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Three Common Microbiological Testing-Methods for Food Products

Food safety
Food safety | Source

Overview

These days, there is increased awareness among consumers for the safety of food products they consume. Consumers' growing demand for fresh and appropriate forms of products has led to the advance of food safety practices in the food industry. Moreover, the relatively high occurrence of outbreaks of foodborne diseases in many countries, including the developed ones, has resulted in increasing concern and intensive investigation of foodborne pathogens. As a result, there is currently an increased demand for the microbiological testing of food products.

Purpose

The purpose of a microbiological testing should be to identify and restrict harmful microorganisms, which can spoilage foods, and ensure safety from foodborne diseases. This means that the responsible one (or the Quality Assurance team in a facility) must establish a though testing procedure to identify all the possible threats, which may lead to one of the two results: pathogen not detected or detected.

Before performing a microbiology test, the analyst should know the necessity, purpose, and primary expectations underlying the test, the predicted certainty of identifying an issue, and possible results that may come out from the test. Accordingly, this will help understand the sampling procedure to be performed, the type of samples to be collected, the particular test method to be used, and appropriate actions to be taken before and after the test results are attained.

Reasons for a Microbiology Test

The typical reasons for microbiological testing should be to:

  • Meet desired specifications for raw material, intermediate, and finished product,
  • Identify risk factors,
  • Process verification, and
  • Confirm that regulatory guidelines are followed.

Why Microbiological Testing?

Although microbiological testing is just one component of the food safety system and does not guarantee 100% product safety, but it is a prerequisite and integral part that must take place to ensure food safety. A microbiological testing can outline important information about a manufacturing process, processing environment, as well as a specific product batch. It also informs whether a sampling/testing procedure is correctly designed and finished following regulatory guidelines or not.

However, one must understand that a microbiological testing cannot determine 100% safety from pathogens, as tests are done using samples, which are only a portion from the food products. With microbiological testing, one can, mostly achieve that no pathogens are detected from the sample and/or, realize the levels of sensitivity and assurance provided by the testing procedures and sampling plans used.

To ensure the optimum food quality, the manufacturers must also establish prerequisite programs including, Hazard Analysis Critical Control Point (HACCP), Good Manufacturing Practices (GMP), Recall Management, Traceability, and Sanitation Practices.

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Common Test Methods

Although a wide range of technologies are used for the identification and verification of microorganisms, but among these technologies, three types of method are commercially popular.

These are:

  1. Culture Media,
  2. Immunoassay, and
  3. Polymerase chain reaction.

Culture Media

A special medium that is used in microbiological laboratories to identify and detect different types of microorganisms by culturing or growing. Usually, a culture medium is composed of different nutrients to enhance the microbial growth.

Traditionally, cultural techniques have been the tests of choice for both ready-to-eat foods and fresh produce. However, today immunoassay and PCR methods are more accepted than cultural methods, because recent developments of newer testing methods and validation studies have demonstrated that cultural methods aren't suitable for all food groups.

Key Features:

  • Different methods are involved in culturing techniques. For identification and detection of microorganisms in cultures, both liquid and solid culture media are employed.
  • Microscopes are usually used to detect microbes in cultures, and biochemical and serological techniques are used to differentiate various organisms.
  • Both qualitative and quantitative results of microorganisms can be obtained using cultural methods. This means a culture media technique not only detects the presence or absence of an organism but also provides information about the number of organisms present in the medium. However, quantitative analysis is only possible using solid culture media, because the individually developing colonies of organisms can be counted only on the surface.
  • Time to attain results can range from twelve hours to more than a week.

Four different types of agar plates showing differential growth depending on bacterial metabolism.
Four different types of agar plates showing differential growth depending on bacterial metabolism. | Source

Immunoassay

Immunoassay can be illustrated as a microbiological test that is used to measure the concentration of a macromolecule in a solution via using an antibody or immunoglobulin. The detected macromolecule from an immunoassay method is in many cases a protein and is widely termed as an "analyte". These analytes in biological liquids -- e.g. urine or serum -- are often measured following immunoassay test methods for various purposes. ELISA (Enzyme-Linked Immuno Sorbent Assay, i.e. “pregnancy test” or “dip stick” type method) is one of the most commercially available immunoassay types.

