The relationship between the antibacterial effect of Nisin and light exposure

As a natural antibacterial peptide, the antibacterial activity of nisin depends on its intact molecular structure (a polypeptide chain containing 5 lanthionine rings). Light, a common environmental factor in food processing and storage, affects its antibacterial efficacy through a "structure destruction-activity attenuation" pathway. This effect is not a simple "inactivation upon light exposure" but is closely related to light wavelength, light intensity, irradiation duration, and coexisting systems (e.g., food matrix, pH value). A detailed analysis can be conducted from three aspects: mechanism of action, influence patterns, and prevention/control strategies.

I. Core Mechanism of How Light Affects Nisin’s Antibacterial Efficacy

The antibacterial activity of nisin stems from its specific spatial structure: the cyclic structures formed by thioether bonds such as lanthionine and β-methyl lanthionine in the molecule are key to its binding to phospholipids in bacterial cell membranes and the formation of transmembrane channels. Once this structure is destroyed, nisin can no longer penetrate bacterial cell membranes or cause the leakage of intracellular nutrients (e.g., potassium ions, amino acids), ultimately losing its ability to inhibit Gram-positive bacteria. Light damages nisin primarily through two pathways:

1. Photooxidation Destroys Thioether Bonds

Light (especially ultraviolet rays and short-wave components of visible light) activates oxygen or photosensitive substances (e.g., pigments, vitamin B2 in food) in the food system, generating reactive oxygen species (ROS, such as superoxide anions and hydroxyl radicals). These ROS preferentially attack the thioether bonds in nisin molecules: the sulfur atoms in thioether bonds are oxidized to sulfoxide or sulfone structures, leading to the breakage of cyclic structures and changes in molecular spatial conformation. For example, when exposed to ultraviolet B (UV-B, wavelength 280–320 nm), the production of ROS increases significantly, causing over 30% of nisin’s thioether bonds to be destroyed within 2–4 hours. Correspondingly, its antibacterial activity (measured by the diameter of the inhibition zone against Staphylococcus aureus) can decrease by 40%–50%.

2. Light-Induced Peptide Bond Breakage Causes Molecular Degradation

High-intensity light (e.g., intense visible light, ultraviolet rays) also acts directly on nisin’s peptide bonds: after peptide bonds absorb photon energy, C-N bonds break, decomposing nisin’s polypeptide chain into small-molecule fragments. These fragments lack intact cyclic structures and thus cannot exert antibacterial effects; some fragments may even bind to other components in food (e.g., proteins, carbohydrates), further reducing the concentration of active nisin in the system. For instance, in dairy products packaged in transparent containers, long-term exposure to strong outdoor light (light intensity > 5000 lux) can cause 25% of nisin’s peptide bonds to break within 7 days, resulting in almost complete loss of antibacterial efficacy.

II. Influence Patterns of Different Light Conditions on Nisin’s Antibacterial Efficacy

Nisin’s sensitivity to light is not fixed but varies significantly with light wavelength, intensity, and duration, and is regulated by the properties of the food matrix. The specific patterns are categorized as follows:

1. Light Wavelength: Short-Wave Light (Ultraviolet Rays) Causes Far Greater Damage Than Long-Wave Light (Visible Light, Infrared Rays)

Light of different wavelengths has different energy levels and thus different abilities to damage nisin’s structure:

Ultraviolet (UV) rays: The main type of light that impairs nisin’s antibacterial activity. Among them, UV-B (280–320 nm) has the highest energy and the strongest destructive effect on thioether bonds and peptide bonds. Studies have shown that under the same light intensity (1000 μW/cm²), 1 hour of UV-B irradiation increases the minimum inhibitory concentration (MIC) of nisin against Listeria from 25 IU/mL to 80 IU/mL, reducing antibacterial activity by 68%. In contrast, UV-A (320–400 nm) is less destructive—under the same conditions, it only increases the MIC to 40 IU/mL, reducing activity by 37%.

Visible light (400–760 nm): Alone, it causes minimal damage to nisin. However, when photosensitive substances (e.g., lycopene, chlorophyll, riboflavin) are present in food, a "photosensitization" effect enhances the destructive impact. For example, in beverages containing riboflavin, 5 days of white light (400–760 nm, intensity 3000 lux) irradiation reduces nisin’s antibacterial activity by 55%, compared to only 18% reduction in the control group without riboflavin.

Infrared rays (> 760 nm): With low energy, they mainly produce thermal effects and barely damage nisin’s molecular structure directly. Their impact on nisin’s antibacterial efficacy is negligible.

2. Light Intensity and Duration: "Dose-Dependent" Destruction—Higher Intensity and Longer Duration Lead to More Significant Activity Attenuation

Light-induced damage to nisin follows a "dose effect": the higher the product of light intensity (photon flux per unit area) and irradiation duration (total light dose), the more significant the attenuation of nisin’s antibacterial activity.

