Evaluation of the effect and mechanism of Nisin in the preservation of fruits and vegetables

As a natural antimicrobial peptide, Nisin offers an effective pathway for replacing chemical preservatives and achieving green preservation in fruits and vegetables. Leveraging its targeted antimicrobial activity, non-residue property, and excellent biocompatibility, Nisin delays decay and deterioration by inhibiting the growth of pathogenic and spoilage microorganisms and regulating the physiological metabolism of fruits and vegetables. Its preservation efficacy is influenced by fruit/vegetable type, microbial contamination profile, application method, and environmental conditions. The core mechanisms revolve around two dimensions: "antimicrobial action" and "physiological regulation," which are detailed as follows:

I. Core Mechanisms in Fruit and Vegetable Preservation

1. Targeted Inhibition of Spoilage and Pathogenic Microorganisms

Fruit and vegetable spoilage is primarily caused by fungi (e.g., Penicillium, Aspergillus, Botrytis cinerea) and bacteria (e.g., Escherichia coli, Salmonella, soft rot bacteria). Nisin inhibits these microorganisms through specific antimicrobial mechanisms, with particularly significant effects against Gram-positive bacteria (G⁺):

Bacterial Inhibition Mechanism: As a cationic antimicrobial peptide, Nisin specifically binds to Lipid II (a peptidoglycan precursor) on the cell membrane of G⁺ bacteria, forming transmembrane pores. This leads to the leakage of intracellular substances such as potassium ions and amino acids, disrupting the proton gradient and osmotic balance, and ultimately causing cell lysis. The minimum inhibitory concentration (MIC) of Nisin against common G⁺ spoilage bacteria (e.g., Bacillus subtilis, lactobacilli, streptococci) and pathogenic bacteria (e.g., Staphylococcus aureus, Listeria monocytogenes) in fruits and vegetables ranges from 0.05 to 5 μg/mL, significantly reducing the total viable count (TVC).

Synergistic Fungal Inhibition Mechanism: Nisin has weak direct inhibitory effects on fungi (e.g., Botrytis cinerea, Penicillium italicum), but exerts antifungal effects by disrupting fungal cell membrane integrity, inhibiting spore germination and mycelial growth, or synergizing with the intrinsic defense system of fruits and vegetables. For example, it can induce the production of phytoalexins, polyphenol oxidase (PPO), and other defense substances in fruits and vegetables to enhance fungal resistance. When combined with acidic environments (pH＜5.5) or plant extracts (e.g., tea polyphenols, carvacrol), Nisin can penetrate the fungal cell wall and improve inhibitory efficacy.

Spore Inhibition Mechanism: Against thermophilic spore-forming bacteria (e.g., Bacillus cereus) that may remain after fruit and vegetable processing, Nisin inhibits peptidoglycan synthesis during spore germination, preventing their proliferation into vegetative cells and reducing the risk of spoilage caused by secondary contamination after high-temperature treatment.

2. Regulating Physiological Metabolism to Delay Quality Deterioration

In addition to its antimicrobial activity, Nisin delays physiological senescence and quality decline by regulating respiration, ethylene synthesis, and the antioxidant system of fruits and vegetables:

Inhibiting Respiratory Intensity: Nisin reduces the activity of respiratory chain-related enzymes (e.g., cytochrome c oxidase) in fruit/vegetable mitochondria, decreasing the consumption of respiratory substrates and delaying the onset of the respiratory climacteric. For example, Nisin treatment reduces the respiratory intensity of strawberries by 20%–30%, minimizing the decomposition of nutrients such as sugars and organic acids.

Delaying Ethylene Synthesis: By inhibiting the activity of key ethylene synthesis enzymes (ACC synthase, ACC oxidase), Nisin reduces ethylene release, delaying the ripening and senescence of fruits and vegetables. For climacteric fruits such as tomatoes and bananas, it can delay coloring and softening, extending shelf life.

Enhancing Antioxidant Capacity: Nisin promotes the synthesis of antioxidant substances (e.g., glutathione (GSH), superoxide dismutase (SOD), catalase (CAT)) in fruits and vegetables, reducing malondialdehyde (MDA) content and minimizing damage to cell membranes caused by reactive oxygen species (ROS). This delays water loss, browning, and texture deterioration.

II. Preservation Efficacy Evaluation in Different Types of Fruits and Vegetables

Nisin’s preservation efficacy varies significantly among fruits and vegetables due to differences in moisture content and epidermal structure. The efficacy and optimal application methods for different types (leafy greens, fruit vegetables, berries, root vegetables) are as follows:

1. Berry Fruits (Strawberries, Blueberries, Grapes, Bayberries)

Characteristics: Thin epidermis, high moisture content (85%–95), susceptible to fungal (gray mold, green mold) and bacterial contamination, with spoilage, water loss, and browning occurring within 2–3 days at room temperature.

