Basics of food microbial testing

October 18, 2023

Table of Contents

  1. What is food microbial testing?
  2. Purpose of performing microbial testing
  3. Microbiological testing procedure
  4. Points to note when choosing a microbial testing method
  5. What is Petrifilm® medium?

What is food microbial testing?

Food microbial testing is a process by which food business operators check the number of bacteria in order to confirm the safety of the food, the presence of microorganisms that can cause food poisoning, and the sanitary conditions.

Food businesses must ensure and confirm food safety throughout the food chain, including raw material procurement, manufacturing, transportation, and storage.

Examples include receiving inspections of raw materials, wiping inspections during the manufacturing process, inoculation inspections during product development (inoculation with specific microorganisms and checking for risks posed by those microorganisms), and storage inspections (inspections in which food is stored under certain conditions). Inspections are conducted to confirm the safety of the food during its storage period), as well as sampling inspections of the final product.

If the test results show that the value exceeds the standard value, we will investigate the cause and take countermeasures, and then conduct a microbial test to confirm whether the sanitary conditions have improved.

Microbiological testing is thus carried out to identify microbiological risks in foods.

Purpose of performing microbial testing

"What are you doing the test for?"

The purpose is to think about this question and be able to answer it clearly.
Simply saying "because it's stipulated by the Food Sanitation Law" or "because I inherited it from my boss or seniors" doesn't mean that I understand the purpose.

Generally, the items (target microorganisms) that should be tested for each food are determined by the Food Sanitation Law. However, since many products are manufactured using a wide variety of ingredients, they are not necessarily covered only by the items stipulated in the Food Sanitation Act. Therefore, if it is determined that there is a risk from the ingredients handled or the manufacturing process, it is necessary to conduct additional inspections on your own.

For example, from the perspective of the risk of pathogenic bacteria, chicken meat has a risk of Salmonella food poisoning and Campylobacter food poisoning, but Campylobacter is not included in the inspection target under the Food Sanitation Act. For this reason, you must determine the need for additional testing in-house based on your objectives.

It is also important to consider the significance of the test results. For example, a bento box is a food that is made up of multiple ingredients, and includes a mixture of heated and non-heated foods (raw vegetables, pickles, etc.). The test results obtained and their meaning will vary greatly depending on whether these are tested together or separately. When designing a test, it is necessary to decide in advance what information will be sought during the test and what decisions will be made based on the results.

Against this background, it is important to have a firm understanding of the purpose of inspection, clarify internal standards, and conduct inspections.

Microbial testing procedure

Generally, microbial testing is performed in the following order: preparation, preparation, inoculation, culture, determination, and cleanup.
The specific process is as follows.

  • Preparation: Prepare yourself, clean the examination room, and prepare the necessary instruments, equipment, culture media, etc.
  • Preparation: Prepare a sample solution to test for microorganisms in the target food.
    • Specifically, food and sterilized dilution water are placed in a special bag, and the microorganisms in the food are squeezed out.
  • Inoculation: Drop the prepared sample solution onto a nutrient-rich medium.
  • Cultivation: The culture medium is placed for a certain period of time under conditions that allow microorganisms to grow, increasing the number of microorganisms.
  • Judgment: Measure the increased microorganisms. Compare the results with the standard values ​​to determine pass/fail.
  • Cleaning up: Cleaning used instruments, equipment, and testing rooms, and disposing of garbage such as culture media.
  • Additionally, there are two main methods for conducting microbial testing.

A test method using an agar medium and a test method using a medium that can be tested quickly and easily.

Testing methods using agar media have been used for a long time, but it takes time to prepare in advance because it requires dissolving a powdered medium in sterile dilution water and pouring it into a Petri dish to solidify.

In addition, agar plates are often complicated to work with and require special care, so a great deal of knowledge and experience is required to perform tests correctly.

Therefore, in recent years, many media have been used that have solved these problems with agar media and can produce test results more quickly and easily.

A typical example is Petrifilm medium.


