pH, water activity, redox potential all informations are covered.

1. History of food microbiology
Dr. Priyanka Ghosh (PhD)

2. Food and Microbes
• Interaction between microorganisms plants and animals
are constant and natural.
• Microorganisms use our food supply as a source of
nutrients for their own growth.
• Result= deterioration of food
• “SPOIL” food = 1. increased their numbers (microbes)
• 2. Utilizing nutrients
• 3. Producing enzymatic changes
• 4. Contributing off flavors
• 5. Breakdown of products/ synthesis of new compounds

3. Sometimes interactions are beneficial and sometimes harmful.
When organisms are
pathogenic, their
association with our
food supply is critical
from a public health
point view
When organisms are
beneficial and helps
to formation of a
desired product which
is good to intake

4. A knowledge of the factors that favor/ inhibit the growth of
microorganisms is essential to an understanding of the
principles of food spoilage and preservation.
Points to remember:
Chief compositional factors of a food that
influence microbial activity:
1.Hydrogen ion concentration.
2.Moisture
3.Oxidation-reduction (O-R) potential
4.Nutrients
5.Presence of inhibitory substances/barriers.

5. Hydrogen-ion concentration(pH)
Every microorganisms has a minimal, a maximal and an optimal pH for their
growth.
As microorganisms have no mechanism for adjusting their internal pH so they are
significantly affected by the pH of food.
Most food are neutral/acidic.
As molds and yeasts are acid tolerant so when foods are acidic. They are more
susceptible to yeasts and molds for spoilage than bacteria.
Most bacteria favored by a pH near neutrality (exception: Proteolytic bacteria
favored high alkaline pH)
pH of a product can be readily determined by a pH meter.
pH affects the rate of growth of microorganisms, the rate of survival during
storage, heating , drying and other form of processing.
Sometimes at the initial stage of that particular food’s pH was restricted or
favorable but after certain growth of the organisms changes the pH of the food and
help other organisms to grow on it.

6. pH and Microbial Growth
The pH range of growth for:
Molds: 1.5- 9.0
Yeast: 2.0-8.5
Gram-positive bacteria: 4.0- 8.5
Gram negative bacteria: 4.5-9.0
Acid-resistant or acid tolerant strains can acquire
resistance to lower pH compared with the other strains of
a species (e.g. Salmonella, Escherichia coli O157: H7)
When the pH in a food is reduced below the lower limit
for growth of a microbial species, the cells not only stop
growing but also lose viability, the rate of which depends
on the extent of pH reduction.

7. Moisture Requirement: The concept of
Water Activity
No water = No growth
Microbes have an absolute demand for water.
This water requirement expressed in terms of water activity aw
WATER ACTIVITY( aw) = ------------------------------------------------------------------------------
The vapor pressure of the solution solutes in water in food
The vapor pressure of the solvent (water usually)
Water activity and relative humidity relation:
aw × 100 = equilibrium relative humidity (ERH) %
ERH
---------
-
100
= aw

8. WATER ACTIVITY VALUES FOOD
0.98 AND AVOBE FRESH MEAT, FISH, FRESH FRUIT,
VEGETABLES, MILK, MOST
BEVERAGES, CANNED FRUITS AND
VEGETABLES
0.93-0.98 EVAPORATED MILK, TOMATO PASTE,
PROCESSED CHEESE, BREAD,
FERMENTED SAUSAGE
0.85-0.93 DRIED BEEF, RAW HAM, AGED
CHEDDAR CHEESE, SWEETENED
CONDENSED MILK
0.60-0.85 DRIED FRUIT, FLOUR, CEREALS, JAM,
JELLIES, NUTS, INTERMEDIATE-
MOISTURE FOODS
BELOW 0.60 CHOCOLATE, CONFECTIONERY,
HONEY, BISCUITS, CRACKERS, POTATO
CHIPS, DRIED EGGS, MILK AND
VEGETABLES
FOODS AND THEIR WATER ACTIVITY

9. FACTORS AFFECT WATER ACTIVITY REQUIREMENT
OF MICROORGANISMS
The kind of solutes employed to reduce the aw.
The nutritive value of the culture medium. The lower the
limiting water activity.
Temperature: Most organisms have the greatest tolerance to
low aw at about optimal temperatures.
Oxygen supply: Growth of aerobes takes place at lower aw
in the presence of air than in its absence, and the reverse is
true of anaerobes.
pH: Most organisms are more tolerant of low aw at pH
values near neutrality than in acid or alkaline media.
Inhibitors: The presence of inhibitors narrows the range of
aw for growth of microorganisms.

10. WATER UNAVAILABILITY
1.Solutes and ions tie up water in solution.
Sugar and salts dissolved in water, osmosis
takes place.
2.Hydrophilic colloids make water
unavailable, agar 3/ 4% in a medium
prevent bacterial growth.
3.Crystallization or hydration of water.

