food microbiology

1. Control of microorganisms in
food
Prepared by
Samira fattah
Assis. Lec.
College of health sciences-HMU
Lecture 7

2. • Although some microorganisms are desirable for the
production of bioprocessed food, many are undesirable
because they can cause food spoilage and food borne
disease.
• For efficient production of bioprocessed food, the
objective is to stimulate growth and increase the viability
of desirable microorganisms.
• In contrast, for spoilage and pathogenic microorganisms,
the objective is to minimize their numbers or completely
eliminate them from food.

3. • Several methods ,individually or in
combinations, are used to achieve these goals
by:
1) Controlling access of the microorganisms in food.
2) physically removing the microorganisms present in
foods.
3) Preventing or reducing the growth of microorganisms
and germination of spores present in foods.
4) Killing microbial cells and spores present in foods.

4. Methods of microorganisms control in
foods

5. 1. Control of access(cleaning and sanitation)
• The main objective of sanitation is to minimize the access of
microorganisms in food from various sources at all stages of
handling.
• Proper sanitation helps reduce the microbial load to desired
levels in further processed food.

6. • For example a low microbial level in raw milk
produced through effective sanitation make it easier
to produce pasteurized milk that meets the microbial
standerd.
• Also proper sanitation increase shelf life of food.
• And also reduce incidence of foodborne disease.

7. • Several groups of sanitizers are approved for use
in food-processing plants:
1) Chlorine-based sanitizers
2) Iodophores
3) Quaternary ammonium compounds
4) H2O2
• They are effective against gram positive and
gram negative bacteria, bacterial spores, viruses,
yeast and molds.

8. 2. Control by physical removal
Microorganisms that have gained access to food can be
controlled by removing them physically, this can be
achieved by different methods:
1) Centrifugation
2) filteriation
3) Trimming
4) Washing
• In general these methods can partially remove
microorganisms from food and help other antimicrobial
steps that follow to become more effective.

9. 3. Control by heat
• The main objective of heating food is to destroy vegetative
cells and spores of microorganisms(bacteria, mold, yeast and
viruses) especially pathogenic and spoilage causing ones.
• This is necessary in order to retain the acceptance and
nutritional qualities of a food.

10. On the basis of temperature and time of heating the food used
to destroy microorganisms, the methods can be divided to:
1) Low-heat processing or pasteurization
the temperature used is below 100⁰ C. the objective is to
destroy all the vegetative cells of the pathogens.

11. • Pasteurization of milk has been used for a long time, two
methods are used:
a) low temperature long time (LTLT)
heating at 145⁰ F(63 ⁰ C) for 30 min.
b) High temperature short time(HTST)
heating at 261 ⁰ F(72 ⁰ C) for 15 sec.

12. 2) high-heat processed foods
The process involve heating foods at or above 100 ⁰ C for a
desired period of time.
The temperature and time of heating are selected on the basis
of product characteristics and the specific microorganisms to
be destroyed.

13. • Commercial sterility is obtained by heating the food at very
high temperature for a short time.
• This process is named as Ultrahigh temperature(UHT)
processing.
• Milk is heated to 150 ⁰C for 2 to3 sec. it can be stored at room
temperature (≤30 ⁰C) and the product have 3-month shelf life.

14. 4. Control by low temperature
• Main objective in low-temperature preservation of food is to
prevent or reduce growth of microorganisms.
• Low temperature also reduce or prevent catalytic activity of
microbial enzymes especially heat-stable proteinases and
lipases.
• Germination of spores is also reduced.

15. • However the death rate of M.O at low temperature , as
compared with that at heat treatment cannot be predicted.
• Also spores are not killed at low temperature, thus food are
not preserved at low temperature in order to kill microbial
cells.

16. • Foods are stored at low temperature in different ways to
extend their shelf life, these methods are used for low-
temperature preservation of foods:
a) Ice chilling
b) Refrigeration
c) Freezing

17. 5. Control by reduced water activity
• The main objectives of reducing water activity in food are to
prevent or reduce the growth of vegetative cells and
germination of spores of M.O.
• Prevention of toxin production by toxigenic molds and
bacteria is also an important consideration.
• Microbial cells(not spores) also suffer reversible injury and
death in food with low water activity, although not in a
predictable manner as in heat treatment.

18. • The water activity of food can be reduced by using one or
more of these methods:
1) Natural dehydration
Is low cost method, water is removed by the heat of sun.
2) Mechanical drying
Foods travel through a tunnel against flow of hot air that
remove the water.

19. 3) Smoking
meat and fish are exposed to low heat and smoke ,the heating
process remove water from the product and smoke is
deposited on the surface at the same time.

20. 6. Control by low pH and organic acids
• The major objective of using weak organic acids is to reduce
the pH of the food to control microbial growth.
• As the pH drops below 5.0, some bacteria become injured or
die.
• However the death rate in low pH is not predictable as in the
case of heat.

21. Acids used:
a) Acetic acid
b) Propionic acid
c) Lactic acid
d) Citric acid
e) Sorbic acid
f) Benzoic acid
-these acids are used in
vigetable pickles, salad dressing, jam,
carbonated drinks and ….etc.

22. 7.Control by modified atmosphere
The growth of aerobes (mold, yeast and bacteria) is reduced,
but under these conditions anaerobic and facultative
anaerobic bacteria can grow unless other techniques are used
to control their growth.

23. Methods are:
a) Vacuum packaging
used for meat products and some types of cheese.
b) Gas flushing
used for fresh and cooked products. the gases usually used
are a mixture of CO2 and N2 with some O2 for packaging red
meats.

24. 8. Control by antimicrobial preservatives
• Antimicrobial chemicals are used in food in relatively small
doses either to kill undesirable M.O. or to prevent their
growth.
• They differ greatly in the ability to act against different M.O.
• Some are narrow spectrum.
• Some are broad spectrum.
• Some act aginst G+ or G – or bacteria spores or viruses.

25. • Those that are capable of killing M.O. are designated as:
- Germicides(kill all types)
- Fungicides
- Bactericides
- Sporicides
- Viricides
• Those that inhibit or retard microbial growth are classified as:
- Fungistatic
- Bacteriostatic

26. • Example of antimicrobial preservatives:
a) nitrite(NaNO2)
b) Sulfer dioxide(SO2)
c) H2O2
d) Ethylenediaminetetraacetate (EDTA)
e) Antibiotics
f) Spices

27. 9. Control by irradiation
• A food is irradiated because of the destructive power of
ionization on M.O. a food harbors.
• Depending on the method used, it can either completely or
partially destroy mold, yeast, bacterial cells and spores.
• In addition , irradiation can destroy worm, insects and larvae
in food.
• Irradiation cannot destroy toxins in food.

28. UV radiation
• M.O. are specially susceptible to UV Light
between 200-280nm.
• Because of low penetration power it has
been used to inactivate M.O. on the surface
of foods(meat, fish and bread) and liquids
such as water and syrups.
• as will as on walls, shelves and equipment in
the food handling and processing area.