The relationship between the antibacterial effect of Nisin and packaging materials

As a natural antimicrobial peptide produced by the fermentation of Streptococcus lactis, Nisin boasts advantages such as high safety (classified as a GRAS-level food additive by the FDA), targeted antimicrobial spectrum (highly effective against Gram-positive bacteria like Listeria monocytogenes and Staphylococcus aureus), and easy degradation without residues. It is thus a crucial alternative to chemical preservatives in the food industry. However, Nisin’s antimicrobial efficacy is not isolated; it is closely linked to the packaging materials that carry or release it. The type, structure, surface properties of the packaging material, and its binding method with Nisin directly affect Nisin’s loading capacity, release rate, stability, and final antimicrobial performance. Starting from the core dimensions of how packaging materials influence Nisin’s antimicrobial efficacy, this article systematically analyzes the compatibility, mechanism of action, and application optimization strategies between different types of packaging materials and Nisin, providing theoretical support for the development of high-efficiency antimicrobial packaging.

I. Core Mechanisms of How Packaging Materials Influence Nisin’s Antimicrobial Efficacy

Nisin exerts its antimicrobial effect by directly interacting with microbial cell membranes (e.g., disrupting membrane integrity, inhibiting cell wall synthesis). Packaging materials regulate the concentration and duration of Nisin in the food microenvironment through a "loading-release" process, and indirectly affect its antimicrobial activity by protecting Nisin from external factors (e.g., light, oxygen, humidity). The core mechanisms can be summarized in three points:

1. Loading Capacity Determines Nisin’s Initial Effective Concentration

The loading capacity of a packaging material for Nisin (the amount of Nisin that can be bound per unit area or volume of material) directly determines the "initial ammunition reserve" of the antimicrobial system. Insufficient loading results in a concentration below the minimum inhibitory concentration (MIC, typically 0.1–10 μg/mL) after release, failing to inhibit microorganisms effectively. Excessive loading, however, may cause Nisin aggregation and inactivation (aggregation of antimicrobial peptides destroys their spatial structure, losing the ability to bind to cell membranes).

The key factors affecting loading capacity are the material’s pore size, specific surface area, and surface charge:

Porous materials (e.g., nanocellulose membranes) achieve high loading through physical adsorption or hydrogen bonding due to their large specific surface area (up to 100–500 m²/g).

Positively charged materials (e.g., chitosan coatings) enhance binding and improve loading stability via electrostatic attraction with negatively charged Nisin (isoelectric point pI ≈ 5.0, negatively charged in neutral environments).

2. Release Kinetics Regulate Nisin’s Long-Term Antimicrobial Efficacy

Nisin must be released from the packaging material to the food surface or interior to contact microorganisms and exert its effect. Too fast a release rate leads to rapid consumption of Nisin in a short time, resulting in an "antimicrobial gap" in the later stage due to insufficient concentration. Too slow a release rate fails to reach the inhibitory concentration quickly, allowing microorganisms to multiply in large numbers initially.

Packaging materials regulate release kinetics through "diffusion resistance" and "binding strength":

Dense synthetic polymer materials (e.g., polyethylene, PE) achieve slow release by restricting the diffusion of Nisin molecules.

Degradable natural polymer materials (e.g., starch, sodium alginate) enable fast diffusion and release of Nisin due to large gaps between molecular chains.

An ideal release profile should be "rapid initial release (quickly reaching the inhibitory concentration) + slow subsequent release (maintaining long-term inhibition)", which requires structural design of the packaging material.

3. Barrier Properties Protect Nisin’s Structural Stability

Nisin’s antimicrobial activity depends on its specific spatial structure (e.g., lanthionine rings). Light, oxygen, high temperature, and high humidity can cause structural damage or oxidative degradation (e.g., oxygen oxidizes sulfhydryl groups in Nisin, breaking disulfide bonds). The barrier properties (oxygen resistance, light resistance, moisture resistance) of packaging materials directly determine Nisin’s storage time and activity retention rate:

Aluminum foil composite films, with excellent oxygen and light resistance, can maintain over 90% of Nisin’s activity after 3 months of storage at 4°C.

