Basic food microbiology

1. MIC 204

2.  Number of microorganisms in food and their
growth kinetic
 Factors that influence microbial activity in
food
 Factors affecting heat resistance (HR) of
microorganisms

3. Numbers of microorganisms in food
• The significance of food contamination depends on the numbers
and types of microorganisms and the opportunity for growth
• Levels of contamination will range from calculated upper limits to
extremely lower limit. Two guide microorganisms were used:
• Escherichia coli representing bacteria
• Sacchromyces cerevisiae representing yeasts
• Upper limit: The maximum level of contamination when the
organism occupies the available space may be calculated:
• 1 or 2 x 1012 bacterial cells /ml or /g
• 1 x 1012 yeasts cells / ml or /g
• Lower limits: The minimum level of contamination found in food
may be less than 1 cell / ml or /g
• Sterile: means the absence of all living organisms

4. Growth kinetics
The growth curve is characterized by 4 basic phases:
1) lag phase
2) exponential phase
3) stationary phase
4) death phase

5. Lag phase
• During this phase the cell numbers do not increase. The microorganisms adapt to the environment
and intra cellular metabolic pathways are established.
Exponential phase
• Microbial numbers increase at the maximum rate allowed by environmental conditions
• The length of this phase varies usually it last for 2 hours, but may extend over several hours.
• The overall process of multiplication involved both the absorption of nutrients from the substrate
into the cell and the removed of waste or catabolic product from the cell into the substrate.
• Thus microbial growth results in physio-chemical changes of the environment. These become great
enough to make the environment unsuitable for further growth.
• In this condition, the microbial population has reached to the stationary phase
Stationary phase
• At this stage, due to nutrient shortage and accumulation of waste products, a few cells die and a
few cells multiply, keeping the living population stable.
Death phase
• The population enters into the death phase where the rate of cell death is higher than the rate cell
multiplication

6. The shape of the growth curve is affected by
factors
a) temperature higher or lower than optimum
b) limitation of nutrients for the microorganism
c)presence of natural antimicrobial compound
or disinfectants

7. Internal
 Nutrient contents
 Water activity
 Redox potential
 pH
 Osmotic pressure
External
 Temperature

8. • Microorganisms require foods for energy, growth and proper cell functioning.
• Nutrients include carbohydrate, protein, lipid, minerals and vitamins
• If nutrients are abundant, growth rate is faster.
• Microbes normally found in food vary greatly in nutrient requirement.
• They are also varied in their ability to utilize complex carbohydrate and production of extracellular
enzymes in order to hydrolyze complex molecules.
• Food is important to generate energy - mainly derived from carbohydrate sources. Other carbon
compounds that can be used as energy source are alcohol, organic acids, amino acids, peptides etc.
• It is also important for growth - mainly derived from protein sources, nitrogen sources, peptides,
amino acids, urea, ammonia etc.
• Vitamins and minerals are important for cell functioning.
Internal
1. Effect of nutrients on bacterial growth rates

9. Water activity: a measure of the availability of water for biological functions.
• Relates to water present in "free" and "bound" forms.
• Bound water: Dissolve solutes
: Not available for biological functions
: Does not contribute to Aw
• Aw in food differ between 0.1 - 0.9
• Aw can be reduced by removing water and can be increased by absorption of water. This is
important in the control of microorganism.
• Microorganisms need water to grow. Free water is necessary for microbial growth especially in
process like transport of nutrients, removal of waste materials and carries out enzymatic reactions.
• Each microbial species has an optimum, maximum and minimum Aw level for growth.
Internal
2. Effect of water activity on bacterial growth rates
Bacteria 0.9
Molds 0.6
Yeasts 0.85

10. • Perishable foods such as milk, meat etc are easily spoilt because of the high Aw.
• As Aw is lowered, the ability to grow will reduce. Most spoilage bacteria able to
grow in Aw0.9
• To prevent spoilage, we need to reduce the water activity by desiccation, freezing
etc.
• Shelf life is extended as more water is reduced.
• Molds and yeast are more tolerant to low water activity than the bacteria. They
play an important role in spoilage of bread and dried foods.
• The most xerophillic bacteria can only withstand up to 0.65 whereas fungi at 0.6
• If Aw is reduced to 0.65 / 0.60 the food cannot spoil for at least one year (if other
parameters are also controlled).

11. • The redox potential is a measure of potential
difference in a system or food.
• It can be generated by a couple reaction:
Oxidation: Loss of electron by a substance (i.e. the
substance is an electron donor) or known as reducing
agent.
Reduction: Gain of an electron by another substance (i.e.
an oxidizing agent). The substance is an electron
acceptor
Internal
3. Effect of redox potential on bacterial growth rates

12. • The redox potential of a food is influenced by its chemical
composition, specific processing treatment given and storage
condition in relation to air (vacuum packed, liquid N2, C02 etc)
• Fresh foods of plant and animal origin are in a reduced state due to
presence of reducing substances such as sugars, ascorbic acids and
sulphydryl group of protein.
• Diffusion of oxygen into these fresh food substances can change the
redox potential of the food.
• Processing food such as heating can also alter the Eh (reduced or
oxidized). Food stored in air can have a wide Eh range (+Mv) than
when stored under vacuum.

