Yeasts and molds play an important role in many industries by utilizing their metabolic and fermentation properties. They are used to produce foods and beverages through processes like brewing, winemaking, bread making, and soy sauce and miso production. Pharmaceutical products are also developed using fungi, such as the antibiotic penicillin. Enzymes employed in industries like food processing are commonly derived from fungal sources. Overall, fungi have been an important part of human civilization for centuries and continue to have widespread applications.

1. Course Title: Principles of Food
Science and Nutrition
Course No: FDSN 101
Credit hours: 2(2+0)
FOOD MICROBIOLOGY- YEASTS &
MOULDS

2. Fungi which are generally not filamentous but
unicellular and ovoid or spheroid and which reproduce
by budding or fission
Yeast fermentations are involved in
the manufacture of foods such as
bread, beer, wines, vinegar, and
surface ripened cheese, and yeasts are
grown for enzymes and for food.
Yeasts are undesirable when
they cause spoilage of
sauerkraut, fruit juices, syrups;
molasses, honey, jellies, meats,
wine, beer, and other foods.

3. MORPHOLOGICAL CHARACTERS
CULTURAL CHARACTERS
PHYSIOLOGICAL CHARACTERS

4. YEAST- MORPHOLOGICAL CHARACTERS
Yeasts are-
• Eukaryotic
• Single-celled microorganisms
• Fungus kingdom
Morphology:
Size: 3–4 µm in diameter (up to 40 µm)
Shape:
• spherical to ovoid
• lemon shaped, pear
• shaped, cylindrical, triangular

6. Physiological Characteristics ofYeast
greater
• Grow best with a plentiful moisture
• Many yeasts grow in the presence of
concentrations of solutes (such as sugar or salt).
• Require less moisture than most bacteria
• Most yeast requires more moisture than molds

7. Types of Yeast on the basis of water activity
Ordinary Yeast
• Do not grow in high
concentrations of solutes
• Can’t grow in low water
activity food materials
• aw : 0.88 to 0.94
Osmophilic Yeast
• Grow in high concentrations of
solutes
• grow in low water activity food
materials
• May grow slowly in media with
an aw as low as 0.62 to 0.65 in
syrups

8. • Osmophilic yeast aw= 0.6 (<
mold and bacteria)
• Optimum Temp: 25-30 0C
• Optimum pH: 4-4.5
• Lack chlorophyll
• Need Carbon source (eg.
Sugar)
• Source of Nitrogen:
Ammonia
Nitrate
Urea
Protein
Peptide
Amino acid
• Yeasts grow best under aerobic
conditions
• Some are anaerobic
Physiological Characteristics of Yeast

9. Growth factors
Vitamins:
• Some yeast can synthesize all vitamins and some very few
• Riboflavin and folic acid is synthesized by all
• Biotin is the limiting factor in growth medium
Minerals:
• Generally sulfur and phosphorus is needed
Physiological Characteristics of Yeast

10. • Colonies:
moist
slimy
dry
wrinkled
• Colony Colour:
Whitish
cream-colored
pink
Cultural characteristics

11. OxidativeYeast
• oxidize organic acids and alcohol
CH3CH2COOH CH3CH2CH2OH
• Grow at the surface as pellicle or
scum
C6H12O6
Fermentative yeast
• Ferment glucose to ethanol and
carbon dioxide
CH3CH2OH+ CO2
• grow throughout the liquid
Cultural characteristics

12. 1. Mycelium
Production of a mycelium,
pseudo mycelium, or no
mycelium
2. Growth:
Film yeast over surface of a
liquid (film yeasts) or
Growth throughout medium
Classification and identification of yeasts

13. 3. Colour of the colony 4. Physiology:
Source of Nitrogen, carbon
Vitamins requirement
Oxidative or fermentative
Classification and identification of yeasts