Comparison of testing process between agar medium and Petrifilm medium

Inspection Method Advance Preparation Preparation Inoculation Culture Judgement Tidying Up
Test method using agar medium Many instruments and equipment are required

Medium preparation is required
Same work for both testing methods (preparing sample liquid from food) Drop the sample solution into the petri dish, then pour in the medium, mix and wait until it solidifies.

About 30 minutes until the medium solidifies.
Requires large incubator It may be difficult to distinguish between microorganisms and food residue There is a lot of equipment and equipment to clean up

There is a lot of garbage
Testing method using Petrifilm medium Less equipment and equipment required

No need to prepare culture medium
Same work for both testing methods (preparing sample liquid from food) Drop the sample solution onto  Petrifilm medium, spread it with a special jig (spreader), and wait until it gels.

About 1 minute until it gels
Can be handled with a small incubator Easy to distinguish from food residue because microorganisms are colored Less equipment and equipment to clean up

Less amount of garbage


As mentioned above, the testing process for both agar and Petrifilm media is not significantly different. However, Petrifilm medium has the following three advantages over testing methods using agar media.

  1. The tools and equipment required for advance preparation are minimal (a small incubator can also be used)
  2. No need for culture medium preparation (ready to use right out of the bag)
  3. Easy and efficient work (inoculation operation is simple). It's easy to judge, and it takes a short time to clean up.)

In this way, the materials that need to be prepared and the skills required will vary depending on the inspection method.
Therefore, it is important to understand in advance that there are differences depending on the testing method.

Points to note when choosing a microbial testing method

There are two points to keep in mind when choosing a microbial testing method. These are "reliability" and "accuracy".

Reliability means that a method is recognized as valid based on scientific evidence, or that it can be proven. No matter how many tests you perform, there is no point unless the method yields correct test results. Therefore, in order to prove that the test method is a method that can obtain correct test results, it is necessary to either obtain certification from an external third-party certification body or prove its validity in-house.

On the other hand, accuracy refers to the degree to which variations in test results between inspectors can be suppressed and stable test results can be obtained. Even if a validated test method is selected, correct results will not be obtained if the tester is unable to use it correctly. Therefore, when selecting an inspection method, it is desirable to consider the ability of the inspector and use a method that is simple and easy for anyone to use.

Generally speaking, it is considered to be difficult to satisfy the above two points using a testing method using an agar medium. When testing using agar media, there are many things that must be followed during the process, and if the tester is not an experienced tester, the test results are likely to vary. Additionally, it is extremely difficult for a company to prove the validity of its testing methods based on scientific evidence.

After understanding these things, you need to select the inspection method that is suitable for your company.

What is Petrifilm medium?

Petrifilm media is a revolutionary dry film-like medium that replaces the agar medium released  in 1984. Unlike agar media, it does not require the work of "melting a powdered medium, pouring it into a petri dish, and solidifying it"; it can be used immediately after being taken out of the bag.

The development of Petrifilm media began with an idea from a scientist working at 3M.

In 1980, a scientist working in 3M's Medical Products Division wanted to spend less time preparing agar plates and more time on more important tasks. Therefore, the scientist also visited other companies' laboratories to confirm the actual situation. As a result, other companies also take up a lot of space to store petri dishes, and the expiry date of flat plates (agar plates that are solidified by pouring them into petri dishes) is short, so they must be disposed of unused. We found that some people were dissatisfied with the agar medium, such as the lack of results.

To solve these problems, the scientist came up with the idea of ​​making the culture medium thin and film-like. However, development was difficult, and many trials and errors were made. Ultimately, a breakthrough was achieved by applying technology from another division within 3M, resulting in the creation of Petrifilm media.

Currently, in addition to its workability and ease of use, which can be used immediately after taking it out of the bag, it has also received high praise worldwide for its reliability, having obtained numerous certifications from third-party organizations. In Japan, it is included in the Food Hygiene Inspection Guidelines, a guideline for food microbial testing methods, and has been adopted by many customers.

Learn More about Petrifilm


Category: Food Safety