Ordinary PE films, with poor oxygen resistance, only retain approximately 60% of Nisin’s activity under the same conditions.

II. Compatibility Study Between Different Types of Packaging Materials and Nisin’s Antimicrobial Efficacy

Different packaging materials vary significantly in composition, structure, and performance, leading to differences in their binding methods with Nisin and their impact on Nisin’s antimicrobial efficacy. Currently, commonly used packaging materials are mainly divided into three categories: natural polymer materials, synthetic polymer materials, and composite packaging materials, with the following compatibility characteristics:

(I) Natural Polymer Packaging Materials: High Compatibility and Rapid Release, Suitable for Short-Term Antimicrobial Needs

Derived from plants, animals, or microorganisms, natural polymer materials (e.g., chitosan, starch, sodium alginate, gelatin) have good biocompatibility, degradability, and abundant surface active groups (e.g., hydroxyl, amino, carboxyl groups). They bind to Nisin primarily through hydrogen bonding, electrostatic interaction, or physical adsorption, and are suitable for short-term (1–2 weeks) food preservation needs. Typical applications include:

1. Chitosan-Based Materials

Chitosan molecules contain a large number of positively charged amino groups, which can bind tightly to negatively charged Nisin via electrostatic attraction, with a loading capacity of 50–100 mg/g (based on chitosan mass). Its antimicrobial advantage lies in "synergistic enhancement": chitosan itself inhibits both Gram-positive and Gram-negative bacteria, and its combination with Nisin broadens the antimicrobial spectrum (e.g., the inhibition rate against Gram-negative bacteria like E. coli increases from 30% with Nisin alone to 75%).

For example: Preparing films by solution casting with Nisin (2% addition) and chitosan for chilled meat packaging. At 4°C, the total bacterial count can be maintained below 10⁵ CFU/g within 10 days (compared to 10⁸ CFU/g in the pure chitosan film group within 10 days), and the film can be completely degraded in soil within 30 days. However, chitosan films have poor moisture resistance (water vapor permeability, WVP ≈ 1.5×10⁻¹⁰ g·m/(m²·s·Pa)). In high-humidity environments, water absorption and swelling cause excessive rapid release of Nisin, resulting in insufficient long-term antimicrobial efficacy (inhibition rate drops from 95% to 50% after 10 days). Cross-linking modification (e.g., cross-linking with glutaraldehyde) is required to improve moisture resistance.

2. Starch-Based Materials

Starch is widely available and low-cost, but pure starch films have poor mechanical properties and weak antimicrobial activity. Adding Nisin significantly enhances their functionality. Hydroxyl groups in starch molecules form hydrogen bonds with Nisin, with a loading capacity of approximately 10–30 mg/g and a fast release rate (over 80% release within 24 hours), making them suitable for short-term preserved foods (e.g., fresh fruits and vegetables).

For example: Corn starch films with 1.5% Nisin added have a 90% inhibition rate against Botrytis cinerea (the main spoilage fungus) on strawberry surfaces. The decay rate of strawberries stored at 4°C for 7 days decreases from 45% (control group) to 12%. However, starch films have poor oxygen resistance (oxygen transmission rate, OTR ≈ 5×10⁻¹³ m³·m/(m²·s·Pa)), which easily causes oxidative degradation of Nisin. They need to be compounded with oxygen-resistant materials (e.g., beeswax) to extend Nisin’s activity retention time.

(II) Synthetic Polymer Packaging Materials: Controlled Release and Strong Barriers, Suitable for Long-Term Antimicrobial Needs

Synthetic polymer materials (e.g., polyethylene (PE), polypropylene (PP), polyethylene terephthalate (PET), polylactic acid (PLA)) have excellent mechanical properties, adjustable barrier properties, and high stability. They bind to Nisin primarily through physical blending or surface coating, and can achieve controlled release of Nisin by regulating the material structure, making them suitable for long-term (1–3 months) food preservation needs. Typical applications include:

1. Polyethylene (PE) and Polypropylene (PP)

PE and PP are the most widely used materials in food packaging, with stable chemical properties but poor compatibility with Nisin. Direct blending easily causes Nisin aggregation (aggregation rate up to 30% or more), affecting release uniformity. Therefore, the "surface coating method" is usually used to load Nisin on the surface of PE/PP:

For example: Coating the surface of PE films with a mixed solution of Nisin (5%–10% concentration) and polyvinyl alcohol (PVA, as an adhesive) via ultrasonic spraying to form an antimicrobial coating with a thickness of 5–10 μm. Nisin is slowly released through the hydration of PVA (approximately 60% release within 7 days). When used for meat packaging, the detection rate of Listeria after 30 days of freezing at -18°C decreases from 100% (control group) to 15%.