14. Yeasts of Industrial importance

15. Genus Saccharomyces
Top Yeast
• Top yeasts are very active
fermenters
• grow rapidly at 20 0C
• The clumping of the cells and the
rapid evolution of CO2 sweep the
cells to the surface, hence the
term top yeast
Bottom yeast
• do not clump
• grow more slowly
• best fermenters at lower
temperatures:10 to 15 0C
• The absence of clumping and the
slower growth and evolution of
CO2 permit the yeast to settle to
the bottom, hence the term
bottom yeast

16. Genus Saccharomyces
• S. cerevisiae var. ellipsoideus is a
high-alcohol-yielding variety used
to produce
industrial alcohol
Wines
distilled liquors
• S. uvarum
bottom yeast
used in making beer
• S.fragilis and S. Lactis
ability to ferment lactose
important in milk or milk
products
• S. rouxii and S. mellis are
osmophilic

17. Genus Zygosaccharomyces
• Osmophilic (high concentrations of sugar)
• Involved in the spoilage of
Honey
Syrups
Molasses
Soy sauce
Wines.
• Zygosaccharomyces nussbaumeri grows in honey
S. rouxii and S. mellis are osmophilic

18. Genus Pichia
• These oval to cylindrical yeasts may form pseudomycelia
• Ascospores are round or hat-shaped
• there are one to four spores per ascus
•A pellicle is formed on liquids
e.g., P. membranaefaciens grows
a pellicle on beers or wines.

19. Genus Debaryomyces
• These round or oval yeasts
• form pellicles on meat brines
• Ascospores have a warty surface.
• D. kloeckeri grows on cheese and sausage.

20. False Yeasts: Fungi Imperfect ??
• Lack Sexual Mode of reproduction
• Example
Genus Torulopsis
Genus Rhodotorula
Genus Candida
Genus Brettanomyces
Genus Kloeckera
Genus Trichosporon
CANDIDA

21. False Yeast-Candida
• Asexual Reproduction by-
budding
Chlamydospores
• spoil foods high in acid and salt
Fruit juice
Pickle
• C. utilis is grown for food and feed
• C. krusei has been grown with
dairy starter cultures for
maintaining the LAB activity
increasing the longevity of
the lactic acid bacteria
• Lipolytic C. lipolytica can spoil
 butter
oleomargarine

22. False Yeast- Torulopsis
• Shape: round to oval
• fermentative yeasts
• Asexual Reproduction:
multilateral budding
• cause trouble in
 breweries
T. sphaerica ferments lactose and
may spoil milk products
• Other species can spoil sweetened
condensed milk
fruit-juice concentrates
acid foods

23. False Yeast- Rhodotorula
• cause discolorations on foods like
Red
Pink- pink areas in sauerkraut
yellow
produce coloured spots on meats

24. MOLD

25. Morphological Characteristics
• multicellular filamentous fungi
• fuzzy or cottony appearance
• The thallus/vegetative body lack
true roots
Stems
leaves

26. Morphological Characteristics
• Hyphae: it is a long, branching filamentous structure in fungi

27. • Mycelium: It is the vegetative part of a fungus consisting of a mass
of branching, thread-like hyphae
Morphological Characteristics

28. Cultural Characteristics
Molds may grow as
loose and fluffy
compact
velvety on the upper
surface
dry and powdery and
wet or gelatinous.

29. • Mycelium may grow coloured
Red
Yellow- P. notatum
Brown- A. flavus
Grey- Mucor, Botrytis cinerea
Black- Cladosporium
 blue-green- P. chrysogenum
Purple
Cultural Characteristics

30. Physiological characteristics
1. Moisture Requirements:
Require less moisture than do most yeasts and bacteria
Growth inhibited below 14 to 15 %
Moisture in flour, dried fruits will greatly delay Mold
growth

31. Physiological characteristics: Temperature requirement
Mesophilic:
• Most of the Mold
• The optimal temperature: 25 to 30
0C
• Some grow well at 35 to 370C
or above, e.g., Aspergillus spp
Thermophilic:
• Very few molds
• High optimal temperature: 60
to 62 0C
• Eg.Thermomyces lanuginosus
Psychrotrophic:
• Grow fairly well at
refrigeration temp
• Only few molds
• Temp: - 5 to - 100C

