YEAST AND MOLDS IN FOOD
The large and diverse group of microscopic foodborne
yeasts and molds (fungi) includes several hundred
species. The ability of these organisms to attack many
foods is due in large part to their relatively versatile
environmental requirements. Yeasts tend to grow
within food and drink matrices in planktonic form and
they tend to ferment sugars, growing well under
anaerobic conditions. Molds, on the other hand, tend
to grow on the surface of objects in the shape of a
visible ‘mycelium’ made up of many cells.
Both yeasts and molds cause various degrees of
deterioration and decomposition of foods. They can
invade and grow on virtually any type of food at any
time. They invade crops such as grains, nuts, beans,
and fruits in fields before harvesting and during
storage. They also grow on processed foods and
food mixtures.
Several foodborne molds, and possibly yeasts, may
also be hazardous to human or animal health because
of their ability to produce toxic metabolites known as
mycotoxins. Even though the generating organisms
may not survive food preparation, the preformed toxin
may still be present. Certain foodborne molds and
yeasts may also elicit allergic reactions or may cause
infections. Although most foodborne fungi are not
infectious, some species can cause infection,
especially in immunocompromised populations, such
as the aged and debilitated, HIV-infected individuals,
and persons receiving chemotherapy or antibiotic
treatment.
This is particularly problematic in plants producing
high sugar/low water activity/low pH products.
Factories producing fruit products, baked goods,
confectionary, and fermented dairy products can be at
real risk from yeast and mold contamination.
Some Important Food Spoilage Yeast and Mold Species
YEASTS
Species
Foods affected
Brettanomyces bruxellensis
Beer and wine, fruit yoghurts
Debaryomyces hansenii
Cured meats and brined products
Saccharomyces cerevisiae
Soft drinks and fruit juices
Zygosaccharomyces bailii
Soft drinks, sauces, fruit juice, wine, ciders and syrups
Zygosaccharomyces rouxii
Confectionery, fruit concentrates
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Some Important Food Spoilage Yeast and Mold Species
MOLDS
Species
Foods affected
Aspergillus flavus
Nuts and oilseeds (mycotoxin producer – aflatoxins)
Aspergillus niger
Fresh fruit and vegetables, nuts and seeds, dried foods
Aspergillus ochraceus
Dried and stored foods (mycotoxin producer – ochratoxin)
Byssoclamys fulva
Canned fruits
Eurotium chevalieri
Cereals and a wide range of processed and stored foods
Fusrium culmorum
Infects cereals, especially barley, in the field (mycotoxin producer – DON and others)
Penicillium aurantiogriseum
Stored fruits and vegetables
Penicillium commune
Cheese
Penicillium digitatum
Citrus fruits
Rhizopus stolonifer
Fruits, tomatoes and peppers
Wallemia sebi
Dried and salted fish and other dried foods
YEAST
Yeast have long been considered the organism of
choice for the production of alcoholic beverages,
bread, and a large variety of industrial products.
All of these products are currently making a huge
impact in the agriculture and food industry.
Traditionally, yeasts have been very important in the
food industry and nowadays it would be almost
impossible to imagine a world devoid of fermented
products such as wine, beer or cheese. Nevertheless,
given their ability to grow at low pH levels, low water activity and even in the presence of some chemical preservatives,
they have become a classic food contaminant causing huge losses to the food industry as well as illnesses to
consumers. Yeasts are slow growing organisms when compared to bacteria. If yeasts and bacteria were placed in
the same optimum environment and both could grow, it is most likely that the faster growing bacteria would quickly
outgrow and outcompete the slower growing yeast, becoming the dominant flora. However, if we move outside the
‘optimum’ growth conditions of most bacteria, into environments that are acidic, or of low water activity (high in
sugar), then the yeasts have advantage and would rapidly overtake the growth of bacteria. It is in these specialist food
niches that the yeast spoilage has become a problem.
MOLDS
Molds have both positive and negative effects on the food industry the
same way that yeasts do. Some molds are perfectly safe to eat and, in
some cases, even desirable (the classic example would be cheese made
with mold, such as blue, Brie, Camembert, and Gorgonzola). Other molds
can be quite toxic and may produce allergic reactions and respiratory
problems, or produce poisonous substances called mycotoxins.
Aspergillus mold, for instance, which is most often found on meat and
poultry (as well as environmentally), can cause an infection called
aspergillosis, which is actually a group of illnesses ranging from mild to
severe lung infections, or even whole-body infections. One of the greatest
concerns regarding mold in food is the mycotoxins that some varieties
produce. One of the most researched mycotoxins is aflatoxin, a
cancer-causing poison.
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ISO 21527-1
Dextrose Chloramphenicol Agar (YGC) (Cat. 1301).
Selective agars for yeasts and molds usually contain
antibiotics to help suppress bacterial growth. Plates
are typically incubated at 25 ºC for 5 to 7 days and
then examined for the presence of yeast and mold
colonies.
There are a number of ISO horizontal methods for
the enumeration of yeasts and molds in foods and
animal feed. ISO 21527:2008 is published in two
parts. Part 1 relates to food and feed samples with
water activity of >0.95, while part 2 specifically
applies to dried and processed foods with reduced
water activity of <0.95.
Individual yeast and mold species can be isolated from
selective agars by subculturing onto a non-selective
agar such as Malt Extract Agar (Cat .1038) or Potato
Dextrose agar (Cat. 1022). Although it may be
possible to recognise some molds at the genus level
simply by colony morphology and the appearance
of conidia and other features under the
microscope, identification at the species level is
very difficult and requires specialised skills and
experience.
These methods typically employ a surface plating
technique, where a known quantity of the sample, or
the initial suspension, is spread over the surface of a
suitable selective agar medium. ISO 21527-1
recommends
Dichloran
Rose
Bengal
Chloramphenicol Agar (DRBC) (Cat. 1160). Other
media used include Oxytetracycline Glucose Yeast
Extract Agar (OGYE) (Cat. 1527) and Yeast Extract
Rose Bengal Agar + Chloramphenicol +
Dichloran (DRBC Agar) (Cat.1160)
(Peptone /Glucose / Monopotassium
Phosphate Magnesium Sulfate /
Chloramphenicol / Rose Bengal / Dichloran
/ Bacteriological Agar)
Rose Bengal inhibits growth
of bacteria and limits the
size and height of
faster-growing ones molds
Dichloran prevents the fast
spreading of mucoraceous
fungi and restricts size of
the colonies of other
Chloramphenicol
is a wide spectrum
antibiotic
Incubation at 25ºC±1 and observed after 3,4 and 7 days
BIBLIOGRAPHY
Valerie Tournas, Michael E. Stack, Philip B. Mislivec,
Herbert A. Koch and Ruth Bandler. BAM: Yeasts, Molds
and Mycotoxins. U.S. Food and Drug Administration
YeastBook. (2011) A comprehensive compendium of
reviews that presents the current state of knowledge of the
molecular biology, cellular biology, and genetics of the yeast
Saccharomyces cerevisiae, Genetics
Barnett, H.L. 1960. Illustrated Genera of Imperfect Fungi,
2nd ed. Burgess, Minneapolis.
Molds on food: Are they dangerous? U.S. Department of
Agriculture
http://www.fsis.usda.gov/Fact_Sheets/Molds_On_Food/i
ndex.asp. Accessed June 30, 2012.
Lodder, J. 1970. The Yeasts, a Taxonomic Study, 2nd ed.
North-Holland Publishing Co., Amsterdam, The
Netherlands.
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