13. • The presence or absence of oxygen in the environment is important in the growth
of microorganisms. Microorganisms can be grouped into categories based on their
requirement on intolerance to oxygen
1) Aerobes
Grow in the presence of air that contains molecular oxygen.
Obligate aerobes require oxygen for growth and carry out aerobic respiration.
2) Microaerophiles
Grow only at reduced concentrations of molecular oxygen - 5% of atmospheric
oxygen concentration (20%)
3) Facultative anaerobes
Can grow in the presence or absence of air. If oxygen is not available, they will
carry out anaerobic respiration
4) Anaerobes
Do not require oxygen for growth, therefore only in the absence of air.
Strict anaerobes are sensitive to oxygen and even to a brief exposure to oxygen
will kill such organisms e.g. Clostridium spp.

14. • Grow of all these microorganisms and their metabolic reactions are
extremely dependent on positive redox potential of food.
• The range of Eh at which different groups of microorganisms grow
are:
• The presence or absence of oxygen and the Eh of food determine
the growth of a particular microbial group in foods. This is
important in microbial spoilage of foods and in desirable
characteristics of fermented foods.
Aerobes Between +500 and +300 mV
Anaerobes Between +100 and -250 mV
Facultative anaerobes Between +300 and -100 mV

15. • Microbial spoilage: putrefaction of meat by Clostridium spp.
• Fermentation: Penicillium spp. in blue cheese

16. Food can be grouped as:
a) Low acids foods (pH > 4.6) Meat, fish, milk and soups
b) High acids foods (pH<4.6) Fruits and juices, vegetables and
salad dressings.
Internal
4. Effect of pH on bacterial growth rates

17. • Microorganisms grow in different pH range:
• Most bacteria have limited pH range. Optimal pH value for growth is near neutral.
• The ability of microorganism to grow well will be inhibited if the pH of the
surrounding is higher or lower than the optimal pH value.
• Most pathogenic bacteria are very sensitive to low pH value. Soft drinks and fruits
are very acidic and not suitable for bacterial growth.
• Proteolytic bacteria can grow in media with high pH.
• Vegetables have higher pH than fruits. Therefore they are subjected to spoilage by
bacteria.
• Molds have a wider pH range. Therefore they are able to tolerate low pH better
than bacteria and yeast. Molds can cause spoilage of soft drinks, fruits and honey.
Molds pH1.5 - pH9.0
Yeast pH2.0 - pH8.5
Bacteria pH6.5 - pH7.5

18. • Osmotic pressure: an internal experience by microorganisms.
• When microorganisms is in sugar or salt solutions, water from
microorganisms will migrate, which cause microorganisms to
experience higher osmotic pressure.
• As the solute concentration is higher, osmotic pressure is higher,
thus more inhibition to growth.
• Most microorganisms are not able to tolerate high osmotic pressure
except the halophiles such as Pseudomonas spp. And Vibrio spp.
(0.5% - 0.3% salt concentration) and also osmpohiles such as
Leuconostoc spp. Spoilage of food with higer sugar and salt content
is usually caused by osmophiles and halophiles respectively.
Internal
5. Effect of osmotic pressure on bacterial growth rates

20. • Temperature can greatly influence the rate of reaction or enzymatic activities.
• The temperature at which food is held will influence the growth rate of
microorganisms, hence the rate of spoilage.
• Microorganisms grow over a wide range of temperature. With respect of
temperature, there are 3 groups of microorganisms:
a) Psychrophiles: Grow best at low temperatures (<20°C), cause spoilage of
refrigerated foods.
b) Mesophiles: Grow best at moderate temperatures (20 - 40°C).
c) Thermophiles:Grow best at high temperatures (> 45/50°C).
Two other important terminologies:
a) Psychrotrophs: Microorganisms that can grow at refrigerated temperature (0 -
5°C).
b) Thermoduric: Microorganisms that can survive pasteurization process.
External
Effect of temperature on bacterial growth rates

21. 1. Time and temperature
2. Type of microorganisms
3. Number of microorganisms
4. pH
5. Water
6. Food ingredient/ medium/ substrate

22. • As time and temperature increase, the death
rate increases
Time and temperature
Temperature ↑
Death ↑
Time required for killing ↓

23. • HR related to the optimal growth temperature of the microorganisms
Thermophiles > Mesophiles > Psycrophiles
• Psychrophiles grow well in refrigeration temperature as temperature is
increased, death occur rapidly.
• Thermophiles - high growth rate at high temperature. If the temperature is
lower than the optimum temperature, growth rate declines.
• Spores are more resistant than the vegetative cells.
• Gram positive bacteria are more resistant than gram negative.
• Molds and yeast are not heat resistant. Killed at 70°C -80°C.
Type of microorganisms

24. • The higher the number of microorganisms, the
higher is the degree of heat resistance due to:
a) Higher production of protective substances
/ protein extracellular component.
b) More varieties of microorganisms with
different heat resistant.
Number of microorganisms

25. • Microorganisms are most resistant at their
optimal pH. If pH is increased or decreased
from the optimum, the HR is reduced.
• High acid food required less heating
• Examples: Effect of pH on B.subtilis spores pH
4.4 - 2 minutes survival time pH 7.6 - 11
minutes survival time
pH

26. • High humidity (more water) - less heat
resistant.
• Less water - more heat resistant.
• More water, better heat penetration.
Denaturation of protein also faster. Therefore
kill microorganisms in shorter time.
Water

27. • A microorganism in food with high fat is more
resistant due to fat protection. Long chain
fatty acids provide better protection than the
short chain.
• Microorganisms with high proteins are more
resistant. Proteins are colloids, which also
provide better protection.
Food ingredient/ medium/ substrate