11. Methods to determine water activity
1. Freezing point determination [feasible only on liquids
food with high aw values. Based on Clausius-
Clapeyron equation]
2. Manometric techniques [ directly measuring the vapor
pressure in the vapor space surrounding a food ]
3. Electric devices [ Lots of devices used mostly sensors
are used which measure relative humidity in the vapor
space surrounding a food based on electrical
resistance]

12. Moisture consideration during growth of
microbes
1. Each organisms has its own characteristic optimal aw and its own range aw
for growth for a given set of environmental conditions ( pH, temperature,
nutritive properties of substrate, contents of inhibitory substances, availability
of free oxygen)
2. An unfavorable aw will result not only in a reduction in the rate of growth but
also in a lowered maximal yield of cells.
3. The more unfavorable the aw of the substrate, greater the delay in initiation of
growth, germination of spores. It is important for preservation.
4. Bacteria require more moisture than yeasts, yeasts more than molds
5. Microbes that can grow in high concentration of solutes, obviously have a low
minimal aw [halophilic bacteria require minimal concentration of dissolved
sodium chloride for growth, Osmophilic yeasts grow best in high
concentration of sugar.

13. Nutrients Contents of the Food
Microorganisms require proteins, carbohydrates, lipids, water,
energy, nitrogen, sulphur, Phosphorus, vitamins and minerals for
growth.
Various foods have specific nutrients that help in microbial
growth.
Foods such as milk, meat and eggs contain a number of nutrients
that are required by microorganisms.
These foods are hence susceptible to microbial spoilage.
Growth factors and inhibitors in food:
Foods can also have some factors that either stimulate growth or
adversely affect growth of microorganisms. An example is the
growth factors in tomatoes that stimulate growth of some

14. Antimicrobial Substances
Antimicrobial substances in food inhibit
microbial growth.
Various foods have inherent antimicrobial
substances that prevent (inhibit) microbial
attack.
Such inhibitors are like lactinin, and anti-
coliform factors in milk and egg white
lysozyme in eggs.

15. Biological Structures
Some foods have biological structures that
prevent microbial entry
For example: meat has fascia, skin and other
membranes that prevent microbial entry
Eggs have shell and inner membranes that
prevent yolk and egg white from infections.

16. Extrinsic Factor
Temperature of storage
Presence and concentration of
gasses in the environment.
Relative humidity of food
storage environment.

17. Temperature
Psychrophilic Mesophilic
Thermophilic
•These grow best at
about 20°C but also
down to -10°C in
unfrozen media.
•Several of microbes
found in soil and
water belong to this
group.
•Bacteria:
Achromobacter,
Flavobacterium,
Pseudomonas,
Micrococcus.
•Molds: Penicillium,
Cladosporium, Mucor
•These organisms
grow between 25°C
and 40°C with an
optimum growth
temperature close to
37°C.
•None of the
mesophilic bacteria
are able to grow 5°C
or above 45°C.
•Most pathogenic
bacteria belong to
this group.
•These grow at temperatures
above 45°C.
•Often their optimum growth
temperatures is between 50°C
and 70°C.
•Growth of some bacteria occur
at 80°C.
•Bacteria in this group are
mainly spore formers and are of
importance in the food industry
especially in processed foods.
•Bacillus stearothermophilus
can survive ultra-high-
temperature treatment (UTH)
of milk (135°C for 2 seconds)

18. Concentration of Gases in Environment
This relates to the presence and concentration of
gases in the food environment.
Various microorganisms require for growth,
either high oxygen tension (aerobic), low oxygen
tension (micro-aerobic) or absence of oxygen
(anaerobic) .
Some microorganisms may grow either in high
oxygen tension or in the absence of oxygen
(facultative anaerobes).

19. Redox-Potential and Microbial Growth
Many anaerobes (obligate or strict anaerobes) can not grow in the presence
of even small amounts of free oxygen as they lack the supreoxide dismutase
necessary to scavenge the toxic oxygen free radicals.
Addition of scavengers such as thiols (e.g. thioglycolate) helps overcome
the sensitivity to these free radicals. Microaerophiles grow better in the
presence of less oxygen.
Growth of microorganisms and their ability to generate energy by the
specific metabolic reactions depend on the redox potential of food.
Molds, yeast, Bacillus, Pseudomonas, Moraxella and Micrococcus genera
are some example that have aerobic species.
Some example of facultative anaerobes are the lactic acid bacteria and
those in the family Enterobacteriaceae, such as Salmonella, Escherichia coli,
Yersinia sp.
The most important anaerobe in food is Clostridium. An example of a
microaerophile is Campylobacter sp. This is important in microbial spoilage
of a food (such as putrification of meat by Clostridium sp under anaerobic
conditions) and to produce desirable characteristics of fermented foods (such
as growth of Penicillium sp in blue cheese under aerobic conditions).

20. Relative Humidity
Relative humidity is the amount of moisture in
the atmosphere or food environment.
Foods with low water activity placed at high
humidity environment take up water, increase
their water activity and gets spoiled easily.
For example: dry grains stored in a environment
with high humidity will take up water and
undergo mold spoilage.