Nisin in the preservation of seafood products

I. Antibacterial Mechanism and Advantages of Nisin in Seafood Preservation

As a natural antibacterial peptide, Nisin (nisin) specifically inhibits common Gram-positive spoilage bacteria (such as bacilli, Listeria) and pathogenic bacteria (such as Staphylococcus aureus) in seafood. Its mechanism involves damaging the Lipid II structure of bacterial cell membranes, causing intracellular substance leakage and inhibiting reproduction. Compared with chemical preservatives (such as sulfites, potassium sorbate), Nisin has the following advantages:

High safety: Classified as a GRAS substance by institutions such as FDA and EFSA, with no risk of chemical residues, suitable for halal and organic seafood products;

Synergistic preservation potential: It can be compounded with natural components such as tea polyphenols, chitosan, and organic acids to form a synergistic system of "biological preservation + barrier protection", making up for its weak inhibition effect on Gram-negative bacteria (such as Pseudomonas, vibrio).

II. Challenges of Seafood Characteristics to Nisin Application

High Water Activity and Complex Microflora

Seafood muscle tissue has a water content of 75% to 85%, and the carried microorganisms include psychrophiles (such as Pseudomonas), halophiles (such as vibrio) and bacilli, among which Gram-negative bacteria account for more than 60%, requiring combination with other antibacterial means for control.

High pH and Enzyme Activity

The muscle pH of most seafood (such as fish and shellfish) rises from weakly acidic (6.0-6.5) to neutral or weakly alkaline (7.0-7.5) after death, while the activity of Nisin decreases significantly under neutral conditions (the antibacterial efficiency at pH 7.0 is more than 50% lower than that at pH 5.0). At the same time, the activity of endogenous proteases and oxidases in seafood increases, accelerating quality deterioration.

Cold Chain Fluctuation and Storage Environment

Seafood is susceptible to temperature fluctuations during fishing and transportation. The stability of Nisin decreases at room temperature (the half-life is about 72 hours at 25°C, and extends to 15 days at 4°C), so preservation technology needs to be matched to maintain activity.

III. Optimization Strategies for Nisin Application Technology

1. Design of Compound System: Enhancing Broad-Spectrum Antibacterial Properties

Nisin + Organic Acid (Lactic Acid/Citric Acid)

Mechanism: Organic acids reduce the environmental pH to 5.0-6.0, which on the one hand enhances the permeability of Nisin to the outer membrane of Gram-negative bacteria, and on the other hand inhibits the activity of endogenous enzymes in seafood. For example, in oyster preservation, soaking treatment with a compound solution of 0.5% lactic acid + 200 mg/L Nisin can reduce the total number of psychrophiles by 3 log levels, and extend the shelf life to 12 days (storage at 4°C).

Precautions: The concentration of organic acids should be controlled at 0.5%-1.0% to avoid damaging the tenderness of seafood; spray or vacuum permeation can be used to improve the uniformity of permeation.

Nisin + Natural Antibacterial Peptide (Chitosan/Lysozyme)

Synergistic effect: Chitosan (1%-2%) forms an antibacterial film to reduce Nisin loss and inhibit fungal growth; lysozyme (50-100 mg/L) 破坏 the cell wall of Gram-positive bacteria, producing a double attack of "membrane-cell wall" with Nisin. For example, in salmon slices, combined treatment (200 mg/L Nisin + 1% chitosan + 80 mg/L lysozyme) can maintain the total viable count <5×10³ CFU/g within 15 days, which is better than single Nisin treatment (viable count <1×10⁴ CFU/g).

Nisin + Tea Polyphenols/Rosemary Extract

Synergy of antioxidant and antibacterial: Tea polyphenols (200-300 mg/L) inhibit lipid oxidation by scavenging free radicals, and can damage the integrity of bacterial cell membranes when compounded with Nisin. In the preservation of hairtail, this compound system keeps the TVB-N (total volatile basic nitrogen) value <20 mg/100g after 10 days of storage at 4°C, meeting the first-class freshness standard.

2. Optimization of Dosage Form and Processing Technology: Improving Nisin Utilization Efficiency

Preparation of Nano Carriers

Chitosan-sodium alginate nanospheres embedding: Nisin is embedded in nanospheres (particle size 50-100 nm), and the stability is improved through electrostatic adsorption and ion cross-linking. For example, the activity retention rate of embedded Nisin in seawater at 4°C for 7 days reaches 85%, which is significantly higher than that of free Nisin (retention rate 50%), and the slow release characteristics can prolong the antibacterial effect.

Liposome embedding technology: Nisin liposomes are prepared by using lecithin, and the peptide chain structure is protected through hydrophobic interaction, which is suitable for fried or high-temperature processed seafood (such as ready-to-eat shrimp). The activity retention rate after sterilization at 121°C is increased from 30% to 60%.