PE/PP has the advantage of strong moisture resistance (WVP ≈ 5×10⁻¹¹ g·m/(m²·s·Pa)), which can reduce the interference of moisture on Nisin release. However, oxygen resistance needs to be improved by compounding (e.g., co-extrusion with EVOH) to avoid Nisin oxidation.

2. Polylactic Acid (PLA)

PLA is a degradable synthetic polymer material with better compatibility with Nisin than PE/PP (ester groups in PLA can form weak hydrogen bonds with amino groups in Nisin). Nisin (1%–3% addition) can be uniformly dispersed in the PLA matrix via melt blending. The crystallinity of PLA can regulate the release rate of Nisin:

Low-crystallinity PLA (crystallinity < 20%) has large gaps between molecular chains, leading to fast Nisin release (85% release within 30 days).

High-crystallinity PLA (crystallinity > 40%) has tight molecular chains, resulting in slow Nisin release (45% release within 30 days).

For example: High-crystallinity PLA films with 2% Nisin added for bread packaging. After 30 days of storage at 25°C, the number of mold colonies in bread decreases by 80% compared to the control group, and the film degrades by 60% in soil within 60 days, balancing antimicrobial properties and environmental friendliness.

(III) Composite Packaging Materials: Synergistic Multi-Performance, Suitable for Complex Food Antimicrobial Needs

A single material can hardly meet the requirements of "high loading, controlled release, and strong barriers" simultaneously. Composite packaging materials (e.g., natural-synthetic polymer composites, multi-layer co-extruded composites, nanocomposites) integrate the advantages of different materials, enabling precise regulation of Nisin’s antimicrobial efficacy. They are suitable for antimicrobial needs of complex foods (e.g., dairy products, aquatic products) that are high in moisture and prone to oxidation. Typical applications include:

1. Chitosan-PLA Composite Films

Chitosan provides high loading capacity (can bind Nisin via electrostatic interaction), while PLA provides strong barrier properties and controlled release performance. Their combination achieves "synergistic optimization":

For example: Coating a chitosan-Nisin layer (Nisin loading capacity 50 mg/g) on the surface of PLA films to form a double-layer composite film. Nisin in the chitosan layer is released rapidly in the early stage (40% release within 24 hours, quickly reaching the inhibitory concentration), and the PLA layer restricts subsequent release (70% total release within 30 days, maintaining long-term inhibition). When used for cheese packaging, the total bacterial count of cheese stored at 4°C for 21 days decreases by 90% compared to the control group, avoiding the high hygroscopicity of single chitosan films and the low loading capacity of single PLA films.

2. Nanocomposite Films (e.g., Nisin-Montmorillonite-PET Composite Films)

Montmorillonite (MMT) is a layered nanomaterial with a large specific surface area (up to 750 m²/g), which can load Nisin via interlayer adsorption (loading capacity up to 100–200 mg/g). Its layered structure can block the penetration of oxygen and light. Preparing nanocomposite films by melt blending Nisin-MMT composites with PET:

MMT not only improves the loading and stability of Nisin (92% Nisin activity retention rate after 3 months of storage at 4°C) but also reduces the oxygen transmission rate of PET by "nanobarrier effect" (OTR decreases by 50% compared to pure PET). When used for juice packaging, the number of acetic acid bacteria in juice stored at room temperature for 14 days decreases by 85% compared to the control group, without affecting the juice flavor.

