FOOD MICROBIOLOGY: FUNGI IN FOOD
YEAST
Single-celled fungus, 3 to 10 micrometres. Reproduces by budding. Ferments sugar to C₂H₅OH and CO₂.
MOULD
Multicellular fungus. Grows as branching threads (hyphae) forming a fuzzy mycelium. Reproduces by airborne spores.
Figure 1: Yeasts are single cells that bud, while moulds are thread-like colonies that spread by spores.
Introduction
Yeast and mould are the two forms of fungi found in food, and they differ in form, growth and the spoilage they cause. Yeasts are single cells that ferment sugars, while moulds are thread-like colonies that grow across food surfaces and spread by spores.
Quick comparison
Quick comparison: Yeast and Mould
| Aspect | Yeast | Mould |
|---|---|---|
| Form | Single-celled fungus | Multicellular fungus growing as hyphae |
| Size | One cell, usually 3 to 10 micrometres | Colonies visible to the eye as fuzzy mats; single hyphae need a microscope |
| Reproduction | Mostly by budding; some by fission; many have a sexual cycle with spores in a sac (ascus) | Mainly by asexual spores (conidia or sporangiospores) carried by air; also by hyphal fragments |
| Oxygen need | Many grow with or without oxygen; Saccharomyces cerevisiae ferments without oxygen | Need oxygen; strict aerobes that grow on surfaces |
| Water activity | Grow down to about aw 0.85 | Grow down to about aw 0.70, so they spoil drier foods |
| Acid tolerance | Grow well in acid foods down to pH 2 or 3 | Grow well in acid foods down to pH 2 or 3 |
| Visible sign | Cloudiness, bubbles, fizz, alcohol smell, swollen containers | Fuzzy coloured patches: green, black, blue or white, from the spores |
| Food examples | Saccharomyces cerevisiae in bread, beer and wine; spoilage yeasts in juice, jam and pickles | Aspergillus on grains and nuts; Penicillium on citrus and cheese; Rhizopus as black fuzz on bread and fruit |
| Toxin risk | Few yeasts make toxins; most food yeasts are harmless | Some moulds make mycotoxins, such as aflatoxins, that survive cooking |
Scientific explanation
Definition
Yeasts are single-celled fungi, eukaryotes with a true nucleus. A yeast cell is larger than a bacterium, usually 3 to 10 micrometres, with a cell wall of glucans and mannans with some chitin. Moulds are multicellular fungi that grow as branching threads called hyphae; a mass of hyphae is a mycelium, the visible fuzzy mat on spoiled bread or fruit.
Working principle
Yeasts ferment sugars: under oxygen Saccharomyces cerevisiae respires and grows fast, and without oxygen it ferments sugars to C₂H₅OH and CO₂, which raises dough and makes alcohol in brewing. In spoilage the same fermentation makes bubbles, alcohol smells and swollen juice bottles. Moulds grow by extending hyphae into and across food and by releasing millions of airborne spores from the tips of aerial hyphae, so one colony can seed a whole storage room.
Apparatus
Both are grown on media such as potato dextrose agar and incubated at 25 to 30 °C. A microscope shows yeast buds and mould hyphae: Aspergillus makes chains of conidia on a swollen head, Penicillium makes brush-like clusters, and Rhizopus makes spores inside a round sporangium on a stalk.
Measurement procedure
Yeast and mould counts use spread or pour plates on a fungal medium, incubated at 25 to 30 °C for several days, with colonies counted in the 30 to 300 range and reported as CFU/g or CFU/ml.
Factors affecting readings
Sugar and acid favour yeasts; dry surfaces and air favour moulds. Temperature, water activity and the medium change the count, and mould colonies spread fast, so plates must be read before colonies merge.
Formulas and calculations
This topic has no standard formula. Counts for yeasts and moulds use the plate count formula, CFU/g = colonies counted divided by (volume plated in ml x dilution factor).
Industrial applications
- Yeasts: bread, beer, wine and fermented milks; yeast extracts and flavour production.
- Moulds: blue-veined and soft-ripened cheese; soy sauce, rice wine and mycoprotein; citric acid and enzyme production.
- Both: spoilage testing of fruit, juice, jam, bread, grain and cheese.
Advantages and limitations
Yeast: advantages are fast fermentation, reliable leavening and alcohol production, and growth in acidic foods; limitations are spoilage of juice, jam, pickles and syrups by gas and alcohol, and yeasty off flavours in yogurt kept too long.
Mould: advantages are cheese ripening, fermented foods and enzyme production, plus visible growth that is easy to spot; limitations are spoilage of bread, fruit and grain, fast spread by airborne spores, and mycotoxins that cooking cannot remove.
FAQ
Is yeast a bacterium?
No. Yeast is a single-celled fungus, a eukaryote with a true nucleus, while bacteria are prokaryotes with no true nucleus.
Why does mould grow on bread but yeast does not spoil it the same way?
Mould needs oxygen and grows on the surface, even in dry food, while yeast needs sugar and water and ferments inside moist, sugary food. Bread crust is dry and open to air, which suits mould.
Why do yeasts spoil fruit juice more than bacteria do?
Fruit juice is acidic, often below pH 4.5, which slows most bacteria, while yeasts tolerate acid well and ferment the sugars.
What is the difference between septate and coenocytic hyphae?
Septate hyphae are divided by cross walls, as in Aspergillus and Penicillium; coenocytic hyphae are one long tube with many nuclei and no cross walls, as in Rhizopus.
References and further reading
FTN subjects used: Food Microbiology (Units 1, 2, 3, 4, 5 and 8) and General Microbiology (Units 1, 2, 3, 4, 5, 6 and 10), from the notes at ~/workspace/food-tech-notes/notes/.
Books used: Adams, M. R. and Moss, M. O., Food Microbiology, Royal Society of Chemistry, 2007 (book on disk at food-tech-notes/books/Food-Microbiology.pdf; Google Books page: https://books.google.com/books?id=BZNKI088iooC). Ray, B. and Bhunia, A., Fundamental Food Microbiology, 5th edition, as cited in the FTN notes. Essentials of Industrial Microbiology (book on disk at food-tech-notes/books/Essentials-of-Industrial-Microbiology.pdf).
Google Books pages used: Food Microbiology by Adams and Moss, https://books.google.com/books?id=BZNKI088iooC (contents and chapter list read on 2026-11-11).
