Microbiological analysis of food products is the use of biological, biochemical, molecular or chemical methods for the detection, identification or enumeration of microorganisms in a material. Here some of the common methods have been described.

1. Microbiological
Examination of Food

2. Why it’s necessary to examine food qualitatively
or quantitatively for microorganisms?
The principal objective of microbiological testing is to
ensure that -
1. The food meets certain statutory standards.
2. The food meets internal standards set by processing
company or external standards required by the
purchaser.
3. The food material entering the factory for
processing are of required standard and/or meet a
standard agreed with supplier.
4. The process control and line sanitation are being
maintained .

3. Following are the Microbiological Test Procedures in Usage -
TOTAL VIABLE COUNT –
• Most common test performed on foods.
• It is also called Standard Plate Count or Aerobic Plate Count.
• Food dilutions are plated on agar based media containing
complex nutrients. (beef extracts, yeast extract, peptone,
growth factors etc.)
• Many plating techniques are employed for enumerating viable
cells.
They are as follows -

4. 1) The Pour Plate Method
• A set of petri dish is inoculated with 1 ml aliquots from food
dilutions.
• Incubation is provided as per the required temperature and
duration.
• After incubation plates containing 30-300 colonies should be
counted.
• From this the number of viable cells per gram of food can be
calculated.

6. 2) The Spread Plate Method
• 0.1 ml dilutions are spread evenly over the surface of pre-poured
and solidified petri dishes, using sterile L-shaped glass rods.
• Colonies observed on the surface can be easily observed and
picked off.

7. 3) The Drop Plate Method
• Specially calibrated pipettes delivering 0.02 ml per drop are
used and five different drops are delivered onto the surface.
• The drops are dried before incubation.

8. 4) The Agar Droplet Method
• Rapid technique used for smaller quantities of materials.
• Dilutions are prepared in molten agar and colonies develop in
the solidified droplets (0.1 ml) during incubation.
• Counting is facilitated by using a projection viewer which
magnifies the droplets.

9. 5) The Spiral Plate Method
• It is a semi-automatic method.
• A machine continuously plates a known volume of sample on
the surface of a rotating agar
plate.
• The amount of sample deposited
decreases as the dispensing stylus
is moved from the centre to the
perimeter of rotating plate.
• Counting can be manually done but laser based automatic
colony counters have been developed specially for use.

10. COUNTING BY MEASUREMENT OF ATP
• A bioluminescent technique has been developed based on
action involving ATP and the Luciferase enzyme derived from
fireflies.
ATP + Luciferase → Light
• The total amount of light produced is directly proportional to
the quantity of ATP present and since all bacteria contain
roughly the same amount of ATP per cell it should be possible
to measure the number of bacterial cells in any sample.

11. COUNTING USING DIRECT EPIFLUORESCENT FILTER
TECHNIQUE
• In this method pre-filtered suspensions of food are
subsequently passed through a fine polycarbonate membrane
filter.
• Bacteria retained on the surface of the filter are then stained
with fluorescent material and enumerated by means of an
epifluorescence microscope.

12. DIRECT MICROSCOPIC COUNT
• It may be sometimes necessary to obtain total number of
microorganisms (viable and non-viable cells) present in food
samples.
• The total number of microbial cells in a given number of
microscopic fields is counted and from this the rough
calculation of total number of organisms present per gram
food can be done.

13.  INDICATOR ORGANISMS
(presence indicates the possibility of poisoning)
eg. Coliforms, Enterococci, Enterobacteriaceae etc.
 FOOD POISONING ORGANISMS
eg. Salmonellas, Staphylococcus aureus, Bacillus aureus,
Clostridium botulinum etc.
 FOOD SPOILAGE ORGANISMS
eg. Pseudomonas, Micrococci, Streptococci etc.

14. References –
H A Modi (2007) Introductory Food Microbiology
Jaipur, India : Aavishkar Publishers

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