During the microbial fermentation process, we sometimes need to adjust the culture medium to a slightly acidic range. So what reagents can be used to adjust the fermentation broth to a slightly acidic environment and have a good buffering effect?

To adjust the culture medium to a slightly acidic environment (usually pH 5.5-6.5) and provide a good buffering effect at the same time, the following buffer reagents or buffer systems can be used:

1. Phosphate Buffer System

• Sodium dihydrogen phosphate (NaH₂PO₄) and disodium hydrogen phosphate (Na₂HPO₄) or potassium dihydrogen phosphate (KH₂PO₄) and dipotassium hydrogen phosphate (K₂HPO₄) can be used to adjust and maintain the pH of the culture medium in the slightly acidic range.

• pH range: The phosphate buffer system has good buffering capacity in the pH range of 5.8-8.0, and the pH can be accurately controlled by changing the ratio of NaH₂PO₄ and Na₂HPO₄.

• Advantages: The phosphate buffer system has high stability, low price, and is suitable for the cultivation of various microorganisms.

Preparation example (adjusted to pH 6.5):

• NaH₂PO₄ (0.1M): 8.1 mL

• Na₂HPO₄ (0.1M): 41.9 mL

• Make the total volume 100 mL

2. Citrate-Phosphate Buffer System

• The combination of citric acid and sodium dihydrogen phosphate (NaH₂PO₄) is an effective acidic buffer system suitable for maintaining pH in an acidic or slightly acidic environment.

• pH range: suitable for pH 2.6-7.6 regulation, good buffering of slightly acidic environment.

• Advantages: It can cover a wider acidic range, which is an ideal choice if you need to maintain a lower slightly acidic condition (such as pH 5.5-6.5).

Preparation example (adjusted to pH 6.0):

• Citric acid (0.1M): 19 mL

• Na₂HPO₄ (0.2M): 31 mL

• Make the total volume 100 mL

3. Acetate Buffer System

• The buffer system of acetic acid and sodium acetate performs well under slightly acidic conditions and is usually used to adjust and maintain pH in the range of 4.0-6.0.

• pH range: Applicable to pH 3.6-5.6, more suitable for experiments that need to maintain a slightly acidic environment.

• Advantages: Applicable to organic acid environments, especially when it comes to specific fermentation products (such as organic acids), the application of acetic acid system is more appropriate.

Preparation example (adjusted to pH 5.5):

• Acetic acid (0.1M): 24 mL

• Sodium acetate (0.1M): 26 mL

• Make the total volume 100 mL

4. MES buffer (2-(N-morpholino)ethanesulfonic acid)

• MES is a commonly used biological buffer, suitable for regulating a relatively stable slightly acidic environment.

• pH range: Mainly suitable for the pH range of 5.5-6.7, with good buffering capacity, very suitable for pH regulation in culture media and microbial fermentation systems.

• Advantages: Provides stable buffering in a narrow acidic range and has no toxic effects on bacteria or fermentation processes.

Preparation example (adjust to pH 6.1):

• Dissolve 19.52 g of MES in 800 mL of distilled water

• Adjust pH to 6.1 with 10 N NaOH

• Add water to 1000 mL

Summary

In slightly acidic environments, buffer systems such as phosphate, citric acid-phosphate, and acetic acid-sodium acetate can effectively regulate pH and maintain good buffering capacity. For general slightly acidic requirements (pH 5.5-6.5), the phosphate buffer system is the most commonly used and stable. If a stronger acidic buffering effect is required, consider using a citric acid-phosphate buffer system or MES buffer.