For example, in chilled meat products packaged in transparent plastic:

Under low-light conditions (indoor natural light, intensity 500 lux), after 14 days of irradiation, nisin’s inhibition rate against Clostridium botulinum decreases from 98% to 75%, still maintaining basic antibacterial efficacy.

Under high-light conditions (simulated outdoor sunlight, intensity 8000 lux), the inhibition rate drops to 40% after only 3 days, failing to control bacterial proliferation; if irradiation continues for 7 days, the inhibition rate falls below 10%, resulting in complete loss of antibacterial activity.

Furthermore, this "dose dependence" is more pronounced in acidic foods—acidic environments (e.g., fruit juices with pH < 4.0) make nisin’s peptide bonds more susceptible to light-induced breakage. Under the same light dose, the activity attenuation rate of nisin in acidic systems is 20%–30% faster than in neutral systems (pH 6.0–7.0).

3. Food Matrix: Fats, Proteins, and Antioxidants Regulate the Extent of Light-Induced Damage to Nisin

Components of the food matrix influence light-induced damage to nisin through "protective" or "synergistic" effects:

Protective effect of fats and proteins: Fats in food (e.g., milk fat in dairy products, animal fat in meat products) can adsorb nisin molecules to form "fat-nisin complexes," reducing direct contact between light and nisin. Proteins (e.g., whey protein, soy protein) bind to nisin via hydrogen bonds, stabilizing its spatial structure and reducing photooxidative damage to thioether bonds. For example, after 4 hours of UV-B irradiation, the activity retention rate of nisin in whole milk is 65%, compared to only 42% in skim milk.

Mitigating effect of antioxidants: Natural or synthetic antioxidants added to food (e.g., vitamin E, tea polyphenols, tert-butylhydroquinone (TBHQ)) can scavenge ROS generated by light, reducing oxidative damage to thioether bonds. Studies have shown that in beverages containing 0.02% tea polyphenols, the activity retention rate of nisin increases from 35% to 62% after 7 days of white light irradiation, significantly alleviating light-induced activity attenuation.

Synergistic effect of metal ions: Transition metal ions (e.g., iron, copper) in food (such as trace iron ions in canned foods) catalyze the generation of ROS under light, accelerating the oxidative degradation of nisin. For example, in fruit and vegetable juices containing 5 mg/L Fe³+, the activity attenuation rate of nisin after 2 hours of UV-A irradiation is 28% higher than that in the control group without Fe³+.

III. Protection Strategies for Nisin’s Antibacterial Efficacy Against Light Influence

To address light-induced damage to nisin, three types of strategies—"blocking light contact," "optimizing food formulation," and "controlling storage environment"—can be adopted to protect its antibacterial activity. Specific measures are as follows:

1. Using Light-Protective Packaging to Block Direct Contact Between Light and Nisin

Selecting packaging materials with light-blocking properties is the most direct and effective protection method. For example:

For transparent beverages and dairy products, use brown or black PET bottles (which can block over 90% of ultraviolet rays and more than 50% of visible light) or add light-blocking coatings (e.g., titanium dioxide coatings) inside transparent packaging.

For chilled meat products and baked goods, use aluminum foil composite packaging (which completely blocks ultraviolet rays and visible light) to prevent light from penetrating the packaging and acting on nisin.

For short-circulation foods (e.g., fresh-cut fruits and vegetables), use opaque kraft paper packaging to balance cost and light-protective effects.

2. Optimizing Food Formulation by Adding ROS Scavengers or Stabilizers

Adjusting food formulations to reduce light-induced damage to nisin:

Add antioxidants (e.g., tea polyphenols, vitamin C, addition level 0.01%–0.05%) to scavenge ROS generated by light and protect nisin’s thioether bonds.

Control the content of metal ions by adding chelating agents (e.g., sodium citrate, addition level 0.02%–0.03%) to bind iron and copper ions, inhibiting their role in catalyzing ROS generation.

Use proteins or polysaccharides (e.g., whey protein, pectin) to form complexes with nisin, enhancing its tolerance to light. For example, adding 0.1% whey protein to fruit juice can increase the activity retention rate of nisin after light irradiation by more than 30%.

3. Controlling Storage and Circulation Environments to Reduce Light Dose Exposure

Minimize nisin’s exposure to light in terms of time and intensity during food storage and circulation:

Avoid direct sunlight during storage; place food in cool, light-protected areas (e.g., warehouse shelves away from windows, refrigerated truck compartments with light-protective designs).

In retail, install ultraviolet-filtering glass in transparent display cabinets or use LED cold light sources (mainly long-wave visible light with low energy) to reduce damage to nisin.

Shorten the time products stay in light environments. For example, fresh-cut fruits and vegetables adopt a circulation model of "light-protective packaging + rapid cold chain" to avoid long-term exposure.