Preservation Efficacy: Soaking or spraying with 0.1–0.5 g/L Nisin solution (5–10 minutes), combined with refrigeration (4℃) and vacuum packaging, extends the shelf life of strawberries from 2–3 days to 7–10 days, and that of blueberries and grapes by 50%–80%. Spoilage rate is reduced by 60%–70%, water loss rate is controlled below 5%, and fruit firmness, color, and total soluble solids (TSS) content are well-maintained without significant off-odors.

Key Points: Combine with antioxidants (e.g., vitamin C, tea polyphenols) to inhibit browning; avoid high Nisin concentrations (>0.5 g/L) to prevent slight bitterness on the fruit surface.

2. Leafy Greens (Spinach, Lettuce, Romaine Lettuce)

Characteristics: Tender leaves, high moisture content, susceptible to bacterial (soft rot bacteria, Escherichia coli) and fungal (downy mildew) infection, with yellowing, wilting, and rotting occurring within 1–2 days at room temperature.

Preservation Efficacy: Soaking in 0.05–0.2 g/L Nisin solution (3–5 minutes), draining, and combining with modified atmosphere packaging (MAP: 5%–10% CO₂, 2%–5% O₂) and refrigeration (0–4℃) extends the shelf life of spinach and lettuce from 1–2 days to 5–7 days. Yellowing rate is reduced by 50%–60%, spoilage rate is controlled below 10%, and losses of chlorophyll and vitamin C are reduced by 30%–40%.

Key Points: Avoid prolonged soaking (>5 minutes) to prevent mechanical damage from excessive water absorption; combining with 0.5%–1% salt enhances Nisin’s bacterial inhibitory efficacy.

3. Fruit Vegetables (Tomatoes, Cucumbers, Bell Peppers)

Characteristics: Climacteric (tomatoes) or non-climacteric (cucumbers, bell peppers) with relatively thick but easily damaged epidermis. Main spoilage microorganisms include fungi (green mold, blue mold) and bacteria (soft rot bacteria).

Preservation Efficacy: Spraying green-ripe tomatoes with 0.1–0.3 g/L Nisin solution and storing at room temperature (20–25℃) extends shelf life from 5–7 days to 10–14 days, delaying reddening and softening. Treating cucumbers and bell peppers with 0.2–0.4 g/L Nisin and refrigerating (10℃) extends shelf life by 40%–60%, reduces spoilage rate by over 50%, and maintains crisp texture and color.

Key Points: Control Nisin concentration (<0.3 g/L) for climacteric fruits to avoid inhibiting normal ripening; avoid low-temperature refrigeration (<8℃) for thermophilic vegetables (cucumbers, bell peppers) to prevent chilling injury, which Nisin can alleviate.

4. Root Vegetables (Carrots, Potatoes, Onions)

Characteristics: Thick epidermis, low moisture content (60%–80), good storage stability but susceptible to fungal (dry rot bacteria, Penicillium) contamination, with sprouting and rotting occurring during room-temperature storage.

Preservation Efficacy: Soaking in 0.1–0.3 g/L Nisin solution (10–15 minutes), air-drying, and storing in well-ventilated conditions at room temperature (15–20℃) extends the shelf life of carrots and potatoes by 30%–50%. Sprouting rate is reduced by 40%–50%, spoilage rate is controlled below 8%, and starch content and firmness of tubers remain stable.

Key Points: Thoroughly air-dry after soaking to prevent mold growth from moist epidermis; combining with 0.5%–1% calcium chloride enhances epidermal cell wall stability and reduces water loss.

III. Key Factors Influencing Nisin’s Preservation Efficacy

1. Intrinsic Characteristics of Fruits and Vegetables

Epidermal Structure: Fruits/vegetables with thick epidermis and intact waxy layers (e.g., apples, oranges) have weak Nisin adsorption and penetration; scratching the epidermis or extending treatment time improves efficacy. Those with thin, wax-free epidermis (e.g., strawberries, spinach) are more sensitive to Nisin, showing better preservation effects.

pH Value: Nisin’s antimicrobial activity is enhanced at fruit/vegetable juice pH＜5.5 (e.g., strawberries, lemons), allowing reduced dosage. At pH＞6.0 (e.g., cucumbers, carrots), Nisin activity declines; increasing concentration or combining with acidulants (e.g., citric acid) is required.

2. Application Method and Concentration

Concentration Regulation: The optimal concentration range is 0.05–0.5 g/L. Concentrations below 0.05 g/L are ineffective, while concentrations above 0.5 g/L may affect flavor (e.g., bitterness) and increase costs.

