1. FACTORS THAT INFLUENCE MICROBIAL
ACTIVITIES IN FOOD
Internal
1. Nutrient contents
2. Water activity
3. Redox potential
4. pH
5. Osmotic pressure
External
1. Temperature

2. LECTURE 4
FACTORS THAT INFLUNCE MICROBIAL
ACTIVITIES IN FOOD
Numbers of microorganisms in food and their growth kinetic
• 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

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

4. • 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
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

5. Internal
1. Effect of nutrients on bacterial growth rates
 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.
Bacteria Nutrient requirement
Yeast ↓
Mold
 They are also vary 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.

6.  Vitamins and minerals are important for cell functioning.
2. Effect of water activity on bacterial growth rates
 Water activity is 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
Cereals, cracker, sugar, salt 0.1-0.2
Honey, chocolate <0.6
Jam, jelly 0.6-0.85
Condensed milk 0.85-0.93
Tomato paste, fruit juice 0.93-0.98
 Aw can be reduced by removing water and can be increased
by absorption of water. This is important in the control of
microorganism.
Aw ↓ Remove of water
Aw ↓ Absorption of water
 Microorganisms need water to grow. Free water is necessary for
microbial growth especially in process like transport of

7. nutrients, removal of waste materials and carry out enzymatic
reactions.
 Each microbial species has an optimum, maximum and
minimum Aw level for growth.
Bacteria 0.9
Molds 0.6
Yeasts 0.85
 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 do not able to grow 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 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).

8. 3. Effect of pH on bacterial growth rates
 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.
 Microorganisms grow in different pH range:
Molds pH1.5 - pH9.0
Yeast pH2.0 - pH8.5
Bacteria pH6.5 - pH7.5
 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.

9. 4. Effect of redox potential on bacterial growth rates
 The redox potential is a measure of potential difference
in a system or food.
 It can be generated by a couple reaction:
a. Oxidation: Loss of electron by a substance (i.e. the
substance is an electron donor) or known as reducing
agent.
b. Reduction: Gain of an electron by another substance
(i.e. an oxidizing agent). The substance is an electron
acceptor
 The redox potential is measured in electrical units of
milivolts designated as Eh.
a) In the oxidized range, the redox potential is + mV
b) In the reduced range, the redox potential is -mV.
 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).

10.  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.
 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

11. 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.
 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:
Aerobes Between +500 and +300 mV
Anaerobes Between +100 and -250 mV
Facultative anaerobes Between +300 and -100 mV
 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.
Microbial spoilage: putrefaction of meat by
Clostridium spp.
Fermentation: Penicillium spp. in blue cheese
5. Effect of osmotic pressure on bacterial growth rates
 Osmotic pressure is an internal experience by microorganisms.

12.  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.
External
1. Effects of temperature on bacterial growth rates
 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

13. 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.

14. FACTORS AFFECTING HEAT RESISTANCE (HR) OF
MICROORGANISMS
1. Time and temperature
• As time and temperature increase, the death rate increases
Temperature ↑
Death ↑
Time required for killing ↓
2. Type of microorganisms
• 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 higher than the optimum temperature, growth
rate declines.
• Spores are more resistant than the vegetative cells.

15. • Gram positive bacteria are more resistant than gram negative.
• Molds and yeast are not heat resistant. Killed at 70°C -80°C.
3. Number of microorganisms
• The higher the number of microorganisms, the higher is the
degree of heat resistance due to:
a) Higher production of protective substances / protein
extrcellular component.
b) More varieties of microorganisms with different heat
resistant.
4. pH
• 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
5. Water
• High humidity (more water) - less heat resistant.
Less water - more heat resistant.

16. • More water, better heat penetration. Denaturation of protein also
faster. Therefore kill microorganisms in shorter time.
6. Food ingredient / medium / substrate
• 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.