Innovation of Processing Methods

Vacuum permeation treatment: In the preservation of squid slices, vacuum permeation (-0.08 MPa, 20 minutes) is used to make Nisin solution (300 mg/L) penetrate into the deep muscle, and the penetration depth is increased by 2 times compared with the soaking treatment (atmospheric pressure), and the antibacterial effect in the central part is increased by 40%.

Combination of spray-coating: First spray Nisin solution (200 mg/L) on shrimp, and then coat a composite film containing 0.5% chitosan to form a dual-effect system of "outer membrane barrier + internal release", which can inhibit the growth of surface and internal microorganisms during storage at 4°C, and extend the shelf life to 15 days.

3. Environmental Control and Cold Chain Matching

pH Regulation and Buffer System

In fish preservation, adding phosphate (0.2%-0.5%) to maintain muscle pH at 6.0-6.5 can not only delay the activity decay of Nisin, but also inhibit endogenous proteases. For example, in the preservation of large yellow croaker, the muscle hardness of the treatment group with 0.3% sodium dihydrogen phosphate + 200 mg/L Nisin is increased by 25% compared with the control group after 14 days of storage at 4°C, and the Nisin activity retention rate reaches 70%.

Precise Control of Cold Chain Temperature

The antibacterial effect of Nisin is positively correlated with temperature (the antibacterial efficiency at 4°C is 2-3 times that at 25°C), so cold storage agents and temperature recorders are needed to ensure that the storage temperature of seafood is stable at 0-4°C. Studies have shown that temperature fluctuation (4°C±2°C) will cause a daily loss of 5% of Nisin activity in sea bass preservation, while the loss rate under constant temperature (4°C±0.5°C) is <1%.

4. Differentiated Applications for Different Seafood Categories

Fish preservation: Taking chilled sea bass as an example, the combined process of "ice-temperature storage (0°C) + Nisin soaking (200 mg/L) + oxygen barrier packaging" can control the total number of colonies below 5×10⁵ CFU/g within 21 days, the TVB-N value <30 mg/100g, and the Nisin residue in fish meat <10 mg/kg (far lower than the safety threshold specified by FAO/WHO).

Shrimp preservation: For whiteleg shrimp, the ice coat treatment (ice layer containing 0.5% compound preservative) of Nisin (150 mg/L) compounded with ε-polylysine (50 mg/L) can inhibit the resuscitation of psychrophiles during frozen storage at -18°C, and the thawing juice loss rate of shrimp meat after 6 months is reduced by 30% compared with the control group.

Shellfish preservation: For oysters, the technology of "Nisin spray (250 mg/L) + chitosan-tea polyphenol coating (1% chitosan + 0.3% tea polyphenols)" is developed, which can block microbial pollution in the air and inhibit the protease secreted by oysters themselves during storage at 4°C, so that the content of umami amino acids (such as glutamic acid) remains stable within 10 days.

IV. Key Problems and Solutions in Practical Applications

Risk of Microbial Drug Resistance

Long-term single use of Nisin may induce bacterial resistance (such as by changing the Lipid II structure). The solution is to adopt the strategy of "intermittent use of Nisin + compound preservatives", and alternately use Nisin and lysozyme for each batch of seafood to reduce the selective pressure of drug resistance.

Influence on Seafood Flavor

High-concentration Nisin (>300 mg/L) may react with free amino acids in seafood to produce a slight bitter taste. Nano-embedding can be used to reduce the concentration of free Nisin, or 0.1%-0.2% sweeteners (such as trehalose) can be added to cover the peculiar smell.

Cost Control

The raw material cost of Nisin is high, and the production cost can be reduced through large-scale fermentation (such as using whey wastewater as the culture medium). At present, the combination of compound technology can control the increase of seafood preservation cost within 0.5-1 yuan/kg.

V. Future Research Directions

Genetic Engineering Modification of Nisin

By mutating the Nisin gene, its antibacterial activity against Gram-negative bacteria is enhanced (such as introducing outer membrane penetrating peptide sequences), and the inhibition efficiency against E. coli has been increased by 40% in the laboratory.

Intelligent Responsive Preservation System

Develop a pH/temperature dual-responsive Nisin release carrier, which can quickly release Nisin when the pH rises or the temperature fluctuates due to seafood spoilage, so as to achieve precise preservation of "release on demand".

Combination with Non-thermal Processing Technology

Combine Nisin with high pressure processing (HPP, 300-400 MPa), use high pressure to damage the bacterial cell membrane and promote Nisin penetration, which can shorten the sterilization time from 30 minutes to 5 minutes in ready-to-eat seafood, while retaining more nutrients.

Conclusion

The application of Nisin in seafood preservation needs to combine product characteristics, microbial ecology and storage conditions, and achieve efficient preservation through compound system, dosage form optimization and cold chain coordination, providing a sustainable solution for green seafood preservation technology.