III. Key Strategies to Optimize the Compatibility Between Packaging Materials and Nisin

To maximize Nisin’s antimicrobial efficacy, it is necessary to optimize packaging materials for their shortcomings. The core strategies focus on "improving loading uniformity, regulating release kinetics, and enhancing barrier protection":

1. Material Surface Modification: Improve Nisin Loading Uniformity

For synthetic polymer materials (e.g., PE, PP) with poor compatibility with Nisin, surface modification methods such as plasma treatment and UV irradiation can introduce active groups (e.g., hydroxyl, carboxyl groups) on the material surface, enhancing interaction with Nisin and reducing aggregation.

For example: After plasma treatment of PE films, the surface hydroxyl content increases by 3 times, Nisin’s loading uniformity increases from 65% to 90%, and the fluctuation range of the release rate narrows from ±20% to ±5%.

2. Structural Design: Achieve "Pulsed Release" of Nisin

By designing multi-layered packaging materials and regulating the release rate of different layers, "rapid initial release + slow subsequent release" can be achieved.

For example: A three-layer film (inner layer: fast-release layer, e.g., chitosan-Nisin; middle layer: controlled-release layer, e.g., low-crystallinity PLA; outer layer: barrier layer, e.g., EVOH). Nisin in the inner layer is released 30% within 12 hours (rapid inhibition), the middle layer controls slow release of Nisin within 30 days (60% total release, maintaining long-term inhibition), and the outer layer prevents oxygen entry to protect Nisin activity. When used for aquatic product packaging, the antimicrobial duration is doubled compared to single-layer films.

3. Barrier Enhancement: Extend Nisin’s Activity Retention Time

For natural polymer materials with poor barrier properties, compounding with oxygen-resistant and light-resistant materials (e.g., aluminum foil, EVOH, beeswax) can improve protection.

For example: After compounding sodium alginate-Nisin films with aluminum foil, oxygen resistance increases by 10 times, and Nisin’s activity retention rate after 3 months of storage at 4°C increases from 55% to 90%, making it suitable for oxygen-sensitive foods (e.g., cooked meat products).

4. Compound Antimicrobial: Broaden Nisin’s Antimicrobial Spectrum

Packaging materials can simultaneously load Nisin and other natural antimicrobial agents (e.g., plant essential oils, lysozyme) to broaden the antimicrobial spectrum through synergistic effects.

For example: Co-loading Nisin (1%) and carvacrol (0.5%) in chitosan-PLA composite films. The inhibition rate against Gram-negative bacteria (e.g., E. coli) increases from 30% with Nisin alone to 85%, making it suitable for foods at high risk of multi-microbial contamination (e.g., ready-to-eat vegetables).

IV. Conclusions and Future Directions

The relationship between Nisin’s antimicrobial efficacy and packaging materials is essentially a synergistic matching of "material properties – Nisin behavior – antimicrobial performance". Specifically, the loading capacity of the packaging material determines Nisin’s initial concentration, the release kinetics determine the long-term antimicrobial efficacy, and the barrier properties determine the activity stability.

Natural polymer materials are suitable for short-term antimicrobial needs, synthetic polymer materials for long-term needs, and composite materials, by integrating multiple properties, for the antimicrobial needs of complex foods. Currently, the compatibility between Nisin and packaging materials still faces challenges such as "poor compatibility with synthetic materials, weak barrier properties of natural materials, and insufficient precision in release regulation". Future breakthroughs need to be made in three aspects:

1. Precise Regulation of Release

Combine computer simulations (e.g., molecular dynamics simulations) to predict the diffusion behavior of Nisin in different materials and design customized release profiles.

2. Intelligent Responsive Release

Develop pH- and temperature-responsive packaging materials (e.g., pH-sensitive chitosan derivatives) to enable Nisin to be released precisely when environmental changes occur due to microbial reproduction (e.g., pH decrease).

3. Green and Low-Cost Preparation

Explore the use of agricultural wastes (e.g., straw starch, shrimp and crab shell chitosan) to prepare packaging materials, which can reduce costs while improving the biocompatibility of Nisin, thereby promoting the industrial application of natural antimicrobial packaging.

By continuously optimizing the compatibility between packaging materials and Nisin, the natural antimicrobial advantages of Nisin can be fully utilized, the use of chemical preservatives can be reduced, and a safe, efficient, and environmentally friendly antimicrobial solution can be provided for the food industry.