Food microbiology : How to choose the best Buffered Peptone Water ?

BPW (Buffered Peptone Water) is an enrichment broth (or diluent) used in most food microbiology laboratories.

When we started writing this article, we thought, “BPW is BPW—they’re all the same! Plus, with supplier certificates of analysis and ISO 11133 tests, there’s no risk.”

Apparently, we weren’t the only ones who thought that:

Well, we were completely wrong!

Through our discussions and experiences, we realized it’s not that simple, and choosing the right BPW is (definitely) not something to take lightly.

BPW is the starting point of the entire analysis. If it doesn’t work properly, the whole analysis that follows can be compromised. This can even lead to the risk of false negatives.

In this article, we’ve gathered as much information as possible to help you choose the most suitable Buffered Peptone Water for your matrices and analyses.

Content :

• What is BPW?
• Uses of BPW
• Different Formats of Buffered Peptone Water
• What is the Composition of BPW?
• Criteria for Choosing BPW
• Special Cases

What is Buffered Peptone Water (BPW)?

Buffered Peptone Water is a non-selective liquid culture medium (a broth) that allows the multiplication (enrichment) of all microorganisms. So we call it an enrichment broth.

Uses of BPW

BPW is commonly used in food microbiology laboratories to detect certain microorganisms, such as Salmonella.

Typically, 25 g of product (matrix) is diluted using a gravimetric diluter with 225 ml of BPW enrichment broth.

This mixture is then homogenized in a lab blender to extract microorganisms from the matrix.

Then the sample is incubated for several hours in an incubator.

After incubation, a portion is taken for analysis using a detection method (culture media, PCR, or others).

BPW can also be used for the detection of Cronobacter sakazakii and the enumeration of Listeria monocytogenes.

It can also be used as a diluent for serial dilutions.

Different Formats of Buffered Peptone Water

Today, food microbiology laboratories can choose from several BPW formats:

• Powder: For preparing BPW on-site, either with a media preparator or an autoclave.
  
• Ready-to-Use (RTU) Bags: Sterile bags available in 3 or 5 liters.
  
• Dehydrated Bags: To be prepared in the laboratory.

Each format comes with its own advantages and disadvantages.

What is the Composition of BPW?

The composition of Buffered Peptone Water is specified in several standards, such as ISO 11133 and ISO 6887-1.

Composition per liter:

• Peptone: 10.00 g
• Sodium chloride: 5.00 g
• Disodium phosphate anhydrous: 3.57 g
• Monopotassium phosphate anhydrous: 1.50 g
• Final pH: 7.2 ± 0.2 at 25 °C

Just four ingredients… it seems simple enough. And yet, if all BPW formulas were truly the same, why would they each have a slightly different color?

“You can follow the same recipe as a Michelin-starred chef… but that doesn’t mean your dish will taste as good! The same goes for BPW.”

A good microbiologist cook !

Everyone we spoke to (suppliers, auditors, microbiologists) agrees there are several variables in BPW production:

• The quality and choice of peptones.
  
• The manufacturing process.
  
• Performance verification of raw materials.

So, it’s not a myth, there are real quality differences between the BPW options on the market. That’s also why prices can vary significantly.

But how do you tell the difference between good BPW and bad BPW?

Criteria for Choosing BPW

As we’ve seen, not all BPWs are created equal. In fact, some suppliers even offer two types:

• Basic BPW
  
• “Enhanced” BPW

So how do you choose the right one?

Your decision will depend on several factors, such as:

• Your matrices: Are they acidic, acidifying, or alkaline?
  
• Your production processes: Will the strains be stressed?
  
• Your protocol: Are you working with large weights (375 g) or a dilution ratio below 1/10?
  
• Your detection methods: Are you using short enrichment times or highly sensitive detection methods?

If you answered “yes” to some of these questions, you’ll need to pay special attention to your choice of BPW! The enhanced version of the BPW might be the right choice for you !

Testing the Buffering Capacity of Enrichment Broth

Some matrices, such as cheese, sauces, or milk powder, can be acidic, acidifying, or alkaline.

If the BPW’s buffering capacity is insufficient, there’s a risk of the broth becoming too acidic or too basic.

This creates suboptimal growth conditions for microorganisms, which can cause issues with short incubation times and sensitive detection methods (risk of false negatives).

Test to Perform:

• Combine BPW and your matrix → Measure pH before and after incubation.

If your BPW’s buffering capacity isn’t sufficient for your matrices, consider using double-buffered BPW options (see below).

Testing the Growth Performances of the BPW

To thoroughly evaluate your BPW, you can test its performance using the ISO 16140-3 protocol. This helps assess the effectiveness of the “golden trio”:

1. The broth itself.
   
2. The matrix being tested.
   
3. The detection method.

Does this mean you need to re-validate every time you switch BPW suppliers?

Given the performance differences between BPWs, the answer is “ideally, yes.” Ouch—we know that’s not what everyone wants to hear!

Certificates of Analysis and ISO 11133

It’s often said that BPW is “good” because it comes with a supplier’s certificate of analysis and has been tested in the lab according to ISO 11133.

However, these are two different things. The certificate of analysis and ISO 11133 only confirm that the received batch meets the required specifications.

They do not guarantee that this BPW formulation will perform well with your matrices and detection methods.

To verify that, you would need to test it using the ISO 16140-3 protocols.

Special Cases for Buffered Peptone Water

BPW + Polysorbate 80

If your matrices contain more than 20% fat, you need to use BPW with polysorbate 80 (Tween 80).

This emulsifying and dispersing agent ensures that microorganisms come into proper contact with the enrichment broth.

Double-Concentration BPW

Most of the time, when referring to “double-concentration BPW,” it actually means double-buffered BPW (only the buffer concentration is doubled).

This is particularly recommended for matrices that are highly acidic or alkaline.

Composition per liter (double-buffered):

• Final pH: 7.2 ± 0.2 at 25 °C
• Peptone: 10.00 g
• Sodium chloride: 5.00 g
• Disodium phosphate anhydrous: 7.14 g
• Monopotassium phosphate anhydrous: 1.50 g

However, ISO 6887-1 specifies that if the powdered preparation already includes all the ingredients, you simply need to double the quantity. In that case, the amount of peptone and sodium chloride will also be doubled.

BPW from the Alternative Method Provider

Most methods validated by AFNOR or Microval or AOAC are compatible with non-proprietary BPWs, meaning laboratories can use any BPW they choose with the certified method.

However, during validation, each provider uses their own BPW. Since BPW performance can vary significantly, it makes sense to prioritize the BPW reference used during the validation of the detection method.

Conclusion

As we’ve seen, not all BPWs are created equal.

Unfortunately, we can’t tell you which one to use, it all depends on your matrices and detection methods.

It’s up to you to conduct a risk analysis and, if necessary, perform a few tests in your lab.

After all, what’s the point of having a Rolls Royce for detection if you don’t give it the right… fuel? 😉