Treatment Methods: Soaking (suitable for berries, leafy greens) ensures uniform efficacy but may cause water absorption damage; spraying (suitable for fruit vegetables, root vegetables) is convenient and reduces mechanical damage; coating (Nisin combined with chitosan, sodium alginate) forms a sustained-release antimicrobial layer for longer-lasting preservation.

3. Storage Environmental Conditions

Temperature: Low temperatures (0–4℃) synergize with Nisin to inhibit microbial growth and fruit/vegetable metabolism, enhancing efficacy. At room temperature, vacuum packaging or MAP compensates for reduced Nisin activity due to higher temperatures.

Humidity: High humidity (85%–95%) reduces water loss and wilting but promotes mold growth; balancing humidity and antimicrobial efficacy is critical. Combining Nisin with desiccants (e.g., silica gel) controls storage humidity.

Gas Composition: MAP (low O₂, high CO₂) inhibits aerobic microbial growth and fruit/vegetable respiration, synergizing with Nisin to extend shelf life—particularly effective for leafy greens and berries.

4. Synergistic Effects of Composite Systems

Nisin’s efficacy is significantly enhanced when combined with other preservatives:

Combination with Plant Extracts: Tea polyphenols, carvacrol, and clove essential oil expand the antimicrobial spectrum, enhance antifungal effects, and improve antioxidant capacity to delay browning.

Combination with Polysaccharides: Chitosan, sodium alginate, and pectin form antimicrobial coatings, extending Nisin retention on the fruit/vegetable surface for sustained efficacy.

Combination with Chemical Preservatives: Potassium sorbate and sodium dehydroacetate reduce chemical preservative dosage by 30%–50%, improving safety and expanding the antimicrobial spectrum.

IV. Application Advantages and Limitations

1. Core Advantages

Natural and Safe: Derived from microbial fermentation, non-toxic, residue-free, and metabolized into amino acids. It meets green food and food safety requirements, replacing chemical preservatives (e.g., sodium benzoate, thiabendazole).

Good Biocompatibility: No antagonism with fruit/vegetable components (proteins, carbohydrates, vitamins), preserving original flavor, color, and nutrients.

Heat Resistance: Retains over 80% activity after heating at 121℃ for 30 minutes, compatible with preprocessing technologies such as blanching and sterilization.

Broad Synergistic Potential: Works with refrigeration, MAP, and coating technologies to enhance preservation efficacy and stability.

2. Limitations

Limited Antimicrobial Spectrum: Weak inhibitory effects on Gram-negative bacteria (e.g., Escherichia coli, Salmonella) and fungi; supplementary measures (composite formulations, acidic environments) are required.

Environmental Sensitivity: Alkaline conditions (pH＞6.0), high temperatures (>50℃), and proteases (endogenous to fruits/vegetables) reduce Nisin activity.

Insufficient Sustained Efficacy: Single Nisin treatment loses activity due to surface moisture evaporation and microbial degradation; coating or packaging extends its action time.

Higher Cost: High-purity Nisin is more expensive than traditional chemical preservatives; optimizing composite systems reduces costs for large-scale applications.

V. Optimized Application Strategies and Future Directions

1. Optimized Application Technologies

Microencapsulation: Encapsulating Nisin in chitosan or sodium alginate improves stability in alkaline and high-temperature environments, enabling controlled release and extending preservation cycles.

Composite Coating Preservatives: Nisin combined with polysaccharides, plant extracts, and antioxidants forms multifunctional coatings with antimicrobial, antioxidant, and water-retention properties, suitable for various fruits and vegetables.

Precision Processing Parameters: Optimizing Nisin concentration, treatment time, and composite ratios based on fruit/vegetable type and storage conditions achieves "personalized preservation."

2. Future Development Directions

Genetic Engineering Modification: Modifying Nisin’s molecular structure via genetic engineering enhances inhibitory effects on fungi and Gram-negative bacteria, expanding the antimicrobial spectrum.

Integrated Synergistic Preservation Systems: Combining Nisin with intelligent packaging (antimicrobial packaging, humidity-controlled packaging) and cold chain logistics builds a full-chain preservation system from post-harvest treatment to end-point sales.

Low-Cost Production Processes: Optimizing Nisin fermentation and purification processes reduces production costs, promoting large-scale application in fruit and vegetable preservation.

Nisin exhibits excellent preservation efficacy in fruits and vegetables by targeting spoilage microorganisms and regulating physiological metabolism, effectively extending the shelf life of berries, leafy greens, fruit vegetables, and root vegetables while reducing spoilage and quality deterioration. Its core advantage of natural safety aligns with green consumption trends, but composite strategies, optimized application methods, and storage conditions are needed to address limitations such as limited antimicrobial spectrum and insufficient stability. With advancements in modification technologies, composite systems, and preservation processes, Nisin is expected to become a core technology in green fruit and vegetable preservation, supporting improved supply chain efficiency and reduced resource waste.