The preservative application of Nisin in cooked food products

As a natural peptide bacteriostatic agent, Nisin is widely used in the processing of ready-to-eat foods such as sauce-braised products, smoked and roasted products, and prepared dishes, thanks to its characteristics of narrow-spectrum high efficiency, safety, non-toxicity, and no impact on product flavor. Its core function is to target and inhibit Gram-positive spoilage bacteria and foodborne pathogens, extend product shelf life, and conform to the clean-label trend in food processing. The preservative application of Nisin in ready-to-eat foods requires systematic design based on product characteristics, covering aspects such as action targets, application schemes, and influencing factors, which are detailed as follows:

I. Preservative Targets and Application Value in Ready-to-Eat Foods

Post-processing ready-to-eat foods are prone to spoilage caused by residual microbial growth and face the risk of foodborne pathogen contamination. Nisin exerts targeted preservative effects to address these issues effectively:

Inhibiting Foodborne Pathogens and Reducing Safety RisksThe main safety hazards of ready-to-eat foods (especially low-temperature ready-to-eat products) stem from Gram-positive bacteria such as Clostridium botulinum, Staphylococcus aureus, and Listeria monocytogenes. Clostridium botulinum can produce lethal botulinum toxin and tends to germinate and proliferate in the low-acid, anaerobic storage environment of ready-to-eat foods. Listeria monocytogenes has cold tolerance and can grow even under refrigerated conditions, making it a key target for prevention and control in ready-to-eat foods. Nisin exerts high-efficiency inhibitory effects on these pathogens by damaging bacterial cell membranes and inhibiting cell wall synthesis. Experimental data show that adding 50–100 mg/kg of Nisin to sauce-braised beef can reduce the count of Listeria monocytogenes by 3–5 log orders under refrigerated conditions; its addition to vacuum-packaged braised chicken legs can completely inhibit the germination of Clostridium botulinum spores, eliminating the risk of toxin production.

Controlling Spoilage Bacteria Proliferation and Extending Shelf LifeSpoilage bacteria such as Bacillus cereus, Bacillus subtilis, and Brochothrix thermosphacta in ready-to-eat foods cause product stickiness, off-flavors, and gas production, which are the main factors shortening shelf life. Nisin exhibits better inhibitory effects against such spore-forming bacteria than chemical preservatives like potassium sorbate, with a particularly prominent inhibitory effect during the spore germination stage. For example, adding Nisin to high-temperature sterilized braised dried tofu can extend the shelf life from 15 days to 30 days under room-temperature storage; in incompletely sterilized smoked sausages, the combination of Nisin and low-temperature sterilization process can prolong the refrigerated shelf life from 7 days to 21 days without altering the product’s color and taste.

Reducing Chemical Preservative Dosage and Meeting Clean-Label RequirementsTraditional ready-to-eat food processing often relies on chemical preservatives such as potassium sorbate and sodium dehydroacetate, while consumers’ demand for "low-additive" and "naturally additive" products is growing. Nisin can replace 30%–50% of chemical preservatives, and its natural fermentation origin meets clean-labeling requirements, helping to enhance product market competitiveness.

II. Application Schemes and Process Essentials in Ready-to-Eat Foods

The preservative effect of Nisin in ready-to-eat foods depends on factors such as dosage, addition timing, and compounding schemes, and targeted schemes should be formulated according to product types and processing technologies:

Compliant Dosage ControlAccording to China’s National Food Safety Standard for the Use of Food Additives (GB 2760-2021), the maximum allowable dosage of Nisin is 0.5 g/kg in cooked meat products and 0.1 g/kg in cooked soybean products. Practical applications require adjustments based on product characteristics:

High-temperature sterilized ready-to-eat foods (e.g., canned braised foods, high-temperature sterilized prepared dishes): Partial Nisin activity is lost during sterilization, so the dosage can be controlled at 200–500 mg/kg;

Low-temperature ready-to-eat foods (e.g., sauce-braised beef, smoked chicken, ready-to-eat salads): No high-intensity heat treatment is required, and a dosage of 50–150 mg/kg is sufficient to achieve ideal bacteriostatic effects;

Cooked soybean products (e.g., braised dried tofu, yuba): The high protein content in the matrix easily binds to Nisin and reduces its activity, so the dosage needs to be close to the limit (80–100 mg/kg).

Optimizing Addition Processes to Improve Activity Utilization RateNisin activity is susceptible to processing conditions, and reasonable addition processes are required to minimize activity loss:

Dissolution method: Nisin is poorly soluble in water but highly soluble in acidic solutions. Before use, it should be dissolved in 0.02 mol/L hydrochloric acid or citric acid solution to prepare a 1%–2% stock solution before addition, avoiding clumping caused by direct addition to ready-to-eat foods;

Addition timing: Priority should be given to adding Nisin in the late marinating stage, before sterilization, or during the cooling phase after sterilization. Adding it in the late marinating stage ensures uniform distribution of Nisin in the product matrix; adding it during the cooling phase after sterilization minimizes Nisin activity damage caused by high temperatures. For example, adding Nisin when braised products are cooled to below 60°C after sterilization can retain over 90% of its activity;

Uniform distribution: For block-shaped ready-to-eat foods (e.g., sauce-braised beef, braised chicken legs), Nisin can be added via injection or immersion to ensure penetration into the product interior; for sauce-braised soup, the Nisin stock solution can be directly added to the soup and stirred evenly, allowing the product to fully absorb Nisin through soaking.