32. Physiological characteristics
Oxygen Requirement
• Aerobic
• They require oxygen for
growth
pH Requirements
• pH- 2 to 9.5
• the majority are favoured
by an acid pH

33. Classification and identification of molds
1. Hyphae
septate or
non-septate
2. Mycelium
clear or
dark
(smoky)
3. Mycelium
coloured or
colourless

35.  Many fungi are useful to humans and have been exploited both
industrially and commercially.
 Societies have utilized fungi for centuries in a wide variety of ways
by capitalizing on the metabolism and metabolites produced.
 The oldest and best-known example is the use of yeasts performing
fermentation in brewing, wine making and bread making.
 Yeasts and other fungi play a critical role in drug production, food
processing, bio-control agents, enzyme biotechnology, as well as
research and development.

36.  The use of yeast (Saccharomyces cerevisae) to make alcohol and carbon
dioxide uses the fermentation process to break down sugars.
 Up to 50% of the sugar can be converted to alcohol, but rarely surpasses
15% because the fungi are sensitive to high concentrations of alcohol.
 In the beer making industry, cereal grains are fermented to make the final
product.
 Wine is composed of fermented grapes while hard cider is essentially
fermented apples.
 Sake is produced by rice fermentation, using Aspergillus oryzae and then
an additional fermentation step utilizing bacteria and yeasts.

37. A mushroom or toadstool
is the fleshy, spore-
bearing fruiting body of a
fungus, typically
produced above ground,
on soil, or on its food
source.

41. Growth of baker’s yeast under aerobic
conditions – maximizes carbondioxide
production which leavens bread
Other microbes are used to make special
breads ( sourdough breads)

42. Pencillium roquefortii
Strong and pungent flavor to
cheese
Ex: Roquefort, Gorgonzola, Stilton
blue and Danish bue

43. Penicillium camembertii
White crust on the outside of
the cheeses known as brie and
cambert
Strong flavors are a result of the fungus
producing methyl ketones

44.  The production of soy sauce and other fermented soybean
products are likely among the largest industries.
 The peoples of Asia have developed a wide variety of
interesting fermented foods, sauces and drinks, using fungi.

45. Aspergillus
Koji, miso, soy bean
paste
Rhizopus
Sufu, cheese, bean
cheese, tempeh

46. • Authentic soy sauce is
fermented in a three-step process
with the Aspergillus oryzae and
Zygosaccharomyces rouxii, as
well as the bacterium
Pediococcus halophilus.

47.  Aspergillus is utilized industrially in a number of ways.
 Most sodas and soft drinks contain citric acid as a main ingredient.
 Citric acid is also used in other drinks, many candies, canned goods, baked
goods, etc.
 It is too expensive to isolate the citric acid from citrus fruits so it is produced
in large-scale fermentation vats utilizing Aspergillus niger.

48. Organic acid Organism
Citric acid Aspergillus niger,
Pencillium sps.
Oxalic acid Aspergillus niger
Itaconic acid Aspergillus terreus
Kojic acid Aspergillus oryzae

49. Currently there are about 1,600 antibiotics commercially
produced and a number of medical drugs are
manufactured using various fungi.
These multi-billion dollar industries include examples
such as anti-cholesterol statins, the antibiotic penicillin,
the immunosuppressant cyclosporins and steroids.

50. Antibiotic Organism
Penicillin Penicillium notatum
Griseofulvin Penicillium griseofulvum
Cephalosporin Cephalosporium acremonium
Clavicin Aspergillus clavatus

51. A large number of enzymes such as protease, invertase,
maltase, catalase, cellulase, amylase, pectinase and glucose
oxidase are produced by fungi.

52. ENZYME ORGANISM
Maltase Mucor rouxii, Rhizopus
japonicus, Aspergillus niger
Glucose oxidase Penicillium chrsogenum
Pectinase, protease, catalase Aspergillus niger
Invertase Yeast