Microbiological examination of food

Microbiological examination of food

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  Microbiological Examination of Food
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  Why it’s necessaryto 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 .
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  Following are theMicrobiological 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 -
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  1) The PourPlate 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.
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  2) The SpreadPlate 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.
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  3) The DropPlate 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.
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  4) The AgarDroplet 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.
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  5) The SpiralPlate 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.
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  COUNTING BY MEASUREMENTOF 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.
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  COUNTING USING DIRECTEPIFLUORESCENT 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.
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  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.
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   INDICATOR ORGANISMS (presenceindicates 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.
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  References – H AModi (2007) Introductory Food Microbiology Jaipur, India : Aavishkar Publishers
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  Thank you