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Food Microbiology 1
Unit 2
Indicators in Food
Microbiology II
Yeasts and Molds
o Grow slowly in the presence of bacteria
o Proliferate when bacterial growth is restricted
For instance in:
 Refrigerated temperatures
High sugar/salt
Dried foods
High Acid
Yeasts
• Five times larger than bacteria
• True yeast reproduce via sexual ascospores
although they can also reproduce asexually
• Sexual reproduction: two yeast cells
fuse to form a zygote, ascospores are
formed within a zygote
Yeasts
Those found in food reproduce asexually by
Budding
Budding: new cell forms as a small outgrowth of
the old cell. The bud gradually enlarges and
eventually breaks away.
Yeasts
• Grow aerobically (in presence of oxygen) and
produce predominantly CO2 (bread makingleavening bread)
• Cannot grow anaerobically but metabolize
carbohydrates to predominantly ethanol (production
of alcohol, beer, wine, industrial; alcohol)
Saccharomyces cervisiae
Wild Yeasts
• Produce a range of undesirable products
(especially in alcoholic fermentations)
Hydrogen Sulfide
Organic Acids
Phenolics
• Gas blowing in vacuum packed foods
Molds
• Filamentous fungi
• Have no roots, stems, leaves or chlorophyll
• Fundamental unit is the hypha, a mass of
hyphae is mycelium
•They reproduce sexually, asexually or by both
Perfect
fungi
Imperfect
fungi
Molds
• Strict aerobes (require O2)
• Grow at low temperatures, low pH and
low water activity
Beneficial Molds
• Provide an indication that the food is not fit for
consumption
• Fermentations
Aspergillus: Oriental fermentations, produces citric
acid & gluconate
Penicillium roquerfortii: Cheese ripeming
Food Safety Indicators
Safety Indicators
• Pathogens usually occur in low numbers and hard to
pin down
• May not necessarily be pathogenic but commonly
associated with the origins of pathogens
• Used as a measure of sanitation and history of
the product
Safety Indicators
• Mesophilic Aerobic Plate Counts
• Bacterial Spore Counts
• Enterobacteriaceae
•Total coliforms
• Fecal coliforms
• E. coli
• Staphylococcus spp
• Mold counts
Mesophiles
Grow best
between 30oC40 oC
Mesophilic Aerobic Plate Counts
• Levels indicate bacteria that can proliferate at
body temperature
• High levels in cooked products or foods
from plant origin could indicate presence of
pathogens
Bacterial Spore Count
Spores (dormant form of the organism) are produced
by Clostridium spp and Bacillus spp when conditions
become unsuitable for growth (heating, freezing,
drying, etc..)
Bacillus Spores
Clostridia Spores
• Spores can remain dormant for years and are
often found in soil and dust
Sporulation
In bacteria, is the process of forming an endospore.
Transition between vegetative growth and sporulation
is a response to external triggers
• Exhaustion of nutrients
• Accumulation of metabolic products
From initiation until spore release can take > 10h
• Not all cells will undergo sporulation
•Typically occurs under sub-optimal growth conditions
Step 1: A vegetative bacterium about to enter the
endospore cycle
Step 2: The nucleoid replicates
Step 3: A spore septum
forms
Step 4: Each nucleoid becomes surrounded
by its own cytoplasmic membrane.
Step 5: The cytoplasmic membrane surrounds the isolated
nucleoid, cytoplasm, and membrane from the previous step
forming a forespore
Step 6: The forespore is completed and the other
molecule of DNA is eventually degraded
Step 7:A thick protective layer of peptidoglycan
called the cortex is synthesized between the
inner and outer forespore membranes.
Step 8: Another protective layer
called the spore coat and
composed of protein is
synthesized
Step 9: Sometimes a final layer called the
exosporium is added. As the vegetative
portion of the bacterium is degraded, the
completed endospore is released
Step 10: With the proper environmental stimuli, the
endospore germinates. As the protective layers of the
endospore are enzymatically broken down, a vegetative
bacterium begins to form and emerge
Step 11: The vegetative bacterium now
begins to divide by binary fission
Step 12
Spores are Very Resistant
Treatment to
inactivate 90%
Spore
Vegetative Cell
Wet Heat
14 mins @ 95oC
< 15 sec @ 65oC
Dry Heat
33 mins @ 120oC 5 mins @ 90oC
10% Hydrogen
Peroxide
40 mins
< 2 mins
Resistance of Endospores
• Spores are packages of DNA
• Structures of spores primarily function to protect
DNA and provide resources for initial germination
events
• DNA of spores is contained within the core
• Spores are the most resistant structures
encountered
Spore Structure
Spore Germination
Spore germination (to become vegetative cell) is
stimulated by activation:
• Mild heat (heat shock) @ 70o C for 10 mins
• High pressure
• Oxidizing or reducing agents
Activation enhances permeability of spores to
germinating agents. If none are present the spore
returns to its dormant state
Germinating Agents
• L-alanine, fructose, glucose
• Once initiated, spore germination cannot be
reversed
• Very rapid: initiation to growing cells < 40 mins
• Not all spores germinate (10% from a super dormant
population)
Significance of Spores
 To inactivate spores, focus on damaging DNA
 Inactivation of spore germination systems result in
spore death
 Spores are commonly used as indicators to verify
sterilization process (thermal treatment, irradiation)
 Significant spores (in terms of food safety)
encountered in foods:
• Clostridium botulinum
• Clostridium perfringens
• Bacillus cereus