-yeasts) 🔗

Not all organisms called “yeasts” are the same. In food microbiology, a useful distinction is drawn between true yeasts and false yeasts (also known as pseudo-yeasts or yeast-like fungi).

True yeasts (Saccharomycetales) 🔗

True yeasts belong to the order Saccharomycetales within the phylum Ascomycota. They are permanently unicellular, reproduce by budding, and – critically – can produce sexual spores called ascospores. The most well-known representative is Saccharomyces cerevisiae, commonly called baker’s yeast or brewer’s yeast. True yeasts are the workhorses of the fermentation industry. They are highly efficient at converting sugars into ethanol and carbon dioxide, which is exactly the biochemistry behind bread, beer, and wine.

False yeasts (pseudo-yeasts) 🔗

False yeasts, or pseudo-yeasts, are fungi that grow in a yeast-like unicellular form but can also develop filamentous (mould-like) structures under certain conditions. They are considered dimorphic – capable of switching between a unicellular budding form and a filamentous hyphal form depending on temperature, nutrient availability, or other environmental cues. Unlike true yeasts, many false yeasts do not produce ascospores. Genera such as Candida and Rhodotorula are often classified as false yeasts. Some members of this group are significant in food spoilage, and certain species like Candida albicans can even cause human infections.

From a food industry standpoint, the distinction matters because true yeasts are generally the species deliberately used in fermentation, while many false yeasts are more commonly encountered as contaminants and spoilage organisms.

The beneficial side: yeasts in food fermentation 🔗

Yeasts have been humanity’s partners in food production for millennia. Archaeological evidence suggests that fermented beverages have been part of human culture for close to 7,000 years. The species at the centre of nearly all major food fermentation processes is Saccharomyces cerevisiae.

Bread making 🔗

In bread production, S. cerevisiae ferments the sugars present in flour dough and releases carbon dioxide gas. This gas gets trapped in the gluten network of the dough, causing it to rise and giving bread its characteristic soft, airy texture. The small amount of ethanol produced during this process evaporates during baking. Bakers control fermentation by adjusting dough temperature, hydration, and proofing time to get the desired volume and crumb structure.

Beer brewing 🔗

Beer production relies on yeast fermenting sugars extracted from malted grains, primarily barley. S. cerevisiae is involved in producing many fermented beverages, including wine, beer, and cider, as well as distilled spirits like rum, whisky, and brandy. In brewing, two broad categories of yeast are used: top-fermenting ale yeasts (S. cerevisiae), which work at warmer temperatures (15-25°C) and tend to produce fruitier flavours, and bottom-fermenting lager yeasts (S. pastorianus), which operate at cooler temperatures (5-10°C) and yield cleaner, crisper profiles.

Winemaking 🔗

In winemaking, yeasts ferment the natural sugars in grape juice (must) into alcohol and carbon dioxide. While spontaneous fermentation driven by wild yeasts present on grape skins still occurs in some traditions, most commercial wineries now inoculate with selected S. cerevisiae strains. These strains are chosen for their ability to tolerate high sugar concentrations, low pH, and rising ethanol levels. They also produce specific flavour compounds – esters, higher alcohols, and fatty acids – that contribute to the wine’s sensory profile.

Other fermented foods 🔗

Beyond bread, beer, and wine, yeasts play important roles in the production of soy sauce, cocoa, coffee, kefir, and kombucha. In many of these products, yeasts work alongside bacteria (especially lactic acid bacteria) to create complex flavour profiles through co-fermentation. The metabolic versatility of yeasts – their ability to ferment a wide range of sugars including glucose, fructose, sucrose, and maltose – makes them suitable for processing diverse raw materials.

The harmful side: yeasts as food spoilage agents 🔗

When yeasts appear uninvited in food products, they become spoilage organisms. Natural yeast populations in raw ingredients and environmental contamination in manufacturing facilities are the main sources of food contamination. Once established, yeasts ferment available sugars, producing ethanol, carbon dioxide, organic acids, and off-flavour compounds that make food unacceptable to consumers.