Key Features:

  • Antibodies are used to detect and identify specific proteins, which are predicted to be unique to the target microorganism.
  • Both qualitative and quantitative results can be possible, but methods are usually detected/not-detected (qualitative) type tests and only some can be quantitative.
  • Methods can only be sensitive when paired with a cultural enrichment.
  • Time to obtain results can range from 24 hours to 48 hours.

A sandwich ELISA. (1) Plate is coated with a capture antibody; (2) sample is added; (3) detecting antibody is added; (4) enzyme-linked secondary antibody is added; (5) substrate is added and conversion is done.
A sandwich ELISA. (1) Plate is coated with a capture antibody; (2) sample is added; (3) detecting antibody is added; (4) enzyme-linked secondary antibody is added; (5) substrate is added and conversion is done. | Source

Polymerase Chain Reaction (PCR)

PCR is a very recent and revolutionary method developed by Dr. Kary Mullis in 1983. Today, it is used in medical and biological research labs as a common and often indispensable technique for a variety of applications.

A PCR test can recognize pieces of DNA or RNA, which are expected to be unique to the target microorganism. PCR is based on using the ability of DNA polymerase and can generate billions of copies of a specific DNA sequence.

Key Features:

  • Selected sections of DNA or RNA can be reproduced using PCR technique.
  • Both qualitative and quantitative results can be possible, but PCR methods are usually detected/not-detected (qualitative) type test and only some can be quantitative.
  • Test methods can be sensitive and rapid, predominantly when paired with a cultural pre-enrichment.
  • Test results can be obtained within 24-48 hours. This time frame includes time for cultural pre-enrichment as well. However, PCR is now done in test tubes, and it takes only a few hours to get the result.
  • Cross-reaction with other non-targeted microorganisms is rare if the test method is validated.

Placing a strip of eight PCR tubes, each containing a 100 μl reaction mixture, into the thermal cycler
Placing a strip of eight PCR tubes, each containing a 100 μl reaction mixture, into the thermal cycler | Source

Sources & Further Reading

  • Downes, Frances Pouch, and Keith Ito, . Compendium of Methods for the Microbiological Examination of Foods. American Public Health Association, 2001.
  • ICMSF. Microorganisms in Foods 2: Sampling for Microbiological Analysis: Principles and Specific Applications. Toronto: University of Toronto Press, 1986.
  • International Commission on Microbiological Specifications for Foods (ICMSF), and Fred S. Thatcher. Microorganisms in foods. 1, 1. Toronto: University of Toronto Press, 1978.
  • Jim Monaghan, Mike Hutchison. "Monitoring microbial food safety of fresh produce." Warwickshire: Food Standards Agency and Horticultural Development Company, 2010.
  • John M. S. Bartlett, David Stirling. PCR Protocols. Humana Press, 2003.
  • Lee W. Riley, M.D., Robert S. Remis, M.D., M.P.H., et al. "Hemorrhagic Colitis Associated with a Rare Escherichia coli Serotype." N Engl J Med, 1983: 681-685.
  • Radomir, ed. Food Quality and Standards - Food Microbiology. Vol. III. Oxford: Eolss Publishers Co. Ltd, 2009.
  • Council, United Fresh Produce Association Food Safety & Technology. Microbiological Testing of Fresh Produce. Washington, DC: United Fresh Produce Association, 2010.

© 2014 Imtiaz Ibne Alam

4 comments

Allan Danns 2 years ago

Given the nuclear radiation leakage into the seas around Japan, how would you test seafood products (particularly fish ) for radiation and for mercury poisoning or contamination? How can one find out if the authorities and merchants respectively in order to preserve economic growth and their profits among other things, respectively, fail to notify consumers?


Abdulghani Ameri 2 years ago

Helpful for students taking a course in the field.


Dr. Matiur Rahman 2 years ago

Very good


James Brown 2 years ago

Thanks :)

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