The antibacterial effect of Nisin in pickled foods

Nisin, a natural peptide bacteriostatic agent produced by the fermentation of Streptococcus lactis, is characterized by narrow-spectrum high efficiency, safety, non-toxicity, and good thermal stability. In pickled foods, it exerts bacteriostatic effects mainly against Gram-positive bacteria (G⁺), effectively inhibiting the proliferation of spoilage bacteria and contamination by foodborne pathogens, extending the shelf life of products, and conforming to the clean-label trend in food processing. Its bacteriostatic efficacy and application essentials are as follows:

I. Core Bacteriostatic Mechanism of Nisin

The bacteriostatic action of Nisin targets the cell membrane of Gram-positive bacteria and destroys bacterial structure and metabolism through three sequential steps:

Adsorption and Binding: The cationic peptide segments in Nisin molecules can bind to anionic phospholipids (e.g., teichoic acid) on the cell membrane of Gram-positive bacteria, anchoring to the membrane surface.

Pore Formation: After binding, the hydrophobic regions of Nisin insert into the phospholipid bilayer of the cell membrane and form transmembrane pores through molecular conformational changes.

Intracellular Substance Leakage: These transmembrane pores cause the outflow of small molecules such as potassium ions and protons from the bacterial cell, disrupting the osmotic balance and proton motive force of the cell membrane. This ultimately inhibits key metabolic processes of the bacterium, including DNA replication and protein synthesis, leading to bacterial death.

It should be noted that Nisin has extremely weak inhibitory effects on Gram-negative bacteria (G⁻), as the lipopolysaccharide layer of their outer membrane prevents Nisin molecules from contacting the cell membrane. However, when compounded with chelating agents such as EDTA, EDTA can disrupt the outer membrane structure of Gram-negative bacteria, enabling Nisin to exert a synergistic bacteriostatic effect.

II. Bacteriostatic Targets and Efficacy of Nisin in Pickled Foods

The spoilage and safety risks of pickled foods (e.g., pickled meat, pickled vegetables, sauce-braised products, pickled fish) mainly stem from Gram-positive bacteria, against which Nisin demonstrates significant inhibitory efficacy:

Against Foodborne Pathogens

Clostridium botulinum: This bacterium produces lethal botulinum toxin and tends to proliferate in low-acid pickled foods (e.g., pickled meat, ham). Adding 50–100 mg/kg of Nisin can effectively inhibit the germination of its spores and the growth of vegetative cells, reducing the risk of toxin contamination.

Staphylococcus aureus: It is prone to overgrowth and enterotoxin production in high-salt pickled foods. The minimum inhibitory concentration (MIC) of Nisin against this bacterium ranges from 0.5 to 5 μg/mL. Its addition can significantly reduce bacterial counts, preventing food poisoning.

Listeria monocytogenes: Its cold tolerance allows it to reproduce even in refrigerated pickled foods. Nisin can inhibit the formation of its cell membrane; at an addition level of 10–20 mg/kg, it can reduce the count of Listeria monocytogenes in refrigerated pickled products by 3–5 log orders.

Against Spoilage Bacteria

Spoilage bacteria such as Bacillus cereus and Bacillus subtilis in pickled foods can cause product stickiness, off-flavors, and gas production. Nisin exhibits better inhibitory effects against such spore-forming bacteria than chemical preservatives (e.g., potassium sorbate), with a particularly prominent ability to inhibit spore germination.

In acidic pickled foods such as pickled vegetables and preserved vegetables, it can inhibit the excessive proliferation of lactic acid bacteria (avoiding over-acidification of products) while not interfering with the normal metabolism of beneficial fermentative bacteria, ensuring stable product flavor.

III. Key Factors Affecting the Bacteriostatic Efficacy of Nisin in Pickled Foods

The bacteriostatic efficacy of Nisin is not constant but is affected by multiple factors such as the formulation and process parameters of pickled foods:

pH Value

Nisin has higher stability and bacteriostatic activity in acidic environments, with optimal efficacy observed at pH 3.0–6.0. The fermentation process of pickled foods naturally lowers the pH value (e.g., the pH of pickled vegetables drops to 3.5–4.5), which can enhance its bacteriostatic effect. In neutral pickled foods (e.g., some sauce-braised products), the addition level of Nisin needs to be appropriately increased to ensure efficacy.