Fruit juices and syrups 🔗

Fruit juices and syrups are highly vulnerable to yeast spoilage because they offer exactly what yeasts need: high sugar content, sufficient moisture, and an acidic pH that yeasts tolerate easily. Signs of yeast spoilage in these products include fizzing or carbonation (from CO₂ production), a slight alcoholic or “winey” smell, cloudiness, and swollen or bulging packaging. Species of Zygosaccharomyces, known for their tolerance to high-sugar and high-acid environments, are among the most problematic spoilage yeasts in fruit-based products.

Dairy products 🔗

Dairy products are also susceptible. Yeasts can ferment lactose or other residual sugars in milk, yoghurt, and cheese, generating off-flavours (described as yeasty, fruity, or alcoholic), gas, and undesirable texture changes. Yeasts are particularly problematic in dairy because some species can grow slowly even at refrigeration temperatures, making it difficult to prevent spoilage through cold storage alone.

Fermented vegetables and other products 🔗

Yeasts are essential in certain fermented vegetable products, but they can also act as spoilage agents that cause significant quality defects. In products like table olives, pickles, and sauerkraut, unwanted yeast growth can produce surface films, softening, off-flavours, and gas. Bakery items with high moisture content, honey, and condiments are also at risk.

Environmental factors that affect yeast growth 🔗

Whether yeasts help or harm food depends largely on the environmental conditions present. Food microbiologists manipulate these factors to either encourage desirable fermentation or prevent spoilage.

Moisture and water activity (aw) 🔗

Yeasts require moisture to grow, but they are more tolerant of low water activity than most bacteria. While most bacteria need a water activity above 0.91, some yeasts and moulds can grow at water activity values as low as 0.80-0.85. Certain osmotolerant species can even thrive in very high-sugar environments like honey, jams, and concentrated syrups. This is why reducing water activity alone may not be sufficient to prevent yeast spoilage in sugar-rich foods.

Temperature 🔗

Most yeasts grow best at moderate temperatures, roughly between 20°C and 30°C. However, many species – particularly those responsible for food spoilage – can grow at refrigeration temperatures (4-10°C), albeit more slowly. Very few yeasts survive above 50-60°C, which is why pasteurisation is effective at eliminating them from beverages and liquid foods. In fermentation, temperature is a critical control point: bakers, brewers, and winemakers carefully regulate it to manage the rate and character of yeast activity.

pH and acidity 🔗

Yeasts are notably acid-tolerant. They tolerate acidic environments with pH values around 3.5 or even lower, which gives them a competitive advantage over most bacteria in acidic foods. This acid tolerance explains why yeasts are the dominant spoilage organisms in fruit juices, wine, pickled products, and fermented foods. It also means that acidification – a strategy effective against many bacteria – often fails to prevent yeast growth.

Oxygen availability 🔗

Yeasts are facultative anaerobes: they can grow in the presence or absence of oxygen. In the presence of oxygen, yeasts respire aerobically and multiply rapidly, producing biomass. In the absence of oxygen (or when oxygen is limited), they switch to anaerobic fermentation, converting sugars mainly into ethanol and carbon dioxide. This metabolic flexibility is what makes yeasts so successful both as fermentation agents and as spoilage organisms. Controlling oxygen exposure – through modified atmosphere packaging, for instance – can help manage yeast growth in some products.

Nutrient availability 🔗

Yeasts feed primarily on simple sugars. Foods with high sugar content (fruits, juices, syrups, honey) naturally support yeast growth. However, yeasts can also utilise organic acids and some amino acids as carbon sources, broadening the range of foods they can spoil. Nitrogen sources, vitamins (particularly B vitamins), and minerals also influence how quickly yeasts multiply.

Key yeast species in the food industry 🔗

A few species stand out for their importance – whether positive or negative – in food processing.

Saccharomyces cerevisiae 🔗

This is the single most important yeast species in food production. It has been employed for thousands of years in baking, brewing, and winemaking due to its efficient sugar fermentation, ethanol tolerance, and adaptability. Different strains have been selected for specific applications – some optimised for rapid CO₂ production in baking, others for flavour development in brewing or winemaking.

