Analyze the physicochemical properties of Nisin

As a natural antibacterial peptide produced by the fermentation of Streptococcus lactis, the physicochemical properties of nisin directly determine its application scope and effectiveness in fields such as food preservation and biological fresh-keeping. Among these properties, solubility, thermal stability, and pH dependence are core characteristics—they are interrelated and regulated by environmental factors, with detailed analysis as follows:

Solubility of Nisin

The solubility characteristics of nisin are closely related to the amino acid composition and charge state in its molecular structure. Nisin’s molecule consists of 34 amino acid residues, including a relatively high proportion of positively charged basic amino acids (e.g., lysine, arginine), which gives it an overall weakly alkaline nature. The key factors affecting its solubility are the pH value and ionic strength of the solvent.

Under neutral to weakly acidic conditions (pH 2.0–7.0), the amino groups (-NH₂) in the Nisin molecule easily bind to protons to form positively charged -NH₃⁺. This charged state enhances its interaction with polar solvents (e.g., water) and reduces intermolecular aggregation caused by hydrophobic interactions, resulting in high solubility. For example, in an acetic acid buffer at pH 4.0, its solubility can reach 20–30 mg/mL, which meets the "uniform dispersion" requirement in most food processing scenarios.

However, when the pH value rises to the alkaline range (pH > 8.0), the basic groups in the molecule become deprotonated, positive charge decreases, and hydrophobic regions are exposed. Molecules tend to form aggregates through hydrophobic bonds and hydrogen bonds, leading to a significant decrease in solubility and even precipitation. In addition, the ionic strength of the solvent also affects solubility: low-concentration salt solutions (e.g., 0.1 mol/L NaCl) can promote Nisin dissolution through the "salting-in effect," while high-concentration salts (e.g., > 1 mol/L NaCl) may reduce solubility by competing for water molecules in the solvent.

It is worth noting that nisin has low solubility in pure water (approximately 1–2 mg/mL). In practical applications, solubility is often improved by adjusting pH (e.g., adding citric acid or lactic acid) or mixing with small amounts of polar solvents (e.g., ethanol) to ensure uniform exertion of its antibacterial activity.

Thermal Stability of Nisin

The heat resistance of nisin exhibits significant "pH dependence," a property that gives it differentiated applicability in foods processed at different temperatures.

Under acidic conditions (pH 2.0–5.0), the secondary structure of the Nisin molecule (e.g., cyclic structures formed by thioether bonds) has high stability, and its thermal denaturation temperature is significantly increased. For example, in fruit juice at pH 3.0, after high-pressure sterilization at 121°C (for 20 minutes), it can retain more than 70% of its antibacterial activity; even under conventional heating (e.g., pasteurization) at 80–100°C, the activity retention rate can exceed 90%. This high stability stems from the resistance of intramolecular hydrogen bonds and thioether bonds to breakage in acidic environments, as well as the reduced intermolecular aggregation and denaturation due to the positive charge state.

As the pH value increases, thermal stability decreases significantly: under neutral conditions (pH 7.0), heating at 80°C for 10 minutes can cause approximately 30% loss of activity; under alkaline conditions (pH > 8.0), even moderate-temperature treatment at 60°C may lead to more than 50% activity loss due to molecular structure depolymerization and cyclic structure breakage.

In addition, heating time and water activity also affect thermal stability: in low-water-activity environments (e.g., dehydrated foods, powdered Nisin formulations), molecular movement slows down, and the rate of thermal denaturation decreases. Therefore, solid Nisin can be stably stored at room temperature for 1–2 years, while in high-water-activity liquid foods, heat sensitivity is further enhanced. This property makes Nisin particularly suitable for thermal processing and preservation of acidic foods (e.g., yogurt, pickles, acidic beverages). For neutral or alkaline foods (e.g., milk, soy products), pH adjustment (e.g., adding small amounts of organic acids) or low-temperature processing techniques are required to reduce activity loss.

pH Dependence of Nisin

pH dependence is a core characteristic that runs through Nisin’s solubility, thermal stability, and antibacterial activity. Essentially, pH value regulates the molecular charge state and structural stability of Nisin.

Beyond the aforementioned effects on solubility and thermal stability, pH value directly determines the exertion of nisin’s antibacterial activity: under acidic conditions, Nisin molecules carry positive charges and easily bind to negatively charged bacterial cell membranes (mainly composed of phospholipids) through electrostatic interactions. They then insert into the cell membrane to form pores, causing leakage of bacterial contents and subsequent bacterial death. Under alkaline conditions, the negative charge on the bacterial cell membrane surface decreases, weakening the binding ability between nisin and the cell membrane; at the same time, the nisin molecule itself depolymerizes, leading to a significant decline in antibacterial activity. For example, at pH 3.0, the minimum inhibitory concentration (MIC) of Nisin against Staphylococcus aureus is approximately 0.1 μg/mL, while at pH 8.0, the MIC can increase to more than 10 μg/mL, resulting in a substantial reduction in antibacterial efficacy.

In addition, pH value affects the duration of nisin’s stability in food: in acidic foods (e.g., yogurt at pH 4.0), nisin can exist stably for 2–3 weeks under refrigeration (4°C) and continuously exert a preservative effect; in neutral foods (e.g., fresh milk at pH 7.0), even under refrigeration, activity may decrease by more than 40% within one week due to slow structural degradation.

This strong pH dependence is both an application advantage of nisin (e.g., high efficiency and stability in acidic foods) and a limitation on its direct application in neutral and alkaline foods. Formulation improvements (e.g., microencapsulation, compounding with other preservatives) are required to expand its pH application range.

In summary, the physicochemical properties of nisin form a synergistic effect centered on "pH regulation": under acidic conditions, it exhibits high solubility, strong thermal stability, and excellent antibacterial activity, adapting to the processing and storage needs of acidic foods; under alkaline conditions, it shows the opposite characteristics. In practical applications, based on parameters such as food pH value, processing temperature, and water activity, the natural antibacterial advantages of nisin should be maximized by adjusting the solvent environment, optimizing processing techniques, or adopting formulation technologies—while avoiding the adverse effects of environmental factors on its properties.