Salinity and Water Activity (Aw)

The high-salt nature of pickled foods reduces water activity, producing a synergistic bacteriostatic effect with Nisin—high salt disrupts the osmotic pressure of bacterial cell membranes, facilitating the formation of transmembrane pores by Nisin. For example, in pickled meat with a salinity of 6%–8%, the bacteriostatic efficacy of Nisin is 20%–30% higher than that in a low-salt environment. However, excessively high salinity (>10%) can cause Nisin molecule aggregation, reducing its solubility and activity.

Temperature and Heat Treatment

Nisin has good thermal stability, retaining more than 70% of its activity after autoclaving at 121℃, making it suitable for the steaming, boiling, and sterilization processes of pickled foods. Heat treatment can damage the structure of bacterial cell membranes, making it easier for Nisin to exert its effects. Therefore, the combination of "heat treatment + Nisin" can significantly reduce the dosage of bacteriostatic agents and lower processing costs.

Food Matrix Components

Components such as proteins and fats in pickled foods can bind to Nisin, reducing its free concentration and impairing its bacteriostatic efficacy. For example, in high-fat pickled meat, part of the Nisin is adsorbed by fat globules, requiring an appropriate increase in addition level (20%–50% higher than that for low-fat products). In contrast, carbohydrates (e.g., sugar, starch) have no significant impact on Nisin activity.

IV. Application Strategies of Nisin in Pickled Foods

To maximize the bacteriostatic efficacy of Nisin, scientific application schemes should be adopted based on product characteristics:

Dosage Control

According to GB 2760 National Food Safety Standard for the Use of Food Additives, the maximum allowable addition level of Nisin is 0.5 g/kg in pickled meat products and 0.1 g/kg in pickled vegetables. In practical applications, it is recommended to adjust the dosage based on the pH and salinity of the product: 50–100 mg/kg for low-acid and high-salt pickled products, and 20–50 mg/kg for high-acid pickled products, which can achieve ideal efficacy.

Compound Application to Enhance Broad-Spectrum Activity

To target Gram-negative bacteria (e.g., Escherichia coli, Salmonella) that may be present in pickled foods, compounding Nisin with EDTA, ε-polylysine, lysozyme, etc., can achieve "synergistic bacteriostasis":

Nisin + EDTA (ratio 2:1): EDTA chelates calcium ions in the outer membrane of Gram-negative bacteria, disrupting outer membrane integrity and allowing Nisin to enter bacterial cells and exert its effects.

Nisin + potassium sorbate: For molds and yeasts in pickled vegetables, potassium sorbate can inhibit fungal growth, complementing Nisin and broadening the antibacterial spectrum.

Optimization of Addition Process

Dissolution Method: Nisin is easily soluble in acidic aqueous solutions. It should be dissolved in 0.02 mol/L hydrochloric acid before use to avoid clumping caused by direct addition to high-salt or high-fat matrices.

Addition Timing: Adding Nisin in the late stage of pickling or during the cooling phase after sterilization can reduce the binding loss of Nisin to the food matrix and avoid activity reduction caused by prolonged high-temperature heating.

V. Application Advantages and Limitations

Advantages

Natural and Safe: Nisin can be decomposed into amino acids by proteases in the human digestive tract, leaving no residues or toxicity, and conforms to the requirements of green food processing.

No Impact on Flavor: Within the recommended addition range, it does not alter the color, texture, or flavor of pickled foods, outperforming chemical preservatives.

Suitable for Clean Labels: It can replace some chemical preservatives, meeting consumers’ demand for "low-additive" foods.

Limitations

Narrow Antibacterial Spectrum: It is only effective against Gram-positive bacteria and needs to be compounded with other bacteriostatic agents.

Relatively High Cost: Fermentatively produced Nisin is more expensive than chemical preservatives, requiring dosage control for large-scale application.