Zygosaccharomyces bailii 🔗

One of the most notorious food spoilage yeasts. Z. bailii can survive and grow in conditions that would kill most other microorganisms – high sugar, high acid, and even in the presence of common preservatives like sorbic acid and benzoic acid. It is a major concern for manufacturers of soft drinks, fruit juices, salad dressings, and sauces.

Candida species 🔗

Several Candida species are found in dairy products, meat, and fermented foods. While some contribute to ripening in certain cheeses, others cause spoilage. Some Candida species can also enter the human body through food and may cause infections, particularly in immunocompromised individuals.

Brettanomyces/Dekkera 🔗

These yeasts are associated with both desirable and undesirable outcomes in the beverage industry. In some Belgian lambic beers, Brettanomyces contributes to the characteristic sour, funky flavour. In wine, however, the same organisms can produce phenolic compounds that create medicinal or barnyard-like off-flavours, making them unwanted contaminants.

Controlling yeast spoilage in the food industry 🔗

Preventing unwanted yeast growth requires a multi-pronged approach:

Good manufacturing practices (GMPs) form the foundation. Proper sanitation of equipment and facilities, clean raw materials, and hygienic handling all reduce the chances of yeast contamination entering the production chain.

Thermal processing, including pasteurisation, effectively kills vegetative yeast cells. This is standard practice for commercially produced fruit juices, dairy products, and many beverages.

Chemical preservatives such as sorbic acid, benzoic acid, and sulphur dioxide inhibit yeast growth. However, some highly resistant species like Z. bailii can tolerate these preservatives, which is why they must be used alongside other control measures.

Modified atmosphere packaging that limits oxygen availability can slow aerobic yeast growth, though it will not prevent fermentation under anaerobic conditions.

Refrigeration slows yeast growth but does not stop it entirely. Proper cold chain management extends shelf life but is most effective when combined with other preservation strategies.

Emerging approaches include the use of biopreservatives – protective microbial cultures or their metabolites that inhibit spoilage yeasts. For instance, certain killer yeasts produce toxins that are lethal to spoilage yeast species while being safe for consumption. This biocontrol strategy is gaining interest as consumers demand fewer chemical additives in food.

Why yeasts matter for food science students and professionals 🔗

Yeasts sit at a unique crossroads in food microbiology. The same fundamental biology – sugar fermentation producing ethanol and CO₂ – can be either a valued industrial process or an unwanted spoilage event. The difference comes down to control: which species is present, in what quantity, and under what conditions.

For food processors, this means understanding yeast physiology is not optional. Selecting the right yeast strain for fermentation, maintaining strict hygiene to prevent contamination, and manipulating environmental factors like temperature, pH, water activity, and oxygen all require a solid grounding in yeast biology. As the food industry continues to move toward cleaner labels and fewer synthetic preservatives, knowledge of yeast ecology and biocontrol methods will become even more valuable.

What do you think? Given that some of the most troublesome spoilage yeasts can resist common preservatives and grow at refrigeration temperatures, what innovative strategies might the food industry develop to better control these organisms while meeting consumer demand for minimally processed products?

References

1. https://www.britannica.com/science/Saccharomyces-cerevisiae
2. https://en.wikipedia.org/wiki/Yeast
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4. https://pmc.ncbi.nlm.nih.gov/articles/PMC7466055/
5. https://pmc.ncbi.nlm.nih.gov/articles/PMC7099199/
6. https://www.mdpi.com/2076-3417/14/24/11698
7. https://pubmed.ncbi.nlm.nih.gov/30056262/
8. https://foodsafety.institute/food-microbiology/micro-organisms-in-food-spoilage/
9. https://link.springer.com/article/10.1007/s00217-021-03888-7
10. https://www.sciencedirect.com/topics/agricultural-and-biological-sciences/alcoholic-fermentation
11. https://www.sciencedirect.com/science/article/abs/pii/S0924224420306968