Compound Application to Broaden Antibacterial SpectrumNisin is only effective against Gram-positive bacteria and has weak inhibitory effects on Gram-negative bacteria (e.g., Escherichia coli, Salmonella), molds, and yeasts that may be present in ready-to-eat foods. Compound application is required to enhance broad-spectrum bacteriostatic activity:

Compounding with EDTA: EDTA can chelate calcium ions in the outer membrane of Gram-negative bacteria, disrupt outer membrane integrity, and allow Nisin to enter bacterial cells and exert its effects. The compound system (ratio 2:1) can significantly improve inhibitory effects against Escherichia coli and Salmonella, making it suitable for sauce-braised products and prepared dishes that are susceptible to Gram-negative bacterial contamination;

Compounding with lactic acid or citric acid: Acidic substances can lower the product pH, enhance Nisin stability and bacteriostatic activity, and inhibit mold growth, making it suitable for low-acidity smoked and roasted products;

Compounding with other natural preservatives: Combining Nisin with ε-polylysine and lysozyme can achieve comprehensive coverage of "Gram-positive bacteria + Gram-negative bacteria + fungi" and meet clean-label requirements, making it suitable for high-end ready-to-eat foods.

III. Key Factors Affecting the Preservative Effect of Nisin in Ready-to-Eat Foods

The bacteriostatic effect of Nisin is influenced by ready-to-eat food matrix components, processing conditions, storage environments, etc., and adverse factors need to be avoided in a targeted manner:

Product Matrix Components

Proteins and fats: Proteins and fats in ready-to-eat foods can bind to Nisin, reduce its free concentration, and weaken the bacteriostatic effect. In high-fat ready-to-eat foods (e.g., fatty cured meat, fried ready-to-eat foods), Nisin activity loss can reach 30%–50%, requiring appropriate dosage increases; high-protein soybean products also need increased dosages to compensate for binding losses;

Salt concentration: Salt in ready-to-eat foods (usually 2%–5% concentration) can produce a synergistic bacteriostatic effect with Nisin—high salt disrupts bacterial cell membrane osmotic pressure, facilitating Nisin to form transmembrane pores. However, excessively high salt concentration (>8%) can cause Nisin molecule aggregation, reduce solubility and activity, so salt concentration should be controlled within a reasonable range.

Processing Temperature and pH Value

Temperature: Nisin has good thermal stability, retaining over 70% of its activity after autoclaving at 121°C for 15 min, making it suitable for steaming, boiling, and sterilization processes of ready-to-eat foods. However, prolonged high-temperature treatment (e.g., autoclaving at 121°C for more than 30 min) can cause significant activity loss, requiring supplementary addition after sterilization;

pH value: Nisin exhibits the highest activity in acidic environments (pH 3.0–6.0). Fermentation or acidification treatment of ready-to-eat foods (e.g., natural fermentation of sauce-braised products) can enhance its bacteriostatic effect; neutral or alkaline ready-to-eat foods (e.g., some braised foods) need appropriate dosage increases or compounding with acidic substances to adjust pH.

Storage Environment

Temperature: Low-temperature storage can significantly extend the preservative validity period of Nisin. Under refrigerated conditions (0–4°C), the activity half-life of Nisin in ready-to-eat foods is 2–3 times longer than that at room temperature;

Packaging method: Vacuum packaging or modified atmosphere packaging (filled with nitrogen/carbon dioxide) can isolate oxygen, reduce Nisin oxidative inactivation, inhibit the growth of aerobic spoilage bacteria, and synergize with Nisin to extend shelf life.

IV. Application Advantages and Limitations

Core Advantages

Safety and no residue: Nisin can be decomposed into amino acids by proteases in the human digestive tract, with no cumulative toxicity, making it safe for consumption by special populations such as infants and pregnant women;

No impact on product flavor: Within the compliant dosage range, Nisin does not alter the color, taste, or aroma of ready-to-eat foods, outperforming chemical preservatives that may cause off-flavors;

Compatibility with multiple processing technologies: It can be combined with high-temperature sterilization, low-temperature sterilization, marination, vacuum packaging, and other processes, with strong compatibility.

Limitations

Narrow antibacterial spectrum: It is only effective against Gram-positive bacteria, requiring compounding with other preservatives to achieve comprehensive preservation;

Relatively high cost: Nisin is more expensive than chemical preservatives, so large-scale applications need to reduce dosage through compounding to control costs;

Activity susceptibility to matrix effects: Significant activity loss occurs in high-protein and high-fat ready-to-eat foods, requiring optimized addition schemes.

V. Typical Application Case

A sauce-braised product enterprise adopted a preservative scheme of "low-temperature sterilization + Nisin + EDTA" in the production of vacuum-packaged sauce-braised beef: 100 mg/kg of Nisin was compounded with 50 mg/kg of EDTA and added in the late marinating stage, followed by low-temperature sterilization at 85°C for 30 min and vacuum packaging. The product’s shelf life under 4°C refrigeration was extended from the original 7 days to 28 days, with no detection of pathogenic bacteria such as Listeria monocytogenes and Staphylococcus aureus. The product flavor showed no significant difference compared with the control group without preservatives, and the dosage of chemical preservatives was successfully reduced by 40%, meeting the market demand for clean labels.