Nisin under extreme pH conditions

Nisin, a natural preservative, is widely used in the food industry. Studying its stability under extreme pH conditions is crucial for expanding its application scope and ensuring food safety. The following briefly outlines the research background, stability performance under different extreme pH conditions, influencing factors, and research methods:

I. Research Background

Nisin exhibits excellent antibacterial properties, effectively inhibiting the growth of various Gram-positive bacteria. However, food systems span a wide pH range, and some processing methods (such as fermentation and pickling) can expose foods to extreme pH environments. Understanding its stability under extreme pH conditions helps determine its applicability in different foods and ensures its antibacterial activity during processing and storage.

II. Stability Performance Under Different Extreme pH Conditions

1. Acidic Conditions

Nisin generally maintains good stability in low pH environments. When the pH is between 2 and 6, its structure remains relatively stable with minimal loss of antibacterial activity. This is because amino acid residues in the nisin molecule are less prone to protonation and deprotonation reactions under acidic conditions, preserving its active conformation. For example, in acidic foods like yogurt and jam, Nisin can sustain its antibacterial effect for extended periods.

2. Alkaline Conditions

Nisin shows poor stability in strongly alkaline environments (pH > 9). Alkaline conditions cause hydrolysis of peptide bonds in its molecule, leading to structural damage and significant reduction in antibacterial activity. Additionally, alkaline environments may promote oxidation, deamination, and other chemical reactions in the molecule, further affecting its stability. For instance, in alkaline foods such as certain alkaline beverages and pastries, Nisin’s antibacterial efficacy may be substantially compromised.

III. Factors Influencing Stability

1. Temperature

Temperature and pH synergistically affect Nisin’s stability. Under extreme pH conditions, high temperatures accelerate degradation reactions. For example, in alkaline conditions, high temperatures increase the hydrolysis rate of peptide bonds, causing rapid loss of Nisin’s antibacterial activity.

2. Ion Strength

Ion strength in solutions influences the charge distribution and solvation around Nisin molecules, thereby affecting stability. In high-ion-strength environments, molecules may aggregate or precipitate, reducing antibacterial activity. Certain ions (e.g., metal ions) may also complex with Nisin, impacting its structure and function.

3. Food Components

Other components in foods can affect Nisin’s stability under extreme pH conditions. Macromolecules like proteins and polysaccharides may interact with Nisin, protecting it from extreme pH damage, while enzymes may degrade Nisin and reduce stability.

IV. Research Methods

1. High-Performance Liquid Chromatography (HPLC)

HPLC is used to separate and detect Nisin and its degradation products, analyzing stability under extreme pH conditions. By comparing chromatograms at different time points, degradation rates and product compositions can be determined.

2. Antibacterial Activity Assays

Methods such as the agar diffusion method and microbroth dilution method are employed to measure Nisin’s antibacterial activity under different extreme pH conditions. Stability is evaluated by comparing its inhibitory effect on indicator bacteria before and after treatment.

3. Structural Analysis Techniques

Techniques like mass spectrometry and nuclear magnetic resonance (NMR) are used to analyze molecular structure changes of Nisin under extreme pH conditions, providing insights into the molecular mechanisms of its stability.